00 multiple-choice questions (MCQs) on Electrical Machines for Junior Engineer (JE) exams, along with answers:

Here are 100 multiple-choice questions (MCQs) on Electrical Machines for Junior Engineer (JE) exams, along with answers:

### Transformers (Q1–Q20)
1. **What is the primary function of a transformer?**
a) Convert DC to AC
b) Change voltage level
c) Generate electrical power
d) Store electrical energy
**Answer: b) Change voltage level**

2. **Core losses in a transformer include:**
a) Copper loss and hysteresis loss
b) Hysteresis loss and eddy current loss
c) Eddy current loss and copper loss
d) Stray loss and friction loss
**Answer: b) Hysteresis loss and eddy current loss**

3. **The efficiency of a transformer is maximum when:**
a) Copper loss = iron loss
b) Copper loss > iron loss
c) Copper loss < iron loss
d) Load current is zero
**Answer: a) Copper loss = iron loss**

4. **The purpose of the conservator tank in a transformer is to:**
a) Cool the oil
b) Provide expansion space for oil
c) Filter impurities
d) Reduce core losses
**Answer: b) Provide expansion space for oil**

5. **Which test gives iron losses of a transformer?**
a) Short-circuit test
b) Open-circuit test
c) Sumpner’s test
d) Load test
**Answer: b) Open-circuit test**

6. **The no-load current in a transformer:**
a) Leads the voltage by 90°
b) Lags the voltage by 90°
c) Is in phase with voltage
d) Has a small wattful component
**Answer: d) Has a small wattful component**

7. **A transformer works on the principle of:**
a) Self-induction
b) Mutual induction
c) Electrostatic induction
d) Electromagnetic induction
**Answer: b) Mutual induction**

8. **The voltage regulation of a transformer is zero at:**
a) Unity power factor
b) Lagging power factor
c) Leading power factor
d) Zero power factor
**Answer: c) Leading power factor**

9. **Buchholz relay protects transformers from:**
a) Overloading
b) External faults
c) Internal faults
d) Overvoltage
**Answer: c) Internal faults**

10. **The dielectric strength of transformer oil is tested to measure:**
a) Viscosity
b) Moisture content
c) Breakdown voltage
d) Acidity
**Answer: c) Breakdown voltage**

11. **An ideal transformer has:**
a) Zero losses
b) 100% efficiency
c) No leakage flux
d) All of the above
**Answer: d) All of the above**

12. **The main reason for laminating the transformer core is to reduce:**
a) Hysteresis loss
b) Eddy current loss
c) Copper loss
d) Stray loss
**Answer: b) Eddy current loss**

13. **The turns ratio of a transformer is 1:2. If the primary voltage is 220V, the secondary voltage is:**
a) 110V
b) 220V
c) 440V
d) 880V
**Answer: c) 440V**

14. **Auto-transformers have:**
a) Two separate windings
b) Single winding
c) Three windings
d) No windings
**Answer: b) Single winding**

15. **Instrument transformers are used for:**
a) Stepping down voltage/current for measurement
b) Isolation
c) Both a and b
d) Power conversion
**Answer: c) Both a and b**

16. **The all-day efficiency of a transformer considers:**
a) Peak load losses
b) Energy losses over 24 hours
c) Iron losses only
d) Copper losses only
**Answer: b) Energy losses over 24 hours**

17. **The phase shift between primary and secondary voltages in a delta-star transformer is:**
a) 0°
b) 30°
c) 45°
d) 60°
**Answer: b) 30°**

18. **The magnetizing current in a transformer:**
a) Supplies iron losses
b) Is independent of load
c) Is about 2-5% of full-load current
d) All of the above
**Answer: d) All of the above**

19. **A step-up transformer:**
a) Increases voltage
b) Decreases current
c) Has more turns in secondary
d) All of the above
**Answer: d) All of the above**

20. **The kVA rating of a transformer depends on:**
a) Frequency
b) Core area
c) Flux density
d) All of the above
**Answer: d) All of the above**

### DC Machines (Q21–Q40)
21. **The back EMF in a DC motor:**
a) Opposes applied voltage
b) Aids applied voltage
c) Is always greater than applied voltage
d) Is zero at full load
**Answer: a) Opposes applied voltage**

