JKSSB | JE ELECTRICAL|100 Multiple-Choice Questions (MCQs) on **Electrical Machines**, covering DC Machines, Transformers, Induction Motors, Synchronous Machines, and Special Machines:

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Here are 100 Multiple-Choice Questions (MCQs) on **Electrical Machines**, covering DC Machines, Transformers, Induction Motors, Synchronous Machines, and Special Machines:

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### **DC Machines (20 Questions)**
1. **What is the function of a commutator in a DC generator?**
A) Converts AC to DC
B) Converts DC to AC
C) Provides mechanical support
D) Reduces eddy currents
**Answer: A**

2. **The back EMF in a DC motor depends on:**
A) Field current only
B) Armature speed only
C) Both field current and armature speed
D) Load torque
**Answer: C**

3. **In a DC shunt motor, speed is:**
A) Directly proportional to armature voltage
B) Inversely proportional to field flux
C) Independent of load
D) Both A and B
**Answer: D**

4. **Which DC motor has the highest starting torque?**
A) Shunt motor
B) Series motor
C) Compound motor
D) Permanent magnet motor
**Answer: B**

5. **Interpoles in DC machines are used to:**
A) Reduce armature reaction
B) Improve commutation
C) Increase efficiency
D) Reduce core losses
**Answer: B**

6. **The direction of rotation of a DC motor can be reversed by:**
A) Reversing armature connections
B) Reversing field connections
C) Either A or B
D) Reversing supply polarity
**Answer: C**

7. **Critical resistance of a DC shunt generator refers to:**
A) Maximum load resistance
B) Field circuit resistance at which voltage builds up
C) Armature resistance
D) Resistance for maximum power transfer
**Answer: B**

8. **Swinburne’s test is used to determine losses in:**
A) DC shunt motors
B) Transformers
C) Induction motors
D) Synchronous motors
**Answer: A**

9. **A DC series motor should never be started at no-load because:**
A) It draws high starting current
B) Speed becomes dangerously high
C) Back EMF becomes zero
D) Torque becomes zero
**Answer: B**

10. **The voltage regulation of a DC generator is given by:**
A) (V_no-load – V_full-load) / V_full-load
B) (V_full-load – V_no-load) / V_no-load
C) (E_g – V) / V
D) (V – E_g) / E_g
**Answer: A**

11. **In a DC machine, armature reaction causes:**
A) Reduced main flux
B) Shift of magnetic neutral axis
C) Sparking at brushes
D) All of the above
**Answer: D**

12. **Lap winding is used in DC machines for:**
A) High voltage, low current
B) Low voltage, high current
C) High speed
D) Low speed
**Answer: B**

13. **The efficiency of a DC machine is maximum when:**
A) Copper losses = Iron losses
B) Copper losses = Stray losses
C) Iron losses = Stray losses
D) Friction losses = Windage losses
**Answer: A**

14. **Compensating windings in DC machines counteract:**
A) Armature reaction
B) Eddy currents
C) Hysteresis loss
D) Temperature rise
**Answer: A**

15. **The torque developed by a DC motor is proportional to:**
A) Armature current × flux
B) Armature voltage
C) Back EMF
D) Speed
**Answer: A**

16. **Residual magnetism is essential for:**
A) Starting a DC shunt generator
B) Speed control of DC motor
C) Commutation
D) Preventing runaway speed
**Answer: A**

17. **Ward-Leonard system is used for:**
A) Speed control of DC shunt motors
B) Voltage control of generators
C) Frequency conversion
D) Power factor correction
**Answer: A**

18. **The most economical method of DC motor speed control is:**
A) Armature resistance control
B) Field flux control
C) Voltage control
D) Frequency control
**Answer: B**

19. **A differentially compounded DC motor has:**
A) Series and shunt fields aiding
B) Series and shunt fields opposing
C) Only series field
D) Only shunt field
**Answer: B**

20. **The core of a DC machine armature is laminated to reduce:**
A) Hysteresis loss
B) Eddy current loss
C) Copper loss
D) Friction loss
**Answer: B**

—

### **Transformers (20 Questions)**
21. **A transformer works on the principle of:**
A) Self-induction
B) Mutual induction
C) Electrostatic induction
D) Electromagnetic conduction
**Answer: B**

22. **The primary purpose of a transformer core is to:**
A) Reduce copper loss
B) Provide mechanical support
C) Provide a low-reluctance flux path
D) Increase efficiency
**Answer: C**

