Cambridge Institute of Technology: Department of Electrical and Electronics Engineering

CAMBRIDGE INSTITUTE OF TECHNOLOGY
K.R. PURAM, BENGALURU-560036
Department of Electrical and Electronics Engineering
QUESTION BANK - MODULE 1
Sl. No. QUESTIONS Marks
1. What is back EMF? Mention its significance.

2. Obtain the expression for maximum power in dc motors.
3. Find the condition for maximum efficiency of a dc motor. 05
4. Draw the symbolic representation of power flow in dc machines.
5. What are the various losses occurring in dc motor?
6. What is the necessity of starter for dc motors?
7. Draw and explain the characteristics of dc shunt motor with relevant equations.
List its applications.
8. With a neat sketch, explain the construction and operation of three point starter.
9. A 200 V dc shunt motor takes 22 A at rated voltage and runs at 1000 rpm. Its field
resistance is 100 Ω and armature circuit resistance including brushes is 0.1 Ω.
Determine the value of additional resistance required in the armature circuit to
reduce the speed to 800 rpm when
i) The load torque is proportional to the speed.
ii) The load torque varies as square of the speed.
10. With a neat diagram, explain the method of speed control for dc shunt motor using
a motor-generator set. 10
11. Derive an expression for the torque in a dc motor. Explain the speed control of dc
series motor by applied voltage control.
12. A 10 kW, 250 V dc shunt motor with an armature resistance of 0.8  and a field
resistance of 275  takes 3.91 A, when running light at rated voltage and rated
speed. Determine the efficiency at rated voltage and speed when machine is
operated as
i) A generator when delivering an output of 10 kW.
ii) A motor drawing an input of 10 kW.
13. With a neat diagram, explain the construction and working of four point starter.
Also mention its advantages and limitations.
14. A series motor with an unsaturated magnetic circuit and 0.5  total resistance,
when running at certain speed takes 60 A at 500 V. If the load torque varies as the
cube of the speed, determine the resistance required to reduce the speed by 25%.
15. With a neat diagram, explain Ward-Leonard method of speed control for dc shunt
motor.
16. A dc series motor drives a load, the torque of which varies as the square of speed.
The motor takes a current of 15 A, when the speed is 600 rpm. Determine the speed
and current when the motor field winding is shunted by a diverter of the same
resistance as that of the field winding.
17. A long shunt compound generator delivers a full load current of 800 A at 500 V.
The shunt field resistance is 100 Ω. The magnetic and mechanical losses combined
together are equal to 12 kW, if the full load efficiency is 92 %. Determine:
i. Armature resistance (Ra) and series field resistance (Rse) given that Ra = 10
2 Rse.
ii. Load corresponding to maximum efficiency.
iii. Maximum efficiency.
18. Explain the construction and operation of four point starter with aid of neat sketch.
QUESTION BANK - MODULE 2
Sl. No. QUESTIONS Marks
1. Draw the characteristics of dc series motor. Mention its applications.

2. A dc series motor of resistance 1.5  runs at 750 rpm at 220 V with a current of
20 A. Find the speed at which it will run when connected in series with 4.5 
resistance and taking same current.
3. What are the advantages and disadvantages of brake test? 05
4. What are the advantages and limitations of Swinburne’s test?
5. What are the advantages and disadvantages of Hopkinson’s test?
6. What are the advantages and disadvantages of regenerative test?
7. A 500 V, dc shunt motor when running on no load takes 5 A. Armature resistance
is 0.5  and shunt resistance is 250 . Determine the output in kW and efficiency
of the motor when running on full load and taking a current of 50 A.
8. With a neat circuit diagram, describe the procedure for conducting Swinburne’s
test on dc motor and obtain expression for its efficiency.
9. With a neat circuit diagram, explain the procedure for conducting load test on dc
shunt motor to determine expression for its efficiency.
10. With a neat circuit diagram, explain the procedure for conducting retardation test
10
on dc shunt motor to determine its moment of inertia by using flywheel.
11. A dc shunt machine runs on no load at 1000 rpm taking 480 W of 120 V. The
shunt field current is 2 A. The armature resistance is 0.1 Ω. Calculate the line
current both as motor and generator at which maximum efficiency occurs. Also
determine the maximum efficiency.
12. With neat circuit diagram, explain the procedure for conducting field test on dc
series machines and obtain the equation for efficiency when machine is made to
work as both generator and motor.
13. With a neat circuit diagram, explain the procedure for conducting no-load test
on dc machine for predetermination of its efficiency and derive the necessary
equations.
14. With a neat circuit diagram, explain the procedure for conducting brake test on dc
motor and obtain the necessary equations.
15. A 4 pole dc shunt motor takes 22 A from 220 V supply. The armature and shunt
field winding resistances are 0.5  and 100  respectively. The armature is lap
connected with 300 conductors, if the flux/pole is 20m wb, determine the speed
and torque developed.
16. With a neat circuit, explain the procedure of conducting Hopkinson’s test on two
10
identical shunt machines and also obtain efficiency equation when working both
as generator and motor.
17. The Hopkinson’s test conducted on two shunt machines has following results on
full load:
Load voltage is 250 V, current drawn from supply excluding field current is 50 A,
motor armature current is 380 A, generator field current is 5 A, motor field current
is 4.2 A. Determine the efficiency of both motor and generator, given Ra to be 0.02
.
18. With the help of circuit diagram, explain the various flux control methods which
can be employed for speed control dc series motor.

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CAMBRIDGE INSTITUTE OF TECHNOLOGY

K.R. PURAM, BENGALURU-560036

Department of Electrical and Electronics Engineering

QUESTION BANK - MODULE 1

Sl. No. QUESTIONS Marks

1. What is back EMF? Mention its significance.

QUESTION BANK - MODULE 2

Sl. No. QUESTIONS Marks

1. Draw the characteristics of dc series motor. Mention its applications.

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