ELECTRICAL

ENGINEERING –EC T32

B.Tech./ ECE/ III SEMESTER

D.KARTHIKEYAN,
AP/EEE

Achariya College of Engineering Technology

(Approved by AICTE and affiliated to Pondicherry University)
An ISO 9001:2008 Certified Institution
Achariyapuram, Villianur, Puducherry – 605 110.
www.acet.edu.in

(For internal circulation only)

 EC T32 - ELECTRICAL ENGINEERING
UNIT - I
Transformers: Principle of operation – Single Phase transformer – Equivalent circuit –Regulation –
Losses and Efficiency – Introduction to 3 phase transformers –Autotransformers – pulse transformers.

UNIT - II
D.C. Machines: Construction, Principles of operation of DC Generators – types -EMFequation – No load
and Load characteristics of series and shunt generators – DC motor –Torque – Speed – Torque
characteristics of series and shunt motors – Speed control andapplication.

UNIT - III
A.C. Machines: Principle of operation of 3-phase Induction Motor – Torque, slipscharacteristics – Speed
control methods – Single-phase Induction motor starting methods –Braking concepts.

UNIT - IV
Special Machines: servo motor – DC and AC servomotors; stepper motors – variablereluctance and
permanent magnet stepper motors; single phase synchronous motor –reluctance motor and hysteresis
motor – universal motor – Linear induction motor(LIM) –linear synchronous motor(LSM).

UNIT - V
Utilisation: Domestic wiring – principle of electrical heating – The law of illumination –Electric lamps –
Photometers – Electroplating – Electric Traction – Air conditioning –Earthing.

Text Books:
1. B.L. Theraja, “Electrical Technology Vol.II AC/DC Machines”, S. Chand, 2008
2. I.J. Nagrath and D.P. Kothari, “Electric machines”, Tata McGraw Hill PublishingCompany Ltd., New
Delhi, 3rd Edition, 2008.

Reference Books:
1. Battacharya S K, “Electrical Machines”, Technical Teachers Training institute”, 2ndedition.2003.
2. J.B.Gupta,“Theory and Performance of Electrical Machines”, J.K.Kataria & Sons, 13thedition, 2004.
3. S.L.Uppal, “Electrical power” Khanna Publications (p) Ltd, Delhi, 2002.
4. G.C.Garg, “Utilisation of Electric power and electric traction” Khanna Publications (p)Ltd, Delhi, 2006.
5. A.Chakrabarti, M.I.Soni, P.V.Gupta,“Textbook on ower sytems engineering”, DhanpatRai, 2008.
 TABLE OF CONTENT

SLNO TITLE PAGE

NUMBER
Syllabus
Unit – I Transformer 1
1.1 Definition of Transformer
1.2 Working principle of Transformer 1
1.3 Types of transformer 2
1.4 Construction 3
1.5 EMF equation of transformer 6
1.6 Transformation ratio of transformer 7
1.7. Voltage ratio of transformer 7
1.8. Turns ratio of transformer 7
1.9. Equivalent circuit of transformer 7
1 . 1 0 Transformer losses 10
1.11. Efficiency of transformer 11
1.12. Condition for maximum efficiency 12
1.13. All day efficiency 13
I
1.14. Auto transformer 13
1.15. Comparison between single three phase and bank of three single
15
phase transformers for three phase system
1.16 Three phase transformer connections 18
Unit-II DC Generator
19
2.1 Introduction to D.C Machines
2.2 Generator principle 19
2.3 Conversion of AC to DC 21
2.4 Constructional details 21
2.5 Types of generators
2.5.1separately excited DC generator
2.5.2 Self-excited DC generators
II 2.5.2.1.Series wound generator
2.5.2.2.Shunt wound DC generators 24
2.5.2.3 .Compound wound DC generator
2.5.2.3.1Short shunt compound wound DC generator
2.5.2.3.2 Long shunt compound wound DC generator

2.6 Emf equation of DC generator 28

2.7 Losses in DC generator 29

2.8 Characteristics of Dc Generator

2.8.1 Magnetization Characteristics
31
2.8.2 Load Characteristics
2.8.2.1Internal Characteristics
2.8.2.2External Characteristics
DC Motor
38
2.9 Construction
2.10 Working principle 39
II 2.11 Types of DC Motors 39
2.12 Characteristics of Dc motor 41
2.13 Applications of Dc Motors 42
 2.14 Necessity of starter for Dc motor 42
2.15 Efficiency of the motor 43
2.16 Efficiency of the generator 43
2.17 Speed control of shunt motor 43
Unit III A C Machines
3.1. Introduction 46
3.2. Constructional details 46
3.3. Working Principle of Induction Motor 48
3.4. Advantages of three phase induction motor 49
3.5. Applications 49
III 3.6. Difference between slip ring and squirrel cage induction motor 49
3.7. Starting method 50
3.8. Speed control methods 53
3.9. Single phase induction motor 58
3.10. Construction and working principle of single phase induction motor 59
3.11. Starting of Single Phase Induction Motors 60
3.12 Slip of induction motor 63
3.13 Torque-Slip characteristics 64
3.14 Electrical braking of an induction motor 67
UNIT IV Special Machines
68
4.1 Introduction

4.2 Types of single-phase motor 68

4.3Types of servo motor
4.3.1 D.C servomotor 68
4.3.2A.C servomotor
IV 4.4 A.C. series motor or universal motor 71
4.5 Unexcited synchronous single phase motor (or) single-phase 73
synchronous motors
4.6Stepper motors 75
4.7 Variable reluctance (VR) stepper motor 77
4.8 Linear induction motor 86
Unit –V Utilization
88
5.1 Laws of Illumination or Luminance
5.2 Types of electric lamps 89
5.3 Arc lamps 91
V 5.4 Discharge lamps 92
5.5 Sodium vapour lamp 92
5.6 High pressure mercury vapour lamp 93
5.7Electroplating 94
5.8Electric traction 94
 UNIT –I

TRANSFORMER

1.1 Definition of Transformer

A transformer is a static machine used for transforming power from one circuit to another without
changing frequency.

1.2 Working principle of Transformer

 It depends upon Faraday's law of electromagnetic induction.

 Mutual induction between two or more winding is responsible for transformation action in an
electrical transformer.
 According to the Faraday's law, "Rate of change of flux linkage with respect to time is directly
proportional to the induced EMF in a conductor or coil".
 One winding is supplied by an alternating electrical source; the alternating current through the
winding produces a continually changing flux or alternating flux that surrounds the winding.
 Some portion of this flux will link with the second winding. As this flux is continually changing in its
amplitude and direction.
 According to Faraday's law of electromagnetic induction, there must be an EMF induced in the
secondary side.
 If the secondary circuit is closed, there must be an electric current flowing through it.
 This is the simplest form of electrical power transformer and this is the most basic of working
principle of transformer.

Fig. 1.1 Flux Distribution in Transformer

 The winding which takes electrical power from the source, is generally known as primary winding
of transformer.
 The winding which gives the desired output voltage due to mutual induction in the transformer, is
commonly known as secondary winding of transformer. Here in our example it is second winding.
 The rate of change of flux linkage depends upon the amount of linked flux with the second
winding.
 So, it is desired to link almost all flux of primary winding to the secondary winding.

1
 Fig. 1.2 Construction of Transformer
 This is done by placing one low reluctance path common to both of the winding. This low
reluctance path is core of transformer, through which maximum number of flux produced by the
primary is passed through and linked with the secondary winding. This is the most basic theory of
transformer.

1.3 Types of transformer

 Step Up Transformer & Step Down Transformer

 Step Up Transformer - Used for stepping up the voltage level of power in transmission network.
 Step Down Transformer - Used for stepping down the voltage level of power in distribution
power network.

 Three Phase Transformer & Single Phase Transformer

 Three Phase Transformer – It is generally used in three phase power system as it is cost
effective than single phase transformer.
 Single Phase Transformer – It is preferable to use bank of three single phase transformer as it
is easier to transport three single phase unit separately than one single three phase unit.
 Electrical Power Transformer, Distribution Transformer & Instrument Transformer
 Electrical Power Transformer – Transformer is generally used in transmission network which is
normally known as power transformer.
 Distribution Transformer – It is used in distribution network and this is lower rating transformer.
 Instrument Transformer – current transformer & potential transformer, we use for relay and
protection purpose in electrical power system and in different instruments in industries are called
instrument transformer.
 Two Winding Transformer & Auto Transformer
 Two Winding Transformer – It is generally used where ratio between high voltage and low
voltage is greater than 2.
 Auto Transformer – It is cost effective where the ratio between high voltage and low voltage is
less than 2.
 Outdoor Transformer & Indoor Transformer
 Outdoor Transformer – Transformers that are designed for installing at outdoor are outdoor
transformer
Indoor Transformer – Transformers designed for installing at indoor are indoor transformers.

1.4 Construction

 Magnetic core and windings (or coils) are the two basic parts of any transformer.
 The core is made of silicon or sheet steel with 4 per cent silicon and laminated to reduce eddy
current loss. It may be in either square or rectangular shape. It has two parts.

2
  The vertical portion on which the coil is wound is called the limb of the core, whereas the top and
bottom horizontal portions are called the yoke.
 Figure 1.2(a) shows the limb and yoke of the core. The permeability of the material used for core
must have high value (µr>1,000) to reduce reluctance of the magnetic path

Fig .1.3 construction of transformer

 The laminations are insulated from each other by a light coat of core plate varnish or by an oxide
layer on the surface. The thickness of lamination is 0.35 mm for a frequency of 50 Hz and 0.5 mm
for a frequency of 25 Hz.
 Transformers are classified into the following three categories based on the relative arrangement
or disposition of the core and the winding
 Core type,
 shell type and
 Spiral core type

1.4.1 Core type

 The complete magnetic circuit of a core-type transformer in the shape of a hollow rectangle
having two limbs. It has a single magnetic circuit. I
 n , I0 is the no-load current and F is the flux produced by it. Number of turns of the primary and
secondary are N1 and N2, respectively.

3
 Fig. 1.4 Schematic view of a core type and shell-type transformer

 The windings surround the core. The coils used are wound and are of cylindrical type having the
general form circular, oval or rectangular.
 Core-type transformer has a longer mean length of core and a shorter mean length of coil turn.
 Core has a small cross section of iron; more number of turns is required because the high flux
may not reach the core.
 Core type is used for high-voltage service, since it has sufficient room for insulation.

Fig. 1.5 Core-type transformer

Fig. 1.6 Different cross sections of Core-type transformer

4
  In small core-type transformers, rectangular cores with rectangular cylindrical coils are used as
shown in Figure (a), whereas circular cylindrical coils are used for large transformers;
 Hence, square cores are preferred as shown in Figure (b). If rectangular cores are used for large
transformers, it becomes wasteful.
 Figure (c) shows the cruciform core, which is an improvement of square core. Figure (d) shows
further core stepping (three-stepped cores) for large transformers resulting in reduced length of
mean turn and copper (Cu) loss.

1.4.2 Shell type

 Shell-type transformer has double magnetic circuit and three limbs.

 Both windings are placed on the central limb. The coils occupy the entire space of windows. The
coils are usually multi-layer disc type or sandwich coils.
 The low-voltage coils are placed nearest to the iron core to reduce the amount of high-voltage
insulation. Core is laminated. Special care is taken to arrange the laminations of the core.
 All the points at alternate layers are staggered properly to avoid narrow air gap at the joint, right
through the cross section of the core.
 The joints are known as overlapped or imbricated joints. The shell-type construction is preferred
for a few high-voltage transformers.
 Since the windings are surrounded by core, natural cooling does not exist. To remove any
winding during maintenance, removal of a large number of laminations is required. Fig shows a
shell-type transformer.

Fig. 1.7 Shell-type transformer

 Due to better provision for mechanical support and bracing of coils in the shell-type
transformer, better resistance to combat high mechanical force is obtained. High mechanical
forces are developed for a high current during short circuit.

1.4.3 Spiral core type

 Fig shows spiral core-type transformer where the core is assembled either of a continuous
strip of the transformer steel wound in the form of a circular or elliptical cylinder or of a group
of short strips assembled to produce the same elliptical shape.
 In this construction, the core flux always follows along the grain of iron. Cold-rolled steel of
high silicon content allows the designer to use higher operating flux densities with lower loss
per kilogram.
 The main advantage of using the higher flux density is that weight/kVA is reduced.

5
 Fig. 1.8 Spiral core-type transformer

1.5 EMF equation of transformer

 In electrical power transformer, one alternating electrical source is applied to the primary winding
and due to this, magnetizing current flowing through the primary winding which produces
alternating flux in the core of transformer.
 This flux links with both primary and secondary windings.
 As this flux is alternating in nature, there must be a rate of change of flux.
 According to Faraday's law of electromagnetic induction if any coil or conductor links with any
changing flux, there must be an induced emf in it.
 As the electric current source to primary is sinusoidal, the flux induced by it will be also
sinusoidal.
 Hence, the function of flux may be considered as a sine function.
 Let,
N1 = Number of turns in primary winding
N2 = Number of turns in secondary winding
Φm = Maximum flux in the core (in Wb) = (Bm x A)
f = frequency of the AC supply (in Hz)

Fig. 1.9 Flux Distribution

As, shown in the figure, the flux rises sinusoidally to its maximum value Φ m from 0. It reaches to
the maximum value in one quarter of the cycle ie., in T/4 sec (where, T is time period of the sin wave of
the supply = 1/f).

Therefore,
average rate of change of flux = Φm /(T/4) = Φm /(1/4f)

average rate of change of flux = 4f Φm (Wb /s).

Now,
Induced emf per turn = rate of change of flux per turn

Therefore,

average emf per turn = 4f Φm (Volts).

Form factor = RMS value / average value

6
 RMS value of emf per turn = Form factor *average emf per turn.

As, the flux Φ varies sinusoidally, form factor of a sine wave is 1.11

Therefore, RMS value of emf per turn = 1.11 x 4f Φm = 4.44f Φm.

RMS value of induced emf in whole primary winding (E 1) = RMS value of emf per turn * Number
of turns in primary winding.

E1 = 4.44f N1 Φm Volts

Similarly, RMS value of induced emf in secondary winding (E2) can be given as
E2 = 4.44f N2 Φm Volts

This is called the emf equation of transformer

1.6 Transformation ratio of transformer

 If K > 1, then the transformer is step up transformer

 If K < 1, then the transformer is step down transformer.

Where

1.7. Voltage ratio of transformer

 Voltage ratio of transformer is expressed as ratio of the primary and secondary voltages of
transformer.

1.8. Turns ratio of transformer

 Ratio of turns is referred as turns ratio of transformer. As the voltage in primary and secondary
of transformer is directly proportional to the number of turns in the respective winding.

1.9. Equivalent circuit of transformer

 The electrical power transformer is an electrical power system equipment, for estimating different
parameters of electrical power system it is required to calculate total internal impedance of an
electrical power transformer, viewing from primary side or secondary side as per requirement.
 This calculation requires equivalent circuit of transformer referred to primary or equivalent circuit
of transformer referred to secondary sides respectively

1.9.1. Equivalent Circuit of Transformer Referred to Primary

Let us consider the transformation ratio

7
 Fig. 1.10 (a) Vector diagram of Transformer

The applied voltage to the primary is V1

Voltage across the primary winding is E1

Total electric current supplied to primary is I1

So the voltage V1 applied to the primary is partly dropped by I1Z1 or I1R1 + j.I1X1before it appears across
primary winding.

The voltage appeared across winding is countered by primary induced emf E1.

So voltage equation of this portion of the transformer can be written as,

The equivalent circuit for that equation can be drawn as below,

Fig. 1.10 (b) Basic Primary Circuit of Transformer

From the vector diagram above, it is found that the total primary current I 1 has two components, one is no
- load component Io and the other is load component I2′.

As this primary current has two components or branches, there must be a parallel path with primary
winding of transformer.

This parallel path of electric current is known as excitation branch of equivalent circuit of transformer.