23. **Transformer oil is used for:**
A) Cooling and insulation
B) Lubrication
C) Reducing eddy losses
D) Increasing flux density
**Answer: A**

24. **The no-load current of 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**

25. **The maximum efficiency of a transformer occurs when:**
A) Copper loss = Iron loss
B) Copper loss = 0
C) Iron loss = 0
D) Load current = 0
**Answer: A**

26. **The turns ratio of a transformer is 1:2. If the primary voltage is 220 V, the secondary voltage is:**
A) 110 V
B) 220 V
C) 440 V
D) 880 V
**Answer: C**

27. **Open-circuit test on a transformer determines:**
A) Copper losses
B) Core losses
C) Total losses
D) Efficiency
**Answer: B**

28. **Short-circuit test on a transformer determines:**
A) Copper losses
B) Core losses
C) Iron losses
D) Hysteresis loss
**Answer: A**

29. **The purpose of a breather in a transformer is to:**
A) Absorb moisture from air
B) Cool the oil
C) Filter oil impurities
D) Reduce noise
**Answer: A**

30. **Auto-transformer is used when:**
A) High efficiency is required
B) Small voltage adjustment is needed
C) Isolation is critical
D) High current is needed
**Answer: B**

31. **The all-day efficiency of a transformer considers:**
A) Peak load only
B) Average load over 24 hours
C) No-load loss
D) Copper loss at full load
**Answer: B**

32. **Leakage flux in a transformer causes:**
A) Voltage drop
B) Reduced efficiency
C) Heating
D) All of the above
**Answer: D**

33. **In a transformer, hysteresis loss is proportional to:**
A) Frequency
B) \( f^{1.6} \)
C) \( f^2 \)
D) \( \sqrt{f} \)
**Answer: A**

34. **Eddy current loss is minimized by:**
A) Using laminated core
B) Using high-resistance core material
C) Reducing core thickness
D) All of the above
**Answer: D**

35. **The phase shift between primary and secondary voltages in a transformer is:**
A) 0°
B) 90°
C) 180°
D) Depends on connection
**Answer: D**

36. **A step-up transformer has:**
A) More primary turns
B) More secondary turns
C) Equal turns
D) No turns ratio
**Answer: B**

37. **The frequency of output voltage in a transformer is:**
A) Half of input frequency
B) Twice the input frequency
C) Same as input frequency
D) Zero
**Answer: C**

38. **Scott connection is used to convert:**
A) Single-phase to three-phase
B) Three-phase to single-phase
C) Three-phase to two-phase
D) DC to AC
**Answer: C**

39. **The kVA rating of a transformer is limited by:**
A) Core losses
B) Copper losses
C) Temperature rise
D) Voltage regulation
**Answer: C**

40. **In a transformer, the flux density is kept low to reduce:**
A) Copper loss
B) Hysteresis loss
C) Stray loss
D) Insulation cost
**Answer: B**

—

### **Induction Motors (20 Questions)**
41. **The slip of an induction motor at standstill is:**
A) 0
B) 0.5
C) 1
D) Infinity
**Answer: C**

42. **The rotor of a 3-phase induction motor is:**
A) Wound type
B) Squirrel cage type
C) Salient pole type
D) Either A or B
**Answer: D**

43. **The starting torque of a squirrel cage induction motor is:**
A) Low
B) High
C) Very high
D) Zero
**Answer: A**

44. **Slip-ring induction motors are used for:**
A) High starting torque
B) Low starting current
C) Speed control
D) All of the above
**Answer: D**

45. **The speed of an induction motor:**
A) Increases with load
B) Decreases with load
C) Remains constant
D) Is independent of load
**Answer: B**

46. **Cogging in induction motors occurs due to:**
A) Low supply voltage
B) High magnetizing current
C) Harmonic synchronization
D) Stator-rotor slot combination
**Answer: D**

47. **The power factor of an induction motor at no-load is:**
A) Unity
B) 0.8 lagging
C) Very low lagging
D) Leading
**Answer: C**

48. **The efficiency of an induction motor is maximum at:**
A) No-load
B) 50% load
C) 75-80% load
D) Full load
**Answer: C**

49. **Star-delta starter reduces starting current by a factor of:**
A) \(\sqrt{3}\)
B) 3
C) \(1/\sqrt{3}\)
D) \(1/3\)
**Answer: D**

50. **Rotor resistance control is applicable to:**
A) Squirrel cage motors
B) Slip-ring motors
C) Synchronous motors
D) DC motors
**Answer: B**

