Electrical Machines-1

UNIT – II

D.C. GENERATORS
INTRODUCTION:
The electrical machines deals with the energy transfer
either from mechanical to electrical form or from electrical to
mechanical form, this process is called electromechanical energy
conversion.
An electrical machine which converts mechanical energy into electrical energy is called an
electric generator while an electrical machine which converts electrical energy into the
mechanical energy is called an electric motor.
Classification of electrical machines

2.1 Principle of operation:

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
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induced emf, where a conductor cuts magnetic flux, dynamically induced emf is produced in it
according to Faraday’s Laws of electromagnetic Induction.( According to Faraday's laws of
electromagnetic induction, whenever a conductor is placed in a varying magnetic field (OR a
conductor is moved in a magnetic field), an emf (electromotive force) gets induced in the
conductor. )
(the faraday’s second law states that the amount of EMF induced is proportional to rate
of change of magnetic flux)This emf causes a current to flow if the conductor circuit is closed.
Hence, two basic essential parts of an electrical generator are
(i) a magnetic field and
(ii) a conductor or conductors which can so move as to cut the flux.(relative motion)
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’.
The conductors placed on the armature are rotated with the help of some external device. Such
an external device is called a prime mover. The commonly used prime movers are diesel
engines, steam engines, steam turbines, water turbines etc. The purpose of the prime mover is
to rotate the electrical conductor as required by Faraday’s laws The direction of induced emf
can be obtained by using Flemings right hand rule.
The magnitude of induced emf = e = BLV sinØ= Em sinØ
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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 = Ndϕ /dt).
• When the plane of the coil is at right angles to lines of flux i.e. when it is in position
1, then flux linked with the coil is maximum, but rate of change of flux linkages is
minimum. Hence, there is no induced emf in the coil.
• As the coil continues rotating further, the rate of change of flux linkages (and hence
induced emf in it) increases, till position 3 is reached where θ = 90º, the coil plane is
horizontal i.e. parallel to the lines of flux. The flux linked with the coil is minimum
but rate of change of flux linkages is maximum. Hence, maximum emf is induced in
the coil at this position.
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• From 90º to 180º, the flux linked with the coil gradually increases but the rate of
change of flux linkages decreases. Hence, the induced emf decreases gradually till in
position 5 of the coil, it is reduced to zero value.
• From 180º to 360º, the variations in the magnitude of emf are similar to those in the
first half revolution. Its value is maximum when coil is in position 7 and minimum
when in position 1. But it will be found that the direction of the induced current is
the reverse of the previous direction of flow.

For making the flow of current unidirectional in the external circuit, the slip-rings are replaced
by split-rings. The split-rings are made out 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.
As before, the coil ends are joined to these segments on which rest the carbon or
copper brushes. It is seen that in the first half revolution current flows along (ABMLCD) i.e. the
brush No.1 in contact with segment ‘a’ acts as the positive end of the supply and ‘b’ as the
negative end. In the next half revolution, the direction of the induced current in the coil has
reversed. But at the same time, the positions of segments ‘a’ and ‘b’ have also reversed with
the result that brush No.1comes in touch with the segment which is positive i.e. segment ‘b’ in
this case. Hence, current in the load resistance again flows from M to L. The waveform of the
current through the external circuit is as shown in below. This current is unidirectional but not
continuous like pure direct current.

• The position of brushes is so arranged that the change over 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.
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• 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.
• Nature of induced emf: The nature of the induced emf for a conductor rotating in
the magnetic field is alternating. As conductor rotates in a magnetic field, the
voltage component at various positions is different. Hence the basic nature of
induced emf in the armature winding in case of dc generator is alternating. To get dc
output which is unidirectional, it is necessary to rectify the alternating induced emf.
A device which is used in dc generator to convert alternating induced emf to
unidirectional dc emf is called commutator.
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2.2 construction of DC machine:
The dc generators and dc motors have the same general construction. In fact, when the
machine is being assembled, the workmen usually do not know whether it is a dc generator
or motor. Any dc generator can be run as a dc motor and vice-versa.

Two major parts required for the construction of DC motor, namely.

Stator – The static part that houses the field windings and receives the supply and,
Rotor – The rotating part that brings about the mechanical rotations.

The main parts of DC Machine (motor or generator) are as follows:

1. Yoke
2. Pole core and pole shoes
3. Pole coil or field coil
4. Armature core
5. Armature winding or conductor
6. Commutator
7. Brushes and bearings
1. Yoke:
It is the outer most covering of the machine
Function
Yoke
o It provide mechanical Support for poles
o It also provide protection to whole machine from dust, moisture etc.
o It also carries magnetic flux produced by the poles
o Yoke is also called as frame.
Material used
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o For small M/C yoke is made of cast iron.
o For large M/C it is made of cast steel or rolled steel.
2. Pole & Pole core
Function
o Pole of a generator is an electromagnet.
o The field winding is winding over pole.
o Pole provides magnetic flux when field winding is excited.
Material used
o Pole core or pole made of cast iron or cast steel.
o It built of these laminations of annealed steel. The laminations is done to reduce the
power lose due to eddy currents
3. Pole Shoe
Function
o It is extended part of pole. It enlarge area of pole
o Due to this enlarged area, flux is spread out in the air gap and more flux can pass
through the air gap to armature.
o
Material used
o It is made of cast iron or cast steed.
o It built of this lamination of annealed steel. the lamination is done to reduce power loss
due to eddy currents
4. Field winding
Function
o It is wound around pole core and called as field coil
o it is connected in series to from field winding
o When Current is passed through field winding it electro
magnetize the poles which produce necessary flux.
Material used
o The material used for field conductor is copper.

5. Armature Core
Function
o It has large number of slots in its periphery
o Armature conductor, are placed in this slots
o It is also provide path of low reluctance to the flux produced by field winding
Material used
o High permeability low reluctance materials such as cast or iron are used for armature
core.
o The lamination is provided so as to reduce the loss due to eddy current.
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6. Armature Winding
Function
o Armature conductor are inter connected to form armature Winding
o When armature winding is rotated using prime mover. the magnetic flux and voltage
gets induced in it
o Armature winding is connected to external circuit
Material used
o It is made of conducting material such as copper.
7. Commutator
Function
o It Convert alternating current induce in the current in a unidirectional current
o It collects the current form armature conductor and pass it load with the help of brushes
o It also provide unidirectional torque for dc motor
Material used
o It is made of a large number of edge shaped segments of hard drawn copper.
o The Segments are insulated from each other by thin layer of mica.
o The Segment of commutator is made of copper and insulating material between
segments is mica.
8. Brushes
Function
o Brushes collect the current from commutator and apply it to external load.
o Brushes wear with time and it is should be inspected regularly.
Material used
o Brushes are made of carbon or graphite it is rectangular in shape.