The resistive and reactive branches of the excitation circuit can be represented as

8
 Fig. 1.10 (c) Equivalent Circuit of Primary side of Transformer

This induced voltage E1transforms to secondary and it is E2

Load component of primary current I2′ is transformed to secondary as secondary current I2. Current of
secondary is I2.

So the voltage E2 across secondary winding is partly dropped by I2Z2 or I2R2 + j.I2X2 before it appears
across load. The load voltage is V2.

The complete equivalent circuit of transformer is shown below.

Fig. 1.10 (d) Equivalent Circuit of Transformer Referred to Primary

The voltage drop in secondary from primary side would be ′K′ times greater and would be written as
K.Z2.I2.
Again I2′.N1 = I2.N2

Since Io is very small compared to I1, it is less than 5% of full load primary current, Io changes the voltage
drop insignificantly.

Hence, ignore the excitation circuit in approximate equivalent circuit of transformer. The winding
resistance and reactance being in series can now be combined into equivalent resistance and reactance
of transformer, referred to primary side.

9
 Fig. 1.10 (e) Approximate Equivalent Circuit of Transformer Referred to Primary

1.9.2. Equivalent Circuit of Transformer Referred to Secondary

In similar way, approximate equivalent circuit of transformer referred to secondary can be drawn.

Fig. 1.10 (f) Approximate Equivalent Circuit of Transformer Referred to Secondary

1 . 1 0 Transformer losses

 Primary copper loss

 Secondary copper loss

 Iron loss

 Dielectric loss

 Stray load loss

10
 These are explained in sequence below.

 Primary and secondary copper losses take place in the respective winding resistances
due to the flow of the current in them. The primary and secondary resistances differ
from their d.c. values due to skin effect and the temperature rise of the windings.

 The iron losses contain two components - Hysteresis loss and Eddy current loss.
The Hysteresis loss is a function of the material used for the core .Ph = KhB1.6f For
constant voltage and constant frequency operation this can be taken to be constant.

 The eddy current loss in the core arises because of the induced emf in the steel
lamination sheets and the eddies of current formed due to it. This again producesa
power loss Pe in the lamination.where t is the thickness of the steel lamination used. As
the lamination thickness is much smaller than the depth of penetration of the field, the
eddy current loss can be reduced by reducing the thickness of the lamination.

 These reduce the eddy current losses in the core.This loss also remains constant due
to constant voltage and frequency of operation. The sum of hysteresis and eddy current
losses can be obtained by the open circuit test.

 The dielectric losses take place in the insulation of the transformer due to the large
electric stress. In the case of low voltage transformers this can be neglected. For constant
voltage operation this can be assumed to be a constant.

 The stray load losses arise out of the leakage fluxes of the transformer. These
leakage fluxes link the metallic structural parts, tank etc. and produce eddy current
losses in them. Thus they take place ’all round’ the transformer instead of a definite
place , hence the name ’stray’. Also the leakage flux is directly proportional to the load
current unlike the mutual flux which is proportional to the applied voltage. Hence this
loss is called ’stray load’ loss

1.11. Efficiency of transformer

Transformer efficiency may be defined as the ratio between Output and Input.

Efficiency = Output/Input

On specified Power factor and load, the Transformer efficiency can be found by dividing its output
on Input.

Transformer has very high efficiency because the losses occur in transformer is very low.

Efficiency = η= Output / Input

Efficiency = η= Output / (Output + Losses) …….… (As Input = Output +Losses)

Efficiency = η= Output / (Output +Cupper Losses + Iron Losses)

In another way

Efficiency = η= Output / Input

11
 Efficiency = η = (Input – Losses) / Input …….… (As Output = Input - Losses)

Efficiency = η = 1 – (Losses /Input)

1.12. Condition for maximum efficiency

Copper Loss = W C = I12. R1 or I22R2

Iron Loss=W I = Hysteresis Loss + Eddy Current Loss

WI = W H + W E

Suppose to Primary Side...

Primary Input = P1 = V1 I1 Cosθ1

Efficiency = η = Output / Input

Efficiency = η = (Input – Losses) / input ….. (As Output = Input - Losses)

Efficiency = η = (Input – Copper losses – Iron Losses)/Input

Efficiency = η = (P1 - W C - W I) / P1

Efficiency = η = (V1 I1 Cosθ1 - I12. R1 - W I)/ V1 I1 Cosθ1

Efficiency = η = 1- (I12. R1 /V1 I1 Cosθ1) – (W I/ V1 I1 Cosθ1)

Or

Efficiency = η = 1- (I1. R1 /V1 Cosθ1) – (W I/ V1 I1 Cosθ1)

Differentiate both sides with respect to I1

Dη/ dI1 = 0 – ( R1 /V1 Cosθ1) + (W I/ V1 I12 Cosθ1)

Dη/ dI1= - ( R1 /V1 Cosθ1) + (W I/ V1 I12 Cosθ1)

For Maximum Efficiency, the value of (Dη/ dI1) should be Minimum i.e.

Dη/ dI1 = 0

The above Equation can be written as

R1 / (V1 Cosθ1) = (W I/ V1 I12 Cosθ1)

Or

W I = I12. R1 or I22R2

Iron Loss = Copper Loss

The value of Output current (I2) on which Maximum efficiency can be gained

I2 = √ (W I/ R2)

(i.e. Copper Loss = Iron Loss)

12
1.13. All day efficiency

The commercial or typical efficiency of a transformer is the ratio of Output and Input in watts

Efficiency = Output (in Watts)/Input (in Watts)

The distribution transformers which supply electrical energy to lighting and other general circuits,
their primary energize for 24 hours, but the secondary windings does not energize all the time. It means
that core loss occurs for 24 hours regularly but copper loss occurs only when transformer is on loaded.

Therefore it realizes the necessity to design a transformer in which the core loss should be low.

As copper loss depends on load, therefore, they should be neglected. In this type of
transformers, we can track their performance only by all day efficiency.

All day efficiency may be also called “Operational efficiency”. On the base of usable energy, we
estimate the all day efficiency for a specific time (During the 24 hours =one day).

All Day Efficiency = Output (in kWh)/Input (in kWh)

1.14. Auto transformer

Auto transformer is a kind of electrical transformer where primary and secondary shares same
common single winding.

In Auto Transformer, one single winding is used as primary winding as well as secondary
winding.

But in two windings transformer two different windings are used for primary and secondary
purpose. A diagram of auto transformer is shown below.

Fig. 1.11 Auto Transformer

The winding AB of total turns N1 is considered as primary winding. This winding is tapped from
point ′C′ and the portion BC is considered as secondary. Let's assume the number of turns in between
points ′B′ and ′C′ is N2.

13
 If V1 voltage is applied across the winding i.e. in between ′A′ and

′C′

Hence, the voltage across the portion BC of the winding, will be,

As BC portion of the winding is considered as secondary, it can easily be understood that value of
constant ′k′ is nothing but turns ratio or voltage ratio of that auto transformer.

When load is connected between secondary terminals i.e.between ′B′ and ′C′, load current I 2starts
flowing. The current in the secondary winding or common winding is the difference of I 2 & I1.

1.14.1. Copper Savings in Auto Transformer

The weight of copper of any winding depends upon its length and cross - sectional area. Again
length of conductor in winding is proportional to its number of turns and cross - sectional area varies with
rated current.

So weight of copper in winding is directly proportional to product of number of turns and rated
current of the winding.

Therefore, weight of copper in the section AC proportional to,

Similarly, weight of copper in the section BC proportional to,

Hence, total weight of copper in the winding of auto transformer proportional to,

14
In similar way it can be proved, the weight of copper in two winding transformer is proportional to,
N1I1 + N2I2⇒ 2N1I1 (Since, in a transformer N1I1 = N2I2)

Let's assume, W a and W tw are weight of copper in auto transformer and two winding transformer
respectively,

∴ Saving of copper in auto transformer compared to two winding transformer,

1.14.2. Advantages of Auto Transformer

For transformation ratio = 2, the size of the auto transformer would be approximately 50% of the
corresponding size of two winding transformer.

1.14.3. Disadvantages of Using Auto Transformer

 Because of electrical conductivity of the primary and secondary windings the lower voltage circuit
is liable to be impressed upon by higher voltage. To avoid breakdown in the lower voltage circuit,
it becomes necessary to design the low voltage circuit to withstand higher voltage.
 The leakage flux between the primary and secondary windings is small and hence the impedance
is low. This results into severer short circuit currents under fault conditions.

1.15. Comparison between single three phase and bank of three single phase transformers for
three phase system

 Single 3 phase transformer costs around 15% less than bank of three single phase transformers.
 Single 3 phase transformer occupies less space than bank of three single phase transformers.

15
  For very big transformer, it is impossible to transport large three phase transformer to the site and
it is easier to transport three single phase transformers which is erected separately to form a
three phase unit.
 Another advantage of using bank of three single phase transformers is that, if one unit of the bank
becomes out of order, then the bank can be run as open delta.

1.15.1. Connection of Three Phase Transformer

 Star-Star Transformer

Fig. 1.12 Star- Star Transformer

Star-star transformer is formed in a 3 phase transformer by connecting one terminal of each

phase of individual side, together.

 The common terminal is indicated by suffix 1. If terminal with suffix 1 in both primary and
secondary are used as common terminal, voltages of primary and secondary are in same phase.
This connection is called zero degree connection or 0° - connection.

 If the terminal with suffix 1 is connected together in HV side as common point and the terminals
with suffix 2 in LV side are connected together as common point, the voltages in primary and
secondary will be in opposite phase. Hence, star-star transformer connection is called 180°-
connection, of three phase transformer.

 Delta-Delta Transformer

In delta-delta transformer, 1 suffixed terminals of each phase primary winding will be connected
with 2 suffixed terminal of next phase primary winding.

Fig. 1.13 Delta- Delta Transformer

 If primary is HV side, then A1 will be connected to B2, B1 will be connected to C2 and C1 will be
connected to A2.

16
  Similarly in LV side, a1 will be connected to b2, b1will be connected to c2 and c1 will be connected
to a2.

 If transformer leads are taken out from primary and secondary 2 suffixed terminals of the winding,
then there will be no phase difference between similar line voltages in primary and secondary.
This delta delta transformer connection is zero degree connection or 0°-connection.

 But in LV side of transformer, if, a2 is connected to b1, b2 is connected to c1 and c2 is connected to

a1. The secondary leads of transformer are taken out from 2 suffixed terminals of LV windings,
and then similar line voltages in primary and secondary will be in phase opposition. This
connection is called 180°-connection, of three phase transformer.

 Star-Delta Transformer

Here in star-delta transformer, star connection in HV side is formed by connecting all the 1
suffixed terminals together as common point and transformer primary leads are taken out from 2 suffixed
terminals of primary windings.

Fig. 1.14 Star- Delta Transformer

 The delta connection in LV side is formed by connecting 1 suffixed terminals of each phase LV
winding with 2 suffixed terminal of next phase LV winding.

 The secondary leads are taken out from 2 suffixed ends of the secondary windings of
transformer.

 The sum of the voltages in delta side is zero otherwise closed delta would mean a short circuit.

 Phase to neutral voltage on the delta side lags by − 30° to the phase to neutral voltage on the
star side.

 Star-delta + 30°-connection is also possible by connecting secondary terminals in following

sequence, a2 is connected to b1, b2 is connected to c1 and c2 is connected to a1. The secondary
leads of transformer are taken out from 2 suffixed terminals of LV windings.

 Delta-Star Transformer

Delta-star transformer connection of three phase transformer is similar to star – delta connection.
If anyone interchanges HV side and LV side of star-delta transformer in diagram, it simply becomes delta
– star connected 3 phase transformer.

17
 Fig. 1.15 Delta- Star Transformer

 Delta-Zigzag Transformer

The winding of each phase on the star connected side is divided into two equal halves in delta zig
zag transformer. Each leg of the core of transformer is wound by half winding from two different
secondary phases in addition to its primary winding.

 Star-Zigzag Transformer

The winding of each phase on the secondary star in a star-zigzag transformer is divided into two
equal halves. Each leg of the core of transformer is wound by half winding from two different secondary
phases in addition to its primary winding.

1.16 Three phase transformer connections

Fig. 1.16 connections of three phase transformer

 The primary and secondary windings of a transformer can be connected in different configuration
as shown to meet practically any requirement. In the case of three phase transformer windings,
three forms of connection are possible: “star” (wye), “delta” (mesh) and “interconnected-star” (zig-
zag).

The combinations of the three windings may be with the primary delta-connected and the
secondary star-connected, or star-delta, star-star or delta-delta, depending on the transformers
use. When transformers are used to provide three or more phases they are generally referred to
as a Polyphase Transformer.

18
 UNIT II

DC GENERATORS

2.1 Introduction to D.C machines

A D.C machine may be operated either as generator or as motor.

When a D.C machine is driven by a prime mover it produces electrical energy and it is called as
generator.

Prime mover D.C

Generator

Fig. 2.1 D.C generator coupled to a prime mover

2.2 Generator principle

 An electrical generator is a machine which converts mechanical energy into electrical energy.
 The energy conversion is based on the principle of the production of dynamically induced emf,
where a conductor cuts magnetic flux, dynamically induced emf is produced in it according to
Faraday’s Laws of electromagnetic Induction.
 This emf causes a current to flow if the conductor circuit is closed. Hence, two basic essential
parts of an electrical generator are
 A magnetic field and
 A conductor or conductors which can so move as to cut the flux.
 The following figure shows a single-turn rectangular copper coil rotating about its
own axis in a magnetic field provided by either permanent magnets or
electromagnets.
 The two ends of the coil are joined to two slip-rings ‘a’ and ‘b’ which are insulated
from each other and from the central shaft.
 Two collecting brushes (of carbon or copper) press against the slip-rings.
 Their function is to collect the current induced in the coil and to convey it to the
external load resistance R. The rotating coil may be called ‘armature’ and the
magnets as ‘field magnets’.
 As the coil rotates in clock-wise direction and assumes successive positions in
the field the, flux linked with it changes. Hence, an emf is induced in it which is
proportional to the rate of change of flux linkages (e = NdØ /dt).

19
Fig. 2.2 Single loop generator with split rings Fig. 2.3 Generated emf waveform

 The position of brushes is so arranged that the changeover of segments ‘a’ and‘b’ from
one brush to the other takes place when the plane of the rotating coil is at right angles to
the plane of the lines of flux. It is so because in that position, the induced emf in the coil is
zero.
 The current induced in the coil sides is alternating as before. It is only due to the
rectifying action of the split-rings (also called commutator) that it becomes unidirectional
in the external circuit.

Fig. 2.4 Single loop generator Fig. 2.5 Split rings

(coil at 180º position )

Fig. 2.6 Output DC waveform

20
2.3 Conversion of AC to DC
Normally a slip ring along with brushes are used to collect the alternating current and supply it to
the external load. For making the flow of current unidirectional in the external circuit, the slip rings are
replaced by split rings. The split rings are made of a conducting cylinder which is cut into two halves or
segments insulated from each other by a thin sheet of mica or some other insulating material.
2.4 Constructional details
The D.C. generator consists of the following parts.
 Yoke or magnetic-frame
 Pole core and pole shoe
 Pole coil or field coil or Exciting coil
 Armature core
 Armature winding
 Commutator
 Brushes
 Bearings.
Yoke
The outer frame or yoke serves double purpose:
 It provides mechanical support for the poles and acts as a protecting cover for the whole
machine.
 It carries the magnetic flux produced by the poles. In small generators where cheapness
rather than weight is the main consideration, yokes are made of cast iron. But for large
machines usually cast steel or rolled steel is employed

Fig. 2.7 dc generator

Pole core and poles shoe
The field magnets consist of pole cores and pole shoes. The pole shoes serve two purposes:
 They spread out the flux in the air gap and also, being of larger cross-section, reduce the
reluctance of the magnetic path.
 They support the exciting coils (or field coils) as shown below.
Pole coil or field coil or exciting coil
 The field coils or pole coils, which consist of copper wire or strip, are former-wound for
the correct dimension. Then, the former is removed and wound coil is put into place over
the core.

