Electrical Machines-II (18EE4DCEM2)

UNIT-1

DC Machines
1.1. Introduction :
DC machines are electrical machines which deal with the conversion of one form of energy to
another. The process of conversion is called as electromechanical energy conversion.
A D.C machine which converts mechanical energy into electrical energy is called a d.c generator.
Basic principle of D.C machine: - A D.C machine works on the principle of Faradays laws of
electromagnetic induction.

When a conductor moves in a magnetic field, voltage is induced in the conductor. (Generator action)

Basic principle of D.C machine as a generator: - Conductor is moved in a magnetic field such
that it cuts across lines of flux, dynamically induced e.m.f is produced.

The magnitude of this induced e.m.f in the conductor is given by the equation

E= Blvsinθ

Where, l=length of the portion of the conductor in the magnetic field

v= velocity of the conductor

B=magnetic flux density

θ= Angle between direction of movement of the conductor in the magnetic field and the
direction of magnetic flux

The e.m.f induced in the conductor causes a current to flow in the conductor if the circuit is closed.
Thus, electrical power develops in the conductor. If the conductor does not move or if it is moved
parallel to the lines of flux, no e.m.f induced in it, and hence no power is generated. Hence it is clear
that, for generation of e.m.f there should be relative motion between the conductor and magnetic
field.

For the generating action must have the following requirements

i) The conductor or coil

ii) The flux
iii) The relative motion between the conductor and flux
The direction of the induced e.m.f is given by Fleming’s right hand rule

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Fleming's right-hand rule (for generators):- shows the direction of induced e.m.f (current) when
a conductor moves in a magnetic field.

The right hand is held with the thumb, first finger and second finger mutually perpendicular to each
other (at right angles)

 The Thumb represents the direction of Motion of the conductor

 First finger represents the direction of the Field or Flux. (north to south)
 The Second finger represents the direction of the induced or generated Current (the direction
of the induced current or e.m.f will be the direction of conventional current; from positive
to negative).

1.2. CLASSIFICATION OF DC MACHINES

Depending on the type of excitation of field winding, there are two basic types of DC machine.

1. Separately excited machine: In this type of machines the field flux is produced by connecting
the field winding to an external source.
2. Self-excited machine: The field flux is produced by connecting the field winding with the
armature in this type. A self-excited machine requires residual magnetism for operation.

Depending on the type of field winding connection DC machines can be further classified as:

 Shunt machine: The field winding consisting of large number of turns of thin wire is usually
excited in parallel with armature circuit and hence the name shunt field winding. This
winding will be having more resistance and hence carries less current.

𝑉
Ish = 𝑅𝑠ℎ

Ia = I + Ish.

where I is the line current

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Eg = 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

 Series machine: The field winding has a few turns of thick wire and is connected in series
with armature.

Ia = I = Ise

Eg = V + Ia (Ra + Rse) + B.C.D +A.R.D.

 Compound machine: Compound wound machine comprises of both series and shunt
windings and can be either short shunt or long shunt, cumulative, differential compound.

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Ia = Ise + Ish

Ise = I

Eg = V + IaRa + IseRse + B.C.D +A.R.D = IshRsh

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𝐸𝑔−𝐼𝑎𝑅𝑎
Ish = 𝑅𝑠ℎ
Ia = Ise
Ia = I + Ish

Eg = V + Ia (Ra + Rse) + B.C.D +A.R.D.

𝑉
Ish = 𝑅𝑠ℎ

 In cumulative compound DC generator both the filuxes are in same direction and it can be
added.

𝟇T = 𝟇sh + 𝟇se

 In differential compound DC generator, the filuxes are in opposite direction and it can be
subtracted.

𝟇T = 𝟇sh - 𝟇se

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Differential Compound DC Generator

Cummulative Compound DC Generator

 In separately excited DC machine, the field winding is connected to a separate DC source.

This type of machine is most flexible as full and independent control of both armature and
field circuit is possible. Figure below shows separately excited DC generator. Permanent
magnet machines also fall in this category.

 If the current direction is from the armature to load then, the machine is known as DC
Generator.
 If current direction is from load towards armature, the machine is known as DC Motor.
1.3. ARMATURE WINDINGS

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Armature winding is an arrangement of conductors distributed in slots provided on the periphery

of the armature. Depending on the way in which the coils are interconnected at the commutator
end of the armature, the windings can be classified as lap and wave windings. Further they can be
classified as simplex and multiplex. The important terms used in armature windings are
given below:
 COIL PITCH/COIL SPAN: represents the span of the coil. It can be represented in terms
of electrical degrees, slots or conductor. For full pitched winding, the span is 1800
electrical or number of slots per pole. A full pitched coil leads to maximum voltage per
coil.

