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1. Definitions
1. DC series motor has high starting torque.
2. For most of the applications DC shunt generator
is more popular than DC series generator.
3. A series generator is provided with diverters and
is diverting its rated current. Now if diverter’s
switch is opened, the terminal voltage will
increase.
4. When load is removed, a series DC motor will run
at excessively high speed.
5. The speed of series DC motor decreases if the flux
in the field winding increases.
6. The direction of rotation of a DC series motor can
be reversed by interchanging the field terminal
only.
7. Connecting a resistance in series with the field
winding, the speed of a DC shunt motor can be
increased above the rated speed.
8. When starting a differentially connected
compound motor, it is desirable to short‐circuit the

2. series field winding to avoid excessive starting
speed.
9. During the normal operation of series motor, if
the field circuit suddenly opens, the motor speed
will increase.
10.Series motor has high starting torque, shunt
motor has constant speed, 3‐phase induction
motor has lower starting torque and synchronous
motor has very poor stability.
11.If series field in cumulatively compound motor
get short‐circuited then armature current and
speed will both increase.
12.The series winding of a cumulatively compound
DC motor is short‐circuited while driving a load at
rated torque. This results in increase in both the
armature current as well as motor speed.
13.In a differentially compound DC motor, if shunt
field suddenly opens, the motor will first stop and
then run in opposite direction as series motor.
14.Series motor has poorest speed regulation.

3. 15.DC series motor is used in buses, trains, trolleys,
hoists and cranes because high starting torque is
required.
16.As the load is increased, the speed of DC shunt
motor will reduce slightly.
17.The speed of DC motor can be varied by varying:
(i) field current, (ii) applied voltage, (iii) resistance
in series with armature.
18.Speed control methods of shunt DC motor are: (i)
flux control method, (ii) Armature control method,
(iii) Voltage control method
19.Speed control methods of series DC motor is flux
control method and its types are: (i) field control,
(ii) Armature diverters, (iii) tapped field control,
(iv) parallel field coils
20.If excitation of DC shunt motor is changed, torque
will change, but power will remain constant.
21.Series motor should not be started at no‐load.

4. 22.A series motor will be preferred for load having
peak value for short short duration and almost
zero for most part of the time.
23.Series DC motor, as compared to shunt and
compound motor, has highest torque at the start
because of its comparatively strong series field.
And if load is removed then it will run at highest
speed.
24.If the speed of shunt DC generator is doubled
keeping flux constant, its generated EMF doubles.
25.Electromagnetic torque and speed are in
opposite directions in the case of shunt
generators.
26.When the supply terminals of DC shunt motor are
interchanged, the motor will run at its normal
speed in the same direction as before.
27.If polarity of connection of the armature and field
winding are reversed simultaneously in a DC shunt
motor, then it will rotate in the same direction.

5. 28. If a DC shunt motor is running with a certain
load. The effect of adding an external resistance in
the field circuit is to increase the motor speed.
29.If the applied voltage of a DC shunt motor is
halved with the load torque doubled, the speed
will be the same and armature current is doubled.
30.A shunt motor is running at its rated speed on
rated load. If the field circuit gets suddenly
opened, the motor will draw very high armature
current but it will continue to drive the load at a
reduced speed.
31.Speed of DC shunt motor has nothing to do with
applied voltage if losses and saturation are
neglected.
32.If a DC shunt motor is working at no‐load and if
shunt field circuit suddenly opens, this will result in
excessive speed, possibly destroying armature due
to excessive centrifugal stresses.
33.As the load of shunt DC motor is increased, its
speed reduces slightly.

6. 34.The highest speed attained by a DC shunt motor
at rated flux is equal to no‐load speed.
35.If the speed of a DC shunt motor increased, the
back emf increases.
36.A shunt DC motor has highest %age increase in
input current for a given %age increase in torque.
37.A shunt DC motor has approx. constant speed.
38.If the supply voltage to a shunt motor is increased
by 25%, the full load speed will decrease.
39.A DC shunt motor has external resistance in
the field circuit & in the armature circuit. The
starting armature current for motor will be
minimum, when is minimum & is maximum.
40.Shunt motor with commutating poles has the
least reduction in speed from no load to rated
load.
41.If the field circuit of shunt DC motor running at
rated speed gets open circuited, then immediately,
speed of motor tend to increase.

