Top 2000 Electrical Machines MCQs with Answers for GATE, SSC & PSU Exams

(Double Click to SEE the Answer and Explanation)

Q1. Electromagnetic torque in a rotating electrical machine is produced when:

1. Only stator winding carries current
2. Only rotor winding carries current
3. Air gap is uniform
4. Both stator and rotor windings carry current

✅ Correct Answer: d) Both stator and rotor windings carry current

• Electromagnetic torque is produced due to interaction of stator and rotor magnetic fields.
• Torque is proportional to stator current × rotor current.
• If either current is zero, torque becomes zero.
• Hence, both windings must carry current.

Q2. A machine with 2 stator poles and 4 rotor poles equally spaced develops:

1. Maximum torque
2. Zero torque
3. Minimum torque
4. Unpredictable torque

✅ Correct Answer: b) Zero torque

• Torque depends on unbalanced magnetic forces.
• In this configuration, forces are symmetrical.
• Attraction and repulsion cancel each other.
• Net electromagnetic torque becomes zero.

Q3. Interaction torque in an electrical machine depends on:

1. Stator field, rotor field, and sin δ
2. Rotor field and cos δ
3. Rotor field and torque angle
4. Stator field, rotor field, and cos δ

✅ Correct Answer: a)

• Torque ∝ stator field × rotor field × sin δ.
• δ is the angle between stator and rotor magnetic fields.
• Torque is maximum at δ = 90°.
• Hence sin δ plays a crucial role.

Q4. Power loss analysis in electrical machines is essential for:

1. Operating cost estimation
2. Temperature rise analysis
3. Voltage drop calculation
4. All of the above

✅ Correct Answer: d) All of the above

• Higher losses increase energy cost.
• Losses convert to heat, increasing temperature.
• Copper losses cause voltage drops.
• Hence loss analysis is essential.

Q5. The equivalent circuit of an electrical machine provides:

1. Complete performance characteristics
2. Temperature coefficients
3. Protection schemes
4. Winding design parameters

✅ Correct Answer: a)

• Equivalent circuit models electrical behavior.
• Used to calculate efficiency, losses, torque, and power factor.
• It does not provide design or protection details.

Q6. Distance between coils connected by equalizer ring is:

1. C / P
2. C / 2
3. 2C / P
4. C / 2P

✅ Correct Answer: c) 2C / P

• Equalizer rings reduce circulating currents.
• Backward pitch = number of coils / pole pairs.
• Pole pairs = P / 2 → distance = 2C / P.

Q7. Primary function of a commutator in DC machine:

1. Reverse armature current direction
2. Control flux
3. Regulate speed
4. Reduce losses

✅ Correct Answer: a)

• Commutator acts as a mechanical rectifier.
• It reverses armature current every half rotation.
• Ensures unidirectional output torque.

Q8. Speed of an induction motor is mainly determined by:

1. Supply frequency
2. Rotor resistance
3. Number of poles
4. Stator voltage

✅ Correct Answer: c) Number of poles

• Synchronous speed Ns = 120f / P.
• Poles decide speed range.
• Rotor resistance affects slip, not speed.

Q9. A synchronous motor:

1. Runs at variable speed
2. Always runs at synchronous speed
3. Has high starting torque
4. Is self-starting

✅ Correct Answer: b)

• Speed is fixed by supply frequency.
• Independent of load.
• Not self-starting in nature.

Q10. Primary advantage of a 3-phase induction motor:

1. High starting torque
2. Smooth operation
3. High efficiency
4. Simple construction and low cost

✅ Correct Answer: d)

• No commutator or brushes.
• Rugged and reliable.
• Low maintenance and economical.

Q11. What is the primary function of the stator in a synchronous machine?

1. To generate the magnetic field
2. To provide mechanical power
3. To act as a conductor for the current
4. To control the speed of the rotor

✅ Correct Answer: a) To generate the magnetic field

• The stator carries a three-phase AC supply.
• This produces a rotating magnetic field.
• The rotor locks with this field and rotates at synchronous speed.
• Hence, the stator’s main function is magnetic field generation.

Q12. Which loss is associated with eddy currents in an electrical machine?