 BACK PITCH/COIL SPAN (Yb): is the distance measured in between the two coil
sides of the same coil at the back end of the armature, the commutator end being the
front end of armature. It can be represented in terms of number of slots or coil sides. Back
pitch also represents the span of coil.
 FRONT PITCH (Yf): is the distance between the two coil sides of two different coil in
connected series at the front end of the armature.
 COMMUTATOR PITCH (YC): is measured in terms of commutator segments between the
two coil ends of a coil.
 SINGLE LAYER WINDING: In this winding one coil is placed in each slot.

 DOUBLE LAYER WINDING: In this winding two or multiples of coil sides are arranged
in two layers in each slot.
 Front pitch, back pitch and commutator pitch are shown in figures 1.7 and 1.8 for lap and
wave windings respectively.

Fig. 1.6

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Fig. 1.7 Fig. 1.8

1.3.1. SALIENT FEATURES OF LAP AND WAVE WINDING :

1. Armature winding is a closed winding. Depending on the type of winding, the closed path
gets divided into number of parallel paths and is available between the positive and negative
brushes.
2. Wave winding is used for high voltage low current machines.
3. Equalizing rings are not required in wave winding where as there are used in lap winding.
4. Lap winding is suitable for low voltage high current machines because of more number of
parallel paths.

In case of lap winding, the number of parallel path (A) = number of poles (P)

In case of wave winding, the number of parallel path (A) = 2 irrespective of number of poles.
𝑍
Each path will have 𝐴 conductors connected in series.

1.4. EQUATION FOR INDUCED EMF:

Let, f = Flux / pole in webers

Change in flux d ϕ = P ϕ webers

Z = Total number armature conductors

= Number of slots x Number of conductors per slot

P = Number of poles

A = Number of parallel paths in the armature.

N = Rotational speed of armature in revolutions per minute (r.p.m)

Time taken to complete one revolution = 60/N sec.

E = e.m.f induced / parallel path in armature.

By Faraday’s law

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For a Simplex Wave-Wound Generator

Number of parallel paths A = 2

For Simplex Lap-Wound Generator:

Number of parallel paths, A = P

Equation (i) becomes

Problems :

1) A 6 pole lap wound dc generator has 51 slots, each slot has 18 conductors. The useful flux
per pole is 35 mWb. Find the generated emf in the armature, if it is driven at a speed of 750
rpm.
ANS : Given: P = 6
A = P (lap wound)
Number of slots = 51
Conductors/slot = 18
Total No. of conductors = 51* 18 = Z
ϕ= 35mwb; N = 750 rpm,
emf generated

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= 401.6 volts.

2) An 8 pole d.c. generator has 650 armature conductors. The flux per pole is 20 mWb. Find
the value of emf generated when the armature is wave wound and is rotating at a speed of
1200 rpm. What must be speed at which the armature is to be driven to generate the same
emf, if the armature is lap wound.
generate the same emf, if the armature is lap wound.

ANS: Given: P = 8;

A = 2 (wave wound)

No. of conductors = 650

ϕ = 20 mWb; N = 1200 rpm,

emf generated

To find the speed of armature, when it is lap wound,

1.5. Armature Reaction

The action of magnetic field set up by armature current on the distribution of flux under main poles
of a DC machine is called the armature reaction.
When the armature of a DC machines carries current, the distributed armature winding produces its
own mmf. The machine air gap is now acted upon by the resultant mmf distribution caused by the
interaction of field ampere turns (ATf) and armature ampere turns (ATa). As a result the air gap flux
density gets distorted.

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Figure (a) Figure (b)

Figure (c) Figure (d)

Figure (a) shows a two pole machine with single equivalent conductor in each slot and the main
field mmf (Fm) acting alone. The axis of the main poles is called the direct axis (d-axis) and the
interpolar axis is called quadrature axis (q-axis). It can be seen from the Figure (b) that ATa is along
the interpolar axis as shown. ATa which is at 900 to the main field axis is known as cross
magnetizing mmf. Figure (b) shows the armature mmf (FA) acting alone.