Uses of laminated armature:

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Although a DC generator provides direct current but induced current in the armature is
alternating in nature. That is why, cylindrical or drum shaped armature core is build up of
circular laminated sheet. Eddy current losses are directly proportional to area of armature
or more precisely the path of motion.
Now consider an armature with single piece of iron.In this case with single piece of
armature. In this Eddy current losses are represented by white lines, Now consider
laminated armature with some laminations (in practical we have each lamination of
around 0.4 mm).
In laminated armature eddy current losses are reduced to very less or '0' quantity. That
is why armature of DC machines (either motor or generator) is laminated.

2.3 .Armature windings:

some basic terms related to armature winding of DC generator.
Conductor: It is the armature conductor which is under the influence of magnetic field
placed in armature.
Turn: the two conductors placed in different slots when connected together forms a
turn
Z= 2XNumber of turns
Coil: the group of turns connected together forms a coil. Two coil sides form a coil
There are two physical types of windings
a) single layer winding: if a slot contains only one coil side then it is called as single layer
winding
b) double layer winding: if a slot contains two coil sides one on the top while other at the
bottom shown below

The armature slots are again classified into two different types
a) open type
b) closed type
2.3.1Armature winding in dc machines:
i. Pole Pitch:
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The pole pitch is defined as peripheral distance between center of two adjacent poles in DC
machine. This distance is measured in term of armature slots or armature conductor come
between two adjacent pole centers. This is naturally equal to the total number of armature
slots divided by number of poles in the machine.pole pitch is equal to total numbers of
armature slots divided by total numbers of poles, this can alternatively referred as armature
slots per pole

ii.Coil Span or Coil Pitch:

Coil span is defined as peripheral distance between two sides of a coil, measured in terms of
number of armature slots between them. That means, after placing one side of the coil in a
particular slot, after how many conjugative slots, the other side of the same coil is placed on
the armature. This number is known as coil span.

NOTE:

o If the coil span is equal to the pole pitch, then the armature winding is said to be full -
pitched. At this situation, two opposite sides of the coil lie under two opposite poles.
Hence emf induced in one side of the coil will be in 180° phase shift with emf induced in
the other side of the coil. Thus, total terminal voltage of the coil will be nothing but the
direct arithmetic sum of these two emfs.
o If the coil span is less than the pole pitch, then the winding is referred as fractional
pitched. In this coil, there will be a phase difference between induced emfs in two sides,
less than 180°. Hence resultant terminal voltage of the coil is vector sum of these two
emfs and it is less than that of full-pitched coil. Fractional pitched windings are
purposely used to effect substantial saving in copper of the end connection and
for improving commutation
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iii.Pitch of Armature Winding:

Back Pitch (Yb)

A coil advances on the back of the armature. This advancement is measured in terms of
armature conductors and is called back pitch. It is equal to the number difference of the
conductor connected to a given segment of the commutator.

Front Pitch (Yf)

The number of armature conductors or elements spanned by a coil on the front is
called front pitch. Alternatively, the front pitch may be defined as the distance between the
second conductor of the next coil which are connected together at the front i.e. commutator
end of the armature. In other words, it is the number difference of the conductors connected
together at the back end of the armature. Both front and back pitches for lap and wave
windings are shown in the figure below

Resultant Pitch (Y)

It is the distance between the beginning of one coil and the beginning of the next coil to
which it is connected. As a matter of precautions, it should be kept in mind that all these
pitches, though normally stated in terms of armature conductors, are also times of armature
slots or commutator bars.

For lap winding Y= Yb - Yf

For Wave Y = Yb +Yf
Commutator Pitch
Commutator pitch is defined as the distance between two commutator segments which
two ends of same armature coil are connected. Commutator pitch is measured in terms of
commutator bars or segment.
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Single Layer Armature Winding
Armature coil sides are placed in the armature slots in different manner. In some
arrangement, each slot is occupied by one side of an armature coil. In other words one coil
sides is placed in each armature slot. This arrangement is referred as single layer winding.

Two Layer Armature Winding

In other types of armature Winding, arrangement every armature slot is occupied by
two coil sides, one on upper half and other on lower half of the slot. The coils in two layers
winding are so placed, that if one side is placed on upper half of the slot then other side is
placed on the lower half of some other slot at a distance of one coil pitch away

2.2.2Types of armature winding:

Armature conductors are connected in a specific manner called as armature winding
and according to the way of connecting the conductors; armature winding is divided
into two types.
1.Lap winding
2.wave winding
1.Lap winding:
In this case, if connection is started from conductor in slot 1 then the connections
overlap each other as winding proceeds, till starting point is reached again. There is overlapping
of coils while proceeding. Due to such connection, the total number of conductors get divided
into ‘P’ number of parallel paths,
where P = number of poles in the machine.
Large number of parallel paths indicates high current capacity of machine hence lap winding is
pertained for high current rating generators.

Duplex Lap Winding

A winding in which the number of parallel path between the brushes is twice the number of
poles is called duplex lap winding.
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2.Wave winding:
In this type, winding always travels ahead avoiding over lapping. It travels like a progressive
wave hence called wave winding. Both coils starting from slot 1 and slot 2 are progressing in
wave fashion. Due to this type of connection, the total number of conductors get divided into
two number of parallel paths always, irrespective of number of poles of machine. As number of
parallel paths is less, it is preferable for low current, high voltage capacity generators
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S.No Lap winding Wave winding
1 Number of parallel paths (A) = poles (P) Number of parallel paths (A) = 2
(always) 2.
2 Number of parallel paths (A) = 2 (always) Number of brush sets required is equal
to two
3 Preferable for high current, low voltage Preferable for highvoltage, low current
capacity generators capacity generators capacity generators
4 Normally used for generators of capacity Preferred for generator of capacity less
more than 500A than 500A

2.3.E.M.F. Equation of a D.C. Generator::

Let
 = flux/pole in Wb P = number of poles
Z = No. of armature conductors = No. of slots * conductors/slot
A = No. of parallel paths = P ... for Lap winding
= 2 ... for Wave winding
N = Speed of armature in r.p.m.
Eg = Generated EMF or EMF/parallel path
According to faraday’s laws of electromagnetic induction principle, average induced EMF (Eg) =
ddt
Where d = Flux cut by a conductor in one revolution = P wb
dt = Time taken to complete one revolution
since N no. of revolutions are made by the generator per minute, no. of revolutions are made by
the generator per sec = N / 60
1
 Time taken to complete one revolution (dt) = = 60/N
(Ns/60)
P P N
Average value of induced EMF / conductor = =
(60/N) 60
The DC generator has Z no. of armature conductors and are divided into A no. of parallel
paths, then no. of conductors per each parallel path is Z/A.
PN Z
 Induced EMF per each parallel path = *
60 A
 ZN P
Induced EMF (or) Generated EMF (Eg) = *
60 A
Where A = No. of parallel paths = P for lap winding
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=2 for wave winding
2.4. Armature Reaction:
In a d.c. generator, the purpose of field winding is to produce magnetic field (called main
flux)field flux. Now if a d.c machine is operating as a dc generator an e.m.f is induced in the
armature when driven by a prime mover. If I act as a motor then it carry some armature current
passes through the armature conductors. Every current carrying conductor will set up its own
magnetic field, here called as armature magnetic field.
“The effect of the armature flux on the distribution of main field flux is called as
armature reaction”
This results in weakening and disorting the main field flux. This distortion and field weakening
takes place in both generators and motors. The action of armature flux on the main flux is known
as armature reaction.