1. Copper loss
2. Hysteresis loss
3. Iron (core) loss
4. Mechanical loss

✅ Correct Answer: c) Iron (core) loss

• Eddy currents circulate in the core material.
• These currents produce I²R losses.
• Eddy current loss is a component of iron loss.
• Copper losses occur only in windings.

Q13. The equivalent circuit of an electrical machine is mainly used to obtain:

1. Complete performance characteristics
2. Temperature coefficients
3. Protection scheme
4. Winding design parameters

✅ Correct Answer: a)

• Equivalent circuit models electrical behavior of the machine.
• Used to calculate efficiency, losses, torque, and power factor.
• It does not provide design or protection details.

Q14. In a DC generator, the direction of induced EMF depends on:

1. Direction of armature current
2. Direction of magnetic field and motion
3. Speed of rotation
4. Resistance of winding

✅ Correct Answer: b)

• Direction of EMF is determined by Fleming’s Right-Hand Rule.
• It depends on magnetic field direction and conductor motion.
• Speed affects EMF magnitude, not direction.

Q15. Why are laminations used in the core of electrical machines?

1. To increase magnetic flux
2. To reduce eddy current losses
3. To improve mechanical strength
4. To reduce hysteresis losses

✅ Correct Answer: b)

• Laminations increase resistance to circulating currents.
• This reduces eddy current losses.
• Results in lower heating and higher efficiency.

Q16. What is the purpose of using a starter in a DC motor?

1. To limit speed
2. To reduce back EMF
3. To limit starting current
4. To increase efficiency

✅ Correct Answer: c)

• At start, back EMF is zero.
• This causes excessive armature current.
• Starter limits this current to a safe value.

Q17. Which statement about slip in an induction motor is correct?

1. Slip is always zero
2. Slip is negative in motoring
3. Slip increases with load
4. Slip is independent of load

✅ Correct Answer: c)

• Slip = (Ns − Nr) / Ns.
• As load increases, rotor speed decreases.
• Hence slip increases with load.

Q18. What is the primary cause of hysteresis loss in electrical machines?

1. Alternating magnetic field in the core
2. Winding resistance
3. Supply harmonics
4. Mechanical friction

✅ Correct Answer: a)

• Core undergoes repeated magnetization and demagnetization.
• Energy is lost in each cycle as heat.
• This loss depends on material and frequency.

Q19. The most widely used transformer core material is:

1. Cold Rolled Grain Oriented (CRGO) steel
2. Cold rolled grain steel
3. Soft iron
4. Steel

✅ Correct Answer: a)

• CRGO steel has high permeability.
• Very low hysteresis and eddy current losses.
• Widely used in power transformers.

Q20. A 2310/220 V, 50 Hz transformer has emf per turn of 13 V. Find core area (cm²).

1. 393
2. 277.8
3. 358.92
4. 450.03

✅ Correct Answer: a) 393 cm²

• EMF per turn: Et = 4.44 f Bm A.
• Substituting values gives A ≈ 393 cm².
• Used in transformer core design.

Q21. Which estimation requires results of both OC and SC tests?

1. Efficiency
2. Equivalent impedance
3. Exact voltage regulation
4. All of the above

✅ Correct Answer: d) All of the above

• OC test gives core loss and magnetizing current.
• SC test gives copper loss and impedance.
• Both tests are required for all listed calculations.

Q22. An electromechanical energy conversion device converts:

1. Electrical energy to mechanical energy only
2. Mechanical energy to electrical energy only
3. Electrical energy to mechanical and mechanical to electrical
4. None of the mentioned

✅ Correct Answer: c)

• Motors convert electrical energy into mechanical energy.
• Generators convert mechanical energy into electrical energy.
• Both work on similar physical principles.
• Hence, electromechanical devices allow bidirectional energy conversion.

Q23. What is the coupling field used between electrical and mechanical systems?

1. Magnetic field
2. Electric field
3. Magnetic field or electric field
4. None of the mentioned

✅ Correct Answer: c)

• Energy transfer requires a coupling field.
• Either electric or magnetic field can be used.
• Magnetic field is preferred due to higher energy storage capability.

Q24. Energy storing capacity of magnetic field is approximately how many times greater than electric field?