Figure (c) shows the practical condition in which a DC machine operates. Both the main flux i.e.,
ATf (Field mmf) and ATa (armature mmf) are existing. Because of both mmf acting simultaneously,
there is a shift in brush axis and crowding of flux lines at the trailing pole tip and flux lines are
weakened or thinned at the leading pole tip. (The pole tip which is first met in the direction of
rotation by the armature conductor is leading pole tip and the other is trailing pole tip)

If the iron in the magnetic circuit is assumed unsaturated, the net flux/pole remains unaffected by
the armature reaction though the air gap flux density distribution gets distorted. If the main pole
excitation is such that the iron is in the saturated region of magnetization (practical case) the increase
in flux density at one end of the poles caused by armature reaction is less than the decrease at the

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other end, so that there is a net reduction in the flux/pole. This is called the demagnetizing effect.
Thus it can be summarized that the nature of armature reaction in a DC machine is

(i) Cross magnetizing with its axis along the q-axis.

(ii) It causes no change in flux/pole if the iron is unsaturated but causes reduction in
flux/pole in the presence of iron saturation. This is termed as demagnetizing effect. The
resultant mmf ‘F’ is shown in figure (d)
The cross magnetizing effect of the armature reaction can be reduced by making the main field
ampere-turns larger compared to the armature ampere-turns such that the main field mmf exerts
predominant control over the air gap. This is achieved by

(i) Introducing saturation in the teeth and pole shoe.

(ii) By chamfering the pole shoes which increases the air gap at the pole tips. This increases the
reluctance to the path of main flux but its influence on the cross-flux is much greater.

(iii) The best and most expensive method is to compensate the armature reaction mmf by a
compensating winding located in the pole-shoes and carrying a suitable current.

1.5.1. COMPENSATING WINDINGS: the armature reaction causes the flux density wave to be
so badly distorted that when a coil is passing through the region of peak flux densities, the emf
induced in it exceeds the average coil voltage. If this emf is higher than the breakdown voltage
across adjacent segments, a spark over could result which can easily spread over the whole
commutator, resulting in the complete short circuit of the armature. Another factor which can cause
severe over voltages to appear between commutator segments is the time variation of armature
reaction and its associated flux due to sudden changes in machine load. If the load on the machine
undergoes fast changes, the armature current Ia and armature flux/pole Φa changes accordingly
resulting in statically induced emf in the coil proportional to dΦa/dt. This voltage along with
dynamically induced emf worsens the conditions if both are additive. i.e., if the load is dropped
from a generator or added to a motor.

The remedy for the above situation is to neutralize the armature reaction ampere-turns by
compensating winding placed in the slots cut out in pole face such that the axis of the winding
coincides with the brush axis as shown in the figure below.
.

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The compensating windings neutralize the armature mmf directly under the pole which is the major
portion because in the interpole region the air gap will be large. The remaining small portion of
armature mmf in the interpole region is neutralized by the interpole windings. The number of
ampere-turns required in the compensating windings is given by
𝑝𝑜𝑙𝑒 𝑎𝑟𝑐
ATcw/pole = ATa (peak) X (𝑝𝑜𝑙𝑒 𝑝𝑖𝑡𝑐ℎ )
𝐼𝑍 𝑝𝑜𝑙𝑒 𝑎𝑟𝑐
= 2𝐴𝑃 X (𝑝𝑜𝑙𝑒 𝑝𝑖𝑡𝑐ℎ )

Compensating windings though expensive, must be provided in machines where heavy overloads
are expected or the load fluctuates rapidly.Ex. Motors driving steel mills.

1.5.2. DEMAGNETISING AMPERE TURN/POLE:

The exact conductors which produce demagnetizing effect are shown in Fig (e),Where the brush
axis is given a forward lead of Ɵ so as to lie along the new axis of M.N.A. The flux produced by
the current carrying conductors lying in between the angles AOC and BOD is such that, it opposes
the main flux and hence they are called as demagnetizing armature conductors.
main flux and hence they are called as demagnetizing armature conductors.

Z=
total
Fig (e) Fig (f) no
of
armature conductors
I = current in each armature conductors
𝐼𝑎
= 2 for wave winding

𝐼𝑎
= 𝑃
for lap winding

Ɵm = forward lead in mechanical or angular deg.

