Course: Electrical Machines I

Unit 1

1. Two single phase transformers with equal turns have impedances of (0.5 + j 3)
ohms and (0.6 + j 10) ohms with respect to the secondary. If they operate in
parallel, determine how they will share total load of 100 kW at pf 0.8 lagging?
2. Explain the construction and working principle of single phase transformer with
neat diagram.
3. Derive the emf equation of single phase transformer.
4. Compare the ideal and practical transformer.
5. Draw and explain the phasor diagram of practical transformer without load and
with load.
6. Develop the equivalent circuit of single phase transformer
7. List the various losses in transformer. Explain in detail.
8. Define voltage regulation and efficiency of transformer.
9. Explain the parallel operation of single phase transformer with neat diagram.

Unit 2

1. Relate the single three phase transformer unit and three single phase transformer
units.
2. Explain with neat diagram i) Concept of polarity & Polarity Test ii) open circuit
and short circuit tests iii) Direct Load Test iv) Sumpner’s test
3. State the I.S. Specifications of transformers and applications.
4. Explain the Concept of routine and type tests.
5. Explain with neat diagram three winding transformers. State its applications.
6. A 200/400 V, 50 Hz single phase transformer on test gave the following readings:

OC (LV) – 200 V, 0.7 A, 70 W

SC (HV) – 15 V, 10 A, 80 W

Determine the voltage regulation at 0.8 pf lagging at full load.

7. Obtain/devise the equivalent circuit of single phase, 4 kVA, 200/400 V, 50 Hz

transformer from the following test results:

OC Test: 200 V, 0.7 A. 70 W

SC Test: 15 V, 10 A, 80 W.
8. List the three phase transformer groups, three phase transformer connections and
factors affecting the choice of connections.
9. Illustrate the delta – delta connection of transformers with neat connection and
phasor diagrams.
10. Illustrate the delta – star connection of transformers with neat connection and
phasor diagrams.
11. Illustrate the star – delta connection of transformers with neat connection and
phasor diagrams.

Unit 3

1. Develop the Output equation with usual notations for single phase and three phase
transformers.
2. Illustrate the Design of small single phase transformers.
3. Describe the estimation of resistance and leakage reactance of transformer,
regulation of transformers.
4. Devise the expression for the radial and axial forces in case of transformaers.
5. Determine the dimensions of the core and window for a 100 kVA, 50 Hz, single
phase core type transformer. A square core is used with D = 1.6 a. Assume Emf
per turn Et of 14 volts, maximum flux density of 1.1 T, window space factor Kw =
0.32, current density δ= 3 A/mm2. Take stacking factor Ks = 0.9.
6. Describe the mechanical forces in transformer with neat diagrams.
7. Explain the stepwise procedure for the design of LV and HV windings.

Unit 4
1. Derive an expression for the EMF equation of the DC Machine.
2. Describe the function of the commutator in a DC machine.
3. How do interpoles and compensating windings improve commutation?
4. What are the remedies to minimize armature reaction?
5. What factors influence the EMF generated in a DC machine?
6. Explain the principle working and operation of a DC machine.
7. A DC shunt motor operates on a 240 V supply. The armature resistance is 0.2 Ω,
and the motor draws 40 A of armature current. Calculate Torque and back EMF
generated by the motor with 300 number of conductors.
8. A 220 V DC shunt motor runs at 1000 RPM when taking an armature current of
30 A. The armature resistance is 0.25 Ω. If the load is reduced and the armature
current drops to 20 A, calculate the new speed of the motor, assuming the flux
remains constant.
 9. A DC motor takes 20 kW from the supply and produces an output of 18 kW. If the
copper losses in the motor are 500 W and iron losses are 700 W, calculate the total
losses and efficiency of the motor.
10. What are the main applications of each type of DC motor?
11. Draw and explain the speed-torque characteristics of a shunt motor.
12. Derive the torque equation of a DC motor.