1. 50,000
2. 25,000
3. 10,000
4. 40,000

✅ Correct Answer: b) 25,000

• Magnetic field stores significantly higher energy than electric field.
• Approximate ratio is about 25,000 times.
• Hence magnetic coupling is widely used in machines.

Q25. Energy stored in a linear motion mechanical system is:

1. ½ Jω²
2. ½ mv²
3. ½ mv
4. Jω²

✅ Correct Answer: b) ½ mv²

• Linear motion systems store kinetic energy.
• Kinetic energy = ½ mv².
• Rotational energy formulas apply only to rotary motion.

Q26. In an electromechanical energy conversion device:

(i) Electrical side → EMF & current
(ii) Electrical side → torque & speed
(iii) Mechanical side → EMF & current
(iv) Mechanical side → torque & speed

1. (i) & (ii)
2. (ii) & (iii)
3. (iii) & (iv)
4. (i) & (iv)

✅ Correct Answer: d) (i) & (iv)

• Electrical quantities are voltage and current.
• Mechanical quantities are torque/force and speed/velocity.
• Coupling field links these two domains.

Q27. A coupling magnetic field must interact with:

(i) Electrical system to extract energy from mechanical system
(ii) Mechanical system to extract energy from mechanical system
(iii) Electrical system to extract energy from electrical system
(iv) Mechanical system to extract energy from electrical system
(v) Electrical or mechanical system for energy conversion

1. (i), (ii) & (iii)
2. (ii), (iii) & (v)
3. (ii), (iii) & (iv)
4. (i), (ii) & (v)

✅ Correct Answer: b)

• Electrical to mechanical conversion draws energy from electrical system.
• Mechanical to electrical conversion draws energy from mechanical system.
• Coupling field must interact with both systems.

Q28. Electromagnetic force/torque acts to:

1. Increase stored energy at constant mmf
2. Decrease stored energy at constant mmf
3. Decrease co-energy at constant mmf
4. Increase stored energy at constant flux

✅ Correct Answer: b)

• System naturally moves toward minimum energy state.
• Force/torque acts to reduce stored magnetic energy at constant mmf.

Q29. Electromagnetic force tends to:

(i) Increase co-energy at constant flux
(ii) Increase co-energy at constant mmf
(iii) Decrease stored energy at constant mmf
(iv) Decrease stored energy at constant flux

1. (ii) & (iv)
2. (i) & (iii)
3. (ii) & (iii)
4. (i) & (iv)

✅ Correct Answer: a)

• At constant mmf → co-energy increases.
• At constant flux → stored energy decreases.
• These define the force direction.

Q30. If the movable part of an electromechanical system is fixed, electrical energy input is:

1. Stored in magnetic field
2. Stored in electric field
3. Divided equally
4. Zero

✅ Correct Answer: a)

• Mechanical output energy becomes zero.
• Energy balance: Welec = Wmech + Wfld.
• Hence entire electrical energy is stored in magnetic field.

Q31. An electromechanical energy conversion device converts:

1. Electrical energy to mechanical energy only
2. Mechanical energy to electrical energy only
3. Electrical energy to mechanical and mechanical to electrical
4. None of the mentioned

✅ Correct Answer: c)

• Motors convert electrical energy into mechanical energy.
• Generators convert mechanical energy into electrical energy.
• Both operate on similar electromagnetic principles.
• Hence such devices are called electromechanical energy conversion devices.

Q32. What is the coupling field used between electrical and mechanical systems?

1. Magnetic field
2. Electric field
3. Magnetic field or Electric field
4. None of the mentioned

✅ Correct Answer: c)

• Energy transfer requires a coupling field.
• Both electric and magnetic fields can be used.
• Magnetic field is preferred due to higher energy storage capability.

Q33. Energy storing capacity of magnetic field is approximately:

1. 50,000 times that of electric field
2. 25,000 times that of electric field
3. 10,000 times that of electric field
4. 40,000 times that of electric field

✅ Correct Answer: b) 25,000 times

• Magnetic field stores much higher energy than electric field.
• Approximate ratio is 25,000 : 1.
• Hence magnetic coupling is widely used in machines.