7. 42.If the supply voltage of DC shunt motor running
at rated speed, rated supply voltage is halved, then
the speed of the motor becomes slightly less than
the rated speed.
43.Direction of DC shunt motor will remain the same
if both the field terminals and armature terminal
are interchanged.
44.Difference between the no load and full load
speed of a DC shunt motor is of the order of 1%.
45.In a DC shunt motor if the supply voltage is
reduced by 15%, full load speed will increase.
46.The field winding of an unloaded DC shunt motor
gets opened while running, then it will stop.
47.The speed of DC shunt motor can be increased by
increasing the resistance in field circuit.
48.If the speed of DC shunt motor is increased, the
back emf of rotor will also increase.
49.If a DC shunt motor is working at full load and if
shunt field suddenly opens, this will make

8. armature to take heavy current, possibly burning
it.
Armature
50.The part which houses the conductors and in
which emf induced is to be utilized is called
armature in any rotating machines. Or
51.The rotating part of a DC machine is known as
armature.
52.All rotating electrical machines have rotating
armature and fixed field.
53.The armature reaction in a DC motor is
attributed to the effect of magnetic field set up by
armature current. In DC machine, the armature
reaction MMF is stationary w.r.t. the stator.
54.The armature current drawn by DC motor is
proportional to torque required.
55.Armature control method provides a constant
torque drive.
56.If be the armature current, then speed of DC
shunt motor is proportional to

9. 57.The armature voltage control of DC motor
provides constant torque drive.
58.In a DC machine, if P is the number of poles, N is
the armature speed in rpm then the frequency of
magnetic reversal will be PN/120.
59.To get a strong magnetic field, the air gap
between stator and armature is kept as small as
possible.
60.Armature flux leakage depends on: (i) length of
air gap, (ii) shape of magnet core, (iii) flux density
used in core and teeth.
61.Starter resistance in a DC motor is connected in
series with the armature.
62.Plugging of DC motors is normally done by
reversing the armature polarity.
63.Factors responsible for decrease in terminal
voltage of DC shunt generator are: (i) Armature
resistance, (ii) Armature current, (iii) Reduction in
field current.

10. 64.The armature torque of the DC shunt motor is
proportional to armature current only.
65.The speed control method of DC machine which
offers minimum efficiency is armature control
method.
66.DC generators generate AC voltage in the
armature.
67.Armature core is made of silicon steel in DC
machines.
68.
Commutator
69.Each commutator segment is connected to the
armature conductor by mean of copper lug.
70.Commutator of a DC machine acts as a full‐wave
rectifier.
71.A DC motor can be easily identified by
commutator.
72.The compensating winding is located on pole
shoe for avoiding the flashover at commutator
surface.

11. 73.Sparking at the commutator of DC motor may
result in: (i) damage to commutator segment, (ii)
damage to commutator insulation, (iii) increased
power consumption.
74.Constant torque is produced due to commutator
in DC motor.
75.Number of commutator segments is equal to
number of armature coils in DC machine.
76.Ripples in direct emf generated in DC generators
are reduced by commutators with large number of
segments.
Interpoles
1. If DC machine has interpole as well as
compensating windings, then both are connected
in parallel with armature windings.
2. For reducing sparking at the commutator,
interpoles are used in DC motors.
3. Function of the interpoles in the DC generators is
to neutralize cross magnetizing effects of the
armature reaction.