21
  When current is passed through these coils, they electro magnetize the poles which
produce the necessary flux that is cut by revolving armature conductors.

Fig. 2.8 Pole coils and pole core

Armature core

Fig. 2.9 (a) Armature core Fig. 2.9 (b) Sectional view of armature core

 It houses the armature conductors or coils and causes them to rotate and hence cut the magnetic
flux of the field magnets.
 In addition to this, its most important function is to provide a path of very low reluctance to the flux
through the armature from a N-pole to a S-pole. It is cylindrical or drum- shaped and is built up of
usually circular sheet steel discs or laminations approximately 0.5 mm thick.
 The slots are either die-cut or punched on the outer periphery of the disc and the keyway is
located on the inner diameter as shown. In small machines, the armature stampings are keyed
directly to the shaft. Usually, these laminations are perforated for air ducts which permit axial flow
of air through the armature for cooling purposes.
 The purpose of using laminations is to reduce the loss due to eddy currents. Thinner the
laminations, greater is the resistance offered to the induced emf, smaller the current and hence
lesser the I2 R loss in the core.
Armature windings
 The armature windings are usually former-wound. These are first wound in the form of flat
rectangular coils and are then pulled into their proper shape in a coil puller. Various conductors of
the
 coils are insulated from each other.

22
 The conductors are placed in the armature slots which are lined with tough insulating material.
This slot insulation is folded over above the armature conductors placed in the slot and is secured
in place by special hard wooden or fiber wedges.
Commutator
 The functions of the commutator are to facilitate collection of current from the armature
conductors, and to convert the alternating current induced in the armature conductors into
unidirectional current in the external load circuit.
 It is of cylindrical structure and is built up of wedge-shaped segments of high-conductivity hard-
drawn or drop forged copper.
 These segments are insulated from each other by thin layers of mica. The number of segments is
equal to the number of armature coils.
 Each commutator segment is connected to the armature conductor by means of a copper lug or
riser.
 To prevent them from flying out under the action of centrifugal forces, the segments have V-
grooves, these grooves being insulated by conical micanite rings.

Fig 2.9 commutato

Fig. 2.10 Commutator

Brushes

 The brushes, whose function is to collect current from commutator, are usually made of
carbon or graphite and are in the shape of a rectangular block.
 These brushes are housed in brush-holders, the brush-holder is mounted on a spindle
and the brushes can slide in the rectangular box open at both ends.
 The brushes are made to bear down on the commutator by a spring. A flexible copper
pigtail mounted at the top of the brush conveys current from the brushes to the holder.
Bearings
Because of their reliability, ball-bearings are frequently employed, though for heavy duties, roller
bearings are preferable. The ball and rollers are generally packed in hard oil for quieter operation.

Fig. 2.11 Brush

Fig. 2.11 Brush

23
Shaft

It is used as an interconnector between a prime mover and the generator.

Fig. 2.12 Cross-section of a typical DC machine

2.5 Types of generators

Permanent magnet DC generator

When the flux in the magnetic circuit is established by the help of permanent magnets then it is
known as Permanent magnet dc generator. It consists of an armature and one or several permanent
magnets situated around the armature. This type of dc generators generates very low power. So, they are
rarely found in industrial applications. They are normally used in small applications like dynamos in motor
cycles.

Fig. 2.13 Permanent magnet DC generator

24
2.5.1Separately excited DC generator

These are the generators whose field magnets are energized by some external dc source such
as battery.
A circuit diagram of separately excited dc generator is shown in figure

Fig. 2.14 separately excited DC generator

Where,
Ia = Armature current
IL = Load current
V = Terminal voltage
Eg = Generated emf
Voltage drop in the armature = Ia × Ra (Ra is the armature resistance)
Let, Ia = IL = I (say)
Then, voltage across the load, V = IRa
Power generated, Pg = Eg×I
Power delivered to the external load, PL = V×I.
2.5.2 Self-excited DC generators

 These are the generators whose field magnets are energized by the current supplied by
themselves. In these type of machines field coils are internally connected with the armature.

 Due to residual magnetism some flux is always present in the poles. When the armature is
rotated some emf is induced. Hence some induced current is produced.

 This small current flows through the field coil as well as the load and thereby strengthening
the pole flux. As the pole flux strengthened, it will produce more armature emf, which cause
further increase of current through the field.

 This increased field current further raises armature emf and this cumulative phenomenon
continues until the excitation reaches to the rated value.

According to the position of the field coils the Self-excited DC generators may be classified as:

 Series wound generators

Shunt wound generators
Compound wound generators

2.5.2.1Series wound generator

In these type of generators, the field windings are connected in series with armature conductors as
shown in figure below. So, whole electric current flows through the field coils as well as the load. As

25
 series field winding carries full load current it is designed with relatively few turns of thick wire. The
electrical resistance of series field winding is therefore very low (nearly 0.5Ω )

Fig. 2.15 Series wound generator

Let,
Rsc = Series winding resistance
Isc = Current flowing through the series field
Ra = Armature resistance
Ia = Armature current
IL = Load current
V = Terminal voltage
Eg = Generated emf
Then, Ia = Isc = IL=I (say)
Voltage across the load, V = Eg -I(Ia×Ra)
Power generated, Pg = Eg×I
Power delivered to the load, PL = V×I
2.5.2.2Shunt wound DC generators

 In these type of DC generators the field windings are connected in parallel with armature
conductors as shown in figure below.

 In shunt wound generators the voltage in the field winding is same as the voltage across
the terminal

Let
Fig. 2.16 Shunt Wound DC generator
Rsh = Shunt winding resistance
Ish = Current flowing through the shunt field
Ra = Armature resistance
Ia = Armature current
IL = Load current

26
 V = Terminal voltage
Eg = Generated emf

Here armature current Ia is dividing in two parts, one is shunt field current Ish and another is load
current IL.
So, Ia=Ish + IL
The effective power across the load will be maximum when IL will be maximum. So, it is required
to keep shunt field current as small as possible. For this purpose the resistance of the shunt field
winding generally kept high (100 Ω) and large no of turns are used for the desired emf.
Shunt field current, Ish = V/Rsh
Voltage across the load, V = Eg-Ia Ra
Power generated, Pg= Eg×Ia
Power delivered to the load, PL = V×IL.
2.5.2.3 Compound wound DC generator

 In series wound generators, the output voltage is directly proportional with load current. In
shunt wound generators, output voltage is inversely proportional with load current.

 A combination of these two types of generators can overcome the disadvantages of both.
This combination of windings is called Compound Wound DC Generator.

 Compound wound generators have both series field winding and shunt field winding. One
winding is placed in series with the armature and the other is placed in parallel with the
armature.

This type of DC generators may be of two types-

 short shunt compound wound generator and

 long shunt compound wound generator.

2.5.2.3.1Short shunt compound wound DC generator

The generators in which only shunt field winding is in parallel with the armature winding as shown in fig.
Series field current, Isc = IL
Shunt field current, Ish = (V+Isc Rsc)/Rsh
Armature current, Ia = Ish + IL
Voltage across the load, V = Eg - Ia Ra - Isc Rsc

Fig. 2.17 Short Shunt Compound Wound DC Generator

27
Power generated, Pg = Eg×Ia
Power delivered to the load, PL=V×IL.

2.5.2.3.2 Long shunt compound wound DC generator

The generators in which shunt field winding is in parallel with both series field and armature winding as
shown in fig.
Shunt field current, Ish=V/Rsh
Armature current, Ia= series field current, Isc= IL+Ish
Voltage across the load, V=Eg-Ia Ra-Isc Rsc=Eg-Ia (Ra+Rsc) [∴Ia=Ics]
Power generated, Pg= Eg×Ia
Power delivered to the load, PL=V×IL

Fig. 2.18 Long Shunt Compound Wound DC Generator

In a compound wound generator, the shunt field is stronger than the series field. When the series
field assists the shunt field, generator is said to be commutatively compound wound. On the other hand if
series field opposes the shunt field, the generator is said to be differentially compound wound.

Fig. 2.19 Current flow in field coils of Compound Wound DC Generator

2.6 EMF equation of DC generator

Let
Φ = flux/pole in Weber
Z = total number of armature conductors= No. of slots x No. of conductors/slot
P = No. of generator poles
A = No. of parallel paths in armature
N = armature rotation in revolutions per minute (r.p.m.)
E = emf induced in any parallel path in armature
Generated emf (Eg) = emf generated in any one of the parallel paths ( E).

28
 Average emf generated/conductor = volt

Now, flux cut/conductor in one revolution dΦ = ΦP Wb

No. of revolutions/second = N/60
Therefore, time for one revolution, dt = 60/N second
Hence, according to Faraday’s Laws of Electromagnetic Induction,

EMF generated/conductor = = = volt

No. of conductors (in series) in one path = Z/A

For a simplex wave-wound generator: A=2

EMF generated/path = × = volt

For a simplex lap-wound generator: A=P, No. of conductors (in series) in one path=Z/P
EMF generated/path = × = volt

2.7 Losses in dc generator

Fig. 2.20 Losses of dc generator

(a) Copper loss

(i) Armature copper loss:

It is power loss in the armature circuit and equal to I a2 Ra . The value of armature current is
decided by the load. This loss is about 30 to 40% of full -load losses.
Where,
Ra- Resistance of armature
Ia – Armature current
(ii) Field copper loss
It is the loss in series or shunt field of generator. Ise2Rse is the field copper loss in case of series
generators, where Rse is the resistance of the series field winding. Ish2Rsh is the field copper loss in case of
shunt generators. It is practically constant in case of shunt generators. This loss is about 20 to 30% of F.L
losses.

(iii) The loss due to brush contact resistance. It is usually included in the armature copper loss.

(b) Magnetic Losses :( also known as iron or core losses)

Due to the rotation of armature in the magnetic field. There are some losses taking place
continuously in the core and are known as iron losses or core losses. It consists of

29
  Eddy current loss and

 Hysteresis loss

Hysteresis loss

This loss is due to the reversal of magnetic flux in the armature core. Every portion of the rotating
core passes under N and S pole alternately. There by alternating S and N polarity respectively. The core
undergoes one complete cycle of magnetic reversal after passing under one pair of poles. If p is the
number of poles and N, the armature speed in r.p.m. Then frequency of magnetic reversals is f= .

The loss depends upon the volume and grade of iron, maximum value of flux density B max and
frequency of magnetic reversals. For normal flux densities (i.e. upto 1.5 wb/m 2), hysteresis loss is given
by steinmetz formula.

Wh = η Bm1.6 f v watts
Where,

η – Steinmet hysteresis co-efficient.

Bm – Maximum flux density in Wb/m 2

f – Frequency of magnetic reversals..

V – Volume of armature core.

Eddy current loss

When the armature core rotates, it also cuts the magnetic flux; hence an e.m.f. is induced in the
body of the core according to the laws of electromagnetic induction. This e.m.f. though small, sets up
large current in the body of the core due to its small resistance, this current is known as eddy current. The
power loss due to the flow of this current is known as eddy current loss. This loss would be considerable
if solid iron core were used. In order to reduce this loss and the consequent heating of the core to a small
value. The core is built up of thin laminations which are stacked and then riveted at right angles to the
path of the eddy currents. These core laminations are insulated from each other by a thin coating of
varnish.

It is found that eddy current loss is given by the following relation:

We = ke B2max f2 t2 v Watts
Where,

ke = Constant

Bmax = Maximum flux density in the core in Wb/m 2

t = Thickness of laminations.

V = Volume of core in m3.

30
 It is seen from the relation that this loss varies directly as the square or the thickness of
laminations, hence it should be kept as small as possible. Another point to note is that W e = f2 but W h = f.
This fact makes it possible to separate the two losses experimentally if so desired.

These losses are constant for shunt and compound wound generators. Because in their case,
field current is approximately constant. Both these losses total up to about 20% to 30% of full load losses.

(c) Mechanical losses

These losses consists of

i.Friction loss at bearings and commutator.

ii.Air friction or windings loss of rotating armature.

These are about 10 to 20 % of full load losses.

(d) Stray losses

The sum of the mechanical and magnetic losses are called stray losses. These are also known
as rotational losses.

Constant and variable losses

Constant losses are sum of the shunt field copper loss and stray losses. They do not vary with
load current

Variable loss is nothing but armature copper loss because it varies with load current.

2.8 Characteristics of dc generator

The d.c. generators have following characteristics in general,

 Magnetization characteristics
 Load characteristics
2.8.1 Magnetization characteristics
This characteristics is the graph of generated no load voltage E against the current If , when
speed of generator is maintained constant. As it is plotted without load with open output terminals it is
also called No load characteristics or Open circuit characteristics.
E0 Vs If is magnetization characteristics
Where, E0 = No load induced e.m.f.
But for generator,
E=

E α Ф with = constant

Also E α If as Ф α If
2.8.2 Load characteristics
These are further divided into two categories
 External characteristics

31
  Internal characteristics
The external characteristics is the graph of the terminal voltage Vt against load current IL
Key Point: While plotting both the characteristics the speed N of the generator is maintained constant.
In most of the cases, the shunt field current is very small as compared with load current IL. Hence in
practice the internal characteristics shows the graph of induced e.m.f. E against load current IL instead of
Ia, neglecting Ish.
Separately excited DC generator characteristics

For a given dc generator it is clear that the induced emf is proportional to the flux and the speed.
lf speed is kept constant, and the flux is varied, the induced emf also varies

Since =

N = Constant
As increases , also increases

 The variation of flux with the induced emf is called the no-load magnetisation curve or saturation
curve or open circuit characteristics (OCC) of the generator. Since the measurement of flux is
difficult,
 The curve is plotted between field current (lf) and induced emf (Eg). Fig 2.21 shows circuit
diagram for O.C.C of a separately excited generator. The prime mover gives the mechanical input
to the d.c. generator

Fig. 2.21 separately excited dc generator

Fig below shows open circuit characteristics of a separately excited dc generator.
When the field current is zero, there is some flux due to residual magnetism and this causes a small
induced emf. it is shown in figure above as OA.

As the field current is increased, the induced emf increases, increasing linearly from A to B. As
the field current is further increased, the increase in flux is much smaller and hence the emf also
increases slowly. At point D saturation has set in and any further increase in field current does not
produce any increase in induced emf.

32
 Fig. 2.22 open circuit characteristics of Separately excited dc generator

Internal and External characteristics

The curve (l) can be drawn, armature current versus no load induced emf. It is for ideal dc
generator only. There is no voltage drop in generator.

2.8.2.1Internal characteristics
This curve is drawn between the emf E actually induced in the armature (after allowing for the
demagnetizing effect of armature reaction) and armature current Ia. Here by increasing the armature
current induced emf E will decrease due to armature reaction. This curve is called internal characteristics
or total characteristics. This is curve (2), indicated in fig.2.23 below.

Fig. 2.23 Internal and external characteristics

2.8.2.2 External characteristics

This curve is drawn between the terminal voltage (voltage across load) and armature current.
Here by increasing armature current or load current, the induced emf again increases due to armature
resistance. This curve is called external characteristics or voltage regulation curve. This is curve (3),
indicated in above figure

DC shunt generator characteristics

 In the discussion on OCC curve above, the field was connected to a separate d.c source.
 We may now ask, if d.c voltage is obtained from the armature of the generator can we use this
voltage to supply the field, thus eliminating the need for a separate d.c source.
 The answer is yes, we may use this voltage for the field.
 The induced emf will depend the field current and at the same time the field current will depend
on the induced emf. It is nothing but self excitation.

33
  Since the armature and field windings are connected in parallel it is called shunt excitation. Figure
below shows dc shunt generator.

Fig. 2.24 dc shunt generator

Here, the generator speed is constant. Fig. 2.25 below shows open circuit characteristics of a dc
shunt generator.