51. **The frequency of rotor current in an induction motor is:**
A) \(s f\)
B) \(f\)
C) \((1-s)f\)
D) \(f/s\)
**Answer: A**

52. **Double-cage induction motors provide:**
A) High starting torque
B) Low starting current
C) Better efficiency
D) All of the above
**Answer: D**

53. **The synchronous speed (\(N_s\)) of an induction motor is given by:**
A) \(120f / P\)
B) \(60f / P\)
C) \(Pf / 120\)
D) \(120P / f\)
**Answer: A**

54. **An induction generator:**
A) Requires DC excitation
B) Self-starts
C) Operates at super-synchronous speed
D) Cannot supply reactive power
**Answer: C**

55. **Deep bar rotors in induction motors improve:**
A) Starting torque
B) Full-load efficiency
C) Power factor
D) Speed regulation
**Answer: A**

56. **The torque-slip characteristic of an induction motor has:**
A) Linear region
B) Constant torque region
C) Breakdown torque
D) All of the above
**Answer: D**

57. **No-load test on an induction motor determines:**
A) Stator copper loss
B) Core loss & friction loss
C) Rotor copper loss
D) Total loss
**Answer: B**

58. **Blocked rotor test gives information about:**
A) Magnetizing reactance
B) Leakage reactance
C) Rotor resistance
D) B and C
**Answer: D**

59. **The direction of rotation of a 3-phase induction motor can be reversed by:**
A) Changing supply phase sequence
B) Changing rotor resistance
C) Changing stator terminals
D) Reducing voltage
**Answer: A**

60. **The frame size of an induction motor indicates:**
A) Power rating
B) Shaft diameter
C) Mounting dimensions
D) Weight
**Answer: C**

—

### **Synchronous Machines (20 Questions)**
61. **The speed of a synchronous motor is:**
A) Variable with load
B) Constant
C) Proportional to frequency
D) Both B and C
**Answer: D**

62. **Synchronous motors are not self-starting because:**
A) Rotor inertia is high
B) Starting torque is zero
C) Back EMF is zero
D) Field excitation is required
**Answer: B**

63. **Damper windings in a synchronous motor help in:**
A) Reducing hunting
B) Providing starting torque
C) Improving power factor
D) Both A and B
**Answer: D**

64. **Under-excited synchronous motor operates at:**
A) Leading power factor
B) Lagging power factor
C) Unity power factor
D) Zero power factor
**Answer: B**

65. **The synchronous impedance of an alternator is:**
A) \(E_{oc}/I_{sc}\)
B) \(V/I\)
C) \(X_s\)
D) \(R_a + jX_s\)
**Answer: A**

66. **Voltage regulation of an alternator is given by:**
A) \((E_{ph} – V_{ph}) / V_{ph}\)
B) \((V_{nl} – V_{fl}) / V_{fl}\)
C) \((E_{ph} – V_{ph}) / E_{ph}\)
D) \((V_{fl} – V_{nl}) / V_{nl}\)
**Answer: A**

67. **Hunting in synchronous machines is reduced by:**
A) Damper windings
B) Increasing excitation
C) Decreasing load
D) Using salient poles
**Answer: A**

68. **In a synchronous generator, the armature reaction is demagnetizing when the load is:**
A) Resistive
B) Inductive
C) Capacitive
D) None
**Answer: B**

69. **The distribution factor (\(K_d\)) for an alternator is:**
A) \(\frac{\sin(m\gamma/2)}{m \sin(\gamma/2)}\)
B) \(\cos(\gamma/2)\)
C) \(\sin(\gamma/2)\)
D) \(\frac{m \sin(\gamma/2)}{\sin(m\gamma/2)}\)
**Answer: A**

70. **Synchronous condensers are used to:**
A) Generate reactive power
B) Improve power factor
C) Stabilize voltage
D) All of the above
**Answer: D**

71. **The rotor of a turbo-alternator is:**
A) Salient pole
B) Cylindrical
C) Slotted
D) Wound type
**Answer: B**

72. **An over-excited synchronous motor draws:**
A) Leading current
B) Lagging current
C) Unity power factor current
D) Zero current
**Answer: A**

73. **The angle \(\delta\) in a synchronous machine is called:**
A) Power factor angle
B) Torque angle
C) Impedance angle
D) Loss angle
**Answer: B**

74. **V-curves of a synchronous motor show the relationship between:**
A) \(I_a\) and \(I_f\)
B) \(I_f\) and power factor
C) Torque and speed
D) Voltage and excitation
**Answer: A**

