Sample Questions for End Semester Examination

Subject: DCMT
EE-251

Module-1
1. A single-phase 100 kVA, 1000 V / 100 V, 50 Hz transformer has a voltage drop of 5%
across its series impedance at full load. Of this, 3% is due to resistance. What is the
percentage regulation of the transformer at full load with 0.8 lagging power factor?

2. A single phase, 50 kVA, 1000 V/100 V two winding transformer is connected as a step-
up autotransformer to supply 1100V. What is the kVA rating of the autotransformer?

3. A single-phase transformer has no-load loss of 64 W, as obtained from an open circuit

test. When a short-circuit test is performed on it with 90% of the rated currents flowing
in its both LV and HV windings, he measured loss is 81 W. What is the load at which
the transformer has maximum efficiency?

4. A single phase transformer has a maximum efficiency of 90% at full load and unity
power factor. What is the efficiency at half load at the same power factor?

5. A single-phase, 2 kVA, 100/200 V transformer is reconnected as an auto-transformer

such that its kVA rating is maximum. What is the new kVA rating?

6. A single-phase 400 V, 50 Hz transformer has an iron loss of 5000 W at the condition.

When operated at 200 V, 25 Hz, the iron loss is 2000 W. When operated at 416 V, 52
Hz, the value of the hysteresis loss divided by the eddy current loss is ________?

7. The core loss of a single phase, 230/115 V, 50 Hz power transformer is measured from
230 V side by feeding the primary (230 V side) from a variable voltage variable
frequency source while keeping the secondary open circuited. The core loss is measured
to be 1050 W for 230 V, 50 Hz input. The core loss is again measured to be 500W for
138 V, 30 Hz input. Determine the hysteresis and eddy current losses of the transformer
for 230 V, 50 Hz input.

8. Two transformers are to be operated in parallel such that they share load in proportion
to their kVA ratings. The rating of the first transformer is 500 kVA and its pu leakage
impedance is 0.05 pu. If the rating of second transformer is 250 kVA, its pu leakage
impedance is?

9. Two single phase transformers A and B have the following parameters.

Transformer A: 400V/200V, 10 kVA, percentage resistance and percentage reactance
are 3% and 4% respectively.
Transformer B: 5 kVA, 400V/200V, percentage resistance and percentage reactance
are 4% and 3% respectively.
These two transformers are connected in parallel and they share a common load
 of 12 kW at a power factor of 0.8 lag. Determine the active and reactive power
delivered by transformer

10. A 10 kVA, 400 V/200 V, single phase transformer with a percentage resistance of 3%
and percentage reactance of 6% is supplying a current of 50 A to a resistive load. The
value of the load voltage is?

Module-2
1. A 3-phase transformer bank consists of three identical 2300/230 V, 15 kVA single-
phase transformers connected in delta/delta. The bank supplies a 20 kVA, unity p.f. 3-
phase load. If one of the single-phase transformer develops a fault, and is removed, the
load carried by each of the two transformers now operating in open delta will be?

2. If the primary line voltage rating is 3.3 kV (Y side) of a 25 kVA. Y−△ transformer (the
per phase turns ratio is 5:1), then the line current rating of the secondary side (in
Ampere) is?

3. Three single-phase transformers are connected to form a delta-star three-phase

transformer of 110 kV/ 11 kV. The transformer supplies at 11 kV a load of 8 MW at
0.8 p.f. lagging to a nearby plant. Neglect the transformer losses. The ratio of phase
currents in delta side to star side is?

4. A 3-phase 900 kVA, 3 kV / 3 kV (Δ/Y), 50 Hz transformer has primary (high voltage

side) resistance per phase of 0.3 Ω and secondary (low voltage side) resistance per phase
of 0.02 Ω. Iron loss of the transformer is 10 kW. The full load % efficiency of the
transformer operated at unity power factor is _______.

5. A three-phase, three winding △ / △ / Y (1.1 kV/6.6 kV/400 V) transformer is energized

from AC mains at the 1.1 kV side. It supplies 900 kVA load at 0.8 power factor lag
from the 6.6 kV winding and 300 kVA load at 0.6 power factor lag from the 400 V
winding. The RMS line current in ampere drawn by the 1.1 kV winding from the mains
is?_____.

6. If the primary line voltage rating is 3.3 kV (Y side) of a 25 kVA. Y−△ transformer (the
per phase turns ratio is 5:1), then the line current rating of the secondary side (in
Ampere) is?
Module-3
1. A solenoid has 2000 turns wound over a length of 0.3 m. The area of cross-section is
1.2×10–3 m2. Around its central section a coil of 300 turns is closely would. If an initial
current of 2A is reversed in 0.25s, find the emf induced in the coil.