Geometrical and Magnetic Neutral Axes

• The geometrical neutral axis (G.N.A.) is the axis that bisects the angle between the
centre line of poles.
• The magnetic neutral axis (M. N. A.) is the axis drawn perpendicular to the mean
direction of the flux passing through the centre of the armature. Clearly, no e.m.f. is
produced in the armature conductors along this axis because then they cut no flux. With
no current in the armature conductors, the M.N.A. coincides with G, N. A. as shown in
Fig. In order to achieve sparkless commutation, the brushes must lie along M.N.A.
Concept of Armature Reaction:
➢ To understand the concept of armature reaction, consider a two pole d.c.generator without
any load. Let the main field flux produced by the main field mmf IfNf is along the phasor
OA=ϕf show below
➢ When the dc machine is loaded current flowing through the armature winding these currents
are shown by dot under S pole and X under N pole. These armature current setup armature
flux shown by vertical line OB = ϕa with field unexcited.
➢ if DC machine working as Generator then it is rotated in clockwise direction , if it act as a
motor it rotates in antilock wise direction.
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➢ The path of armature flux is perpendicular to the main field flux in other words the path of
the armature flux cross the main field flux path. Thus the effect of armature flux on the
main field flux is entirely cross magnetizing and it is for the reason that the flux created by
by the armature mmf is called cross flux.
➢ When current flowing through the armature and field winding the resultant flux distribution
is obtained by superimposing the two fluxes shown in fig.3
➢ It is seen that the armature flux aids the main flux at the upper end of the N-pole and lower
end of the S –pole, therefore at these two pole ends (tips) the armature flux strengthens the
main field flux. Mean while at the remaining two tips the armature flux weakens the main
field flux . if there is no magnetic saturation then the amount of strengthening and weakening
of main flux are equal and resultant flux remains unaltered from its no load. But in practical
magnetic saturation does not occour as a consequence the strengthening effect is less
compared to the weakening effect and resultant flux decrese from the no load value.this is
called as DEMAGNETISING EFFECT
➢ The GNA(Geometric Neutral Axis ) is always along the q-axis of dc machine.MNA (magnetic
neutral axis) is perpendicular to the resultant field flux.
➢ The MNA at no laod coincides with the GNa at no load. When the DC machine is loaded the
MNA is shifted from GNA. This shift depends on the magnitude of armature current. Thus
greater the magnitude of armature current more will be shifting of the MNA from GNA.
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2.4.1.EFFECTS OF ARMATURE REACTION::

The effect of armature flux on main field flux has two important effects
I. Distortion of main field flux
II. Net reduction in the main field flux
Distortion of the main field flux gives rise to increase in iorn losses, poor commutation and
sparking at commutator.
i) iron losses
The iron losses in the teeth and pole shoes are determined by the maximum value of flux
density at which they work. Due to distortion in main field flux the maximum density at load
increases above no load. Thus more iron losses are observed on load than no load.roughly iorn
losses take 1.5 times its value from no load.
ii) Commutation:
For proper commutation the commutator segments are shorted by brushes and the emf
in them is zero. Brushes are placed along the GNA, due to armature reaction the GNA shift
from the q axis. So the coils undergoing commutation have some emf indeced at the brushes
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which delays the reversal of armature current in short circuited coils this results in sparking
and poor commutation at brushes.
iii) Sparking:
if the density of magnetic flux is increased and due to poor commutation sparking occurs at
the commutator.
iv)cost of field winding
Due to demagnetizing and cross magnetizing effect the net magnetic flux reduces. Hence in
a generator the EMF generated decrease and in motor torque decrease. To meet the demand of
load it has to be increased ,to increase this we have to give extra number of turns to meet the
requirement thus the cost of winding increase
2.4.2Methods Of Reducing Armature Reaction::
The armature effect can be reduced by the following methods
a. High Reluctance pole tips
b. Reduction in armature flux
c. Strong magnetic field system
d. Interlopes
e. Compensating winding
High Reluctance pole tips:
The armature reaction causes the distortion in main field flux. This can be reduced if the
reluctance of the path of the cross-magnetizing field is increased. The armature teeth and air
gap at pole tips offer reluctance to armature flux. Thus by increasing length of air gap, the
armature reaction effect is reduced. If reluctance at pole tips is increased it will reduce
distorting effect of armature reaction. By using special construction in which leading and
trailing pole tip portions of laminations are alternately omitted.
Reduction in armature flux
If the armature flux is decreased the armature reaction can be reduced. this is done by
increase the reluctance in magnetic field system this is made by using field pole laminations
having several rectangular holes punched in them,
Strong magnetic field system
During designing the main field flux is designed to be maximum to have proper operation
Interpoles:
The armature reaction causes shifting the magnetic neutral axis. Therefore there will be
some flux density at brush axis which produces emf in the coil undergoing commutation.This
will lead to delayed commutation. Thus the armature reaction at brush axis must be
neutralized. This requires another equal and opposite mmf to that of armature mmf.This can be
applied by interpoles which are placed at geometric neutral axis at midway between the main
poles.
Compensating winding
Compensating Windings The cross-magnetizing effect of armature reaction may cause
trouble in d.c. machines subjected to large fluctuations in load. In order to neutralize the cross
magnetizing effect of armature reaction, a compensating winding is used.
It is connected in series with armature in a manner so that
the direction of current through the compensating conductors in
any one pole face will be opposite to the direction of the current
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through the adjacent armature conductors]. Let us now calculate the
number of compensating conductors/ pole face. In calculating
the conductors per pole face required for the compensating
winding, it should be remembered that the current in the
compensating conductors is the armature current Ia whereas
the current in armature conductors is
Ia/A where A is the number of parallel paths.
Zc = No. of compensating conductors/pole face
Za = No. of active armature conductors
Ia = Total armature current
Ia/A = Current in each armature conductor

The use of a compensating winding considerably increases the cost of a machine and is justified
only for machines intended for severe service e.g., for high speed and high voltage machines

2.4.3.Calculation of Demagnetizing Ampere-Turns Per Pole (ATd/Pole)

It is sometimes desirable to neutralize the demagnetizing ampere-turns of armature
reaction. This is achieved by adding extra ampere-turns to the main field winding. We shall now
calculate the demagnetizing ampere-turns per pole (ATd/pole).
Let Z = total number of conductors
I = current in each armature conductor
= Ia/2 for simplex wave winding
=Ia/P for simplex lap winding
θm = forward lead in mechanical degree
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Note. When a conductor passes a pair of poles, one cycle of voltage is generated. We say one
cycle contains 360 electrical degrees. Suppose there are P poles in a generator. In one
revolution, there are 360 mechanical degrees and 360 *P/2 electrical degrees.
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2.4.4.Cross-Magnetizing Ampere-Turns Per Pole (ATc/Pole)
We now calculate the cross-magnetizing ampere-turns per pole (ATc/pole).