12. 4. The brushes on commutator remain in contact
with conductor which lies under interpole regain in
DC generators.
5. Interpoles winding is connected in series with
armature winding in DC generators.
6. In DC generators interpole field coils are
connected in series with armature winding.
Interpoles helps the commutation process.
7. If residual magnetism of a shunt generator is
destroyed accidently, it may be restored by
connecting its shunt field to a battery.
8. The sole purpose of commentator in a DC
generator is to convert the induced alternating
voltage into unidirectional pulses.
9. All rotating electric machines are basically
electro‐mechanical converters.
10.V×B=E is the equation of the electrical machine
which converts mechanical energy into electrical
energy.

13. 11.The maximum number of brushes which may be
used in an electrical machine is equal to either 2 or
number of poles.
12.A DC generator has 6 poles. A physical brush shift
of 6 degree means as electrical brush shift of 18
degree.
13.For satisfactory commutation of DC machines the
brushes should be: (i) of proper grade & size, (ii)
smoothly run in the holders, (iii) smooth
concentric and properly undercut
14.Brushes for commutators for 220V DC generator
are generally made of electro‐graphite.
15.With the help of brushes & commutators, the
unidirectional torque is produced in a DC motor.
16.The number of brushes in commutators depends
on amount of current to be collected.
17.Voltage drop is the least in metal graphite
brushes in DC machines.
18.In a loaded DC generator if the brushes are given
a shift from the interpole axis in the direction of

14. rotation, then the commutation will deteriorate
which will result in fall of terminal voltage.
19.The yoke in a small DC machine is made of cast‐
iron.
20.The yoke of DC machine is always made of
magnetic material.
21.Compensating winding in a DC machine is placed
on yoke in the pole faces.
Losses
22.Hysteresis losses in a DC generator are basically
due to reversal of magnetism of the armature
core. It is proportional to N, f
23.Windage loss is proportional to square of
armature speed in a DC generator.
24.Iron losses in a DC machine take place in
armature rotor.
25. Core losses in a DC machine occur in both
armature and pole faces.

15. 26.The efficiency of a DC shunt generator is
maximum when the armature copper losses are
equal to constant losses.
27.Armature copper loss is likely to have highest
proportion of losses in DC generator.
28.Armature copper loss varies significantly with
load current in DC generator.
29.Using material of low hysteresis co‐efficient for
armature core material will result in low hysteresis
losses.
30.Iron losses in DC generator take place in armature
rotor.
31.Brush contact loss depends on load while stray
load losses depend on square of load.
32.Eddy current losses in a DC machine are
proportional to , .
33.Hysteresis loss in a DC machine is given by .
where V is the volume of iron part.
34.A DC machine has maximum efficiency when
variable losses are equal to constant losses.

16. 35.In DC machines, constant loss is composed of
friction Windage and iron loss and field circuit loss.
36.If a DC generator operates at constant speed,
variable loads then the losses which would be
most significant are copper losses.
37.Iron losses occur in the yoke in DC motor.
38.In DC machines, maximum losses occur due to
copper losses.
39.Hysteresis and eddy current losses depend on
flux magnitude.
40.No‐load rotational losses depend on rotor
rotation.
41.In DC generator mechanical losses have least
proportion.
42.Iron losses cause excessive heating of core, rise in
temperature and low efficiency.
43.If ‘t’ is the thickness of lamination then eddy
current loss in generator are proportional to
44.If is the maximum flux density then eddy
current loss is proportional to

17. 45.The hysteresis loss is proportional to f which is
frequency of magnetic reversal in DC generators.
46.Copper loss varies with load in DC generator.
47.Windage loss doesn’t vary with load nor with flux
density.
48.Mechanical loss, core loss and copper loss
dissipate in the form of heat.
49.Eddy current loss are significantly reduced by
laminating the core of DC generator.
50.Total losses in a well‐designed DC generator of 10
kW will be nearly 500W.
51.Mechanical losses are primary function of speed.
52.Iron losses are independent of load variation.
53.Sum of iron losses and mechanical loss is called
stray loss in DC machine.
Different Tests
54.Hopkinson’s test is a useful method for testing
the efficiency of DC machine.
55.Swinburne test is the most economical in finding
the no‐load losses of a large DC shunt motor.