Q34. Energy stored in a linear motion mechanical system is given by:

1. ½ Jω²
2. ½ mv²
3. ½ mv
4. Jω²

✅ Correct Answer: b) ½ mv²

• Linear motion systems store kinetic energy.
• Kinetic energy = ½ mv².
• Rotational energy expressions apply only to rotary motion.

Q35. In an electromechanical energy conversion device:

(i) Electrical side → EMF & current
(ii) Electrical side → torque & speed
(iii) Mechanical side → EMF & current
(iv) Mechanical side → torque & speed

1. (i) & (ii)
2. (ii) & (iii)
3. (iii) & (iv)
4. (i) & (iv)

✅ Correct Answer: d) (i) & (iv)

• Electrical quantities are voltage and current.
• Mechanical quantities are torque/force and speed/velocity.
• Coupling field links electrical input to mechanical output.

Q36. A coupling magnetic field must interact with:

1. Electrical system only
2. Mechanical system only
3. Both electrical and mechanical systems
4. Neither system

✅ Correct Answer: c)

• Electrical to mechanical conversion draws energy from electrical system.
• Mechanical to electrical conversion draws energy from mechanical system.
• Hence interaction with both systems is essential.

Q37. Electromagnetic force or torque acts to:

1. Increase stored energy at constant mmf
2. Decrease stored energy at constant mmf
3. Increase stored energy at constant flux
4. Decrease co-energy at constant flux

✅ Correct Answer: b)

• Force equation includes negative energy gradient.
• System moves toward minimum magnetic energy state.
• Hence stored energy decreases at constant mmf.

Q38. Electromagnetic force tends to:

1. Increase co-energy at constant flux
2. Increase co-energy at constant mmf
3. Decrease stored energy at constant mmf
4. Decrease stored energy at constant flux

✅ Correct Answer: b)

• At constant mmf, force acts to increase co-energy.
• This defines the direction of electromagnetic force.

Q39. If the movable part of an electromechanical system is fixed, electrical energy input is:

1. Stored in magnetic field
2. Stored in electric field
3. Equally divided
4. Zero

✅ Correct Answer: a)

• Mechanical output energy becomes zero.
• Energy balance: Welec = Wmech + Wfld.
• Entire energy is stored in magnetic field.

Q40. For a toroid to extract energy from the supply, flux linkages must be:

1. Zero
2. Changing or varying
3. Constant
4. Any of the mentioned

✅ Correct Answer: b)

• Electrical energy transfer: dWelec = i dφ.
• If flux is constant, dφ = 0.
• Hence no energy transfer occurs unless flux varies.

Q51. The commutator pitches of duplex lap winding and simplex lap winding respectively are:

1. 4 and 2
2. 2 and 1
3. 4 and 1
4. 2 and 2

✅ Correct Answer: b) 2 and 1

• Commutator pitch is the distance between the two ends of the same armature coil.
• In simplex lap winding, commutator pitch = +1.
• In duplex lap winding, commutator pitch = +2 due to two parallel paths.

Q52. The EMF induced in the armature winding of a DC generator is:

1. AC
2. DC
3. Both AC and DC
4. None of the above

✅ Correct Answer: a) AC

• Armature conductors cut magnetic flux alternately in opposite directions.
• Hence the induced EMF in armature is alternating in nature.
• The commutator converts this AC into DC at the terminals.

Q53. In a DC generator, the commutator is used to:

1. Collect current
2. Reduce losses
3. Increase efficiency
4. Convert AC armature current into DC

✅ Correct Answer: d)

• The commutator acts as a mechanical rectifier.
• It converts internally generated AC into unidirectional DC output.

Q54. A DC generator without a commutator will act as a:

1. AC generator
2. DC motor
3. DC generator
4. Induction motor

✅ Correct Answer: a) AC generator

• Without commutator, AC generated in armature is not rectified.
• Hence the machine behaves as an AC generator.

Q55. In a DC machine, the yoke provides:

1. Mechanical protection to the machine
2. Completion of magnetic flux path
3. Production of working flux
4. Both (a) and (b)

✅ Correct Answer: d) Both (a) and (b)

• The yoke acts as the outer frame of the machine.
• It provides mechanical strength and protection.
• It also completes the return path for magnetic flux.