(found as above)
Cross-magnetizing ampere-turns/pole are

2.5.Commutation:
The fig shows the schematic diagram of 2-pole lap-wound generator. There are two
parallel paths between the brushes. Therefore, each coil of the winding carries one half (Ia/2 in
this case) of the total current (Ia) entering or leaving the armature
Note that the currents in the coils connected to a brush are either all towards the brush
(positive brush) or all directed away from the brush (negative brush). Therefore, current in a
coil will reverse as the coil passes a brush. This reversal of current as the coil passes & brush is
called commutation. The reversal of current in a coil as the coil passes the brush axis is called
commutation.

When commutation takes place, the coil undergoing commutation is short circuited by
the brush. The brief period during which the coil remains short circuited is known as
commutation period Tc. If the current reversal is completed by the end of commutation
period, it is called ideal commutation. If the current reversal is not completed by that time, then
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sparking occurs between the brush and the commutator which results in progressive damage to
both.
2.5.1.Ideal commutation
Let us discuss the phenomenon of ideal commutation (i.e., coil has no inductance) in one
coil in the armature winding shown in Fig.. For this purpose, we consider the coil A. The brush
width is equal to the width of one commutator segment and one mica insulation.
Suppose the total armature current is 40 A. Since there are two parallel paths, each coil
carries a current of 20 A
• In Fig (i), the brush is in contact with segment 1 of the commutator. The commutator
segment 1 conducts a current of 40 A to the brush; 20 A from coil A and 20 A from the
adjacent coil as shown. The coil A has yet to undergo commutation.

• As the armature rotates, the brush will make contact with segment 2 and thus short-circuits
the coil A as shown in Fig. (ii). There are now two parallel paths into the brush as long as the
short-circuit of coil A exists. Fig (ii) shows the instant when the brush is one-fourth on
segment 2 and three-fourth on segment 1. The brush again conducts a current of 40 A; 30 A
through segment 1 and 10 A through segment 2. Note that current in coil A (the coil
undergoing commutation) is reduced from 20 A to 10 A.
• Fig. (iii) shows the instant when the brush is one-half on segment 2 and one-half on segment
1. The brush again conducts 40 A; 20 A through segment 1 and 20 A through segment 2 (Q
now the resistances of the two parallel paths are equal). Note that now. current in coil A is
zero.
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• Fig. (iv) shows the instant when the brush is three-fourth on segment2 and one-fourth on
segment 1. The brush conducts a current of 40 A; 30 A through segment 2 and 10 A through
segment 1. Note that current in coil A is 10 A but in the reverse direction to that before the
start of commutation.
• Fig. (v) shows the instant when the brush is in contact only with segment 2. The brush again
conducts 40 A; 20 A from coil A and 20 A from the adjacent coil to coil A. Note that now
current in coil A is 20 A but in the reverse direction. Thus the coil A has undergone
commutation. Each coil undergoes commutation in this way as it passes the brush axis. Note
that during commutation, the coil under consideration remains short circuited by the brush.

Above Fig. shows the current-time graph for the coil A undergoing commutation. The
horizontal line AB represents a constant current of 20 A upto the beginning of commutation.
From the finish of commutation, it is represented by another horizontal line CD on the
opposite side of the zero line and the same distance from it as AB i.e., the current has exactly
reversed (- 20 A). The way in which current changes from B to C depends upon the
conditions under which the coil undergoes commutation. If the current changes at a uniform
rate (i.e., BC is a straight line), then it is called ideal commutation as shown in Fig.Under

NOTE:
The ideal commutation cannot be attained in practice. This is mainly due to the fact that the
armature coils have appreciable inductance. When the current in the coil undergoing
commutation changes, self-induced e.m.f. is produced in the coil. This is generally called
reactance voltage. This reactance voltage opposes the change of current in the coil
undergoing commutation.
such conditions, no sparking will take place between the brush and the commutator

This reactance voltage makes commutation occurs more slowly than it would be under ideal
commutation.

This is illustrated in Fig. The straight line RC represents the ideal commutation whereas
the curve BE represents the change in current when self-inductance of the coil is taken into
account. Note that current
CE (= 8A in 2.9) is flowing from the commutator segment 1 to the brush at the instant
when they part company. This results in sparking just as when any other current carrying circuit
is broken. The sparking results in overheating of commutator brush contact and causing
damage to both..
 Electrical Machines-1

2.5.2.Calculation of Reactance Voltage::

Reactance voltage = Coefficient of self-inductance ´ Rate of change of current
When a coil undergoes commutation, two commutator segments remain short circuited by the brush.
Therefore, the time of short circuit (or commutation period Tc) is equal to the time required by the
commutator to move a distance equal to the circumferential thickness of the brush minus the thickness
of one insulating strip of mica.
Let Wb = brush width in cm;
Wm = mica thickness in cm
V = peripheral speed of commutator in cm/s

The commutation period is very small, say of the order of 1/500 second
Let the current in the coil undergoing commutation change from + I to - I (amperes) during the
commutation. If L is the inductance of the coil, then reactance voltage is given by
𝑑𝑖 2𝑖
Ec = Lc 𝑑𝑡 or Ec = Lc𝑇𝑐
2.5.3.Methods of Improving Commutation
To make the commutation satisfactory we have to make sure that the current flowing
through the coil completely reversed during the commutation period attains its full value.
There are three main methods of improving commutation. These are
1.Resistance commutation
2.E.M.F. commutation
3.Compensating windings
1.Resistance Commutation
In this method of commutation we use high electrical resistance brushes for getting spark
less commutation. This can be obtained by replacing low resistance copper brushes with high
resistance carbon brushes.
 Electrical Machines-1

We can clearly see from the picture that the current IC from the coil C may reach to the brush in
two ways in the commutation period. One path is direct through the commutator segment b
and to the brush and the 2nd path is first through the short-circuit coil B and then through the
commutator segment a and to the brush. When the brush resistance is low, then the current
IC from coil C will follow the shortest path, i.e. the 1st path as its electrical resistance is
comparatively low because it is shorter than the 2nd path.
When high resistance brushes are used, then as the brush moves towards the commutator
segments, the contact area of the brush and the segment b decreases and contact area with
the segment a increases. Now, as the electrical resistance is inversely proportional to the
contact area of then resistance Rb will increase and Ra will decrease as the brush moves. Then
the current will prefer the 2nd path to reach to the brush. Thus by this method of improving
commutation, the quick reversal of current will occur in the desired direction.