22. **The direction of rotation of a DC motor is determined by:**
a) Fleming’s left-hand rule
b) Fleming’s right-hand rule
c) Lenz’s law
d) Faraday’s law
**Answer: a) Fleming’s left-hand rule**

23. **DC generators work on the principle of:**
a) Self-induction
b) Mutual induction
c) Electromagnetic induction
d) Static induction
**Answer: c) Electromagnetic induction**

24. **Armature reaction in a DC machine causes:**
a) Neutral plane shift
b) Sparking at brushes
c) Reduced main flux
d) All of the above
**Answer: d) All of the above**

25. **Commutation in DC machines is improved by:**
a) Interpoles
b) Compensating windings
c) High-resistance brushes
d) All of the above
**Answer: d) All of the above**

26. **A DC shunt motor is best suited for:**
a) Constant speed applications
b) Variable speed applications
c) High starting torque
d) Low starting torque
**Answer: a) Constant speed applications**

27. **The speed of a DC motor is:**
a) Directly proportional to back EMF
b) Inversely proportional to flux
c) Both a and b
d) None of the above
**Answer: c) Both a and b**

28. **Critical resistance of a DC shunt generator refers to:**
a) Armature resistance
b) Field resistance
c) Load resistance
d) Critical speed
**Answer: b) Field resistance**

29. **Residual magnetism in a DC generator is essential for:**
a) Starting
b) Voltage build-up
c) Speed regulation
d) Efficiency
**Answer: b) Voltage build-up**

30. **Wave winding in DC machines is used for:**
a) High current, low voltage
b) Low current, high voltage
c) High current, high voltage
d) Low current, low voltage
**Answer: b) Low current, high voltage**

31. **The number of parallel paths in a lap-wound DC machine is equal to:**
a) Number of poles
b) Number of brushes
c) Number of slots
d) Number of commutator segments
**Answer: a) Number of poles**

32. **In a DC motor, if the field winding is disconnected while running, the motor will:**
a) Stop immediately
b) Run at normal speed
c) Run at dangerously high speed
d) Draw low current
**Answer: c) Run at dangerously high speed**

33. **The starting torque of a DC series motor is:**
a) Low
b) Medium
c) High
d) Zero
**Answer: c) High**

34. **The efficiency of a DC machine is maximum when:**
a) Variable losses = constant losses
b) Copper losses = iron losses
c) Friction losses = windage losses
d) Armature losses = field losses
**Answer: a) Variable losses = constant losses**

35. **Dummy coils in DC machines are used to:**
a) Reduce losses
b) Improve commutation
c) Provide mechanical balance
d) Increase efficiency
**Answer: c) Provide mechanical balance**

36. **The function of interpoles is to:**
a) Reduce armature reaction
b) Improve commutation
c) Increase speed
d) Reduce losses
**Answer: b) Improve commutation**

37. **A DC compound motor has:**
a) Only shunt winding
b) Only series winding
c) Both shunt and series windings
d) No windings
**Answer: c) Both shunt and series windings**

38. **The speed of a DC shunt motor can be increased by:**
a) Increasing armature resistance
b) Decreasing armature resistance
c) Increasing field resistance
d) Decreasing field resistance
**Answer: c) Increasing field resistance**

39. **The emf equation of a DC generator is:**
a) \( E = \frac{P\phi NZ}{60A} \)
b) \( E = \frac{P\phi N}{60A} \)
c) \( E = \frac{\phi NZ}{60A} \)
d) \( E = \frac{P\phi Z}{60A} \)
**Answer: a) \( E = \frac{P\phi NZ}{60A} \)**

40. **In a DC machine, carbon brushes are used to:**
a) Reduce friction
b) Collect current from commutator
c) Provide insulation
d) Reduce losses
**Answer: b) Collect current from commutator**

### Induction Motors (Q41–Q60)
41. **The slip of an induction motor is 5%. If the synchronous speed is 1500 rpm, the rotor speed is:**
a) 1425 rpm
b) 1500 rpm
c) 1575 rpm
d) 750 rpm
**Answer: a) 1425 rpm**