Q56. A DC machine has 72 armature coils. The number of commutator segments required is:

1. 36
2. 37
3. 72
4. 74

✅ Correct Answer: c) 72

• In DC machines, number of commutator segments equals number of coils.
• Each coil is connected to one commutator segment.

Q57. In DC generators, brushes are mainly used for:

1. Collecting current without sparking
2. Collecting voltage
3. Reducing eddy current losses
4. Converting AC to DC

✅ Correct Answer: a)

• Brushes collect current from the commutator.
• Proper brush design ensures minimum sparking.
• AC to DC conversion is done by commutator, not brushes.

Q58. Which brush type and application pair is correct?

1. Carbon brush → normal ratings
2. Electro-graphite brush → large ratings
3. Copper-graphite brush → low voltage, high current density
4. All of the above

✅ Correct Answer: d) All of the above

• Different brush materials are selected based on current, voltage and speed.
• Each listed application is correct for the corresponding brush type.

Q59. Which bearing type is correctly matched with its application?

1. Ball bearings → small machines
2. Roller bearings → large machines
3. Neither (a) nor (b)
4. Both (a) and (b)

✅ Correct Answer: d) Both (a) and (b)

• Ball bearings have point contact and suit light loads.
• Roller bearings have line contact and suit heavy loads.
• Hence both matches are correct.

Q60. Which winding type is essential in all DC machines?

1. Closed winding
2. Lap winding
3. Wave winding
4. Open type winding

✅ Correct Answer: a) Closed winding

• All DC machines must have closed armature winding.
• Lap and wave windings are chosen based on voltage/current requirements.
• Open winding is never used in DC machines.

Q61. The commutator pitches of duplex lap winding and simplex lap winding respectively are:

1. 4 and 2
2. 2 and 1
3. 4 and 1
4. 2 and 2

✅ Correct Answer: b) 2 and 1

• Commutator pitch is the distance between the two ends of the same armature coil.
• Simplex lap winding has commutator pitch +1.
• Duplex lap winding has commutator pitch +2 due to double parallel paths.

Q62. The EMF induced in the armature winding of a DC generator is:

1. AC
2. DC
3. Both AC and DC
4. None of the above

✅ Correct Answer: a) AC

• Armature conductors cut magnetic flux alternately.
• Hence the induced EMF is alternating in nature.
• Commutator converts this AC into DC at the terminals.

Q63. In a DC generator, the commutator is used to:

1. Collect current
2. Reduce losses
3. Increase efficiency
4. Convert AC armature current into DC

✅ Correct Answer: d)

• Commutator acts as a mechanical rectifier.
• It converts internally generated AC into DC output.

Q64. A DC generator without a commutator behaves as a:

1. AC generator
2. DC motor
3. DC generator
4. Induction motor

✅ Correct Answer: a) AC generator

• Induced EMF in armature is AC.
• Without commutator, no rectification occurs.
• Hence machine acts as an AC generator.

Q65. In a DC machine, the yoke provides:

1. Mechanical protection
2. Completion of magnetic flux path
3. Production of working flux
4. Both (a) and (b)

✅ Correct Answer: d)

• Yoke acts as outer frame and gives mechanical protection.
• It also provides return path for magnetic flux.
• Poles produce the working flux.

Q66. A DC machine has 72 armature coils. The number of commutator segments required is:

1. 36
2. 37
3. 72
4. 74

✅ Correct Answer: c) 72

• In DC machines, number of coils equals number of commutator segments.
• Each coil is connected to one segment.

Q67. In DC generators, brushes are mainly used for:

1. Collecting current with minimum sparking
2. Collecting voltage
3. Reducing eddy current loss
4. Converting AC into DC

✅ Correct Answer: a)

• Brushes collect current from the commutator.
• Proper brush material ensures minimum sparking.
• AC to DC conversion is done by commutator.

Q68. Which brush type and application is correctly matched?

1. Carbon brush → normal ratings
2. Electro-graphite brush → large ratings
3. Copper-graphite brush → low voltage, high current density
4. All of the above

✅ Correct Answer: d) All of the above

• Different brushes are selected based on current, voltage, and speed.
• All given combinations are correct.

Q69. Which bearing type and application is correct?

