Electrical machines are fundamental for every electrical engineer aiming for competitive exams, core company placements, or PSU interviews. This post compiles the Top 75 Electrical Machines Interview Questions and Answers explained with clear examples to strengthen your technical preparation. Whether you are revising concepts of transformers, motors, or generators, these 75 most asked questions will help you build a strong foundation and confidence for your upcoming interviews. Let’s explore these important Q&A and upgrade your knowledge systematically.
1. What is an electrical machine?
Answer: It is a device that converts mechanical energy to electrical energy (generator) or electrical energy to mechanical energy (motor) or transfers electrical energy (transformer).
2. What is the main difference between a motor and a generator?
Answer:
- Motor: Converts electrical energy into mechanical energy.
- Generator: Converts mechanical energy into electrical energy.
Example: Ceiling fan (motor), hydro generator in dams.
3. Define transformer.
Answer: A static device that transfers electrical energy between two or more circuits through electromagnetic induction.
Example: Step-down transformer in phone chargers.
4. Why is a transformer rated in kVA not in kW?
Answer: Because it is independent of power factor; losses depend on voltage and current, not load power factor.
5. State the types of electrical machines.
Answer:
- Static machines (transformer)
- Rotating machines (motors and generators)
6. What is an amateur reaction?
Answer: Effect of magnetic field produced by armature current on the distribution of flux under main poles of generator/motor.
7. Explain EMF equation of a transformer.
Answer:
E=4.44×f×N×ϕmE = 4.44 \times f \times N \times \phi_mE=4.44×f×N×ϕm
where f is frequency, N number of turns, and φm maximum flux.
8. What is slip in an induction motor?
Answer:
Slip=Ns−NrNs×100%Slip = \frac{Ns – Nr}{Ns} \times 100 \%Slip=NsNs−Nr×100%
Where Ns is synchronous speed and Nr is rotor speed.
Example: For Ns=1500 rpm, Nr=1450 rpm, slip=3.3%.
9. Why is an induction motor called an asynchronous motor?
Answer:
An induction motor is called an asynchronous motor because its rotor does not rotate at the same speed as the rotating magnetic field (synchronous speed) produced by the stator.
Explanation:
- In an induction motor:
- The stator produces a rotating magnetic field at synchronous speed (Ns).
- The rotor always rotates at a speed (N<sub>r</sub>) less than the synchronous speed to induce relative motion (slip) between the stator field and rotor conductors.
- This relative speed (slip) is necessary to induce electromotive force (EMF) in the rotor according to Faraday’s Law, which produces torque.
- The stator produces a rotating magnetic field at synchronous speed (Ns).
Key reason for “asynchronous” name:
- Since N<sub>r</sub> < Ns, the motor’s operation is not synchronous with the stator field, hence it is called asynchronous.
10. What are losses in a transformer?
Answer:
- Core (iron) loss: hysteresis + eddy current loss
- Copper loss: I2RI^2RI2R loss in windings.
11. Explain synchronous speed.
Answer:
Ns=120fPNs = \frac{120f}{P}Ns=P120f
Where f is frequency, P is poles.
Example: For 50Hz, 4-pole motor, Ns=1500 rpm.
12. Why are starters used in motors?
Answer: To limit high starting current and provide protection during startup.
Example: DOL starter for small motors.
13. What is commutation?
Answer:
Commutation is the process of reversing the direction of current in the armature conductors as they pass from one brush to another in a DC machine (generator or motor).
Explanation:
- In DC machines, the armature conductors rotate and the current direction must reverse every half rotation to maintain unidirectional torque (in motors) or unidirectional output voltage (in generators).
- This reversal is automatically done by the commutator segments and brushes.
14. Why is a DC motor used for traction?
Answer: Due to high starting torque and easy speed control.
15. Difference between squirrel cage and slip ring induction motor.
Answer:
Squirrel Cage | Slip Ring |
Rotor bars short-circuited | Rotor has windings + slip rings |
No external resistance | External resistance can be added |
Low starting torque | High starting torque |
16. What is voltage regulation in a transformer?
Answer: Change in secondary voltage from no-load to full-load expressed as a % of full-load voltage.