2. A coil of 160 turns of cross-sectional area 250 cm2 rotates at an angular velocity of 300
radian/sec about an axis parallel to the plane of the coil in a uniform magnetic field of
0.6 Wb/m2. What is the maximum emf induced in the coil? If the coil is connected to a
resistance of 2 ohm, what is the maximum torque that has to be delivered to maintain
its motion?
3. A circular copper disc 10 cm in radius rotates at 20π rads–1 about an axis through its
centre and perpendicular to the disc. A uniform magnetic field of 0.2T acts
perpendicular to the disc. (a) Calculate the potential difference developed between the
axis of the disc and the rim. (b) What is the induced current, if the resistance of 2Ω is
connected in between axis and rim of the disc?

Module-4 and Module-5 (combined)

1. A separately excited DC generator has an armature resistance of 0.1 Ω and negligible
armature inductance. At rated field current and rated rotor speed, its open-circuit
voltage is 200 V. When this generator is operated at half the rated speed, with half the
rated field current, an un-charged 1000 μF capacitor is suddenly connected across the
armature terminals. Assume that the speed remains unchanged during the transient. At
what time (in microsecond) after the capacitor is connected will the voltage across it
reach 25 V?

2. A 8 pole DC generator has a simplex wave wound armature containing 32 coils of 6

turns each. If the flux per pole is 0.06Wb. The machine is running at 250rpm. The
induced armature voltage is?

3. A DC shunt generator delivers 45 A at a terminal voltage of 220 V. The armature and

the shunt field resistances are 0.01 Ω and 44 Ω respectively. The stray losses are 375
W. The percentage efficiency of the DC generator is?

4. A separately excited dc motor has an armature resistance Ra=0.05Ω.Ra=0.05Ω. The

field excitation is kept constant. At an armature voltage of 100 V, the motor produces
a torque of 500 Nm at zero speed. Neglecting all mechanical losses, the no-load speed
of the motor (in radian/s) for an armature voltage of 150 V is?

5. A 200 V DC series motor, when operating from rated voltage while driving a certain
load, draws 10 A current and runs at 1000 r.p.m. The total series resistance is 1Ω1Ω.
The magnetic circuit is assumed to be linear. At the same supply voltage, the load torque
is increased by 44%. The speed of the motor in r.p.m. is?

6. A 220 V DC series motor runs drawing a current of 30 A from the supply. Armature
and field circuit resistance are 0.4ΩΩ and 0.1ΩΩ, respectively. The load torque varies
as the square of the speed. The flux in the motor may be taken as being proportional to
the armature current. To reduce the speed of the motor by 50%, the resistance in ohms
that should be added in series with the armature is?

7. A separately excited DC generator supplies 150 A to a 145 V DC grid. The generator

is running at 800 RPM. The armature resistance of the generator is 0.1ΩΩ. If the speed
of the generator is increased to 1000 RPM, the current in amperes supplied by the
generator to the DC grid is?

8. A 220 V, 10 kW, 900 rpm separately excited DC motor has an armature

resistance Ra=0.02Ω.Ra=0.02Ω. When the motor operates at rated speed and with rated
terminal voltage, the electromagnetic torque developed by the motor is 70 Nm.
Neglecting the rotational losses of the machine, the current drawn by the motor from
the 220 V supply is?
9. A 120 V DC shunt motor takes 2 A no load. It takes 7 A on full load while running at
1200 rpm. The armature resistance is 0.8 ΩΩ, and shunt field resistance is 240 ΩΩ. The
no load speed, in rpm, is?

10. DC motor has the following specifications: 10 HP, 37.5 A, 230V; flux/pole = 0.01 Wb,
number of poles = 4, number of conductors = 666, number of parallel paths = 2.
Armature resistance = 0.267ΩΩ. The armature reaction is negligible and rotational
losses are 600W. The motor operates from a 230V DC supply. If the motor runs at
1000 rpm, the output torque produced in (in Nm) is?

11. A shunt-connected DC motor operates at its rated terminal voltage. Its no-load speed is
200 radians/second. At its rated torque of 500 Nm, its speed is 180 radian/second. The
motor is used to directly drive a load whose load torque TLTL depends on its rotational
speed ωrωr(in radians/second), such that TLTL=2.78×ωr×ωr. Neglecting rotational
losses, the steady-state speed (in radian/second) of the motor, when it drives this load
is?