18. 56.To determine the stray losses in case of shunt
motors & generators retardation test is used.
57.Back‐to‐back test on DC machines needs
minimum of two machines.
58.Hopkinson’s test on DC motor is conducted at full
load.
59.In Ward Leonard method of speed control, the
minimum number of machine needed is three.
60.Ward Leonard control is basically voltage control
method. Its disadvantage are: high initial cost, (ii)
high maintenance cost, (iii) low efficiency ay light
load
61.Field’s test can be conducted on other than shunt
machines.
62.Only one motor is required in Brake test.
63.Swinburne test cannot be used for series motors.
64.Retardation test is used to find the stray loss in
shunt DC machines.
65.Field test will be suitable for testing two similar
DC motors of large capacity.

19. 66.The main disadvantage of Hopkinson’s test is that
it requires two identical shunt machines.
67. Ward‐Leonard system of speed control is not
recommended for constant speed operations.
68.Braking test on DC motors is usually restricted to
small HP motors.
…...……………………………………………………………………………
1. Polarity of DC generator can be reversed by
reversing field current and also the direction of
rotation.
2. In DC generator relation between commercial
efficiency , electrical efficiency and
mechanical efficiency is given by = .
3. When paralleling of two compound generators
are needed then equalizer connections are
required.
4. The function of a starter in a DC machine is to
avoid excessive current at starting.

20. 5. A cumulatively compound DC generator supplies
15A at 240V. Now if the series field winding is
short circuited, the terminal voltage will reduce
below 240V.
6. In a DC machine, torque and induced EMF are
produced both in motor and generator.
7. The mechanical power developed by a DC motor
is maximum when the back EMF is equal to half
the applied voltage.
8. A self‐excited DC shunt motor is running at no
load at 1000 rpm. If the terminal voltage is
reduced 50% of its original value, the no‐load
speed of the rotor will be 1000 rpm (same)
9. The rotor is keyed to the shaft in DC motor.
10.If DC series motor having rating of 230V is
connected to AC supply of 230V then it will run
with less efficiency.
11.Direction of induced EMF can be found by
applying Fleming’s right hand rule.
12.

21. 13.The ratio of back EMF to supply EMF is an
indication of efficiency of DC motor.
14.The voltage equation of DC motor is V= +
15.Maximum power is achieved in a DC motor, when
supply voltage is equal to double of back EMF.
16.The speed of a DC motor is directly proportional
to back EMF.
17.A 150V DC motor with back 142V has armature
resistance 1 Ω, then armature current is 8A.
18.Keeping load current & flux of DC motor constant
if voltage applied across armature is increased 4%,
the speed of motor will increase by 4%.
19.A DC motor develops a torque off 100N‐m at
1000 rpm. Now if rpm is 1200 then torque will be
120N‐m.
20.The starting resistance of DC motor is usually
small.
21.The back emf of a DC motor depends on field
flux.

22. 22.When the direction of power flow reverses, a
cumulatively compound motor becomes a
differentially compound generator.
23.The output power of any electrical motor is taken
from the coupling mounted on the shaft.
24.If the field current is reduced to half, keeping
constant torque load & rated excitation then the
speed of motor will become slightly less than
double.
25.A DC motor draws high current at the time of
starting because =0 (back emf).
26.A 10HP series DC motor uses a diverter to control
the speed. For a constant load torque, the speed
will be minimum when the diverter resistance is
infinite.
27.Two DC series motors connected in series draw
current I from supply and run at speed N when the
same two motors are connected in parallel taking
current I from the supply, the speed of each motor
will be 4N.