Fig. 2.25 open circuit characteristics of a dc shunt generator

 Initially the field current is zero, but emf is induced in the generator due residual magnetism. Due
to this voltage field current increases and emf also increases.
 The emf and field current progressively increase till it reaches point where field current is just
sufficient to produce the voltage. There is no further increase in field current or induced emf.
 This curve can be drawn from field current and induced emf. This curve is open circuit
characteristics.
Internal and external characteristics (or) load characteristics
Fig.2.26 below shows the connections for a d.c shunt generator. The field current Ish, the armature
current Ia, and the load current IL are related by the equation.

Ia = Ish + IL

The armature has to supply both the load and field circuits.

34
 Fig. 2.26 The connections for a d.c shunt generator

Once the generator has built up to the specified voltage on no load, it may be loaded. What
happens as we increase the load on the generator?

The load current IL increases and this implies that the generator current (armature current) also
increases. It will also be found that while the induced emf Eg remains constant (if speed and field current
are constant), the actual voltage available at the generator terminals for supplying the load reduces.

This voltage is called terminal voltage and there are several reasons that cause its reduction.

 As we start loading, Ia increases causing a drop of voltage in the

resistance of the armature Ra.
 Drop in the brush contact resistance.
 Drop due to armature reaction. [When the load current flows in
the armature conductors, a flux is produced.
 The armature is now under the influence of two fluxes. The main
flux produced by the field and armature flux produced by the
current flow

The above three factors cause a decrease in the induced emf. The above discussion can be expressed
by the following equation.
V = Eg - (Drop in armature resistance + Brush drop + Drop due to armature reaction
Figure below shows internal and external characteristics of dc shunt generator.

Fig. 2.27 Internal and external characteristics of dc shunt generator

35
  The curve (l) shows ideal dc generator i.e., by increasing load current the terminal voltage should
be constant. There is no drop in the armature. i.e Eg = V.
 The curve (2) shows internal characteristics, here the drop is due to the armature reaction. The
curve can be drawn for load current versus E.
 The curve (3) shows external characteristics. Here the drop is due to the armature resistance. By
increasing the load current the terminal voltage decreases. It is shown in above figure.
 V = E - Ia Ra

DC Series generator characteristics

The connection for the dc series generator is shown in figure below.
In this case, it is easily seen that
Ia = Ise = IL
Fig.2.28 below shows O.C.C, internal and external characteristics of series generator.
 The curve (l) shows open circuit characteristics. This curve can be obtained by disconnecting the
field winding from the machine and excited by separate d.c source.
 The curve (2) shows internal characteristics. Here the drop is due to armature reaction. By
increasing the load current the induced emf E decreases.
 The curve (3) shows external characteristics. Here the drop is due to armature resistance and
series field resistance. The terminal voltage decreases as
V=E-

Fig. 2.28 O.C.C, internal and external characteristics of series generator

Compound generator
compound generator consists of series field and shunt field windings. Below shows external
characteristics of compound generator.

Fig. 2.29 External characteristics of compound generator

36
Flat compound generator
 In a shunt generator we have seen that the terminal voltage falls on loading, whereas in a series
generator the terminal voltage increases with load.
 A compound generator has both shunt and series fields and if the drop in flux in the shunt field is
exactly compensated for by the rise in flux in series field, then it is possible to have constant
voltage characteristics as shown in figure above (curve l).
 This is called flat compound or level compound generator.
 Here by increasing the load current the terminal voltage is almost constant.
o Eg = V
Over compound generator
Curve (2) shows the characteristics of over compound generator. Here the series field excitation
is more than shunt field. Therefore by increasing the load current, the terminal voltage also increases. It is
known as over-compound generator.
V > Eg
Under compound generator
Curve (3) shows the characteristics of under compound generator. Here the series field excitation
is less than the shunt field. Therefore by increasing the load current, the terminal voltage decreases. It is
shown in figure. It is known as under-compound generator.
V<Eg

37
 DC MOTORS
2.9 Construction
DC Motor converts electrical energy (DC) into mechanical energy.
 Stator – The static part that houses the field windings and receives the supply and,
 Rotor – The rotating part that brings about the mechanical rotations.
 Yoke of dc motor.
 Poles of dc motor.
 Field winding of dc motor.
 Armature winding of dc motor.
 Commutator of dc motor.
 Brushes of dc motor.

Yoke of DC Motor
 The magnetic frame or the yoke of dc motor made up of cast iron or steel and forms an
integral part of the stator or the static part of the motor.
 Its main function is to form a protective covering over the inner sophisticated parts of the
motor and provide support to the armature.
 It also supports the field system by housing the magnetic poles and field winding of the
dc motor.

Poles of DC Motor
 The magnetic poles of DC motor are structures fitted onto the inner wall of the yoke with
screws. The construction of magnetic poles basically comprises of two parts namely,
 The pole core and the pole shoe stacked together under hydraulic pressure and then
attached to the yoke.
 The pole core is of small cross sectional area and its function is to just hold the pole shoe
over the yoke, whereas the pole shoe having a relatively larger cross-sectional area
spreads the flux produced over the air gap between the stator and rotor to reduce the
loss due to reluctance.
 The pole shoe also carries slots for the field windings that produce the field flux.

Field winding of DC Motor

 The field winding of dc motor are made with field coils (copper wire) wound over the slots
of the pole shoes in such a manner that when field current flows through it, then adjacent
poles have opposite polarity are produced.
 The field winding basically form an electromagnet, that produces field flux within which the
rotor armature of the dc motor rotates, and results in the effective flux cutting.

 The armature winding of dc motor is attached to the rotor, or the rotating part of the
machine, and as a result is subjected to altering magnetic field in the path of its rotation
which directly results in magnetic losses.
 For this reason the rotor is made of armature core, that’s made with several low-hysteresis
silicon steel lamination, to reduce the magnetic losses like hysteresis and eddy current loss
respectively.

38
  These laminated steel sheets are stacked together to form the cylindrical structure of the
armature core.
 The armature core are provided with slots made of the same material as the core to which
the armature winding made with several turns of copper wire distributed uniformly over the
entire periphery of the core.

Lap winding
In this case the number of parallel paths between conductors A is equal to the number of poles P.i.e A = P

Wave winding
Here in this case, the number of parallel paths between conductors A is always equal to 2 irrespective of the
number of poles. Hence the machine designs are made accordingly.

 The commutator of dc motor is a cylindrical structure made up of copper segments stacked

together, but insulated from each other by mica.
 Its main function as far as the dc motor is concerned is to commute or relay the supply from the
mains to the armature winding housed over a rotating structure through the brushes of dc motor.

 The brushes of dc motor are made with carbon or graphite structures, making sliding contact over
the rotating commutator.
 The brushes are used to relay the current from external circuit to the rotating commutator form where
it flows into the armature winding.
 So, the commutator and brush unit of the dc motor is concerned with transmitting the power from the
static electrical circuit to the mechanically rotating region or the rotor.
2.10 Working principle
A DC Motor works on the principle that “whenever a current carrying conductor is placed in a magnetic
field, it experiences a force”. The magnitude is given by
F = B.I.L
where, F = Force in Newton
B = Flux density in Weber/ meter2

I = Current in amperes flowing through the conductor

L = Length of the conductor in meters

The direction of force is given by Fleming’s left hand rule
2.11 Types of DC Motors
Depends upon the field winding connected to the armature
 DC Shunt Motor
 DC Series Motor
 DC Compound Motor
Cumulative Compound Motor
 Long shunt
 Short shunt
Differential Compound Motor
 Long shunt
 Short shunt

39
 DC shunt motor

Fig. 2.30 DC shunt motor

Ish = V/Rsh and
Ia = I– Ish.
where Iis the line current
Eb= V - IaRa– B.C.D – A.R.D
where B.C.D is brush contact drop(1 V/brush)
A.R.D is the armature reaction drop
Dc series motor

Fig. 2.31 DC shunt motor

Ia = I = Ise

Eb= V - Ia(Ra +Rse) – B.C.D – A.R.D.

Dc compound motor

Fig. 2.32 DC compound motor

40
 2.12 Characteristics of Dc motor
 Electrical characteristics (Ta / Ia characteristics)
 N/Ia characteristics.
 Mechanical characteristics (N/Ta characteristics)
2.12.1 Characteristics of Dc shunt motor

Fig. 2.33 Characteristics of DC shunt motor

2.12.2 Characteristics of Dc compound Motor

In the cumulative compound motor as Ia increases, flux i n c r e a s e s but the shunt field current
Ish and Ish remain constant and total flux increases.

Fig. 2.34 Characteristics of Dc compound Motor

41
2.13 Applications of Dc Motors
 DC Shunt Motor: When constant speed is required DC shunt motors are used.
Example: Lathes, Centrifugal pumps, fans, drilling machines. etc.
 DC Series Motor: For high starting torque we prefer DC series motor. Example: Electric
traction, electric locomotive, cranes, hoists, conveyors etc.
 DC Compound Motor: When we require constant speed and high starting torque
Cumulative compound motors are preferred. Example: shears, punches, coal cutting machine,
elevators, conveyors, printing presses etc. Differential compound motors have no practical
applications (being unstable).

2.14 Necessity of starter for Dc motor

. Three point starter

Fig. 2.35 Three point starter

To avoid excess inflow of current in the armature, some resistance is included at the starting.
• L, F, A are the three terminals of the starter, to which external connections are made.
• R is the starting resistance, which is divided into various studs.
• To start the motor, close the supply switch and the brass arm L is moved to the right to touch
stud no.1 of R.
• It also touches the brass arc, thus current will flow through shunt field winding as well as the
armature.
• The motor starts rotating, the starting arm is moved gradually and completely when the speed
is above 50% of its rated speed.
• Speed can be increased by field rheostat Rh if required.
• As long as motor is running and the supply is on, the brass arm L is held in the ON position
by the electromagnet E

42
 There are two protective devices in the starter, one is the electromagnet, (hold on coil).
• Under running condition when the power fails electromagnet E de-energizes and the spring
S attached to the brass arm pulls back to OFF position.
• The electromagnet E also prevents the motor from reaching dangerously high speed, when
the field circuit is opened under running condition.
The second protective device is an electromagnet M which, is known as the “over load
release”
(over load protection).

• When the current increases beyond the rated value, M attracts D; thereby short circuiting the
electromagnet E. The electromagnet E gets de- energized and hence the arm L is pulled
back to OFF position.
• This starter is normally used for starting D.C shunt motors.

2.15 Efficiency of the Motor

As pointed out earlier, for efficiency calculation of motor, first calculate the input power and then
subtract the losses to get the output mechanical power as shown below,

2.16 Efficiency of the generator

For generator start with output power of the generator and then add the losses to get the input
mechanical power and hence efficiency as shown below,

Condition for maximum efficiency

maximum efficiency occurs when copper loss = core loss, where, copper loss is the variable loss and
is a function of loading while the core loss is practically constant independent of degree of loading.
This condition can be stated in a different way: maximum efficiency occurs when the variable loss is
equal to the constant loss of the transformer.
2.17 Speed Control of shunt Motor
We know that the speed of shunt motor is given by:

43
  Varying armature resistance.
 varying field resistance
2.17.1 Ward Leonard control system
 Ward Leonard method of speed control is used for controlling the speed of a DC motor. It is a basic
armature control method. This control system is consisting of a dc motor M_1 and powered by a DC
generator G.
 In this method the speed of the dc motor (M_1) is controlled by applying variable voltage across its
armature. This variable voltage is obtained using a motor-generator set which consists of a motor
M_2(either ac or dc motor) directly coupled with the generator G.
 It is a very widely used method of speed control of DC motor.

Fig. 2.36 Ward leonard control system

 The speed of motor M1 is to be controlled which is powered by the generator G. The shunt field of the
motor M1 is connected across the dc supply lines. Now, generator G is driven by the motor M 2.
 The speed of the motor M2 is constant. When theoutput voltage of the generator is fed to the motor
M1 then the motor starts to rotate.
 When the output voltage of the generator varies then the speed of the motor also varies. Now
controlling the output voltage of the generator the speed of motor can also be controlled.
 For this purpose of controlling the output voltage, a field regulator is connected across the generator
with the dc supply lines to control the field excitation. The direction of rotation of the motor M 1 can be
reversed by excitation current of the generator and it can be done with the help of the reversing
switch R.S. But the motor-generator set must run in the same direction.

Advantages of ward leonard system

 It is a very smooth speed control system over a very wide range (from zero to normal
speed of the motor).
 The speed can be controlled in both the direction of rotation of the motor easily.
 The motor can run with a uniform acceleration.
 Speed regulation of DC motor in this ward Leonard system is very good.

44
Disadvantages of ward leonard system

 The system is very costly because two extra machines (motor-generator set) are
required.
 Overall efficiency of the system is not sufficient especially it is lightly loaded.

Application of ward leonard system

 . This speed control system is mainly used in colliery winders, cranes, electric
excavators, mine hoists, elevators, steel rolling mills and paper machines etc.

45
 UNIT III

A.C MACHINES

3.1. Introduction

 One of the most common electrical motor used in most applications which is known as induction
motor. This motor is also called as asynchronous motor because it runs at a speed less than
synchronous speed.
 Synchronous speed is the speed of rotation of the magnetic field in a rotary machine and it
depends upon the frequency and number poles of the machine.
 An induction motor always runs at a speed less than synchronous speed because the rotating
magnetic field which is produced in the stator will generate flux in the rotor which will make the
rotor to rotate, but due to the lagging of flux current in the rotor with flux current in the stator, the
rotor will never reach to its rotating magnetic field speed i.e. the synchronous speed.
 There are basically two types of induction motor that depend upon the input supply - single phase
induction motor and three phase induction motor.

3.2. Constructional details

The Induction Motor comprises of two major parts:

 Stator
 Rotor

3.2.1. Stator

The stator of the three phase induction motor consists of three main parts:

 Stator frame
 Stator core
 Stator winding or field winding
1) Stator frame
 It is the outer most part of the three phase induction motor.
 Its main function is to support the stator core and the field winding.
 It acts as a covering and provide protection and mechanical strength to all the inner
parts of the machine.
 The frame is either made up of die cast or fabricated steel.
 The frame of three phase induction motor should be very strong and rigid as the air
gap length of three phase induction motor is very small, otherwise rotor will not
remain concentric with stator which will give rise to unbalanced magnetic pull.
2) Stator core
 The main function of the stator core is to carry alternating flux.
 In order to reduce the eddy current losses the stator core is laminated.
 This laminated type of structure is made up of stamping which is about 0.4 to 0.5 mm
thick.
 All the stamping are stamped together to form stator core, which is then housed in
stator frame.
 The stamping is generally made up of silicon steel, which reduces the hysteresis
loss.

46
3) Stator winding or field winding
 The slots on the periphery of stator core of the three phase induction motor carries three
phase windings.
 This three phase winding is supplied by three phase ac supply.
 The three phases of the winding are connected either in star or delta depending upon
which type of starting method is used.
 The winding wound on the stator of three phase induction motor is also called field
winding and when this winding is excited by three phase ac supply it produces rotating
magnetic field.

Fig. 3.1 Cross Section of Induction Motor

3.2.2. Rotor

 There are two types of rotors used in Induction motors

a) Squirrel Cage rotor
b) Slip ring or wound rotor
a. Squirrel Cage Rotor
 The rotor of the squirrel cage three phase induction motor is cylindrical in shape and has
slots on its periphery.
 The slots are not made parallel to each other but are skewed as the skewing prevents
magnetic locking of stator and rotor teeth and makes the working of motor more smooth
and quieter.
 The squirrel cage rotor consists of aluminum, brass or copper bars. This aluminum, brass
or copper bars are called rotor conductors and are placed in the slots on the periphery of
the rotor.
 The rotor conductors are permanently shorted by the copper or aluminum rings called
the end rings. In order to provide mechanical strength these rotor conductor are braced to
the end ring.
 The absence of slip ring and brushes make the construction of Squirrel cage three phase
induction motor very simple and robust.