ρ is the resistivity of the conductor l is the length of the

conductor.
A is the cross-section of the conductor (here is this description it is used as contact area)

2. E.M.F. Commutation::
The main reason of the delay of the current reversing time in the short circuit coil during
commutation period is the inductive property of the coil. In this type of commutation, the
reactance voltage produced by the coil due to its inductive property, is neutralized by
producing a reversing emf in the short circuit coil during commutation period.
Reactance Voltage:
The voltage rise in the short circuit coil due to inductive property of the coil, which opposes
the current reversal in it during the commutation period, is called the reactance voltage.
We can produce reversing emf in two ways
i.By brush shifting.
ii.By using inter-poles or commutating poles.

i)Brush Shifting Method of Commutation

In this method of improving commutation the brushes are shifted forward direction for
the DC generator and in backward direction for the motor for producing the sufficient reversing
emf for eliminating the reactance voltage. When the brushes are given the forward or
backward lead then it brings the short circuit coil under the influence of the next pole which is
of the opposite polarity. Then the sides of the coil will cut the necessary flux form the main
 Electrical Machines-1
poles of opposite polarity for producing the sufficient reversing emf. This method is rarely used
because for best result, with every variation of load, the brushes have to be shifted.

ii.Method Of Using Inter-Pole

In this method of commutation some small poles are fixed to the yoke and placed between
the main poles. These poles are called inter-poles. Their polarity is same as the main poles
situated next to it for the generator and for the motor the polarity is same as the main pole
situated before it. The inter-poles induce an emf in the short circuit coil during the
commutation period which opposes reactance voltage and give spark-less commutation.

3.Compensating Windings
This is the most effective mean of eliminating the problem of armature reaction and flash
over by balancing the armature mmf. Compensating windings are placed in slots provided in
pole faces parallel to the rotor (armature) conductors.
The major drawback with the compensating windings is that they very costly. Their use is
mainly for large machines subject to heavy overloads or plugging and in small motors subject to
sudden reversal and high acceleration.
 Electrical Machines-1

3.1. classification of DC Machines:

For the operation of DC machine we require two fluxes (filed flux and armature flux). To produce them
we are using two windings namely Armature winding and Field winding. Based on the interconnection
of these windings
Each DC machine can act as a generator or a motor. Hence, this classification is valid for
both: DC generators and DC motors. DC machines are usually classified on the basis of their field
excitation method. This makes two broad categories of dc machines;
(i) Separately excited
(ii) Self-excited.

Separately 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.

Let Ia = Armature current

IL = Load current
V = Terminal voltage
Eg = Generated emf Let, Ia = IL = I (say)
The voltage drop across armature = Ia × Ra
Then, voltage across the load, V = Ira
Power generated, Pg = Eg×I
Power delivered to the external load, PL = V×I

iii.Self-excited DC Generators
 Electrical Machines-1
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…
A. Series wound generators
B. Shunt wound generators
C. Compound wound generators

a.Series Wound Generator

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

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
b. Shunt 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, Rsh = Shunt winding resistance
Ish = Current flowing through the shunt field
Ra = Armature resistance
Ia = Armature current
IL = Load current
V = Terminal voltage Eg = Generated emf
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
 Electrical Machines-1
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

C. 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
Short Shunt Compound Wound DC Generator:
The generators in which only shunt field winding is in parallel with the armature winding as
shown inSeries 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
Power generated, Pg = Eg×Ia
Power delivered to the load, PL=V×IL
 Electrical Machines-1

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 figure.
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
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

3.2. Build-Up of E.M.F:

3.2.1 Voltage Build-Up in a shunt Generator:

One of the simplest forms of ‘self-excited’ generator is the shunt-wound machine, the
connection diagram (without load) of which is shown in Fig. 32. The manner in which a self-excited
generator manages to excite its own field and build a D.C. voltage across its armature is described with
reference to Fig.
in the following steps
• Assume that the generator starts from rest, initially let it have some residual flux.
• As the prime-mover rotates the generator armature and the speed approaches rated speed,
the voltage due to residual magnetism and speed increases.

• At rated speed, the voltage across the armature due to residual magnetism is small, E 1 as
shown in the figure. But this voltage is also across the field circuit whose resistance is R f Thus,
the current which flows in the field circuit I1, is also small.
 Electrical Machines-1
• When I1 flows in the field circuit of the generator ,an increase in m.m.f results. That is I fTf
increase.
• Voltage E2 is now impressed across the field, causing a large current I2 to flow in the field
circuit. I2Tf is an increased m.m.f., which produces generated voltage E3·
• E3 yields I3 in the field circuit, producing E4. But E4 causes I4 to flow in the field producing
E5 ;and so on, up to E8, the maximum value,…
• The process continues until that point where the field resistance line crosses. the
magnetization curve in Fig. Here the process stops. The induced voltage produced, when
impressed across the field circuit, produces a current flow that in turn produces an induced
voltage of the same magnitude. Eg. as shown in the figure.

Critical Field Resistance for a Shunt Generator

The voltage build up in a shunt generator depends upon field circuit resistance. If the field
circuit
resistance is R1 (line OA), then generator will build up a voltage OM as shown in Fig. If the field circuit
resistance is increased to R2 (tine OB), the generator will build up a voltage OL, slightly less than OM. As
the field circuit resistance is increased, the slope of resistance line also increases. When the field
resistance line becomes tangent (line OC) to O.C.C., the generator would just excite.
If the field circuit resistance is increased beyond this point (say line OD), the generator will fail
to excite. The field circuit resistance represented by line OC (tangent to O.C.C.) is called critical field
resistance RC for the shunt generator. It may be defined as under: The maximum field circuit resistance
 Electrical Machines-1
(for a given speed) with which the shunt generator would just excite is known as its critical field
resistance.

Note:: The maximum field circuit resistance (for a given speed) with which the shunt generator would just
excite is known as its critical field resistance

3.2.4.Reasons for Failure of Self-excited Shunt Generator to Build-up Voltage.

Remedial Measures
The reasons why a self-excited generator may fail to build-up voltage are given below:
1.No residual magnetism:.
The start of the build-up process requires some residual magnetism in the magnetic circuit of
the generator. If there is little or no residual magnetism, because of inactivity or jarring in transportation,
no voltage will be generated that can produce field current.
To overcome this difficulty, a separate source of direct current is applied to the field for a short
pedal, of time and then removed. The magnetic field should now be sufficient to allow the voltage to
build-up. The application of a separate source of direct current to the field is called ‘flashing the field’.
2. Field connection reversed.
The voltage generated due to residual magnetism is applied to the field. Current should flow in
the field coils in such a direction as to produce lines of flux in the same direction as the residual flux. If the
field connections are reversed, the lines of flux produced by the current flow will oppose the residual flux
so that the generated voltage will decrease rather than increase.
when the field circuit is dose so, In this instance it is necessary to reverse the field connections
with respect to the armature.
3.Field circuit resistance too. high.
 Electrical Machines-1
A field circuit resistance greater than critical value will prevent an appreciable build-up. At no
load, resistance greater than the critical may be caused by the following:
• Open field circuit connection:
The effects of an open circuit are apparent. The field circuit resistance is much greater than
the critical value; hence generator will not build- up.
• Dirty commutator:
A dirty commutator does not permit good contact between the brushes and the commutator.
This poor contact shows up as a high resistance to the flow of current in the field circuit and produces the
same effect as a high field circuit resistance