12. A 4-pole, separately excited, wave wound DC machine with negligible armature
resistance is rated for 230 V and 5 kW at a speed of 1200 rpm. If the same armature
coils are reconnected to forms a lap winding, what is the rated voltage (in volts) and
power (in kW) respectively at 1200 rpm of the reconnected machine if the field circuit
is left unchanged?

13. With an armature voltage of 100 V and rated field winding voltage, the speed of a
separately excited DC motor driving a fan is 1000 rpm, and its armature current is 10 A.
The armature resistance is 1ΩΩ. The load torque of the fan load is proportional to the
square of the rotor speed. Neglecting rotational losses, the value of the armature voltage
(in Volt) which will reduce the rotor speed to 500 rpm is?

14. A 15 kW, 230 V dc shunt motor has armature circuit resistance of 0.4 Ω and field circuit
resistance of 230 Ω. At no load and rated voltage, the motor runs at 1400 rpm and the
line current drawn by the motor is 5 A. At full load, the motor draws a line current of
70 A. Neglect armature reaction. The full load speed of the motor in rpm is?

15. A 250 V dc shunt machine has armature circuit resistance of 0.6 Ω and field circuit
resistance of 125 Ω. The machine is connected to 250 V supply mains. The motor is
operated as a generator and then as a motor separately. The line current of the machine
in both the cases is 50 A. The ratio of the speed as a generator to the speed as a motor
is?

16. The no-load speed of a 230 V separately excited dc motor is 1400 rpm. The armature
resistance drop and the brush drop are neglected. The field current is kept constant at
rated value. The torque of the motor in Nm for an armature current of 8 A is?

17. A separately excited 300 V DC shunt motor under no load runs at 900 rpm drawing an
armature current of 2 A. The armature resistance is 0.5 Ω and leakage inductance is
0.01 H. When loaded, the armature current is 15 A. Then the speed in rpm is?
 18. A 220 V, 15 kW, 1000 rpm shunt motor with armature resistance of 0.25Ω; has a rated
line current of 68 A and a rated field current of 2.2 A. The change in field flux required
to obtain a speed of 1600 rpm while drawing a line current of 52.8 A and a field current
of 1.8 A is?

19. A 220 V, DC shunt motor is operating at a speed of 1440 rpm. The armature resistance
is 1.0 Ω and armature current is 10 A. If the excitation of the machine is reduced by
10%, the extra resistance to be put in the armature circuit to maintain the same speed
and torque will be?

20. A separately excited DC motor runs at 1500 rpm under no-load with 200 V applied to
the armature. The field voltage is maintained at its rated value. The speed of the motor,
when it delivers a torque of 5 Nm, is 1400 rpm. The rotational losses and armature
reaction are neglected. For the motor to deliver a torque of 2.5 Nm at 1400 rpm the
armature voltage to be applied is?

21. A 240 V, dc shunt motor draws 15 A while supplying the rated load at a speed of 80
rad/s. The armature resistance is 0.5 Ω and the field winding resistance is 80 Ω.The
external resistance to be added in the armature circuit to limit the armature current to
125% of its rated value is?
Sample Objective Questions
Negative voltage regulation of a transformer indicates that the load is
(a) capacitive only
(b) inductive only
(c) inductive or resistive
(d) none of the above

The function of breather in a transformer is

(a) to provide oxygen inside the tank
(b) to cool the coils during reduced load
(c) cool the transformer oil
(d) to arrest flow of moisture when outside air enters the transformer.

In a star-star transformer with ungrounded neutral, the third harmonic voltages are present in
(a) line voltage
(b) phase voltage
(c) both in line voltage and phase voltage
(d) none of the above

In a delta-delta transformer, the third harmonic currents are absent in

(a) line current
(b) phase current
(c) both in line voltage and phase voltage
(d) none of the above

The effect of 3rd harmonics in three phase transformers

(a) Neutral oscillation
(b) Over voltage stress
(c) Both (a) and (b)
d) None of the above

In an open delta connection, the utilization factor is

(a) 0.577
(b) 0.866
(c) 0.924
(d) 1

Distribution transformer are generally connected in

A. DZ6
B. Dd0
C. Dy11
D. Dd6

Scott-connected transformer is used to convert

A. single phse to three phase
B. three phase to single phase
C. three phase to two phase
D. two phase to single phase

Equalizer rings are present in which type of windings?

a) Lap
b) Wave
c) Forgleg
d) None of the above

OCC or no-load characteristics are not obtained for which type of DC generators?
a) Series
b) Shunt
c) Separately excited
d) None of the above