47
 Fig. 3.2 Squirrel Cage Rotor

b. Slip ring wound rotor

 In this type of three phase induction motor the rotor is wound for the same number of
poles as that of stator but it has less number of slots and has less turns per phase of a
heavier conductor.
 The rotor also carries star or delta winding similar to that of stator winding.
 The rotor consists of numbers of slots and rotor winding are placed inside these slots.
 The three end terminals are connected together to form star connection.
 The three ends of three phase windings are permanently connected to the slip rings.
 The external resistance can be easily connected through the brushes and slip rings and
hence used for speed control and improving the starting torque of three phase induction
motor.
 The brushes are used to carry current to and from the rotor winding.
 At starting, the resistance are connected in rotor circuit and is gradually cut out as the
rotor pick up its speed.
 When the motor is running the slip ring are shorted by connecting a metal collar, which
connect all slip ring together and the brushes are also removed. This reduces wear and
tear of the brushes.

Fig. 3.3 Slip Ring Rotor

3.3. Working Principle of Induction Motor

 When a 3 phase supply is given to the stator winding, rotating magnetic field will generate in the
coil due to flow of current in the coil which rotates at the synchronous speed
 As the rotor winding in an induction motor are either closed through an external resistance or
directly shorted by end ring.

48
  The flux from the stator will cut the coil in the rotor and since the rotor coils are short circuited,
according to Faraday's law of electromagnetic induction, electric current will start flowing in the
coil of the rotor.
 The relative velocity between the rotating flux and static rotor conductor is the cause of electric
current generation
 When the current will flow, another flux will get generated in the rotor.
 Now there will be two fluxes, one is stator flux and another is rotor flux and the rotor flux will be
lagging to the stator flux.
 Due to this, the rotor will exert a torque which will make the rotor to rotate in the direction of
rotating magnetic flux.
 So the speed of the rotor will be depending upon the ac supply and the speed can be controlled
by varying the input supply.
 The rotor speed should not reach the synchronous speed produced by the stator.
 If the speeds equals, there would be no such relative velocity, so no emf induction in the rotor,
and no current would be flowing, and therefore no torque would be generated.
 The difference between the stator (synchronous speed) and rotor speeds is called the slip.

3.4. Advantages of three phase induction motor

 Self-starting.
 Less armature reaction and brush sparking because of the absence of commutators and brushes
that may cause sparks.
 Robust in construction.
 Economical.
 Easier to maintain.

3.5. Applications

 Lathes
 Drilling machine
 Fan
 Blower
 Printing machines
 Hoists
 Cranes
 Elevator

3.6. Difference between slip ring and squirrel cage induction motor

Sl.No Slip Ring or Phase Wound Induction Motor Squirrel Cage Induction Motor

Construction is complicated due to Construction is very simple

1
presence of slip ring and brushes

The rotor winding is similar to the stator The rotor consists of rotor bars which are
2
winding permanently shorted with the help of end rings

We can easily add rotor resistance by Since the rotor bars are permanently shorted, it’s
3
using slip ring and brushes not possible to add external resistance

4 Due to presence of external resistance Staring torque is low and cannot be improved

49
 high starting torque can be obtained

5 Slip ring and brushes are present Slip ring and brushes are absent

Frequent maintenance is required due to Less maintenance is required

6
presence of brushes

The construction is complicated and the The construction is simple and robust and it is
7 presence of brushes and slip ring makes cheap as compared to slip ring induction motor
the motor more costly

This motor is rarely used only 10 % Due to its simple construction and low cost. The
8
industry uses slip ring induction motor squirrel cage induction motor is widely used

Rotor copper losses are high and hence Less rotor copper losses and hence high
9
less efficiency efficiency

Speed control by rotor resistance method Speed control by rotor resistance method is not
10
is possible possible

Slip ring induction motor are used where Squirrel cage induction motor is used in lathes,
11 high starting torque is required i.e in hoists, drilling machine, fan, blower printing machines
cranes, elevator etc etc

3.7. Starting methods

Starting methods of an Induction motor is used to

 Reduce heavy starting currents and prevent motor from overheating.

 Provide overload and no-voltage protection

There are many methods to start 3-phase induction motors. Some of the common methods are;

 Direct On-Line Starter (DOL)

 Star-Delta Starter
 Auto Transformer Starter
 Rotor Impedance Starter
 Power Electronics Starter

3.7.1. Direct On- Line Starter (DOL)

 The Direct On-Line (DOL) starter is the simplest and the most inexpensive of all starting
methods and is usually used for squirrel cage induction motors.
 It directly connects the contacts of the motor to the full supply voltage.
 The starting current is very large, normally 6 to 8 times the rated current.
 The starting torque is likely to be 0.75 to 2 times the full load torque.
 In order to avoid excessive voltage drops in the supply line due to high starting currents,
the DOL starter is used only for motors with a rating of less than 5KW.

50
 Fig. 3.4 Power Circuit

 K1M-Main Contactor
 The DOL starter consists of a coil operated contactor K1M controlled by start and stop
push buttons.
 On pressing the start push button S1, the contactor coil K1M is energized from line L1.
 The three mains contacts (1-2), (3-4), and (5-6) are closed.
 The motor is thus connected to the supply. When the stop push button S2 is pressed, the
supply through the contactor K1M is disconnected.
 Since the K1M is de-energized, the main contacts (1-2), (3-4), and (5-6) are opened. The
supply to motor is disconnected and the motor stops.

3.7.2. Star-Delta Starter

 The star delta starting is a very common type of starter and extensively used, compared
to the other types of the starters.
 This method achieves low starting current by first connecting the stator winding in star
configuration, and then after the motor reaches a certain speed, throw switch changes
the winding arrangements from star to delta configuration.
 By connecting the stator windings, first in star and then in delta, the line current drawn by
the motor at starting is reduced to one-third as compared to starting current with the
windings connected in delta.
 Since the torque developed by an induction motor is proportional to the square of the
applied voltage, star- delta starting reduced the starting torque to one – third that
obtainable by direct delta starting.

K2M Main Contactor

K3M Delta Contactor

K1M Star Contactor

F1 Thermal Overload Relay

51
 Fig. 3.5 Star Delta Starter

3.7.3. Auto Transformer Starter

 The operation principle of auto transformer method is similar to the star delta starter
method.
 The starting current is limited by (using a three phase auto transformer) reducing the
initial stator applied voltage.
 The auto transformer starter is more expensive, more complicated in operation and
bulkier in construction when compared with the star – delta starter method.
 But an auto transformer starter is suitable for both star and delta connected motors, and
the starting current and torque can be adjusted to a desired value by taking the correct
tapping from the auto transformer.

Fig. 3.6 Auto Transformer Starter

3.7.4. Rotor Impedance Starter

 This method allows external resistance to be connected to the rotor through slip rings and
brushes.
 Initially, the rotor resistance is set to maximum and is then gradually decreased as the
motor speed increases, until it becomes zero.
 The rotor impedance starting mechanism is usually very bulky and expensive when
compared with other methods.
 It also has very high maintenance costs. Also, a heat is generated through the resistors
when current runs through them.
 The starting frequency is also limited in this method.

52
  However, the rotor impedance method allows the motor to be started while on load.

Fig. 3.7 Rotor Impedance Starter

3.8. Speed control methods

Synchronous speed

Where f = frequency and P is the number of poles

The speed of induction motor is given by,

Where N is the speed of rotor of induction motor,

NS is the synchronous speed,
S is the slip.

The speed control of three phase induction motor from stator side are further classified as

 V / f control or frequency control

 Changing the number of stator poles

 Controlling supply voltage

 Adding rheostat in the stator circuit

The speed controls of three phase induction motor from rotor side are further classified as

 Adding external resistance on rotor side

 Cascade control method

 Injecting slip frequency emf into rotor side

53
3.8.1. Speed Control from Stator Side

1. V / f control or frequency control

 Whenever three phase supply is given to three phase induction motor rotating magnetic
field is produced which rotates at synchronous speed given by

 In three phase induction motor emf is induced by induction similar to that oftransformer
which is given by

Where K is the winding constant, T is the number of turns per phase and f is frequency.
 Now if we change frequency, synchronous speed changes but with decrease in
frequency flux will increase and this change in value of flux causes saturation of rotor and
stator cores which will further cause increase in no load current of the motor .
 So, it’s important to maintain flux , φ constant and it is only possible if we change voltage
.
 If we decrease frequency flux increases but at the same time if we decrease voltage flux
will also decease causing no change in flux and hence it remains constant.
 So, here we are keeping the ratio of V/ f as constant.

2. Controlling supply voltage

 The torque produced by running three phase induction motor is given by

 In low slip region (sX)2 is very small as compared to R2 . So, it can be neglected. So
torque becomes

 Since rotor resistance, R2 is constant so the equation of torque further reduces to

 We know that rotor induced emf E2 ∝ V. So, T ∝ sV2.

 If we decrease supply voltage torque will also decrease.
 But for supplying the same load, the torque must remains the same and it is only possible
if we increase the slip and if the slip increases the motor will run at reduced speed .
 This method of speed control is rarely used because small change in speed requires
large reduction in voltage, and hence the current drawn by motor increases, which cause
over heating of induction motor.

54
 Fig. 3.8 Torque- Speed Curve for stator voltage control

3. Changing the number of stator poles

The stator poles can be changed by two methods

 Multiple stator winding method

 Pole amplitude modulation method (PAM)

1. Multiple stator winding method

 In this method of speed control of three phase induction motor, the stator is provided by
two separate winding.
 These two stator windings are electrically isolated from each other and are wound for two
different pole numbers.
 Using switching arrangement, at a time, supply is given to one winding only and hence
speed control is possible.
 Disadvantages of this method are that the smooth speed control is not possible.
 This method is more costly and less efficient as two different stator winding are required.
 This method of speed control can only be applied for squirrel cage motor.
2. Pole amplitude modulation method (PAM)
 In this method of speed control of three phase induction motor the original sinusoidal mmf
wave is modulated by another sinusoidal mmf wave having different number of poles.
Let f1(θ) be the original mmf wave of induction motor whose speed is to be controlled
f2(θ) be the modulation mmf wave
P1 be the number of poles of induction motor whose speed is to be controlled
P2 be the number of poles of modulation wave

Therefore by changing the number of poles we can easily change the speed of three phase
induction motor.

55
 4. Adding rheostat in the stator circuit

 In this method of speed control of three phase induction motor rheostat is added in the
stator circuit due to this voltage gets dropped.
 In case of three phase induction motor torque produced is given by T ∝ sV22.
 If we decrease supply voltage torque will also decrease.
 But for supplying the same load , the torque must remains the same and it is only
possible if we increase the slip and if the slip increase motor will run At reduced speed

3.8.2. Speed Control from Rotor Side

1. Adding external resistance on rotor side

 In this method of speed control of three phase induction motor external resistance are
added on rotor side. The equation of torque for three phase induction motor is

 The three phase induction motor operates in low slip region .In low slip region term
(sX)2becomes very small as compared to R2. So, it can be neglected . and also E2 is
constant. So the equation of torque after simplification becomes,

 Now if we increase rotor resistance R2, torque decreases but to supply the same load
torque must remains constant.
 So, we increase slip, which will further results in decrease in rotor speed.
 Thus by adding additional resistance in rotor circuit we can decrease the speed of three
phase induction motor.
 The main advantage of this method is that with addition of external resistance starting
torque increases but this method of speed control of three phase induction motor also
suffers from some disadvantages :
 The speed above the normal value is not possible
 Large speed change requires large value of resistance and if such large value of
resistance is added in the circuit it will cause large copper loss and hence reduction in
efficiency
 .Presence of resistance causes more losses
This method cannot be used for squirrel cage induction motor

Fig. 3.9 Torque- Speed Curve for Rotor Resistance Control

56
2. Cascade control method

 In this method of speed control of three phase induction motor, the two three phase
induction motor are connected on common shaft and hence called cascaded motor.
 One motor is the called the main motor and another motor is called the auxiliary motor.
 The three phase supply is given to the stator of the main motor while the auxiliary motor
is derived at a slip frequency from the slip ring of main motor
Let NS1 be the synchronous speed of main motor
NS2 be the synchronous speed of auxiliary motor
P1 be the number of poles of the main motor
P2 be the number of poles of the auxiliary motor
F is the supply frequency
F1 is the frequency of rotor induced emf of main motor
N is the speed of set and it remains same for both the main and auxiliary motor as both
the motors are mounted on common shaft
S1 is the slip of main motor

The auxiliary motor is supplied with same frequency as the main motor i.e

Now put the value of

Now at no load , the speed of auxiliary rotor is almost same as its synchronous speed i.e
N = NS2

Now rearrange the above equation and find out the value of N, we get,

 This cascaded set of two motors will now run at new speed having number of poles (P 1 +
P2).
 In the above method the torque produced by the main and auxiliary motor will act in same
direction, resulting in number of poles (P1 + P2).
 Such type of cascading is called cumulative cascading.
 There is one more type of cascading in which the torque produced by the main motor is
in opposite direction to that of auxiliary motor. Such type of cascading is called
differential cascading; resulting in speed corresponds to number of poles (P1 - P2).

In this method of speed control of three phase induction motor, four different speeds can be
obtained

57
  When only main induction motor work, having speed corresponds to NS1 = 120 F / P1

 When only auxiliary induction motor work, having speed corresponds to NS2 = 120 F
/ P2

 When cumulative cascading is done, then the complete set runs at a speed of N =
120F / (P1+ P2)

 When differential cascading is done, then the complete set runs at a speed of N =
120F / (P1- P2)

3. Injecting slip frequency emf into rotor side

 When the speed control of three phase induction motor is done by adding resistance in
rotor circuit, some part of power called, the slip power is lost as I2R losses.
 Therefore the efficiency of three phase induction motor is reduced by this method of
speed control.
 This slip power loss can be recovered and supplied back in order to improve the overall
efficiency of three phase induction motor and this scheme of recovering the power is
called slip power recovery scheme and this is done by connecting an external source of
emf of slip frequency to the rotor circuit.
 The injected emf can either oppose the rotor induced emf or aids the rotor induced emf.
 If it oppose the rotor induced emf, the total rotor resistance increases and hence speed
decreases and if the injected emf aids the main rotor emf the total resistance decreases
and hence speed increases.
 Therefore by injecting induced emf in rotor circuit the speed can be easily controlled.

3.9. Single phase induction motor

3.9.1Why Single Phase Induction Motor is not Self Starting?

 We know that the ac supply is a sinusoidal wave and it produces pulsating

magnetic field in uniformly distributed stator winding.

 Since pulsating magnetic field can be assumed as two oppositely rotating

magnetic fields, there will be no resultant torque produced at the starting and due
to this the motor does not run.

 After giving the supply, if the rotor is made to rotate in either direction by external
force, then the motor will start to run.

 This problem has been solved by making the stator winding into two winding, one
is main winding and another is auxiliary winding and a capacitor is fixed in series
with the auxiliary winding.

 This will make a phase difference when electric current will flow through the both
coils. When there will be phase difference, the rotor will generate a starting
torque and it will start to rotate.

 The direction of rotation of induction motor can easily be changed by changing

the sequence of three phase supply, i.e. if RYB is in forward direction, the RBY
will make the motor to rotate in reverse direction in the case of three phase motor
but in single phase motor, the direction can be reversed by reversing the
capacitor terminals in the winding.

58
 3.9.2. Advantages

 Simple in construction
 Cheap in cost
 Reliable
 Easy to repair and maintain.

3.9.3. Applications

 Vacuum cleaner
 Fans
 Washing machine
 Centrifugal pump
 Blowers
 Washing machine
 Small toys

3.10. Construction and working principle of single phase induction motor

3.10.1. Stator of Single Phase Induction Motor

 The stator of the single phase induction motor has laminated stamping to reduce eddy
current losses on its periphery.
 The slots are provided on its stamping to carry stator or main winding.
 In order to reduce the hysteresis losses, stamping are made up of silicon steel.
 When the stator winding is given a single phase ac supply, the magnetic field is produced
and the motor rotates at a speed slightly less than the synchronous speed N swhich is
given by

 The single phase induction motors are mostly provided with concentric coils. As the
number of turns per coil can be easily adjusted with the help of concentric coils, the mmf
distribution is almost sinusoidal.
 Except for shaded pole motor, the asynchronous motor has two stator windings namely
the main winding and the auxiliary winding. These two windings are placed in space
quadrature with respect to each other.