4.speed less than critical speed:

With no external resistance in the field circuit if the self excited generator fails to build up, it
may be due to the armature speed being less than the critical speed .
To overcome this speed of armature is increased upto critical speed.
Critical Speed (NC)
The critical speed of a shunt generator is the minimum speed below which it fails to excite.
Remedy
In case the generator is started up for the first time, it may be that no voltage will be built up either because
the poles
have no residual magnetism or the poles have retained some residual magnetism but the field winding
connections are
reversed so that the magnetism developed by the field winding on start has destroyed the residual
magnetism and the
machine can not “build up”. In both the cases, the field coils must be connected to a dc source (a
storagebattery) for a
short while to magnetise the poles. The application of external source of direct current to the field is called
flashing of
the field.
→Conditions for Voltage Build-Up of a Shunt Generator:
The necessary conditions for voltage build-up in a shunt generator are:
• There must be some residual magnetism in generator poles.
• The connections of the field winding should be such that the field current strengthens the residual
magnetism.
• The resistance of the field circuit should be less than the critical resistance. In other words, the
speed of the generator should be higher than the critical speed
3.3.Characterstics Of DC Generators:
The operating characteristics of DC generators are the relationship between the basic quantities related to
 Electrical Machines-1
Generator operation graphically. The basic quantities are terminal voltage V T , armature current Ia, excitation
current If .
The characteristics are
1. Open Circuit Characteristic (O.C.C.) (E0/If)
2. Internal Or Total Characteristic (E/Ia)
3. External Characteristic (V/IL)

Open Circuit Characteristic (O.C.C.) :

Open circuit characteristic is also known as magnetic characteristic or no-load saturation
characteristic. This characteristic shows the relation between generated emf at no load (E0) and the field
current (If) at a given fixed speed. The O.C.C. curve is just the magnetization curve and it is practically similar
for all type of generators. The data for O.C.C. curve is obtained by operating the generator at no load and
keeping a constant speed. Field current is gradually increased and the corresponding terminal voltage is
recorded.

Internal Or Total Characteristic (E/Ia):

An internal characteristic curve shows the relation between the on-load generated emf (Eg) and the
armature current (Ia). The on-load generated emf Eg is always less than E0 due to the armature reaction. Eg
can be determined by subtracting the drop due to demagnetizing effect of armature reaction from no-load
voltage E0. Therefore, internal characteristic curve lies below the O.C.C. curve.

External Characteristic (V/IL):

An external characteristic curve shows the relation between terminal voltage (V) and the load current
(IL). Terminal voltage V is less than the generated emf Eg due to voltage drop in the armature circuit.
Therefore, external characteristic curve lies below the internal characteristic curve. External characteristics
are very important to determine the suitability of a generator for a given purpose. Therefore, this type of
characteristic is sometimes also called as performance characteristic or load characteristic.

3.3.1.Characterstics of Seperately excited Generator:

In a separately excited DC generator, the field winding is excited by an external independent source.
There are generally three most important characteristic of DC generator:

I.Magnetic or Open Circuit Characteristic :

The curve which gives the relation between field current (If) and the generated voltage (E0) in the
armature on no load is called magnetic or open circuit characteristic of a DC generator. The plot of this curve
 Electrical Machines-1
is practically same for all types of generators, whether they are separately excited or self-excited. This curve
is also known as no load saturation characteristic curve of DC generator.

Let us consider a separately excited DC generator giving its no load voltage E0 for a constant field
current. If there is no armature reaction and armature voltage drop in the machine then the voltage will
remain constant. Therefore, if we plot the rated voltage on the Y axis and load current on the X axis then the
curve will be a straight line and parallel to X-axis as shown in figure below. Here, AB line indicating the no
load voltage (E0).

When the generator is loaded then the voltage drops due to two main reasons-
1. Due to armature reaction,
2. Due to ohmic drop (IaRa).
II. Internal or Total Characteristics:

The internal characteristic of the separately excited DC generator is obtained by subtracting

the drops due to armature reaction from no load voltage. This curve of actually generated voltage (E g) will be
slightly dropping. Here, AC line in the diagram indicating the actually generated voltage (E g) with respect to
load current. This curve is also called total characteristic of separately excited DC generator.

III. External Characteristic of Separately Excited DC Generator

The external characteristic of the separately excited DC generator is obtained by subtracting

the drops due to ohmic loss (Ia Ra) in the armature from generated voltage (Eg).
Terminal voltage(V) = Eg - Ia Ra.
This curve gives the relation between the terminal voltage (V) and load current. The external characteristic
curve lies below the internal characteristic curve. Here, AD line in the diagram below is indicating the
change in terminal voltage(V) with increasing load current. It can be seen from figure that when load current
increases then the terminal voltage decreases slightly. This decrease in terminal voltage can be maintained
easily by increasing the field current and thus increasing the generated voltage. Therefore, we can get
constant terminal voltage.
 Electrical Machines-1
Applications:
→Because of their ability of giving wide range of voltage output, they are generally used for testing purpose
in the laboratories.
→Separately excited generators operate in a stable condition with any variation in field excitation. Because
of this property they are used as supply source of DC motors, whose speeds are to be controlled for various
applications. Example- Ward Leonard Systems of speed control.

3.3.2.Characterstics of DC series generators:

In these types of generators the field windings, armature windings and external load circuit all are connected
in series
as shown in figure below.

Therefore, the same current flows through armature winding, field winding and the load. Let, I = I a = Isc = IL
Here, Ia = armature current
Isc = series field current
IL = load current
There are three most important characteristics of series wound DC generator which is discussed below

Magnetic or Open Circuit Characteristics::

The curve which shows the relation between no load voltage and the field excitation current is
called magnetic or open circuit characteristic curve. As during no load, the load terminals are open circuited,
there will be no field current in the field since, the armature, field and load are series connected and these
three make a closed loop of circuit. So, this curve can be obtained practically be separating the field winding
and exciting the DC generator by an external source.
Here in the diagram below AB curve is showing the magnetic characteristic of series wound DC
generator. The linearity of the curve will continue till the saturation of the poles. After that there will be no
further significant change of terminal voltage of DC generator for increasing field current. Due to residual
magnetism there will be a small initial voltage across the armature that is why the curve started from a point
A which is a little way up to the origin O.
 Electrical Machines-1