42. **The starting torque of a squirrel-cage induction motor is:**
a) Low
b) High
c) Very high
d) Zero
**Answer: a) Low**

43. **The rotor frequency of a 3-phase induction motor is:**
a) Equal to supply frequency
b) \( s \times \text{supply frequency} \)
c) \( (1-s) \times \text{supply frequency} \)
d) Zero
**Answer: b) \( s \times \text{supply frequency} \)**

44. **The synchronous speed of a 4-pole, 50 Hz induction motor is:**
a) 1500 rpm
b) 1000 rpm
c) 750 rpm
d) 3000 rpm
**Answer: a) 1500 rpm**

45. **Double-cage rotors are used to improve:**
a) Efficiency
b) Power factor
c) Starting torque
d) Speed regulation
**Answer: c) Starting torque**

46. **Star-delta starters are used for induction motors to:**
a) Reduce starting current
b) Increase starting torque
c) Improve efficiency
d) Reduce losses
**Answer: a) Reduce starting current**

47. **Crawling in induction motors is caused by:**
a) Harmonics
b) Overloading
c) Low voltage
d) High slip
**Answer: a) Harmonics**

48. **The power factor of an induction motor at no load is:**
a) Zero
b) 0.2–0.3 lagging
c) Unity
d) Leading
**Answer: b) 0.2–0.3 lagging**

49. **The maximum torque of an induction motor is:**
a) Inversely proportional to rotor reactance
b) Independent of rotor resistance
c) Proportional to square of voltage
d) All of the above
**Answer: d) All of the above**

50. **Slip rings are used in:**
a) Squirrel-cage motors
b) Wound-rotor motors
c) Synchronous motors
d) DC motors
**Answer: b) Wound-rotor motors**

51. **An induction motor is analogous to:**
a) Transformer with shorted secondary
b) DC shunt motor
c) Synchronous motor
d) None
**Answer: a) Transformer with shorted secondary**

52. **The stator of a 3-phase induction motor is fed from a balanced 3-phase supply. The rotating magnetic field rotates at:**
a) Synchronous speed
b) Rotor speed
c) Half synchronous speed
d) Zero speed
**Answer: a) Synchronous speed**

53. **The direction of rotation of an induction motor can be reversed by:**
a) Changing supply phase sequence
b) Changing supply frequency
c) Reducing voltage
d) Increasing slip
**Answer: a) Changing supply phase sequence**

54. **Deep-bar rotors provide:**
a) Low starting torque
b) High starting torque
c) High efficiency
d) Low cost
**Answer: b) High starting torque**

55. **The efficiency of an induction motor is about:**
a) 50–60%
b) 60–70%
c) 80–90%
d) 95–99%
**Answer: c) 80–90%**

56. **No-load test on an induction motor gives:**
a) Stator copper loss
b) Rotor copper loss
c) Iron loss and friction loss
d) Total losses
**Answer: c) Iron loss and friction loss**

57. **Blocked rotor test on an induction motor gives:**
a) No-load losses
b) Short-circuit impedance
c) Rotor resistance
d) Magnetizing reactance
**Answer: b) Short-circuit impedance**

58. **Synchronous speed (Ns) is given by:**
a) \( N_s = \frac{120f}{P} \)
b) \( N_s = \frac{120P}{f} \)
c) \( N_s = \frac{60f}{P} \)
d) \( N_s = \frac{f}{120P} \)
**Answer: a) \( N_s = \frac{120f}{P} \)**

59. **The frame of an induction motor is made of:**
a) Silicon steel
b) Cast iron
c) Copper
d) Aluminum
**Answer: b) Cast iron**

60. **The slip at which maximum torque occurs is:**
a) \( s = \frac{R_2}{X_2} \)
b) \( s = \frac{X_2}{R_2} \)
c) \( s = \frac{R_2}{X_1} \)
d) \( s = \frac{X_1}{R_2} \)
**Answer: a) \( s = \frac{R_2}{X_2} \)**

### Synchronous Machines (Q61–Q80)
61. **Synchronous motors are not self-starting because:**
a) Rotor inertia is high
b) Starting torque is zero
c) Back EMF opposes
d) No slip
**Answer: b) Starting torque is zero**