3.10.2. Rotor of Single Phase Induction Motor

 The construction of the rotor of the single phase induction motor is similar to the squirrel
cage three phase induction motor.
 The rotor is cylindrical in shape and has slots all over its periphery.
 The slots are not made parallel to each other but are bit skewed as the skewing prevents
magnetic locking of stator and rotor teeth and makes the working of induction motor more
smooth and quieter.
 The squirrel cage rotor consists of aluminium, brass or copper bars.
 These aluminium or copper bars are called rotor conductors and are placed in the slots
on the periphery of the rotor.
 The rotor conductors are permanently shorted by the copper or aluminium rings called
the end rings.

59
  The absence of slip ring and brushes make the construction of single phase induction
motor very simple and robust.

3.10.3. Working Principle of Single Phase Induction Motor

 According to double field revolving theory, any alternating quantity can be resolved into
two components, each component have magnitude equal to the half of the maximum
magnitude of the alternating quantity and both these component rotates in opposite
direction to each other.
 For example - a flux, φ can be resolved into two components

 Each of these components rotates in opposite direction i. e if one φ m / 2 is rotating in

clockwise direction then the other φm / 2 rotates in anticlockwise direction.
 When a single phase ac supply is given to the stator winding of single phase induction
motor, it produces its flux of magnitude, φm.
 According to the double field revolving theory, this alternating flux, φ m is divided into two
components of magnitude φm /2. Each of these components will rotate in opposite
direction, with the synchronous speed, Ns.
 Let us call these two components of flux as forward component of flux, φ f and backward
component of flux, φb.
 The resultant of these two component of flux at any instant of time, gives the value of
instantaneous stator flux at that particular instant.

 Now at starting, both the forward and backward components of flux are exactly opposite
to each other.
 Also both of these components of flux are equal in magnitude. So, they cancel each other
and hence the net torque experienced by the rotor at starting is zero. So, the single
phase induction motors are not self starting motors.

3.11. Starting of Single Phase Induction Motors

The single phases induction motors are classified based on the method of starting method.
 Split phase induction motor
 Capacitor start inductor motor
 Capacitor start capacitor run induction motor
 Shaded pole induction motor

3.11.1. Split Phase Induction Motor

 In addition to the main winding or running winding, the stator of single phase induction
motor carries another winding called auxiliary winding or starting winding.
 A centrifugal switch is connected in series with auxiliary winding. The purpose of this switch is
to disconnect the auxiliary winding from the main circuit when the motor attains a speed up to
75 to 80% of the synchronous speed.
 The running winding is inductive in nature.To create the phase difference between the two
winding and this is possible if the starting winding carries high resistance.

60
  Irun is the current flowing through the main or running winding,
 Istart is the current flowing in starting winding and VT is the supply voltage.

Fig. 3.10 Split Phase Induction Motor

 For highly resistive winding the current is almost in phase with the voltage and for highly
inductive winding the current lag behind the voltage by large angle.
 The starting winding is highly resistive so, the current flowing in the starting winding lags
behind the applied voltage by very small angle and the running winding is highly inductive
in nature so, the current flowing in running winding lags behind applied voltage by large
angle.
 The resultant of these two current is IT. The resultant of these two current produce
rotating magnetic field which rotates in one direction.
 In split phase induction motor the starting and main current get splitted from each
other by some angle so this motor got its name as split phase induction motor.

Applications of Split Phase Induction Motor

 Split phase induction motors have low starting current and moderate starting torque. So
these motors are used in fans, blowers, centrifugal pumps, washing machine grinder etc.
These motors are available in the size ranging from 1 / 20 to 1 / 2 KW.

3.11.2. Capacitor Start IM and Capacitor Start Capacitor Run IM

Fig. 3.11 Capacitor Start Induction Motor

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  The working principle and construction of Capacitor start inductor motors and capacitor
start capacitor run induction motors are almost the same.
 In order to produce rotating magnetic field there must be some phase difference.
 In case of split phase induction motor we use resistance for creating phase difference but
here we use capacitor for this purpose.
 The current flowing through the capacitor leads the voltage. So, in capacitor start
inductor motor and capacitor start capacitor run induction motor we are using two
winding, the main winding and the starting winding.
 With starting winding we connect a capacitor so the current flowing in the capacitor i.e
Ist leads the applied voltage by some angle, φst.
 The running winding is inductive in nature so, the current flowing in running winding lags
behind applied voltage by an angle, φm.
 Now there occur large phase angle differences between these two currents which
produce a resultant current, I and this will produce a rotating magnetic field. Since the
torque produced by these motors depends upon the phase angle difference, which is
almost 90°. So, these motors produce very high starting torque.
 In case of capacitor start induction motor, the centrifugal switch is provided so as to
disconnect the starting winding when the motor attains a speed up to 75 to 80% of the
synchronous speed but in case of capacitor start capacitors run induction
motor there is no centrifugal switch so, the capacitor remains in the circuit and helps to
improve the power factor and the running conditions of single phase induction motor.

Application of Capacitor Start IM and Capacitor Start Capacitor Run IM

 These motors have high starting torque hence they are used in conveyors, grinder, air
conditioners etc. They are available up to 6 KW.

3.11.3. Shaded Pole Single Phase Induction Motors

Fig. 3.12 Shaded Pole Induction Motor

 The stator of the shaded pole single phase induction motor has salient or projected
poles. These poles are shaded by copper band or ring which is inductive in nature. The
poles are divided into two unequal halves. The smaller portion carries the copper band
and is called as shaded portion of the pole.
 When a single phase supply is given to the stator of shaded pole induction motor an
alternating flux is produced. This change of flux induces emf in the shaded coil. Since this
shaded portion is short circuited, the current is produced in it in such a direction to
oppose the main flux.
The flux in shaded pole lags behind the flux in the unshaded pole. The phase difference
between these two fluxes produces resultant rotating flux.

62
  When the flux changes its value from zero to nearly maximum positive value.
 When the flux remains almost constant at its maximum value.
 When the flux decreases from maximum positive value to zero.

Advantages of Shaded Pole Motor

 Very economical and reliable.

 Construction is simple and robust because there is no centrifugal switch.

Disadvantages Of Shaded Pole Induction Motor

 Low power factor.

 The starting torque is very poor.
 The efficiency is very low as, the copper losses are high due to presence of copper band.
 The speed reversal is also difficult and expensive as it requires another set of copper
rings.

Applications of Shaded pole motor

 Due to their low starting torques and reasonable cost these motors are mostly employed
in small instruments, hair dryers, toys, record players, small fans, electric clocks etc. These
motors are usually available in a range of 1/300 to 1/20 KW.

3.12 Slip of induction motor

 When have seen that rotor rotates in the same direction as that of R.M.F. but in steady
state attains a speed less than the synchronous speed.
 The difference between the two speeds i.e. synchronous speed of R.M.F. ( Ns ) and
rotor speed (N) is called slip speed. This slip speed is generally expressed as the
percentage of the synchronous speed.
 So slip of the induction motor is defined as the difference between the synchronous
speed (Ns) and actual speed of rotor i.e. motor (N) expressed as a friction of the
synchronous speed (Ns).
This is also called absolute slip or fractional slip and is denoted as 's'.Thus

The percentage slip is expressed as,

In terms of slip, the actual speed of motor (N) can be expressed as,

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 3.13 Torque-Slip characteristics
 As the induction motor is located from no load to full load, its speed decreases hence
slip increases. Due to the increased. load, motor has to produce more torque to satisfy
load demand.
 The torque ultimately depends on slip as explained earlier.
 The behavior of motor can be easily judged by sketching a curve obtained by plotting
torque produced against slip of induction motor.
 The curve obtained by plotting torque against slip from s = 1 (at start) to s = 0 (at
synchronous speed) is called torque-slip characteristics of the induction motor. It is
very interesting to study the nature of torque- slip characteristics.
We have seen that for a constant supply voltage, E2 is also constant. So we can write
torque equations as,

Now to judge the nature of torque-slip characteristics let us divide the slip range (s = 0 to s = 1)
into two parts and analyse them independently.
i) Low slip region
2
In low slip region, 's' is very very small. Due to this, the term (s X 2) is so small as compared to
R2 2 that it can be neglected.

 Hence in low slip region torque is directly proportional to slip. So as load increases,
speed decreases, increasing the slip. This increases the torque which satisfies the load demand.
 Hence the graph is straight line in nature.
 At N = Ns , s = 0 hence T = 0. As no torque is generated at N = Ns, motor stops if it tries
to achieve the synchronous speed. Torque increases linearly in this region, of low slip values.
ii) High slip region
In this region, slip is high i.e. slip value is approaching to 1. Here it can be assumed that the term
R2 2 is very very small as compared to (s X2)2. Hence neglecting from the denominator, we get

 So in high slip region torque is inversely proportional to the slip. Hence its nature is like
rectangular hyperbola.
 Now when load increases, load demand increases but speed decreases. As speed decreases,
slip increases. In high slip region as T α 1/s, torque decreases as slip increases.
 But torque must increases to satisfy the load demand. As torque decreases, due to extra
loading effect, speed further decreases and slip further increases.
 Again torque decreases as T α 1/s hence same load acts as an extra load due to reduction in

64
 torque produced. Hence speed further drops.
 Eventually motor comes to standstill condition. The motor cannot continue to rotate at any point
in this high slip region. Hence this region is called unstable region of operation.
So torque - slip characteristics has two parts,
 Straight line called stable region of
operation
 Rectangular hyperbola called unstable region of operation.

 In low slip region, as load increases, slip increases and torque also increases linearly.
 Every motor has its own limit to produce a torque.
 The maximum torque, the motor can produces as load increases is Tm which occurs at s = sm.
So linear behavior continues till s = sm.
 If load is increased beyond this limit, motor slip acts dominantly pushing motor into high
slipregion.
 Due to unstable conditions, motor comes to standstill condition at such a load.
 Hence i.e. maximum torque which motor can produce is also called breakdown torque or
pull out torque.
 So range s = 0 to s = s m is called low slip region, known as stable region of operation.
Motor always operates at a point in this region.
 And range s = s m to s = 1 is called high slip region which is rectangular hyperbola, called
unstable region of operation.
 Motor cannot continue to rotate at any point in this region.
At s = 1, N = 0 i.e. start, motor produces a torque called starting torque denoted as
Tst. The entire torque - slip characteristics is shown in the Fig. 3.13

Full load torque

Fig. 3.13 Torque speed characteristics

 When the load on the motor increases, the torque produced increases as speed decreases
and slip increases. The increases torque demand is satisfied by drawing motor current from
the supply.
 The load which motor can drive safely while operating continuously and due to such load, the

65
 current drawn is also within safe limits is called full load condition of motor. When current
increases, due to heat produced the temperature rise.
 The safe limit of current is that which when drawn for continuous operation of motor, produces
a temperature rise well within the limits. Such a full load point is shown on the torque-slip
characteristics torque as TF.L.
 The interesting thing is that the load on the motor can be increased beyond point C till
maximum torque condition.
 But due to high current and hence high temperature rise there is possibility of damage of
winding insulation, if motor is operated for longer time duration in this region i.e. from point C
to B. But motor can be used to drive loads more than full load, producing torque upto maximum
torque for short duration of time. Generally full load torque is less than the maximum
torque.
 So region OC upto full load condition allow motor operation continuously and safely from the
temperature point of view. While region CB is possible to achieve in practice but only for
short duration of time and not for continuous operation of motor.
 This is the difference between full load torque and the maximum or breakdown torque. The
breakdown torque is also called stalling torque.
 TFull load < Tm
Generating and braking region
 When the slip lies in the region 0 and 1 i.e. when 0 ≤s ≤1, the machine runs as a motor
which is the normal operation.
 The rotation of rotor is in the direction of rotating field which is developed by stator
currents. In this region it takes electrical power from supply lines and supplies
mechanical power output.
 The rotor speed and corresponding torque are in same direction.

Fig. 3.14 Regions of torque - slip characteristics

When the slip is greater than 1, the machine works in the braking mode.
 The motor is rotated in opposite direction to that of rotating field. In practice two of the stator
terminals are interchanged which changes the phase sequence which in turn reverses the

66
 direction of rotation of magnetic field.
 The motor comes to quick stop under the influence of counter torque which produces braking
action. This method by which the motor comes to rest is known as plugging.
 Only care is taken that the stator must be disconnected from the supply to avoid the rotor to
rotate in other direction
 To run the induction machine as a generator, its slip must be less than zero i.e. negative.
 The negative slip indicates that the rotor is running at a speed above the synchronous speed.
When running as a generator it takes mechanical energy and supplies electrical energy from
the stator.
 Thus the negative slip, generation action takes place and nature of torque - slip characteristics
reverses in this generating region.
 The Fig.3.14 shows the complete torque - slip characteristics showing motoring, generating
and the braking region.

3.14 Electrical braking of an induction motor

 Dynamic or Rheostatic Braking
 D.C. Dynamic Braking
 Plugging
 Regenerative Braking

67
 UNIT-IV
SPECIAL MACHINES
4.1 Introduction
 As the name suggests, these motors are used on single-phase supply. Single phase motors are the
most familiar of all electric motors because they are extensively used in home appliances, shops,
offices etc.
 It is true that single phase motors are less efficient substitute for 3-phase motors but 3-phase power
is normally not available except in large commercial and industrial establishments.
4.2 Types of single-phase motors
Single-phase motors are generally built in the fractional-horsepower range and may be classified into
the following four basic types:
Single-phase induction motors
 split-phase type
 capacitor type
 shaded-pole type
A.C. series motor or universal motor
Repulsion motors
 Repulsion-start induction-run motor
 Repulsion-induction motor
Synchronous motors
 Reluctance motor
 Hysteresis motor
Universal motor
4.3 Types of servo motor

There are two types in servomotor, those are

 Dc servomotor
 Ac servomotor

4.3.1 D.C servomotor

A dc motor that is used in servo mechanism is called a dc servomotor. The three main types of dc
servomotors are,

 Field controlled dc servomotor

 Armature controlled dc servomotor
 Permanent magnet armature controlled dc servomotor

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 1. Field controlled dc servomotor

Fig. 4.1 Field controlled dc servomotor.

 Here the field winding is controlled by an electronic amplifier. The armature is supplied by a constant
current source. The error voltage represents the difference between the measured signal and the
desired signal the torque produced by this motor is zero when no field excitation is supplied by the dc
error amplifier.

Since armature current is always constant, the torque is directly proportional to the field flux and also
it is directly proportional to field current upto saturation (Tα Φ*Ia). If the polarity of the field is reversed, the
motor direction reverses. The control of field current by this method is used only in small servomotors
because of the following reasons.

 It is undesirable to supply a large and fixed armature current as would be required for large dc
servomotor,
 The time constant of highly inductive field circuit is very long. Therefore the dynamic response of
motor will be slow.
2. Armature controlled dc servomotor

Fig. 4.2 Armature controlled dc servomotor.

69
  Here the armature circuit is controlled by an electronic amplifier. The field winding is supplied by a
constant current source.

 The error voltage represents the difference between the measured signal and the desired signal.

 A sudden large or small change in armature voltage produced by an error signal will cause an almost
immediate response in torque.

 It is because the armature circuit is essentially resistive compared to the highly inductive field circuit
of field controlled dc servomotor.

 If the error signal and the polarity of the armature voltage are reversed the motor reverses its
direction.

3. Permanent magnet armature controlled dc servomotor

Fig. 4.3 Permanent magnet (PM) armature controlled dc servomotor

 It uses permanent magnets for constant field excitation instead of a constant current source.

 These type of motor use either alnico or ceramic magnet to produce the magnetic field.

 Permanent magnets have several advantages over wound field servomotors including increased
efficiency, reduced frame size and high accelerating torque.

 The speed of a PM dc servomotor is varied by changing the voltage applied to the armature.
Conventional wound field motor are often equipped with interpoles inorder to improve commutation
and minimise armature reaction.