Which type of winding is preferred for high current and low voltage machines?
a) Lap
b) Wave
c) Both Lap and Wave
d) None

Which type of winding is preferred for high voltage and low current machines?
e) Lap
f) Wave
g) Both Lap and Wave
h) None

Which type of DC motor are never operated under no-load?

a) Series
b) Shunt
c) Compound
d) Separately excited

In speed control of DC machines by varying armature voltage control which of the following
parameters is constant?
a) Speed
b) Torque
c) Power
d) None of these

The function of compensating windings in DC machine is to

a) Nullify the effect of armature reaction
b) Weaken the main filed flux
c) Nullify the inter-pole flux
d) None of these

Non-linear commutation in DC machine occurs due to

a) Inter-poles
b) Inductance of the coil
c) Brush defect
d) None of these

As compared to Delta-Delta bank,the capacity of VV bank in percentage is

A. 57.7
B. 66.7
C. 50
D. 86.6

Ward-Leeonard system of speed control is not recommend for

A. wide speed range
B. constant speed operation
C. frequent motor reversal
D. very low speed

One of the main advantage of Swinburne's test

A. applicable for both shunt and compound motor
B. needs one running test
C. economical and convenient
D. ignores any change in iron loss

The most economical method of finding no load losses of a large DC shunt motor
A. Hopkinson's test
B. Swinburne's test
C. Ward Leonard Ttest
D. None
The generator has poorest voltage regulation
A. series
B. shunt
C. compound
D. over compound

A transformer in open circuit condition draws _ current.

A. full load current
B. no load current
C. normal current
D. no current

Under constant load the speed of the DC motor affected by

A. field flux
B. armature current
C. back emf
D. field flux and back emf

Between no load and full load, which motor develops the least torque
A. series
B. shunt
C. cumulative compounded
D. Differentially compounded

In a DC generator, the effect of armature flux on the main field flux is

A. reduce it
B. distort it
C. reverse it
D. distort and reverse

Relation between Electrical angle and Mechanical Angle is

A. Electrical angle = Mechanical Angle
B. Mechanical angle=Electrical angle/pair of poles
C. Mechanical angle/pair of poles=Electrical angle
D. Electrical angle =2* Mechanical Angle

What is back pitch

A. Z/p+1
B. Z/p-1
C. Z/p+2
D. Z/p-2

Number of parallel path for lap winding is

A. two
B. three
C. no. of poles
D. none of these

Why lamination is used in transformer

A. reduce Cost
B. reduce heat
C. reduce eddy current loss
D. none of these

Why CRGO steel is used in transformer

A. reduce Cost
B. reduce heat
C. reduce eddy current loss
D. reduce Hysteresis loss

The distance between the beginnings of two consecutive turns of windings

A. back pitch
B. front pitch
C. resultant pitch
D. average pitch

A 4pole, 2 layer D.C lap winding with 20 slots and 1 conductor per layer. The number of
commutator bar is
A. 80
B. 20
C. 40
D. 60

What causes the heating of armature in DC machines?

A. Eddy current
B. Unequal strength of magnetic poles
C. operation above rated voltage and below normal speed
D. All of the above option

The series motor is suitable for driving

A. Lethes
B. Cranes and Hoists
C. Shares and Punches
D. Machine tools

A transformer having 1000 primary turns is connected to 250 V ac supply, for a secondary
voltage of 400 V, the number of secondary turns should be
A. 1600
B. 250
C. 400
D. 1250

A 400 Hz transformer is operated at 50 Hz, the KVA rating is

A. reduced to 1/8
B. reduced to 8 times
C. unexpected
D. increases 64 times

Why D.C series motor is not recommended to start without any load
A. Armature will burn
B. Insulation gradually decay
C. Speed will be dangerously high
D. Voltage will be high

Which one of the following loss is directly related with the frequency?
A. eddy current loss
B. frictional loss
C. copper loss
D. Windage loss

Armature reaction of DC generator is associated with

A. demagnetising effect
B. cross-magnetising effect
C. both A and B
D. none of the above

Compensating winding is used to reduce

A. demagnetising effect
B. cross-magnetising effect
C. both a & b
D. none of the above

The number of commutator segment is equal to

A. number of poles
B. number of brushes
C. number of coils
D. number of slots

The equivalent circuit of the transformer is composed of how many electrical circuits
A. 1
B. 2
C. 3
D. 4