75. **In a synchronous motor, if \(\delta = 0\):**
A) Motor is under-loaded
B) Motor is over-loaded
C) Motor is at no-load
D) Motor stalls
**Answer: C**

76. **The maximum power output of an alternator occurs when:**
A) \(\delta = 0^\circ\)
B) \(\delta = 45^\circ\)
C) \(\delta = 90^\circ\)
D) \(\delta = 180^\circ\)
**Answer: C**

77. **Zero power factor method (Potier triangle) determines:**
A) Armature leakage reactance
B) Synchronous reactance
C) Voltage regulation
D) Core loss
**Answer: A**

78. **A synchronous motor can be started by:**
A) DC motor coupling
B) Induction motor principle
C) Reducing frequency
D) All of the above
**Answer: D**

79. **The armature of a synchronous machine carries:**
A) AC only
B) DC only
C) Both AC and DC
D) Pulsating DC
**Answer: A**

80. **The efficiency of a synchronous motor is higher than an induction motor due to:**
A) Absence of slip
B) DC excitation
C) Lower copper loss
D) All of the above
**Answer: D**

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### **Special Machines & Miscellaneous (20 Questions)**
81. **Stepper motors are used for:**
A) Precise position control
B) High-speed applications
C) Power generation
D) High torque applications
**Answer: A**

82. **BLDC motors have:**
A) Brushes and commutator
B) Electronic commutation
C) Slip rings
D) Wound rotor
**Answer: B**

83. **Hysteresis motors operate on the principle of:**
A) Eddy currents
B) Hysteresis loss
C) Reluctance torque
D) Back EMF
**Answer: B**

84. **Universal motors can run on:**
A) AC only
B) DC only
C) Both AC and DC
D) Pulsed DC
**Answer: C**

85. **Reluctance motors are a type of:**
A) Synchronous motor
B) Induction motor
C) DC motor
D) Universal motor
**Answer: A**

86. **The main advantage of a permanent magnet synchronous motor (PMSM) is:**
A) No field winding
B) High efficiency
C) Compact size
D) All of the above
**Answer: D**

87. **Servomotors are designed for:**
A) High torque at low speeds
B) Precise angular motion
C) Closed-loop control
D) All of the above
**Answer: D**

88. **Linear induction motors (LIM) produce:**
A) Rotational motion
B) Linear motion
C) Oscillatory motion
D) Static torque
**Answer: B**

89. **The core loss in an electrical machine includes:**
A) Hysteresis + Eddy current loss
B) Copper loss
C) Friction loss
D) Windage loss
**Answer: A**

90. **Skewing of rotor slots in induction motors reduces:**
A) Magnetic locking
B) Noise
C) Harmonics
D) All of the above
**Answer: D**

91. **Buchholz relay protects transformers from:**
A) Overload
B) Internal faults
C) Lightning
D) Overvoltage
**Answer: B**

92. **In a transformer, tap changers are used to adjust:**
A) Frequency
B) Power factor
C) Output voltage
D) Efficiency
**Answer: C**

93. **The dielectric strength of transformer oil is tested using:**
A) Flash point test
B) BDV test
C) Acidity test
D) Viscosity test
**Answer: B**

94. **The harmonic order most dominant in transformer core saturation is:**
A) 3rd
B) 5th
C) 7th
D) 2nd
**Answer: A**

95. **The cooling medium in ONAN transformer is:**
A) Oil + Air (Natural convection)
B) Water
C) Hydrogen
D) SF6 gas
**Answer: A**

96. **The insulation class “F” can withstand temperatures up to:**
A) 105°C
B) 130°C
C) 155°C
D) 180°C
**Answer: C**

97. **The unit of magnetic flux density is:**
A) Weber
B) Tesla
C) Henry
D) Gauss
**Answer: B**

98. **Lenz’s law relates to:**
A) Electromagnetic induction
B) Magnetic hysteresis
C) Electrostatics
D) Kirchhoff’s law
**Answer: A**

99. **The eddy current loss in a magnetic core is proportional to:**
A) \(f\)
B) \(f^2\)
C) \(f^{1.5}\)
D) \(\sqrt{f}\)
**Answer: B**

100. **The stacking factor for laminated cores accounts for:**
A) Insulation thickness
B) Air gaps between laminations
C) Core losses
D) Flux leakage
**Answer: B**

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**Note:** These questions cover fundamental concepts, design principles, operation, and applications of electrical machines. Ideal for exams, interviews, and quick revisions.