 The high coercive force of permanent magnet materials used in PM motors eliminates the need for
interpoles due to their excellent commutation characteristics.

 PM servomotors are used in a wide variety of application including electric vehicles and cordless
power tools. The popularity of these motor is due to their high efficiency, compact design and good
commutation.

4.3.2A.C servomotor

An ac motor that is used in servomechanism is called an ac servomotor. Most ac servomotors

induction motors.

70
Construction

Fig. 4.4 Two phase ac servomotor.

 The squirrel cage rotor has high resistance and low inertia. The stator has two distributed windings
which are displaced from each other by 90º (electrical). One stator winding called main winding is
excited by a fixed ac voltage Vm .

 The other stator winding called control winding is fed by the output voltage V a of an amplifier. The
voltage Vmand Va must be in synchronism which means that they must be derived from the same ac
source. They must be mode to have a phase difference of 90º by connecting a suitable capacitor in
series with the main winding as shown in figure

Operations

 With zero error voltage, the squirrel cage rotor is at standstill. A small error signal is amplified by the
ac amplifier and fed to the control winding.

 The servomotor rotates with a speed proportional to this voltage and in a direction so as to reduce the
error signal. The motor ceases to rotate with zero error signal is produced at the control winding.

 Servomotors are designed to produce large valves of torque low speed.

 Since these motors are widely used in position control devices, it is important that the torque reduced
at high speeds to prevent the motor from overshooting its desired position.

 To reduce overshooting the squirrel cage rotor is made longer and thinner than in the ordinary
induction motor.

 The rotor bars have a high value of resistance that help to prevent the inductive reactance of the
motor affecting the torque. As a result, the torque of the motor decreases as the speed increases.

4.4 A.C. series motor or universal motor

Construction
The construction of en A.C. series motor is very similar to a D.C. series motor except that above
modifications are incorporated. such a motor can be operated either on A.C. or D.C. supply and the resulting
torque-speed curve is about the same in each case. For this reason, it is sometimes called a universal motor.

71
 Fig. 4.5 Universal Motor
Operation
 When the motor is connected to an A.C. supply, the same alternating current flows through
the field and armature windings.
 The field winding produces an alternating flux Φ
armature to produce a torque.
 Since both armature current and flux reverse simultaneously, the torque always acts in the
same direction. It may be noted that no rotating flux is produced in this type of machines.
 The principle of operation is the same as that of a D.C. series motor.
Characteristics
The operating characteristics of an a.c. series motor are similar to those of a D.C. series motor.
 The speed increases to a high value with a decrease in load. In very small series motors, the
losses are usually large enough at no load that limits the speed to a definite value (1500 -
15,000 r.p.m.).
 The motor torque is high for large armature currents, thus giving a high starting torque.
 At full-load, the power factor is about 90%. However, at starting or when carrying an
overload, the power factor is lower.
Applications
The fractional horsepower A.C. series motors have high-speed (and corresponding small size) and
large starting torque. They can, therefore, be used to drive:
 high-speed vacuum cleaners
 sewing machines
 electric shavers
 drills
 Machine tools etc.

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4.5 Unexcited synchronous single phase motor (or) single-phase synchronous motors
Very small single-phase motors have been developed which run at true synchronous speed. They do
not require D.C. excitation for the rotor. Because of these characteristics, they are called unexcited single-
phase synchronous motors.
The most commonly used types are:
 Reluctance motors
 Hysteresis motors
The efficiency and torque-developing ability of these motors is low; the output of most of the
commercial motors is only a few watts.
Reluctance motor
 It is a single-phase synchronous motor which does not require D.C. excitation to the rotor. Its
operation is based upon the following principle:
 “Whenever a piece of ferromagnetic material is located in a magnetic field; a force is exerted
on the material, tending to align the material so that reluctance of the magnetic path that
passes through the material is minimum”.
Construction
A reluctance motor (also called synchronous reluctance motor) consists of:
 a stator carrying a single-phase winding along with an auxiliary winding to produce a
synchronous-revolving magnetic field.
 a squirrel-cage rotor having unsymmetrical magnetic construction. This is achieved by
symmetrically removing some of the teeth from the squirrel cage rotor to produce salient
poles on the rotor. As shown in the figure shown below 4 salient poles have been produced
on the rotor. The salient poles created on the rotor must be equal to the poles on the stator.

Fig. 4.6 Reluctance motor

Note that rotor salient poles offer low reluctance to the stator flux and, therefore, become strongly
magnetized.
Operation
 When single-phase stator having an auxiliary winding is energized, a synchronously-revolving field is
produced. The motor starts as a standard squirrel-cage induction motor and will accelerate to near its
synchronous speed.
 As the rotor approaches synchronous speed, the rotating stator flux will exert reluctance torque on
the rotor poles tending to align the salient-pole axis with the axis of the rotating field. The rotor
assumes a position where its salient poles lock with the poles of the revolving field [as shown in the

73
 fig above]. Consequently, the motor will continue to run at the speed of revolving flux i.e., at the
synchronous speed.
 When we apply a mechanical load, the rotor poles fall slightly behind the stator poles, while
continuing to turn at synchronous speed. As the load on the motor is increased, the mechanical angle
between the poles increases progressively. Nevertheless, magnetic attraction keeps the rotor locked
to the rotating flux. If the load is increased beyond the amount under which the reluctance torque can
maintain synchronous speed, the rotor drops out of step with the revolving field. The speed, then,
drops to some value at which the slip is sufficient to develop the necessary torque to drive the load by
induction-motor action.
Characteristics
 These motors have poor torque, power factor and efficiency.
 These motors cannot accelerate high-inertia loads to synchronous speed.
 The pull-in and pull-out torques of such motors are weak.
Despite the above drawbacks, the reluctance motor is cheaper than any other type of synchronous motor.
They are widely used for constant-speed applications such as timing devices, signaling devices etc.
Hysteresis motor
It is a single-phase motor whose operation depends upon the hysteresis effect i.e., magnetization
produced in a ferromagnetic material lags behind the magnetizing force.
Construction
It consists of:
 a stator designed to produce a synchronously-revolving field from a single-phase supply. This is
accomplished by using permanent-split capacitor type construction. Consequently, both the windings
(i.e., starting as well as main winding) remain connected in the circuit during running operation as well
as at starting. The value of capacitance is so adjusted as to result in a flux revolving at synchronous
speed.
 a rotor consisting of a smooth cylinder of magnetically hard steel, without winding or teeth.
Operation
 When the stator is energized from a single-phase supply, a synchronously revolving field (assumed in
anti-clockwise direction) is produced due to split-phase operation.
 The revolving stator flux magnetizes the rotor. Due to hysteresis effect, the axis of magnetization of
rotor will lag behind the axis of stator field by hysteresis lag angle a as shown in fig. given below.
Thus the rotor and stator poles are locked. .
 After reaching synchronism, the motor continues to run at synchronous speed and adjusts its torque
angle so as to develop the torque required by the load.
Characteristics
 A hysteresis motor can synchronize any load which it can accelerate, no matter how great the
inertia. It is because the torque is uniform from standstill to synchronous speed.
 Since the rotor has no teeth or salient poles or winding, a hysteresis motor is inherently quiet
and produces smooth rotation of the load.
 The rotor takes on the same number of poles as the stator field. Thus by changing the
number of stator poles through pole-changing connections, we can get a set of synchronous
speeds for the motor.

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 Fig. 4.7 Hysteresis motor
Applications
Due to their quiet operation and ability to drive high-inertia toads, hysteresis motors are particularly
well suited for driving
 electric clocks
 timing devices and tape-decks
 From-tables and other precision audio-equipment.
4.6 Stepper motors

Introduction

 A stepper motor is a brushless DC motor whose rotor rotates in a discrete angular

displacements when its stator windings are energized in a programmed manner.

 Rotation occurs because of magnetic interaction between rotor poles and poles of the
sequentially energized stator winding.

 The rotor has no electrical windings, but has salient and / or magnetized poles.

 From the definition it is clear that a stepper motor is a digital actuator whose input is in the
form of digital signals and Whose output is in the form of discrete angular rotation.

 As the angular rotation is dependent on the number of input pulses, the motor is suitable for
controlling the position by controlling the number of input pulses.

 Thus they are identically suited for open loop position and speed control.

Types of stepper motors

There is a large variety of stepper motors, which can be divided into the following three basic categories.

 Variable reluctance (VR) stepper motor

 Permanent magnet (PM) stepper motor

 Hybrid stepper motor

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(a) Variable reluctance stepper motor

 It is the most basic type of stepper motor. The VR stepper motor has stator and rotor. The
stator windings are wound on the stator poles. The rotor carries no windings. Rotor poles are
of a Ferromagnetic material.

 The VR stepper motor is shown in fig4.8. The rotor is a salient pole type. This motor may be
single stack or multi stack type.

 This is called variable reluctance motor because the reluctance of the magnetic circuit formed
by the rotor and stator teeth varies with the angular position of the rotor.

 The direction of motor rotation is independent of the polarity of the stator current.

Fig. 4.8 Variable reluctance stepper motor

(b) Permanent magnet stepper motor

The permanent magnet stepper motor is shown in the Fig.4.9. lt has also wound stator poles (i.e.,)
around the poles, the exciting coils are wound. The stator is multipolar.

Fig. 4.9 Permanent magnet stepper motor

The rotor is generally cylindrical and rotor poles are permanently magnetized. The direction of motor
rotation depends on the polarity of the stator current.

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(c) Hybrid stepper motor

 A hybrid stepper motor combines the features of the variable reluctance and permanent
magnet stepper motors. This is the most popular type of stepper motor.

 It has wound stator poles and permanently-magnetized rotor poles as shown in Fig.4.10

 The important feature of the hybrid motor is its rotor structure.

 A recent type uses a disc rotor which is magnetized axially to give a small stepping angle and
low inertia.

Fig. 4.10 Hybrid stepper motor

4.7 Variable reluctance (Vr) stepper motor

 The variable-reluctance stepping motor or VR motor for short is so called because the
reluctance of the magnetic circuit formed by the rotor and the stator teeth varies with the
angular position of the rotor.

 The principle of operation of a VR stepper motor is based on the property of flux lines to
occupy the low reluctance path. The stator and rotor therefore get aligned such that the
magnetic reluctance is minimum. A variable reluctance stepper motor can be of

 Single-stack type

 Multi-stack type

Single-stack variable reluctance stepper Motor

Construction

 The single stack variable reluctance stepper motor has no permanent magnet either on the rotor or
the stator. lt has salient pole (or tooth) stator and rotor.

 The stator has concentrated windings placed over the stator poles (teeth). while the rotor has no
windings. The rotor is a slotted structure. made from ferromagnetic material.

 Both the stator and rotor are made up of high quality magnetic materials having very high
permeability, so that the exciting current required is very small.

 The stator winding phase number depends on the connection of stator coils.

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  The number of poles on the stator and rotor are different. This gives the motor to have,

 The self starting capability

 The ability of bidirectional rotation

In general, if the number of stator poles are and the number of rotor poles are then for a ‘q’
phase motor. the rotor poles ( ) in terms of and q are given by.

= ( ) …..(2.1)

Where, q = Number of phases

Fig. 4.11 Variable reluctance stepper motor

 According to the constructional requirement, the number of phases can vary from two to six.

 We know that each and every stator pole carries a field coil or exciting coils. Because of the
presence of even number of poles, the exciting coils of opposite poles are connected in
series.

 The two coils are connected such that their mmf get added. The combination of two coils is
known as phase winding.

 Consider a single stack variable reluctance stepper motor as shown in Fig. with the three
phases a, b, c.

 In this figure. coils A and are connected in series to form a phase winding.

 This phase winding is connected to a DC source with the help of a semiconductor switch
as shown in the Winding arrangement and electrical connection circuit of VR stepper motor
Fig.

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 Fig. 4.12 Winding arrangement and electrical connection of VR stepper motor

 Similarly B and , C and , are connected to the same source through the semiconductor
switches and respectively.

 So, phase a consists of A and coils, phase b consists of B and , and phasé c consists
of C and coils. Since each phase has its own independent switch (like , and ), the
current flow through the winding can be controlled.

 The opposite pairs of stator coils (A and ) are connected such that when one pole or teeth
becomes a north (N )-pole, the other one becomes south pole (S).

Principle of operation

 The operation is based on variable reluctance principle. (i.e.,) various reluctance positions of
rotor with respect to stator. When there is no current in stator windings, the rotor is
completely free to rotate.

 When any one phase of the stator is excited, it produces its magnetic field whose axis lies
along the poles, the phase around which is excited.

 Then the rotor moves in such a direction, so as to achieve minimum reluctance position.

 Such a position means a position, where axis of magnetic field of stator matches with the axis
passing through any two poles of the rotor.

 The motor has following modes of operation

Mode I : One-phase-on or full step operation

 In this mode of operation of stepper motor, only one phase is energized at any time. The
Fig.4.13(a) shows the position of the rotor, when switch , has been closed for energizing
phase ‘a’. A magnetic field with its axis along the stator poles of phase a is created.

 The rotor is, hence, attached into a position of minimum reluctance with diametrically
opposite rotor teeth 1 and 3 lining up with the stator teeth l and 4 (A, ) respectively.

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 Fig. 4.13 One-phase-on or Full-step operation of variable reluctance stepper motor of

single stack type

 Closing the switch energizes phase ‘b’ and opening of switch , de-energizes the phase ‘a’. Now,
the rotor poles 1 and 3, 2 and 4 experience torques in opposite direction. When the rotor and stator
teeth are not in alignment, in that phase the magnetic reluctance is large.

 The torque experienced by 1 and 3 is in clockwise direction and that of 2 and 4 is in counter
clockwise (CCW) direction. The later is more than the former. As a result, the rotor makes an angular
displacement of 30° in counter clockwise direction so that B and , (stator teeth 2 & 5) are in
alignment with rotor poles 2 & 4. This position is shown in Fig4.13 (b).

 Similarly, when the switch is closed after opening , phase ‘c’ is energized which causes the rotor
teeth l and 3 to line up with (i. e. , stator teeth 3 & 6) as in Fig4.13.(c). The rotor rotates through
an additional angle of 30° in the CCW direction.

 Thus as the phases are excited in the sequence a, b and c, the rotor turns with a step of 30° in
counterclockwise direction. The direction of rotation can be reversed by reversing the switching
sequence of the phases (i_e.,) a, c and b etc., Next if is opened and is closed again, the rotor
teeth 2 and 4 will align with stator teeth 1 and 4 ( ) respectively thereby making the rotor run
through a further angle of 30° as shown in Fig4.13.(d). By now the total angle turned is 9O°.

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 Table 4.1 Truth table of VR stepper motor for CCW and CW

 The direction of rotation depends on the sequence in which the phase windings are
energized and is independent of the direction of currents through the phase winding.

 This mode of operation is known as one-phase-on mode or full-step operation and is the
simplest and widely used way of making the motor step.

 The stator phase switching truth tables are shown in Table. for counter clockwise and
clockwise directions respectively.

Mode II: 2-phase-on mode

 In two-phase-on mode, two stator phases are excited simultaneously. When two phase windings like
ab, bc, ca, again ab etc., are energized, the rotor experiences torques from both phases and comes
to rest at a mid-way between the two adjacent full-step positions.

 When the phases a and b are energized. The rotor rotates in counter clockwise direction by an angle
of Ө = 15° as shown in Fig.4.14(a).

 Then, the phase windings b and c are energized and rotor rotates another 30° in counter clockwise
direction. Now, the angle between axis of stator and rotor teeth 1-3 is equal to 45°. It is shown in
Fig4.14.(b).

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 Fig. (a) Fig. (b)

Fig. (c) Fig. (d)

Fig. 4.14 Two phase on mode operation

 Next, the phase windings c and a are energized and the rotor rotates further 30° in counter clockwise
direction. This is shown in Fig4.14.(c) and during this condition, the angle between axis of stator
and the rotor teeth 1-3 is equal to 75°.