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4Ɵm
Total no of armature conductors in between angles AOC & BOD = XZ
360
2Ɵm
2Z=1turn, total no of turns in these angles = XZ
360

2Ɵm
Demagnetizing amp turns/pair of poles = X ZI
360

Ɵm
Therefore ATd/pole = 360 X ZI

1.5.3. CROSS MAGNETIZING AT/POLE

In the figure (f) Conductors lying in between the angles BOC and DOA are carrying the
current in such a way that the direction of the flux is downwards i.e., at right angles to the
main flux. This results is the distortion in the main flux. Hence, these conductors are called
cross magnetizing or distorting ampere conductors.
𝑍
Total armature conductors/pole = 𝑃
2Ɵm
Demagnetizing conductors / pole = Z 360
𝑍 2Ɵm
Therefore cross magnetizing conductors/pole = ( -Z )
𝑃 360
1 2Ɵm
=Z(𝑃- )
360
1 Ɵm
Cross magnetizing ampere turns/pole = ATc/pole = Z I( 2𝑃 - )
360
For neutralizing the demagnetizing effect number of extra turns required:
𝐴𝑇𝑑
Number of extra turns/pole= for shunt generator
𝐼𝑠ℎ
𝐴𝑇𝑑
= for series generator
𝐼𝑎

2Ɵ𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐𝑎𝑙
Note: Ɵmechanical = 𝑃

When a DC generator is loaded, voltage drop occurs due to

1. The armature reaction - which causes the voltage drop because of demagnetization.
2. Armature circuit resistance - causes voltage drop in the armature.
Hence the terminal voltage of the DC generator will be less than the No-load induced emf

Examples:
1. A 6 pole, 148 A DC shunt generator has 480 conductors and is wave wound. Its field current is
2 A. Find the demagnetising and cross magnetising amp turns/pole at full load if
(i) The brushes are at the geometrical neutral axis (GNA)
(ii) The brushes are shifted from GNA by 50 electrical
(iii) The brushes are shifted from GNA by 50 mech.

Solution : Given P = 6, Ia = 148 A, Z = 480, A = 2, If = 2 A

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I = Ia /2 = 148/2 = 74 A

(i) When the brushes are on GNA, Ɵm = 0

Ɵm
ATd/pole = 360 X ZI = 0

1 Ɵm 480 X 74
ATc/pole = Z I( 2𝑃 - )= = 2960
360 2𝑋6
2Ɵ𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐𝑎𝑙
(ii) When brushes sre shifted from GNA by 50 electrical, , Ɵmechanical = 𝑃
2𝑋5
= = 1.660
6
Ɵm 480 𝑋148 𝑋 1.66
ATd/pole = 360 X ZI = = 327.57
360
1 Ɵm 1 1.66
ATc/pole = Z I( 2𝑃 - ) = 480 X 148 [ 12 - ] = 5590
360 360
(iii) When brushes are shifted from GNA by 50 mech,

Ɵm 480 𝑋148 𝑋 5
ATd/pole = 360 X ZI = = 986.66
360
1 Ɵm 1 5
ATc/pole = Z I( 2𝑃 - ) = 480 X 148 [ 12 - 360 ] = 4933.33
360

1.6. Commutation
The process of reversal of current in the short circuited armature coil is called ‘Commutation’. This
process of reversal takes place when coil is passing through the interpolar axis (q-axis), the coil is
short circuited through commutator segments. Commutation takes place simultaneously for ‘P’ coils
in a lap-wound machine and two coil sets of P/2 coils each in a wave-wound machine.

The process of commutation of coil ‘B’ is shown below. In figure (a) coil ‘B’ carries current from
left to right and is about to be short circuited in figure (b) brush has moved by 1/3 rd of its width
and the brush current supplied by the coil are as shown. In figure (c) coil ‘B’ carries no current as
the brush is at the middle of the short circuit period and the brush current in supplied by coil C and
coil A. In figure (d) the coil B which was carrying current from left to right carries current from
right to left. In fig (e) spark is shown which is due to the reactance voltage. As the coil is embedded
in the armature slots, which has high permeability, the coil possess appreciable amount of self-
inductance. The current is changed from +I to –I. So due to self-inductance and variation in the
current from +I to –I, a voltage is induced in the coil which is given by L dI/dt. Fig (f) shows the
variation of current plotted on the time axis. Sparking can be avoided by the use of interpoles or
commutating-poles.