Internal Characteristic of Series Wound DC Generator

The internal characteristic curve gives the relation between voltage generated in the armature and the load
current.
This curve is obtained by subtracting the drop due to the demagnetizing effect of armature reaction from
the no load voltage. So, the actual generated voltage ( Eg) will be less than the no load voltage (E0). That is
why the curve is slightly dropping from the open circuit characteristic curve. Here in the diagram below OC
curve is showing the internal characteristic or total characteristic of the series wound DC generator
External Characteristic of Series Wound DC Generator
The external characteristic curve shows the variation of terminal voltage (V) with the load current ( I L).
Terminal voltage of this type of generator is obtained by subtracting the ohomic drop due to
armature resistance (Ra) and series field resistance ( Rsc) from the actually generated voltage ( Eg).
Terminal voltage V = Eg - I(Ra + Rsc)
The external characteristic curve lies below the internal characteristic curve because the value of terminal
voltage is less than the generated voltage. Here in the figure OD curve is showing the external characteristic
of the series wound DC generator.
• It can be observed from the characteristics of series wound DC generator, that with the increase in
load the terminal voltage of the machine increases. But after reaching its maximum value it starts to
decrease due to excessive demagnetizing effect of armature reaction.
• if load is increased, the field current, armature current is increased as they are in series with load.
But due to saturation, there will be no further significance raise of magnetic field strength even with
increase in load. But due to increased armature current, the affect of armature reaction increases
significantly which causes significant fall in load voltage. If load voltage falls, the load current is also
decreased proportionally since current is proportional to voltage as per Ohm’s law .
 Electrical Machines-1
• So, increasing load, tends to increase the load current, but decreasing load voltage, tends to
decrease load current. Due these two simultaneous effects, there will be no significant change in
load current in dotted portion of external characteristics of series wound DC generator. That is
why series DC generator is called constant current DC generator.
Applications:
o They are used for general lighting.
o They are used to charge battery because they can be made to give constant output voltage.
o They are used for giving the excitation to the alternators.
o They are also used for small power supply.

3.3.3.Characterstics of DC shunt generators:

In shunt wound DC generators the field windings are connected in parallel with armature conductors as
shown in figure below. In these type of generators the armature current Iadivides in two parts. One part is the
shunt field current Ish flows through shunt field winding and the other part is the load current I L goes through
the external load.

Magnetic or Open Circuit Characteristics::

This curve is drawn between shunt field current(Ish) and the no load voltage (E0). For a given excitation
current or field current, the emf generated at no load E0 varies in proportionally with the rotational speed of
the armature. Here in the diagram the magnetic characteristic curve for various speeds are drawn.

Due to residual magnetism the curves start from a point A slightly up from the origin O. The upper
portions of the curves are bend due to saturation. The external load resistance of the machine needs to be
maintained greater than its critical value otherwise the machine will not excite or will stop running if it is
already in motion. AB, AC and AD are the slops which give critical resistances at speeds N 1, N2 and N3. Here,
N 1 > N 2 > N 3.
 Electrical Machines-1
Critical Load Resistance of Shunt Wound DC Generator is the minimum external load resistance which is
required to excite the shunt wound generator.

Internal Characteristic of Shunt Wound DC Generator

The internal characteristic curve represents the relation between the generated voltage Egand the load
current IL. When the generator is loaded then the generated voltage is decreased due to armature reaction.
So, generated voltage will be lower than the emf generated at no load. Here in the figure below AD curve is
showing the no load voltage curve and AB is the internal characteristic curve.

External Characteristic of Shunt Wound DC Generator

AC curve is showing the external characteristic of the shunt wound DC generator. It is showing the variation
of terminal voltage with the load current. Ohmic drop due to armature resistance gives lesser terminal
voltage the generated voltage. That is why the curve lies below the internal characteristic curve.

The terminal voltage can always be maintained constant by adjusting the of the load terminal. When the load
resistance of a shunt wound DC generator is decreased, then load current of the generator increased as
shown in above figure. But the load current can be increased to a certain limit with (upto point C) the
decrease of load resistance. Beyond this point, it shows a reversal in the characteristic. Any decrease of load
resistance, results in current reduction and consequently, the external characteristic curve turns back as
shown in the dotted line and ultimately the terminal voltage becomes zero. Though there is some voltage due
to residual magnetism.
We know, Terminal voltage
 Electrical Machines-1
Now, when IL increased, then terminal voltage decreased. After a certain limit, due to heavy load current and
increased ohmic drop, the terminal voltage is reduced drastically. This drastic reduction of terminal voltage
across the load, results the drop in the load current although at that time load is high or load resistance is
low.
That is why the load resistance of the machine must be maintained properly. The point in which the
machine gives maximum current output is called breakdown point. point C in the picture).
Applications:
o They are used for supplying field excitation current in DC locomotives for regenerative breaking.
o This types of generators are used as boosters to compensate the voltage drop in the feeder in various
types of distribution systems such as railway service.
o In series arc lightening this type of generators are mainly used

3.3.4.Compound Generator Characteristics

In a compound generator, both series and shunt excitation are combined as shown in Fig. (3.13). The
shunt winding can be connected either across the armature only (short-shunt connection S) or across
armature plus series field (long-shunt connection G). The compound generator can be cumulatively
compounded or differentially compounded generator.

The latter is rarely used in practice. Therefore, we shall discuss the characteristics of cumulatively-
compounded generator. It may be noted that external characteristics of long and short shunt compound
generators are almost identical.

External characteristic
Fig. shows the external characteristics of a cumulatively compounded generator. The series excitation aids
the shunt excitation. The degree of compounding depends upon the increase in series excitation with the
increase in load current
 Electrical Machines-1

(i) If series winding turns are so adjusted that with the increase in load current the terminal voltage
increases, it is called over-compounded generator. In such a case, as the load current increases, the
series field m.m.f. increases and tends to increase the flux and hence the generated voltage. The
increase in generated voltage is greater than the IaRa drop so that instead of decreasing, the terminal
voltage increases as shown by curve A.
(ii) If series winding turns are so adjusted that with the increase in load current, the terminal voltage
substantially remains constant, it is called flat-compounded generator. The series winding of such a
machine has lesser number of turns than the one in over-compounded machine and, therefore, does
not increase the flux as much for a given load current. Consequently, the full-load voltage is nearly
equal to the no-load voltage as indicated by curve B
(iii) If series field winding has lesser number of turns than for a flat-compounded machine, the
terminal voltage falls with increase in load current as indicated by curve C m Fig. (3.14). Such
a machine is called under-compounded generator.
Applications of Compound Wound DC Generators
Among various types of DC generators, the compound wound DC generators are most widely used
because of its compensating property. Depending upon number of series field turns, the cumulatively
compounded generators may be over compounded, flat compounded and under compounded. We can get
desired terminal voltage by compensating the drop due to armature reaction and ohmic drop in the in the
line. Such generators have various applications.

o Cumulative compound wound generators are generally used for lighting, power supply purpose and
for heavy power services because of their constant voltage property. They are mainly made over
compounded.
o Cumulative compound wound generators are also used for driving a motor.
o For small distance operation, such as power supply for hotels, offices, homes and lodges, the flat
compounded generators are generally used.
o The differential compound wound generators, because of their large demagnetization armature
reaction, are used for arc welding where huge voltage drop and constant current is required.
 Electrical Machines-1