23. 28.The starter for a DC motor also provides
protection against damage: (i) due to short circuit
in the equipment, (ii) from long term overload, (iii)
from excessive starting currents.
29.Direct online starters are not suitable for starting
large DC motors because large voltage drop may
occur in the supply mains.
30.The direction of rotation of the motor is usually
reversed by revering the connection of the
generator field terminals in Ward Leonard method.
31.The most economical method of electrical
braking of DC motor is regenerative braking.
32. Regenerative braking in shunt motors is used
when the load has overhauling characteristics.
33.The nominal power printed on the name plate of
any motor signifies the output power at the shaft.
34.In an overload DC motor, main danger arises due
to overheating of windings.
35.The maximum permitted temperature in DC
motor windings is usually 40 degree to 50 degree.

24. 36.The generated emf and the current are in the
opposite direction in case of DC motors.
37.Stepper motor is used in digital control system.
38.Universal motor is used in hand drills, single
phase induction motor is use in ceiling fans while
in cassette tape PM DC motor is used.
39.A 4 pole DC generator is running at 1500 rpm, the
frequency of current in the armature would be 50
Hz.
40.Speed control by variation of field flux results in
constant power drive while speed control by
varying the armature circuit resistance offers
constant torque drive.
41.The relation between electrical ( ) and
mechanical ( ) degree is given by: = (P/2)
where P represents poles of machine.
42.A DC cumulatively compound generator which is
supplying power to infinite bus will become
differentially compound motor with the direction

25. and speed same if mechanical power supply form
the prime mover fails.
43.A DC shunt generator builds up to a voltage of
220V at no‐load while running at its rated speed. If
the speed is raised by 25%, then voltage will build
up to level between 220V & 1.25 times of 220V.
44.Two identical lossless series motors connected in
series across a DC supply voltage run at speed of
& . Then the ratio of their output powers will
be :
45.In DC machine, the field system has to be
provided on stator, unlike synchronous machine
where in it could be on any member.
46.A 2 pole series motor with its two field coils
connected in series runs at a speed of 500 rpm.
Now if field coils are reconnected in parallel and
assuming that torque is constant and the magnetic
circuit is unsaturated, the new speed will be 1000
rpm.

26. 47.Laws of electromagnetic induction (Faraday and
Lenz’s) are summarized in: e = ‐dφ/dt.
48.In armature controlled separately excited DC
motor drive with closed‐loop speed control, an
inner current loop is useful because it limits the
peak current of the motor to the permissible value.
49. An electric motor with “constant output power”
will have torque‐speed characteristics in the form
of a rectangular hyperbola.
50.The efficiency of motor operating at 1500 rpm
and drawing current 3.5A with armature resistance
of 0.8 Ω and no‐load current 1.5A at 25V is 48%.
51.For same HP rating and full load speed
differentially compound motor has poor starting
torque.
52.If the supply voltage for DC motor is increased,
full load current will decrease. Operating speed or
starting torque will not decrease.
53.Air gap in DC motor is more than induction
motor.

27. 54.In variable speed operation DC motor is preferred
over AC motor.
55.Before saturation in DC motor α where
is torque & is armature current.
56.DC motor is used where high starting torque and
wide speed range control is required.
57.The speed of motor falls from 1100 rpm at no
load to 1050 rpm at rated load. The speed
regulation of the motor is (1100‐1050)/1050
=1/21= 4.76%
58.The speed regulation of DC motor is: = (No load
speed ‐ full load speed)/full load speed
59.If back emf and speed both are doubled, the
torque remains unchanged.
60.At the instant of starting, when DC motor is put
on supply, it behaves like low resistance circuit.
61.DC series motors are not used where load is
constant or frequency changing or constant
operating speed is required.

28. 62.If terminals of DC motor are interchanged, this is
called plugging braking which gives highest torque
braking.
63.Thyristor can be used to control the speed of
motor.
64.DC series machine has field consisting of few
number of turns of thick wire.
65.Torque developed in DC motor depends on: (i)
magnetic field, (ii) Active length and number of
conductors, (iii) current flow through the
conductors
66.DC generators are designed for maximum
efficiency around full load.
67.DC generators are installed near load centers to
reduce corona losses.
68.Dynamic braking is generally used for shunt,
series and compound motors.
69.A weaker commutating field is needed at low
speed than at high speed in variable speed motor.