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Fig (a)

Fig (b)

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Fig (c)

Fig (d)

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Fig (e)

Fig (f)

1.6.1. VALUE OF REACTANCE VOLTAGE:

𝑑𝑖
Reactance voltage = co-efficient of self-inductance X rate of change of current = L𝑑𝑡
Time of short circuit = Tc = (time required by commutator to move a distance equal to the
circumferential thickness of brush)–(one mica insulating strip).
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Let Wb= brush width in cm

Wm = width of mica insulation in cm

V = peripheral velocity of commutator segments in cm/sec.

𝑊𝑏−𝑊𝑚
Then Tc = sec
𝑉
Total change in current = I - (-I) = 2I

2𝐼
Therefore self-induced or reactance voltage = L 𝑇𝑐 for linear commutation

2𝐼
= 1.11 L 𝑇𝑐 for sinusoidal commutation
If brush width is given in terms of commutator segments, then commutator velocity should be
converted in terms of commutator segments/seconds.

1.6.2. METHODS OF IMPROVING COMMUTATION:

There are two methods of improving commutation. They are (i) resistance commutation(ii) E.M.F
commutation.

Fig (g)

(i) Resistance commutation: In this method low resistance copper brushes are replaced by
high resistance carbon brushes. From the fig (g). It is seen that when current ‘I’ from coil
‘C’ reaches the commutator segment ‘b’, it has two parallel paths opened to it. The first
path is straight from bar ‘b’ to the brush and the other is via short circuited coil B to bar
‘a’ and then to brush. If copper brushes are used the current will follow the first path
because of its low contact resistance. But when carbon brushes having high resistance are
used, then current ‘I’ will prefer the second path because the resistance r1 of first path
will increase due to reducing area of contact with bar ‘b’ and the resistance r2 of second
path
decreases due to increasing area of contact with bar ‘c’. Hence carbon brushes help in
obtaining sparkles commutation. Also, carbon brushes lubricate and polish commutator.
But, because of high resistance the brush contact drop increases and the commutator has
to be made larger to dissipate the heat due to loss. Carbon brushes require larger brush
holders because of lower current density.

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(ii) E.M.F commutation: in this method, reactance voltage which is the cause for sparking is
neutralized by producing an emf which is in opposite direction to that of reactance
voltage, so that the reactance voltage is completely eliminated. The reversing emf may
be produced in two ways (i) by giving a forward lead sufficient enough to bring the short
circuited coil under the influence of next pole of opposite polarity or (ii) by using inter
poles or compoles. The second method is commonly employed.

1.6.2.1.INTERPOLES OR COMPOLES :

These are small poles fixed to the yoke and placed in between the main poles as shown in figure,
they are wound with few turns of heavy gauge copper wire and are connected in series with the
armature so that they carry full armature current. Their polarity in case of generator is that of the
main pole ahead in the direction of rotation. The function of interpoles is (i) to induce an emf which
is equal and opposite to that of reactance emf thereby making commutation sparkles. (ii) Interpoles
neutralize the cross magnetizing effect of armature reaction in fig. ‘OF’ represents mmf due to main
poles and ‘OA’ represents the cross magnetizing mmf due to armature. ‘BC’ represents mmf due to
Interpoles and is in opposite to that of ‘OA’ resulting in the cancellation of cross magnetization.

1.6.2.2. EQUILISER RINGS

Equalizer rings are used in connection with the lap winding. It is the characteristics of lap winding
that all conductors in any parallel path will be under one pair of poles. If the fluxes from all poles
are exactly the same, then emf induced in each parallel path is same and carries the same current. If
there is any inequality in the flux/pole due to slight variations in the air gap or in the magnetic

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properties of steel, there will be imbalance of emf in various parallel paths resulting in unequal
distribution of current at the brushes. This leads to poor commutation. By connecting together a
number of symmetrical points on armature winding which would be at equal potential, the
difference in brush current will be minimized. The equalizer conductors which are in the form of
copper rings are connected to equi-potential points on the backside of the armature. Such rings are
called as ‘Equalizer rings’.

Hence, the function of equalizer rings is to avoid unequal distribution of current at the brushes
thereby helping to get sparkles commutation.

Equalizer rings are not used in wave wound armatures because there is no imbalance in the emf of
the two parallel paths. This is due to the fact that armature conductors are distributed under all
poles. Hence even if there are inequalities in the flux/pole they will affect all the paths equally.