62. **Damper windings in a synchronous motor are used for:**
a) Starting
b) Preventing hunting
c) Both a and b
d) Reducing losses
**Answer: c) Both a and b**

63. **The synchronous speed of a 6-pole, 50 Hz motor is:**
a) 1000 rpm
b) 1500 rpm
c) 3000 rpm
d) 500 rpm
**Answer: a) 1000 rpm**

64. **A synchronous motor operates at:**
a) Lagging power factor
b) Leading power factor
c) Unity power factor
d) Any power factor
**Answer: d) Any power factor**

65. **Under-excited synchronous motors operate at:**
a) Leading power factor
b) Lagging power factor
c) Unity power factor
d) Zero power factor
**Answer: b) Lagging power factor**

66. **The power factor of a synchronous motor can be controlled by:**
a) Changing field excitation
b) Changing load
c) Changing supply voltage
d) Changing frequency
**Answer: a) Changing field excitation**

67. **Hunting in synchronous machines is reduced by:**
a) Damper windings
b) Flywheels
c) Both a and b
d) Increasing load
**Answer: c) Both a and b**

68. **The armature of a synchronous machine is usually on the:**
a) Rotor
b) Stator
c) Both
d) None
**Answer: b) Stator**

69. **The voltage regulation of an alternator is:**
a) \( \frac{E_0 – V}{V} \times 100\% \)
b) \( \frac{V – E_0}{V} \times 100\% \)
c) \( \frac{E_0 – V}{E_0} \times 100\% \)
d) \( \frac{V – E_0}{E_0} \times 100\% \)
**Answer: a) \( \frac{E_0 – V}{V} \times 100\% \)**

70. **The distribution factor for an alternator winding is:**
a) \( \frac{\sin\left(\frac{m\gamma}{2}\right)}{m \sin\left(\frac{\gamma}{2}\right)} \)
b) \( \frac{m \sin\left(\frac{\gamma}{2}\right)}{\sin\left(\frac{m\gamma}{2}\right)} \)
c) \( \frac{\sin\left(\frac{\gamma}{2}\right)}{m \sin\left(\frac{m\gamma}{2}\right)} \)
d) \( \frac{m \sin\left(\frac{m\gamma}{2}\right)}{\sin\left(\frac{\gamma}{2}\right)} \)
**Answer: a) \( \frac{\sin\left(\frac{m\gamma}{2}\right)}{m \sin\left(\frac{\gamma}{2}\right)} \)**

71. **In alternators, rotating field is preferred because:**
a) Easy insulation
b) Low voltage
c) High power
d) All of the above
**Answer: d) All of the above**

72. **Synchronous impedance method for voltage regulation:**
a) Overestimates regulation
b) Underestimates regulation
c) Is accurate
d) None
**Answer: a) Overestimates regulation**

73. **The frequency of an alternator is given by:**
a) \( f = \frac{PN}{120} \)
b) \( f = \frac{120P}{N} \)
c) \( f = \frac{120N}{P} \)
d) \( f = \frac{PN}{60} \)
**Answer: a) \( f = \frac{PN}{120} \)**

74. **In a synchronous motor, V-curves represent:**
a) \( I_a \) vs. \( I_f \)
b) \( I_f \) vs. load
c) Power factor vs. \( I_f \)
d) Torque vs. speed
**Answer: a) \( I_a \) vs. \( I_f \)**

75. **A synchronous condenser improves power factor by:**
a) Absorbing reactive power
b) Supplying reactive power
c) Supplying active power
d) Reducing losses
**Answer: b) Supplying reactive power**

76. **The number of slip rings in a 3-phase synchronous motor is:**
a) 1
b) 2
c) 3
d) 4
**Answer: b) 2**

77. **The armature reaction in an alternator at lagging power factor is:**
a) Magnetizing
b) Demagnetizing
c) Cross-magnetizing
d) None
**Answer: b) Demagnetizing**

78. **Synchronous motors are used for:**
a) Constant speed drives
b) Power factor correction
c) High starting torque
d) Both a and b
**Answer: d) Both a and b**