 If the phases a and b are again energized, the rotor rotates further 30° in counter clockwise direction.
Now the total angle of rotation by the teeth 1-3 of rotor from the stator axis becomes 105°. It
is shown in the Fig4.14.(d).

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 Table 4.2 Truth table of VR stepper motor in 2 phase on mode for CCW and CW

 The truth table for this two-phase-on mode operation in counter clockwise and clockwise direction are
depicted in Table respectively.

 The two phase-on mode provides greater holding torque and the torque developed by the stepper
motor is more than that due to single-phase-on mode of operation.

 The big characteristic difference between the single-phase-on and two-phase-on operations appears
in the transient response as shown in Fig4.15.(a) and 4.15(b).

 It is evident from the figures that the oscillation damps more quickly in the two-phase-on- drive mode
than in the case of single-phase-on-mode.

 So the two-phase-on mode operation of VR stepper motor produces much better damped single step
response than the one-phase-on mode of operation.

Fig. 4.15 (a) Single phase excitation Fig. 4.15 (b) Two phase excitation

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Mode - III - half-step operation

 The half step operation or half stepping can be obtained by exciting the three phases in the
Sequence a, ab, b, bc, c etc.,
 This mode of operation is nothing but alternate one-phase-ON and two-phase ON modes of
operation of stepper motor.
 This method of excitation is also known as wave excitation. In this mode, the rotor can rotate
each step angle 15° (i.e.,) half the full-step angle. Hence, this mode of is called half-step
operation.
 At first, only the phase winding ‘a’ is energized the rotor position is Ө = 0° as shown in Fig.
4.16 (a). Then the phase windings a and b are energized, now the rotor rotates for an angle
of 15° in counter clockwise direction.
 The angle between the stator axis , and the rotor poles 1 - 3 becomes 15° as shown in
Fig.4.16(b).

Fig.4.16 (a)

Fig.4.16(b) Fig4.16(c)
Fig. 4.16 Half step operation

 Next. the phase winding b is only excited and ‘a’ is de-energized. The rotor rotates further 15°
in counter clockwise direction making total angle Ө = 30°. lt is shown in Fig.4.16(c). Next, the
phase windings b and c are energized and the rotor rotates further 15°.
 The truth table for half-step operation mode in counter clockwise and clockwise directions are
given in Table. respectively. It is realized that in this half stepping mode of operation, the step

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 angle is halved ( = 15° ), thereby doubling the resolution. In addition to this, the

continuous half stepping produces smoother shaft rotation

.
Table 4.3 Truth table of VR stepper motor half stepping mode for CCW and CW

Mode -IV - Micro stepping

 The micro stepping of stepper motor means, the step angle of the VR stepper motor will be
very small. It is also known as mini-stepping.
 It utilizes two phases simultaneously as in 2 phase-on mode but with the two currents
deliberately made unequal [unlike in half-stepping, where the two phase currents have to be
kept equal].
 The current in phase a is held constant and the current flow through b phase winding is
increased in very small increments until maximum current is reached.
 The in phase A is then reduced to zero using the increments. In this way, the resultant step
becomes very small.
 “This method of modulating currents through the stator windings so as to obtain of stator
magnetic field through a small angle to get very small step angle is known as micro stepping”.
The micro stepping provides smooth low speed operation and high resolution.
Through the micro stepping, the step angle is reduced by a factor of

A further reduction in step angle can be achieved by increasing the number of

poles of the stator and rotor or by adopting different constructions such as,
 Using reduction gear stepper motor.
 Using multi stack variable reluctance stepper motor.

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 Table 4.4 Comparison between VR and PM stepper motor

4.8 Linear induction motor

 The linear induction motor works on the same principle as that of normal induction motor
with difference that instead of rotational movement,
 the rotor moves linearly. If the stator and rotor of the induction motor are made flat then it
forms the linear induction motor.
 The flux produced by the flat stator moves linearly with the synchronous speed from one
end to the other. The synchronous speed is given by,
vs = 2wf
where, vs = Linear Synchronous Speed (m/s)
w = Width of one pole pitch (m)
f = Frequency of supply (Hz)
It can be seen that the synchronous speed is independent of number of poles but depends only one
width of pole pitch and supply frequency. The schematic of linear induction motor is shown in the Fig. 4.17

Fig. 4.17 Linear induction motor

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  The flux moves linearly and forces the rotor to move in straight line in the same directions. In
many of the practical applications the rotor plate is a stationary member whereas stator
moves.
 The analysis of linear machines is nearly same as that of rotating machines. All the angular
dimensions and displacements are displaced by linear ones and torque is replaced by the
force.
 The expressions for machine parameters are derived analogously and the results are similar
in form. Some of the typical results are as given below,

 The linear induction motors are widely used in transportation fields i.e. in electric trains. The stator is
mounted on the moving vehicle and a conducting stationary rotor forming the rails.
 The induced currents in the rail not only force the stator to move but also provide magnetic levitation
in which the train floats in air above the track.
 This mechanism proves better for high speed transportation without the difficulties associated with
wheel-rail interactions present in conventional rail transport.
 Thus the trains may have speed of about 300 km/hr. A powerful electromagnet fixed underneath the
train moves across the rails which are conducting.
 The induces the currents in the rail which provides levitation so that the train is pushed up above the
track in the air.
 The operation of such system is automatic and the system is reliable and safe.
 Linear motors also find application in the machine tool industry and in robotics where linear motion
is required for positioning and for operation of the manipulators.
 In addition to this,reciprocating compressors can also be driven by the linear machines.

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 UNIT –V

UTILIZATION

5.1 Laws of illumination or luminance

The illumination of a surface depends upon the following factors

 E is directly proportional to the luminous intensity of the source

 Inverse square law

The illumination of a surface is inversely proportional to the square of the distance of the surface from the
source

Eα

Proof: Consider a surface area A1 and A2 at resistances r1 and r2 from the point source S of the luminous
intensity I and normal to the rays as shown in fig 5.1. Let the solid angle subtended will be ω.

Total luminous flux radiated = Iω lumens

Illumination of the surface area A1

E1 = = lumens per unit area

Similarly for A2

E1 = = lumens per unit area

Fig. 5.1 Inverse square law

 Hence the illumination of the surface is inversely proportional to the square of the distance
between the surface and the light source provided the distance is large and surface is regarded
as point source

Lambert’s cosine law

 According to this law E is directly proportional to the cosine of angle made by the normal to the
illuminated surface with the direction of the incident flux.

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  As Shown in fig(d), Let F be the flux incident onthe surface of area A when in position 1. When
the surface is turned back through an angle θ, then flux incident on it is F cos θ. Hence,
illumination of the surface when in position 1 is

E1 = , but when in position 2, E1 = ,

Therefore E2 =E1 cosθ

Combining all these factors together we get E =

Fig. 5.2 Lambert’s cosine Law

5.2 Types of electric lamps

Incandescent lamps

 It consists of a glass globe completely evacuated and a fine wire known as filament within it.

 The glass globe is evacuated to prevent the oxidation and convection currents of the filament and
also prevent the temperature being lowered by radiation

 Materials commonly used for incandescent lamps: the materials used for making filaments of
incandescent lamps are carbon, tantalum and tungsten.

1. Carbon

 Resistivity, ρ=1000 to 7000 micro Ω cm

 Temperature coefficient, α = -0.0002 to -0.0008
 Melting point =3500ºC
 Density = 1.7 to 3.5
 To prevent the blackening of the bulb, the working temperature is 1800 ºC
 The commercial efficiency of filament lamp is about 4.5 lumens per watt app.
2. Tantalum

 ρ=12.4 micro Ω cm
 Temperature coefficient, α = 0.0036
 Melting point =2996 ºC
 Density = 16.6
 The efficiency is low such as 2 lumens per watt. So it is not used much now-a-days

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3. Tungsten

 ρ=5.6micro Ω cm
 Temperature coefficient, α = 0.0045
 Melting point =3400 ºC
 Density = 19.3
 The efficiency when worked at 2000 ºC in an evacuated bulb is 18 lumens per watt. This metal is
most widely for the purpose.
 The efficiency of the gas filled “coiled coil” tungsten filament is about 30 lumens per watt
The ideal material for the filament of the incandescent lamps in one which have the following properties:

 Heating melting point

 Low vapour pressure

 High resistivity

 Low temperature co-efficient

 Ductility

 Sufficient mechanical Strength to withstand vibrations during use

Fig (5.3) shows the construction of modern coiled-coil gas filled filament lamp

Fig. 5.3 Gas filled filament lamp

Aging Facts

With the passage of light output of incandescent lamp decreases due to following two reasons

 Evaporation of the filament tends to cause the bulb to blacken

 Evaporation makes the filament slowly decrease in diameter which means the resistance of the
diameter increases. Therefore the old filament draws less current and operate at low temperature
which reduces its light output.
Fig (5.4) and Fig (5.5) Shows a coiled filament and coiled-coil filament respectively

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 Fig. 5.4 coiled filament Fig. 5.5 coiled-coil filament

Effects of voltage variation

When an incandescent lamp is subjected to voltage different from normal voltage tis operating
characteristics are affected.

The efficiency of a lamp increases with increase in voltage owing to increase in temperature and is
proportional to square of the voltage.

The various relationships are

 Lumens output α (v)3.55

 Power consumption α (v)1.55
 Luminous efficiency α (v)2
Advantages of incandescent lamps

 Direct operation on standard distribution voltage

 Operating power factor unity
 Good radiation characteristics in the aluminous range
 No effect of surrounding air temperature
 Availability in various shapes and shades
5.3 Arc lamps

 These lamps are used in search lights, projection lamps and other special purpose lamps like
those in flash cameras.

 In an arc lamp electric current is made to flow through two electrodes in contact with each other
which are drawn apart.

The various forms of Arc lamps are

 Carbon arc lamp

 Flame arc lamp
 Magnetic arc lamp

Fig. 5.6 Carbon arc Lamp

 The Carbons rods used with A.C. supply are of the same size as that used with D.C. supply

91
  The craters in the arc are at the positive and negative rods are of the same size while with D.C.
supply the positive crater is bigger and gives 85 percent light at a temperature of 3500ºC
 The positive electrode gets consumed earlier than the negative electrode if the size of the former
is same as the later. Hence the positive electrode is of twice the diameter than that that of the
negative.
 A resistance is used to stabilize the arc
 The voltage drop across the arc is about 60 V and supply voltage is upto 100 V

5.4 Discharge lamps

 In all discharge lamps, an electric current is passed through a gas or vapour which render it
luminous. In this process of producing light by gaseous conduction, the most commonly used
elements are neon, mercury and sodium vapours.
 The colours of light produced depends on the nature of gas or vapour
Types of discharge lamps

Discharge lamps are of the following two types:

Type-1

Those lamps in which colour of lights is the same as produced by the discharge through the gas or
vapour

Example: Sodium vapour, mercury vapour lamp etc.

Type-2

Those lamps which use the Phenomenon of fluorescence; these are known as fluorescent lamp. In these
lamps, the discharge through the vapour produces ultra-violet waves which causes fluorescence in
certain materials called as phosphor. The inside of the fluorescent lamp coated with a phosphor which
absorbs in-visible ultra-violet rays and radiate visible rays.

Example: Fluorescent mercury-vapour tube.

Demerits

 High initial cost

 Poor power factor
 Starting, being somewhat difficult, require starters
 Time is needed to attain full brilliancy
 They are suitable for particular position
5.5 Sodium vapour lamp
Construction

 This type of lamp is of low luminosity, so the length of this lamp is large.

 So the length of this lamp is very large. To get the required length it is made in the form of a U-
tube.

 Two oxide-coated electrodes are sealed with the ends.

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  Fig 5.7 shows the connection diagram. Capacitor is connected to improve the power factor which
will become low by using poor regulation transformer.

Fig. 5.7Sodium vapour lamp

Working

 Before the lamp starts working the sodium is in the form of a solid deposited on the sides of the
tube walls.

 In the beginning when the switch is on, it operates as a low pressure neon lamp with pink colour.
The lamp gets warmed, sodium is vaporized and it radiates yellow light.

 In order to start the discharged lamp, a striking voltage of 380V is required for 40 W lamp and
450V for 100 W lamps.

 These voltages are obtained from a high reactance transformer or auto-transformer. At no load
the voltage is very high which falls down as the lamp starts giving light,

 Since the regulation of transformer is poor. This lamp should be vertical otherwise sodium will
blacken the inside of the tube.

5.6 High pressure mercury vapour lamp

Construction

 It consists of two bulbs.

 An arc tube containing the electric discharge and outer bulb which protects the arc-tube
from changes in temperature.

 The inner tube is made of quartz and the outer bulb oh hard glass.

 The arc tube contains a small amount of mercury and argon gas .

Working

 When the supply is switched on, initial discharge for the few seconds is established in the
argon gas between the auxiliary starting electrode and then in argon between the two
main electrodes.

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  The heat produced due to this discharge through the gas is sufficient to vaporize
mercury.

 Consequently, pressure inside the arc-tube increases to about one to two atmospheres
and p.d. across the main electrodes grows from about 20 to 150 v, the operation taking
about 5 to 7 minutes.

 During this time, discharge is established through the mercury vapours which emit
greenish-blue light.

Fig. 5.8 High pressure mercury vapour lamp

These lamps are used for general industrial lighting, railway yards, ports, work areas etc.

5.7 Electroplating
The process of electroplating is theoretically same as electro refining - only difference is that, in
place of graphite coated cathode we have to place an object on which the electroplating has to be done.
Let's take an example of brass key which is to be copper-platted by using copper electroplating.
Copper electroplating

+ +
We have already stated that copper sulfate splits into positive copper ion (Cu ) and
negative sulfate ion (SO4) in its solution. For copper electroplating, we use copper
sulfate solution as electrolyte, pure copper as anode and an object (a brass key) as
cathode.
 The pure copper rod is connected with positive terminal and the brass key is connected
with negative terminal of a battery.
 While these copper rod and key are immersed into copper-sulfate solution, the copper
rod will behave as anode and the key will behave as cathode.
5.8 Electric traction

Introduction
 By electric traction is meant locomotion in which the driving (or tractive) force is obtained
from electric motors. It is used in electric trains, tramcars, trolley buses and diesel-electric
vehicles etc.
 Electric traction has many advantages as compared to other non-electrical systems of
traction including steam traction.

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 Traction Systems
Broadly speaking, all traction systems may be classified into two categories:
(a) Non-electric traction systems
They do not involve the use of electrical energy at any stage. Examples are: steam engine drive
used in railways and internal-combustion-engine drive used for road transport.
(b) Electric traction systems
They involve the use of electric energy at some stage or the other. They may be further
subdivided into two groups :
 First group consists of self-contained vehicles or locomotives. Examples are : battery-electric
drive and diesel-electric drive etc.
 Second group consists of vehicles which receive electric power from a distribution network fed at
suitable points from either central power stations or suitably-spaced sub-stations.
Examples are: railway electric locomotive fed from overhead ac supply and tramways and
trolley buses supplied with dc supply.
Advantages of Electric Traction
As compared to steam traction, electric traction has the following advantages:
 Cleanliness. Since it does not produce any smoke or corrosive fumes, electric traction is
most suited for underground and tube railways. Also, it causes no damage to the buildings
and other apparatus due to the absence of smoke and flue gases.
 Maintenance Cost. The maintenance cost of an electric locomotive is nearly 50% of that for a
steam locomotive. Moreover, the maintenance time is also much less.

Disadvantages of Electric Traction

 The most vital factor against electric traction is the initial high cost of laying out overhead
electric supply system. Unless the traffic to be handled is heavy, electric traction becomes
uneconomical.
 Power failure for few minutes can cause traffic dislocation for hours.
 Communication lines which usually run parallel to the power supply lines suffer from electrical
interference. Hence, these communication lines have either to be removed away from the rail
track or else underground cables have to be used for the purpose which makes the entire
system still more expensive.
 Electric traction can be used only on those routes which have been electrified. Obviously, this
restriction does not apply to steam traction.

95