Parallel Operation of D.C. Generators

In a d.c. power plant, power is usually supplied from several generators of small ratings connected in parallel
instead of from one large generator. This is due to the following reasons:
(i)Continuity of service
If a single large generator is used in the power plant, then in case of its breakdown, the whole plant will be
shut down. However, if power is supplied from a number of small units operating in parallel, then in case of
failure of one unit, the continuity of supply can be maintained by other healthy units.
(ii)Efficiency
Generators run most efficiently when loaded to their rated capacity. Electric power costs less per kWh when
the generator producing it is efficiently loaded. Therefore, when load demand on power plant decreases, one
or more generators can be shut down and the remaining units can be efficiently loaded.
(iii)Maintenance and repair
Generators generally require routine-maintenance and repair. Therefore, if generators are operated in
parallel, the routine or emergency operations can be performed by isolating the affected generator while
load is being supplied by other units. This leads to both safety and economy.
(iv)Increasing plant capacity
In the modern world of increasing population, the use of electricity is continuously increasing. When added
capacity is required, the new unit can be simply paralleled with the old units.
(v)Non-availability of single large unit
In many situations, a single unit of desired large capacity may not be available. In that case a number of
smaller units can be operated in parallel to meet the load requirement. Generally a single large unit is more
expensive.
Connecting Shunt Generators in Parallel
1. The generators in a power plant are connected in parallel through bus-bars. The bus-bars are heavy
thick copper bars and they act as +ve and –ve terminals. The +ve terminals of the generators are
connected to +ve side of the busbar and negative terminals to the negative side of bus-bars.
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2. To connect the 2 generators with the 1 existing working generators, first we have to bring the speed
of the prime mover of the 2nd generator to the rated speed. At this point switch S 4 is closed
3. The circuit breaker V2 (voltmeter) connected across the open switch S2 is closed to complete the
circuit. The excitation of the generator 2 is increased with the help of field rheostat till it
generates voltage equal to the voltage of bus-bars.
4. The main switch S2 is then closed and the generator 2 is ready to be paralleled with existing
generator. But at this point of time generator 2 is not taking any load as its induced e.m.f. is equal to
bus-bar voltage. The present condition is called floating, that means ready for supply but not
supplying current to the load.
5. In order to deliver current from generator 2, it is necessary that its induced e.m.f. E should be greater
than the bus-bars voltage V. By strengthening the field current, the induced e.m.f. of generator 2
could be improved and the current supply will get started. To maintain bus-bar voltage, the field of
generator 1 is weakened so that value remains constant.
Field current I given by

Load Sharing of Parallel Connected DC Generators:

The load gets shifted to another generator by adjusting induced e.m.f., but in modern power plant,
everything has been done by “sychroscope” which gives instruction to governor of the prime mover. Let us
suppose that two generator having different load voltage. Then the load sharing between these generators
will be
 Electrical Machines-1

The value of current output depends upon the values of E1 and E3 which could be managed by field rheostats
to keep the bus-bars voltage constant.
Precautions During Parallel Connection
o The specification of each generator is different from one another. When they are synchronized
together, their speed are locked into the overall speed of the system.
o The entire load of the system should be distributed in all the generators.
o There should be a controller for keeping check on parameters of the engine. This can be done with
modern digital controllers which are available in market.
o Voltage regulation in the whole system plays an important role. In case of voltage drop in one unit
compare with other units, end up bearing the whole voltage load of the system of parallel
generators.
o While connecting terminals to the bus-bars, extra precaution should be made. If generator is
connected with wrong polarity of the bar, it may result to a short circuit.
The parallel arrangement of DC series generators:
The two DC series motors are disconnected from the supply mains and are connected in parallel to the
resistors. During braking the motors continue to run due to kinetic energy of train and act as self excited dc
series generators generating emfs. The kinetic energy is converted in to heat and dissipated in the resistors.
This is how DC series generators work while operating in parallel.
Without Equalizing Bar:
Let two DC machines (I and II) of resistance R (the armature and field resistance), induced emfs E1 and E2
operate in parallel, as shown in the first figure.
When the induced emfs E1 and E2 are equal they will share the equal load. There is a problem when
the induced emfs are different in two machines. When one of the induced emf becomes greater than another
induced emf a circulating current will flow. If E1 becomes slightly greater than E2, then a current i will
circulate in clockwise direction, as shown by dotted lines in the first figure. The magnitude of the circulating
current i will be (E1-E2)/R .
Now total current supplied by machine I will be (I + i) and of machine II (I - i). So the series field
current of machine I increases, also characteristic of a DC series generator is a rising characteristics. Thus the
induced emf of machine I (E1) will rise and induced emf of machine II will fall. Thus the difference of the two
induced emfs E1 am E2 will increase, which will cause further increase in circulating current. Thus the effect is
cumulative and if there were no fuse or automatic switch in the circuit, the current it machine II will be
reversed. This would reverse the direction of induced emf E2 and resultant emf in the circuit would be [E - (-
E2)] i.e. (E1 + E2) and circulating current (E1+ E2)/2R.
Thus the two emfs will then act in series around a circuit of very low resistance and conditions are
virtually those of a short-circuit on the two machines resulting in damage of the machines. We have to avoid
this short circuit problem for the parallel operation of DC series generators.
 Electrical Machines-1

With Equalizing Bar:

Here comes the use of equalizer bars in the parallel operation of DC series generators. Thepossibility
of reversal of either machine can be prevented by preventing the flow of circulating current produced due to
inequalities of induced emfs of the machines through the series field winding.
This aim can be achieved by connecting a heavy copper bar of negligible resistance across the two
machines as shown in the figure. Now the circulating current does not affect the field winding, but it get
confined to the armature and the equalizing bars. Now if the armature current increases, the terminal voltage
drop occurs and the original condition is restored.

Compound Generators in Parallel:

Under-compounded generators also operate satisfactorily in parallel but over compounded

generators will not operate satisfactorily unless their series fields are paralleled. This is achieved by
connecting two negative brushes together as shown in Fig. (i). The conductor used to connect these brushes
is generally called equalizer bar. Suppose that an attempt is made to operate the two generators in Fig. (ii) in
 Electrical Machines-1
parallel without an equalizer bar. If, for any reason, the current supplied by generator 1 increases slightly, the
current in its series field will increase and raise the generated voltage.
This will cause generator 1 to take more load. Since total load supplied to the system is constant, the
current in generator 2 must decrease and as a result its series field is weakened. Since this effect is
cumulative, the generator 1 will take the entire load and drive generator 2 as a motor. Under such conditions,
the current in the two machines will be in the direction shown in Fig. (ii). After machine 2 changes from a
generator to a motor, the current in the shunt field will remain in the same direction, but the current in the
armature and series field will reverse. Thus the magnetizing action, of the series field opposes that of the
shunt field. As the current taken by the machine 2 increases, the demagnetizing action of series field
becomes greater and the resultant field becomes weaker. The resultant field will finally become zero and at
that time machine 2 will short circuit machine 1, opening the breaker of either or both machines.
When the equalizer bar is used, a stabilizing action exists? and neither machine tends to take all the
load. To consider this, suppose that current delivered by generator 1 increases [See Fig. (i)]. The increased
current will not only pass through the series field of generator 1 but also through the equalizer bar and series
field of generator 2. Therefore, the voltage of both the machines increases and the generator 2 will take a
part of the load.