29. 70.The mechanical power developed by DC motor is
equal to product of back emf and armature
current.
71.The variable resistor shunting the field of a DC
series motor is called a field diverter.
72.In Brake test only one motor is required.
73.Low cost is not an advantage of DC motor over
AC.
74.The function of field regulators for compound
motors is to control the flux.
75.Motor is made to run as generator in
regenerative braking.
76.Induced emf is given by Blv where B=flux density,
l= length of conductor, v= velocity of conductor.
77.For generating large currents on DC generators,
Lap winding is generally preferred.
78.The purpose of providing dummy coils in the
generators is to provide mechanical balance for
the rotor.

30. 79.Reversing the field current, the polarity of DC
generator can be reversed.
80.The maximum number of equalizers rings are
2Z/P in lap wound DC generator where Z=number
of conductor, P = number of poles
81.Eddy current are induced in the pole shoe of a DC
machine due to relative rotation between field and
armature.
82.Equalizer rings are required in case armature in
lap wound.
83.In DC machine, short circuited field coil will result
in: (i) odour of burning insulation, (ii) unbalanced
magnetic pull, producing vibrations, (iii) reduction
of generated voltage, for which excitation has to
be increased to maintain the voltage.
84.Compensating winding help in commutation.
85.In DC generators the magnetic field is produced
by either electromagnetic or permanent magnet.
86.For low voltage, high current lap winding is used
in DC generators.

31. 87.Actual flux distribution in DC generators depends
on : (i) size of air gap, (ii) shape of pole shoe, (iii)
clearance between tips of the adjacent pole shoes
88.DC series generator is used as booster to
maintain constant voltage at the load end of the
feeder.
89.The number of mechanical degree and electrical
degree will be the same when number of poles is 2
in the DC generators.
90.Permanence is the reciprocal of reluctance.
91.The emf generated in a DC generator is directly
proportional to : (i) flux/pole, (ii) speed of
armature, (iii) number of poles
92.When there is no load on generator, the
magnetic neutral axis coincides with the
geometrical neutral axis.
93.If shunt generator at 1000 rpm has emf 200V,
then at 1200 rpm, the emf would be 240V i.e.
direct relation.

32. 94.If a DC generator fails to build up, the probable
cause could not be field resistance less than the
critical resistance. And it could be imperfect brush
contacts, fault shunt connection tending to reduce
the residual magnetism or no residual magnetism
in the generator.
95.If emf induced in armature is 600V in shunt DC
generator. Armature resistance is 0.1 Ω and
current is 200A then terminal voltage is 580V.
96.In DC generator, the critical resistance is the
resistance of field.
97.The reason of short circuit in armature winding
could be: (i) insulation failure between two
commutators bars, (ii) insulation failure between
two turns of a coil, (iii) two or more turns of the
coil getting grounded
98.Number of tapping for each equalizer ring is
equal to number of pole pairs.
99.A DC generator can be considered as rotating
amplifier

33. 100. Lap winding is composed of any even number
of conductors.
101. If resistance of field winding is increased in DC
generator, then the output voltage will decrease.
102. Voltage remains constant irrespective of the
load in flat compound generator.
103. An equalizer bar is used when two DC series
generators are running in parallel, so that machine
passes approx. equal current to the load.
104. Shunt generators are preferred for parallel
operation.
105. A DC generator work on the principle of
Faraday’s low of electromagnetic induction.
106. A series generator can self‐excite only if the
load current is not zero.
107. A shunt generator can self‐excite only if the
resistance of the field circuit is less than critical
value.

34. 108. Terminal voltage of a series generator is 150V
when load current is 5A. If load current is 10A,
then terminal voltage will be greater than 150V.
109. If the open circuit voltage of a compound
generator is 250V, then at full load the terminal
voltage may be greater or less than 250V.
110. In case of DC generator 1% regulation is
usually preferred.