17. Explain no-load test of transformer.
Answer: Measures core loss by applying rated voltage to primary with secondary open.
18. What is a short circuit test of a transformer?
Answer: Measures copper loss by shorting secondary and applying reduced voltage on primary to circulate rated current.
19. What is the efficiency of a transformer?
Answer:
η=OutputOutput+Losses×100%\eta = \frac{Output}{Output + Losses} \times 100 \%η=Output+LossesOutput×100%
Usually between 95-99%.
20. Why can’t the transformer work on DC supply?
Answer: Because emf induction requires changing magnetic field; DC produces constant flux and will cause core saturation and high current.
21. What is Lenz’s law?
Answer: Induced emf always opposes the cause producing it.
Example: Back emf in motors.
22. Define back emf in DC motor.
Answer: Voltage induced in armature winding opposing the applied voltage.
23. Why is the single-phase motor not self-starting?
Answer: Because it does not produce a rotating magnetic field at startup.
24. What is pole pitch?
Answer: The distance between two adjacent poles in terms of armature slots.
25. What are the types of DC generators?
Answer:
- Separately excited
- Self-excited: series, shunt, compound
26. Explain Fleming’s Left-Hand Rule.
Answer: For motors – thumb (force), forefinger (field), middle finger (current) mutually perpendicular.
27. Explain Fleming’s Right-Hand Rule.
Answer: For generators – thumb (motion), forefinger (field), middle finger (induced emf).
28. What is a synchronous motor?
Answer: A motor that runs at synchronous speed, used for power factor correction.
29. What is hunting in a synchronous motor?
Answer: Oscillatory motion of rotor due to sudden load changes.
30. Why is capacitor used in single-phase motors?
Answer: To create phase difference and produce a rotating magnetic field.
31. Explain plug braking.
Answer: Reversing motor connections to produce torque opposite to rotation for braking.
32. What is regenerative braking?
Answer: Motor acts as generator during braking, feeding power back to source.
33. Define transformer tap changing.
Answer: Adjusting turns ratio to regulate output voltage.
34. What is leakage flux?
Answer: Flux that does not link both primary and secondary windings.
35. Why is an auto transformer more efficient?
Answer: Because part of power is transferred conductively, reducing losses.
36. Difference between core type and shell type transformer.
Answer:
Core Type | Shell Type |
Windings surround core | Core surrounds windings |
Simple construction | More complex |
Used for high voltage | Used for low voltage, high current |
37. Explain magnetizing current.
Answer: Current required to produce flux in transformer core.
38. What is an alternator?
Answer: A synchronous generator producing AC power.
39. Define voltage regulation of alternators.
Answer: Rise in terminal voltage when load is removed at constant excitation.
40. What are V-curves of synchronous motors?
Answer: Graph between armature current and field current, showing minimum current at unity PF.
41. What are the types of losses in DC machines?
Answer:
- Copper loss (I2RI^2RI2R loss)
- Iron loss (hysteresis + eddy current)
- Mechanical loss (friction + windage)
42. What is critical field resistance in a DC generator?
Answer: Minimum field circuit resistance for building up rated voltage at a given speed.
43. What is the critical speed in a DC generator?
Answer: Minimum speed below which generator fails to build up voltage.
44. Explain armature reaction effect in DC motor.
Answer: Distorts main field flux causing poor commutation and reduced efficiency.
45. What is an equalizer bar in DC machines?
Answer: Connects armatures in parallel to balance load sharing in compound generators.
46. What is the function of interpoles?
Answer: Improve commutation by producing a flux that counteracts armature reaction.
47. Define load commutated inverter.
Answer: An inverter where commutation of thyristors is achieved by load parameters, used in synchronous motor drives.
48. What is the pitch factor in alternators?
Answer:
Kp=cos(α/2)K_p = \cos(\alpha/2)Kp=cos(α/2)
where α is the angle of short pitching; it reduces harmonics.