Example:

1. Calculate the reactance voltage for a dc machine having the following particulars;

Speed = 900 rpm

No. of commutator segments = 55

Brush Width = 1.74 commutator segment

Inductance of each coil = 0.153 mH

Current in each coil = 27 A

Assume linear commutation. Neglect mica insulation.

Solution : Given, N = 900 rpm, 55 segments, Wb = 1.74 segments, Wm = 0,

L = 0.153mH, I = 27 A

ns = 900 /60 = 15 rps

one revolution, 64 segments get covered

V = peripheral speed in seg /sec

= no. of rev per sec X total seg.

= 15 X 55 = 825 seg/sec
𝑊𝑏−𝑊𝑚 1.74− 0
Tc = = = 2.109 X 10-3 sec
𝑉 825

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I = current through conductor = IL / A = 27A

For linear commutation,

2𝐼 0.153 𝑋 10−3 𝑋 2 𝑋27
Self-induced emf, E = = L 𝑇𝑐 = = 3.917
2.109 𝑋10−3

1.7. CHARACTERISTICS OF D.C GENERATORS

The three important characteristics of DC generator are

1. Open circuit characteristic or Magnetization curve or No – load saturation Curve

Open circuit characteristic is the relation between the No-load generated emf in the armature, and
the field exciting current at a fixed speed. It is the magnetization curve for the material of electro-
magnets. It is same for separately excited or self-excited machine.

2. Internal or total characteristic

This characteristic curve gives the relation between the emf generated in the armature and the
armature current.

3. External characteristic

This gives the relation between the terminal voltage and the load current. This characteristic takes
into account the voltage drop due to armature circuit resistance and the effect of armature reaction.
This characteristic is of importance in judging the suitability of generator for a particular purpose.
This characteristic is also referred to as performance characteristics or voltage regulating curve.

1.7.1. Open circuit characteristics E Vs If :

The circuit diagram for obtaining the OCC is shown in figure above, Irrespective of the type of the
DC machine, namely, shunt, series, compound, the shunt field winding is disconnected and excited
from an external source.
𝑃𝜙𝑍𝑁
Induced emf E = 60𝐴
If the speed is constant, then E = K𝟇 where K is a constant.

As iron is unsaturated under low excitation current condition,

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emf increases as flux increases with increase in excitation

current. As the field current increases further the iron starts
saturating, the emf will not increase proportionately as the flux is
not varying proportionately with the current. This is shown by
the knee ‘pq’ of the characteristic curve shown in figure. A
further increase in field current leads to saturation of iron and the
flux remains almost constant and hence the induced emf will
also remain constant. This is shown by the region ‘qr’ in the figure.

1.7.1.1. CRITICAL RESISTANCE FOR SHUNT GENERATOR

When the armature is rotating with armature open circuited, an emf is induced in the armature
because of the residual flux. When the field winding is connected with the armature, a current flows
through the field winding ( in case of shunt field winding, field current flows even on No-load and
in

case of series field winding only with load) and produces additional flux. This additional flux
along with the residual flux generates higher voltage. This higher voltage circulates more current
to generate further higher voltage. This is a cumulative process till the saturation is attained. The
voltage to which it builds is decided by the resistance of the field winding as shown in the figure
below. If field circuit resistance is increased such that the resistance line does not cut OCC like
‘om’ in the figure , then the machine will fail to build up voltage to the rated value. The slope of
the air gap line drawn as a tangent to the initial linear portion of the curve represents the
maximum resistance that the field circuit can have beyond which the machine fails to build up
voltage. This value of field circuit resistance is called critical field resistance. The field circuit is
generally designed to have a resistance value less than this so that the machine builds up the
voltage to the rated value.

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 Electrical Machines-II (18EE4DCEM2)

1.7.1.2. CONDITIONS FOR VOLTAGE BUILD UP OF A GENERATOR

Following are the conditions necessary for the voltage build-up of a self-excited generator.

(i) Residual magnetism must be present.

(ii) For the given direction of rotation, the field coils must be properly connected to the
armature so that the flux produced by the field current reinforces the residual flux.
(iii) Its field resistance must be less than the critical field resistance.