SwineBurn test is carried out to determine

A. copper loss
B. Core loss
C. both a and b
D. none of the above

Hopkinson's test is carried out to determine

A. copper loss
B. Core loss
C. both a and b
D. none of the above

In case of step-down transformer, the transformation ratio 'K' will be

A. greater than 1
B. less than 1
C. 1
D. none of the above
The maximum efficiency of a 100KVA transformer having an iron loss of 400 Kw and full
load Cu loss of 1600KW occurs at what KVA rating
A. 50
B. 133.3
C. 75
D. 177.7

Saving of copper in auto transformer depends on (K=turns ratio)

A. K2
B. 2-K
C. 1-K
D. K

If Cu loss of a transformer at 1/8th of full load is 800W, then it's full load Cu loss would be
A. 5600
B. 6400
C. 375
D. 429

A transformer efficiency is low as

A. secondary output is low
B. transformer losses are high
C. fixed loss is high in proportion with output
D. Cu loss is small

The all day efficiency of a transformer does not depends on

A. It's Cu loss
B. The amount of load
C. The duration of load
D. insulation

The essential condition for parallel operation of two single phase transformer is that
A. polarity
B. KVA rating
C. voltage ratio
D. % impedance

An auto transformer having a transformation ratio of 0.8 supplies a load of 3KW, the power
transferred conductively from primary to secondary is in Kw
A. 0.6
B. 2.4
C. 1.5
D. 2.7

The advantage of auto transformer over ordinary transformer is

A. saving of copper
B. to reduce current
C. to get variable voltage
D. both a and b
 The capacity of open delta is how many times of delta delta bank
A. 0.532
B. 0.557
C. 1.2
D. 0.5

Distribution transformer forms the connection of

A. DZ6
B. Dd0
C. Dy11
D. Dd6

Series parallel connection of D.C motor is associated with

A. speed control
B. torque improvement
C. power factor improvement
D. regulation improvement

The speed of a D.C motor can be controlled by varying

A. flux per pole
B. armature resistance
C. applied voltage
D. all the above

The advantage of Ward Leonard system of speed control is

A. it can be used for small motors
B. it has high efficiency
C. gives smooth, sensitive and wide range speed control
D. it uses flywheel

The armature torque of a D.C motor is the function of

A. field flux
B. armature current
C. speed
D. field flux and armature current both

Sample short answer type Questions

1 Draw the no-load phasor diagram of transformer.

2 Draw the equivalent circuit of single phase transformer.
3 Derive the expression of voltage regulation of single phase transformer.
4 Derive the condition for maximum efficiency of single phase transformer.
5 What is all day efficiency of a transformer?
6 What are necessary condition for parallel operation of transformers?
7 What are the advantages of autotransformer over two winding transformers?
8 What are the different phasor groups of transformers?
9 Draw the winding connection and phasor diagram of following vector groups;
(i) Dy11
(ii) Dz6
(iii) Yd1
 (iv) Yy6

10 Explain the origin of harmonics in three phase transformers?

11 What are the detrimental effects of third harmonics in a transformer?
12 Explain the behaviour of third harmonics in star and delta connected systems.
13 Explain the behaviour of third harmonics in core type and shell type transformers.
14 Explain the phenomenon of oscillating neutral.
15 Draw and explain the operation of open delta (V-V) connected transformers?
16 Draw and explain the operation of Scott connected transformers?
17 What is tertiary winding? Why it is connected in delta?
18 How can Yd11 and Dy1 transformers be connected in parallel?
19 Draw the phasor diagram to explain the ratio error of current transformer.
20 Why the CT secondary is never kept open circuited?
21 Distinguish between transformer emf and rotational emf.
22 What is the relation between electrical angle and mechanical angle? Explain.
23 State and explain Fleming’s Left hand rule.
24 What is the function of commutator in a DC machine?
25 What is the function of brush in a DC machine?
26 What are functions of compensating windings?
27 What are the functions of inter-poles?
28 What is armature reaction?
29 Explain the phenomenon of commutation in DC machines.
30 Draw the open circuit characteristics of DC shunt generator.
31 What the necessary conditions are for build-up in DC shunt generator?
32 Draw the external characteristics of DC shunt generator.
33 What are different types of compound generators?
34 Draw the external characteristics of compound generators?
35 Why DC series machines are never operated under no-load?
36 What are the methods of speed control of DC motor?
37 Draw the torque-speed characteristics of DC shunt motor.
38 Draw the torque-speed characteristics of DC series motor.
39 Explain Ward Leonard’s test of speed control of DC motors.
40 What are the advantages and limitations of Swinburne’s test?
41 What are the advantages and limitations of Hopkinson’s test?