49. What is the distribution factor?
Answer:
Kd=sin(mβ/2)msin(β/2)K_d = \frac{\sin(m\beta/2)}{m \sin(\beta/2)}Kd=msin(β/2)sin(mβ/2)
where m is slots per pole per phase, β is slot angle. It accounts for distributed windings.
50. Define salient pole and non-salient pole.
Answer:
- Salient pole: Poles projecting out, used for low-speed machines (hydro alternators).
- Non-salient (cylindrical) pole: Uniform air gap, used for high-speed (turbo alternators).
51. Explain Damper winding in synchronous motor.
Answer: Bars embedded in pole faces to prevent hunting and allow starting as an induction motor.
52. What is cogging in an induction motor?
Answer: Locking of rotor due to alignment of stator and rotor teeth (magnetic locking).
53. What is crawling in an induction motor?
Answer: Tendency to run at 1/7th synchronous speed due to harmonics in flux.
54. Define double cage induction motor.
Answer: Rotor with two cages (inner high resistance, outer low resistance) for high starting torque and good running efficiency.
55. What is a synchronous condenser?
Answer: Synchronous motor running without mechanical load to improve power factor.
56. Define load angle in synchronous motor.
Answer: Angle between rotor pole axis and stator rotating magnetic field axis; increases with load.
57. What is a slip ring?
Answer: Conductive rings used to transfer power/signals to and from rotating part, e.g. in slip ring induction motors.
58. Why are brushes used in DC machines?
Answer: To collect current from the commutator (generator) or supply current to the armature (motor).
59. What is Hopkinson’s test?
Answer: Back-to-back test for testing two identical DC machines to determine efficiency.
60. Explain retardation test.
Answer: Used to find moment of inertia and rotational losses in motors by observing deceleration.
61. What is Swinburne’s test?
Answer: No-load test on DC shunt machine to estimate efficiency at different loads.
62. Why is the transformer core laminated?
Answer: To reduce eddy current losses by increasing resistance to circulating currents.
63. Define hysteresis loss.
Answer: Energy loss due to reversal of magnetization in core material.
Example: Proportional to frequency and area of B-H loop.
64. What is eddy current loss?
Answer: Loss due to circulating currents induced in core; reduced by laminations.
65. What is the field weakening method?
Answer: Speed control by reducing flux (field current) to increase speed in DC motors.
66. Why does an induction motor have poor starting torque compared to a DC motor?
Answer: Due to low rotor resistance and absence of armature reaction compensation.
67. What is a universal motor?
Answer: A motor that can run on both AC and DC supply.
Example: Mixer grinder motors.
68. Why is the capacitor start capacitor run motor better?
Answer: Provides high starting torque and better running efficiency with improved power factor.
69. Define synchronous reactance.
Answer:
Xs=Xl+XaX_s = X_l + X_aXs=Xl+Xa
Where X_l is leakage reactance and X_a is armature reaction reactance.
70. What is per unit system?
Answer: Normalization technique expressing quantities as a fraction of base value for simplification.
71. What is an auto transformer starter?
Answer: Provides reduced voltage starting for induction motors using auto transformer taps.
72. What is a shaded pole motor?
Answer: Single-phase motor with shaded copper rings for starting torque; used in small fans.
73. Define torque-slip characteristic of induction motor.
Answer: Shows torque increases with slip up to a maximum (breakdown torque) then decreases.
74. What is vector control in motors?
Answer: Control strategy using field-oriented control to achieve independent torque and flux control in AC motors.
75. Explain brushless DC motor.
Answer: Uses electronic commutation instead of brushes; high efficiency, used in drones and EVs.
Mastering electrical machines is crucial for success in electrical engineering interviews and professional practice. These Top 75 Electrical Machines Interview Questions and Answers with examples provide you with conceptual clarity, practical insights, and confidence to face any technical round effectively. Keep revising these questions regularly, understand the logic behind each answer, and enhance your problem-solving skills to stand out in your interviews and future projects.
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