79. **The torque angle of a synchronous motor is the angle between:**
a) Stator and rotor poles
b) Voltage and current
c) \( E_f \) and \( V \)
d) Flux and current
**Answer: c) \( E_f \) and \( V \)**

80. **The efficiency of a synchronous motor is about:**
a) 50–60%
b) 60–70%
c) 80–90%
d) 95–99%
**Answer: c) 80–90%**

### Special Machines & Miscellaneous (Q81–Q100)
81. **A universal motor can run on:**
a) AC only
b) DC only
c) Both AC and DC
d) None
**Answer: c) Both AC and DC**

82. **Stepper motors are used for:**
a) Precise position control
b) High speed
c) High torque
d) Low cost
**Answer: a) Precise position control**

83. **Hysteresis motors are used in:**
a) Toys
b) Tape recorders
c) Electric clocks
d) Pumps
**Answer: c) Electric clocks**

84. **The capacitor in a single-phase induction motor is used to:**
a) Improve power factor
b) Provide starting torque
c) Reduce noise
d) Increase speed
**Answer: b) Provide starting torque**

85. **In a shaded-pole motor, the shading coil is placed on:**
a) Stator poles
b) Rotor
c) Commutator
d) Slip rings
**Answer: a) Stator poles**

86. **The starting torque of a reluctance motor is:**
a) Low
b) High
c) Zero
d) Very high
**Answer: a) Low**

87. **A servo motor is designed for:**
a) High inertia
b) Precise control
c) Low cost
d) High voltage
**Answer: b) Precise control**

88. **The principle of a linear induction motor is based on:**
a) Rotating magnetic field
b) Travelling magnetic field
c) Static magnetic field
d) Pulsating field
**Answer: b) Travelling magnetic field**

89. **In a permanent magnet DC motor, torque is proportional to:**
a) Armature current
b) Square of current
c) Speed
d) Voltage
**Answer: a) Armature current**

90. **The core losses in rotating machines are:**
a) Hysteresis and eddy current losses
b) Friction losses
c) Copper losses
d) Stray losses
**Answer: a) Hysteresis and eddy current losses**

91. **The insulation class “F” can withstand temperatures up to:**
a) 105°C
b) 130°C
c) 155°C
d) 180°C
**Answer: c) 155°C**

92. **The cooling method “TEFC” stands for:**
a) Totally Enclosed Fan Cooled
b) Totally Enclosed Forced Cooled
c) Tube Enclosed Fan Cooled
d) Tube Enclosed Forced Cooled
**Answer: a) Totally Enclosed Fan Cooled**

93. **The most efficient electrical machine is:**
a) Transformer
b) DC motor
c) Induction motor
d) Synchronous motor
**Answer: a) Transformer**

94. **The unit of magnetic flux density is:**
a) Weber
b) Tesla
c) Henry
d) Ampere
**Answer: b) Tesla**

95. **Lenz’s law relates to:**
a) Direction of induced EMF
b) Magnitude of induced EMF
c) Both a and b
d) None
**Answer: a) Direction of induced EMF**

96. **The stacking factor for laminated cores is about:**
a) 0.5
b) 0.9
c) 1.0
d) 1.5
**Answer: b) 0.9**

97. **The yoke of a DC machine carries:**
a) Half the flux per pole
b) Full flux per pole
c) No flux
d) Leakage flux
**Answer: a) Half the flux per pole**

98. **The power developed by a DC motor is given by:**
a) \( VI_a \)
b) \( E_b I_a \)
c) \( V I_a \cos \phi \)
d) \( \frac{E_b I_a}{746} \)
**Answer: b) \( E_b I_a \)**

99. **The synchronous speed for a 50 Hz, 12-pole machine is:**
a) 600 rpm
b) 500 rpm
c) 400 rpm
d) 300 rpm
**Answer: b) 500 rpm**

100. **In a transformer, the flux is:**
a) Inversely proportional to voltage
b) Proportional to voltage
c) Proportional to current
d) Independent of voltage
**Answer: b) Proportional to voltage**

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These MCQs cover key concepts in transformers, DC machines, induction motors, synchronous machines, and special machines, aligned with JE-level exams. Use them for practice!