1.7.1.3. CRITICAL SPEED

Critical speed of a generator is that speed for which the field circuit resistance be comes the
critical field resistance.
𝐵𝐶 𝑁𝑐
From the figure below, 𝐴𝐶 = 𝑁

𝐵𝐶
Critical speed Nc = xN Where N is the full speed
𝐴𝐶

Relation between induced emf and terminal voltage will provide an insight into the performance of
the machine. The terminal voltage of the machine under loaded condition reduces from the no-load
induced emf value because of the armature circuit voltage drop and armature reaction. Further the
contact drop of the brushes will have to be taken into account. Usually a brush contact drop of 1volt
is considered for a brush. This will be constant throughout the operating range of thee machine.

1.7.2. LOAD CHARACTERISTICS or EXTERNAL CHARACTERISTICS

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Figure (a) Figure (b)

The circuit diagram for obtaining the load characteristics is shown in figure (a)
The generator is brought to its rated speed and rated voltage is built up and loaded gradually. The
plot of terminal voltage vs load current gives the external characteristic which is shown in the
Figure(b). There are three reasons for the drop in voltage (‘ab’ in figure (b)) as the load increases.
1) Armature resistance drop
2) Armature reaction drop
3) Drop in terminal voltage due to 1 and 2 results in decreased field current which further
reduces the induced emf.
The portion ‘ab’ on the external characteristic curve is the working part of the generator. If the load
current is increased further, it is found that the terminal voltage rapidly decreases and comes to zero
which is nothing but short circuit. The drastic decrease in terminal voltage ‘bc’ is due to the severe
armature reaction for the large load current and increased armature resistance drop. Thus any
increase in load current beyond the point ‘b’ results in terminal voltage drastically reducing though
there would be some induced emf due to residual magnetism.

Fig (c) Fig (d)

1.7.3. INTERNAL OR TOTAL CHARACTERISTICS

Internal characteristics give the relation between induced emf E and armature current Ia. In a DC
shunt generator Ia = IL + If and E = V + Ia Ra. Hence, E vs Ia curve can be obtained from V vs IL
characteristic curve as shown in the above figure (c ). In this figure- ‘ab’ represents the external
characteristics. The field resistance ‘OB’ is drawn. The horizontal distances from ‘oy’ line to line
‘OB’ give the values of field currents for different terminal voltages. If these distances are added
horizontally to the external characteristic curve ‘ab’, then we get the curve for total armature current,
i.e., ‘ac’. The armature resistance drop line ‘or’ is plotted. For any armature current ‘ok’ armature
voltage drop IaRa = ‘mk’. If these drops are added to the ordinates of the curve ‘ac’, then we get the
internal characteristic curve ‘at’.
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 Electrical Machines-II (18EE4DCEM2)

If the load resistance is decreased then armature current increases upto certain value and then, any
decrease in any further decrease in load resistance results in decrease in terminal voltage which is
shown in figure (d). This is due to excessive demagnetization of main poles.

1.7.3.1. SERIES GENERATOR CHARACTERISTICS

In series generator field

windings are in series with
the generator. Hence, they
carry full load current.

Figure (e) figure (f)

As the load is increased, Ia increases and hence generated emf also increases as shown by the curve
‘Ob’ in figure ( f). If the excitation current necessary to neutralize the demagnetizing effect of
armature reaction at full load is added, we get the internal characteristics. If IaRa drop is subtracted
from the internal characteristics, we get the external characteristics.

It will be noticed that the series generator has the rising voltage characteristics but at high load
currents the voltage starts decreasing due to excessive demagnetization effects of armature reaction.

1.7.3.2. CHARACTERISTICS OF COMPOUND GENERATOR

The figure (g) shows the characteristics of compound generator. If

the series field ampere turns are such as to produce the same voltage
at rated load as at no-load then the generator is said to be flat
compounded. If the series field ampere turns are such that the rated
load voltage is greater than the no-load voltage, then the generator is
said to be over compounded. If the rated load voltage is less than
the no-load voltage, then the generator is said to be under compounded. Fig (g)

USES OF DC GENERATOR

(i) Shunt Generators with field regulators are used for lighting and power supply purposes. They
are also used for charging batteries because their terminal voltage is almost constant.
(ii) Series Generators are not used for power supply because of their rising characteristics but
their rising characteristic suits to be used as boosters.

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 Electrical Machines-II (18EE4DCEM2)

(iii) Compound Generators maintain almost constant terminal voltage over a large range of load.
Hence they are used where large load is suddenly thrown on and off. Also they are used for
power supplies.

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