Unit 5

1. Explain the construction and working principle of a servo motor.

2. What are the different types of power converters used in servo motor control?
Explain with diagrams.
3. What are the torque-speed characteristics of a servo motor? Explain with the help
of a diagram.
4. Analyze the importance of servo motors in robotics and automation systems.
5. What are the different types of power converters used in servo motor control?
Explain with diagrams.
6. What types of power converters are used to control BLDC motors? How do they
improve performance?
7. Explain the types of BLDC motors and their applications in various industries.
8. Draw the torque-speed characteristics of a BLDC motor and explain its
significance.
9. Explain the construction and principle of operation of a Permanent Magnet
Synchronous Motor (PMSM).
10. Discuss the torque-speed characteristics of a PMSM and how they influence motor
performance.
11. Derive the EMF equation for a PMSM and explain the parameters involved.

Unit 6

1. What are the main applications of switched reluctance motors in the automotive
and industrial sectors? Explain why SRMs are suitable for these applications.
2. Explain the construction and working principle of a stepper motor.
3. Explain the torque-speed characteristics of a switched reluctance motor with the
help of a diagram.
4. What are the major applications of stepper motors in industries? Provide examples
where precision motion is essential.
5. Discuss the various performance characteristics of switched reluctance motors and
how they influence their selection for different applications.
 6. Explain the performance characteristics of a synchronous reluctance motor and
compare it with other AC motors.
7. Draw the phasor diagram for a synchronous motor with armature current in phase
with the applied voltage.
8. Explain the operation of a synchronous motor.
9. Describe the operation of a synchronous motor with constant load and variable
excitation.
10. Draw and explain the equivalent circuit of a synchronous machine
11. What are the main applications of switched reluctance motors in the automotive
and industrial sectors? Explain why SRMs are suitable for these applications.
12. Explain the torque-speed characteristics of a switched reluctance motor with the
help of a diagram.
Question bank for Unit Test

Unit 4
1. Sketch the relevant characteristics and conclude that the DC series motor is used
for traction.
2. Sketch the neat connection diagram of DC Series motor and give the reasons why
few turns with thick wires are used for field winding in DC series motor.
3. Sketch the neat diagrams of armature flux distribution, resultant flux distribution
and conclude the effects of armature reaction.
4. Explain the functions, material used and technology used for parts of dc machine
with the help of neat diagram.
5. Derive the e.m.f equation of DC generator for a simplex lap and simplex wave
wound DC generator.
6. Write a short note on commutation.
7. State the methods of improving commutation. Explain EMF commutation.
8. With the help of neat diagram explain the two effects of armature reaction
9. Explain operating principle of DC generator

Unit 5

1. Describe field and armature controlled DC Servo Motor with schematic diagram.
2. Differentiate between a servo motor and conventional motor used for industrial
application
3. What is a two-phase ac servo motor? Describe its construction and working.
4. Illustrate the working, types, characteristics and applications of servo motor.
5. Explain the construction and electronic commutation of BLDC Motor
6. Explain types of BLDC motor with the help of circuit diagram and waveforms.
7. Relate conventional DC, BLDC motor and PMSM motor.
 8. Describe the construction and working of PMSM
9. Explain different PMSM rotor configurations with neat diagrams.
10. Derive the EMF and Torque equation of PMSM
11. Illustrate the construction and working principle of PMSM with neat diagrams.
12. Draw Torque speed characteristics to explain PMSM

Unit 6

1. Explain the single stack variable reluctance stepper motor with neat diagram,
switching circuit and truth table.
2. Define the terms 1) Step angle 2) Pull in Torque 3) Detent Torque.
3. Explain the two phase ON mode of excitation of stepper motor with neat sketche,
truth table.
4. Correlate the static, dynamic characteristics and applications of stepper motor.
5. Explain construction and working of synchronous reluctance motor
6. Correlate the performance characteristics and applications of synchronous
reluctance motor.
7. Explain construction and Operating Principle of Switched Reluctance Motor.
8. Draw the phasor diagram of synchronous reluctance motor and derive the torque
equation.
9. Compare Switched reluctance Motor and Synchronous reluctance Motor
10. Correlate the performance characteristics and applications of switched reluctance
motor.