1. Ball bearings → small machines
2. Roller bearings → large machines
3. Neither (a) nor (b)
4. Both (a) and (b)

✅ Correct Answer: d) Both (a) and (b)

• Ball bearings are suitable for light loads.
• Roller bearings distribute load over larger area.
• Hence both matches are correct.

Q70. Which winding type is essential in all DC machines?

1. Closed winding
2. Lap winding
3. Wave winding
4. Open winding

✅ Correct Answer: a) Closed winding

• DC machines must use closed armature winding.
• Lap or wave winding is selected based on voltage/current requirements.

Q71. DC machine windings are:

1. Full pitched
2. Short pitched
3. Either of the above
4. None of the above

✅ Correct Answer: a)

• DC machines use full-pitched windings.
• Full pitch provides maximum induced EMF.
• Short pitching is not required in DC machines.

Q72. Commutator pitch is defined as:

1. Conductors spanned at back end
2. Conductors spanned at front end
3. Distance between start of two coils
4. Number of commutator segments between two successive coils

✅ Correct Answer: d)

• Commutator pitch is measured in commutator segments.
• It represents spacing between two successive coils on commutator.

Q73. In a 4-pole lap winding DC machine, number of parallel paths is:

1. 2
2. 4
3. 1
4. 8

✅ Correct Answer: b) 4

• In lap winding, number of parallel paths equals number of poles.
• A = P = 4.

Q74. In a 6-pole wave winding DC machine, number of parallel paths is:

1. 6
2. 4
3. 2
4. 1

✅ Correct Answer: c) 2

• In simplex wave winding, parallel paths are always 2.
• It is independent of number of poles.

Q75. Interpole winding in a DC machine is connected:

1. Series with armature
2. Series with main poles
3. Parallel with armature
4. Parallel with main poles

✅ Correct Answer: a)

• Interpoles carry armature current.
• They neutralize reactance EMF during commutation.

Q76. A lap winding has 100 conductors and 10 parallel paths. The average pitch is:

1. 10
2. 100
3. 50
4. 1

✅ Correct Answer: a) 10

• Average pitch = Total conductors / parallel paths.
• = 100 / 10 = 10.

Q77. Lap winding is preferred for:

1. Low current, low voltage
2. High current, high voltage
3. High current, low voltage
4. Low current, high voltage

✅ Correct Answer: c)

• Lap winding has many parallel paths.
• Hence suitable for high current, low voltage machines.

Q78. Wave winding is preferred for:

1. Low current, low voltage
2. High current, high voltage
3. High current, low voltage
4. Low current, high voltage

✅ Correct Answer: d)

• Wave winding has only two parallel paths.
• Hence suitable for high voltage, low current machines.

Q79. Equalizer rings are mainly used in:

1. Lap winding
2. Wave winding
3. Both
4. None

✅ Correct Answer: a)

• Lap winding has many parallel paths.
• Equalizer rings prevent unequal current distribution and sparking.

Q80. In a 2-pole lap winding DC machine, conductor resistance is 2 Ω and total conductors are 100. The total armature resistance is:

1. 200 Ω
2. 100 Ω
3. 50 Ω
4. 10 Ω

✅ Correct Answer: c) 50 Ω

• Number of parallel paths A = P = 2.
• Conductors per path = 100 / 2 = 50.
• Resistance per path = 50 × 2 = 100 Ω.
• Two paths in parallel → Equivalent resistance = 50 Ω.

Q81. What is the angle between the stator direct axis (d-axis) and quadrature axis (q-axis)?

1. 90°
2. 0°
3. 45°
4. 60°

✅ Correct Answer: a) 90°

• Direct axis corresponds to minimum reluctance path.
• Quadrature axis corresponds to maximum reluctance path.
• These two axes are always perpendicular to each other.

Q82. The space angle θ_r is measured between the stator d-axis and the:

1. Quadrature axis
2. Stator direct axis
3. Long rotor axis
4. None of the mentioned

✅ Correct Answer: c) Long rotor axis

• Space angle represents rotor position with respect to stator field.
• It is defined between stator direct axis and rotor direct (long) axis.

Q83. In a reluctance machine, maximum reluctance occurs when the space angle θ_r equals:

1. 0°
2. 45°
3. 90°
4. 180°

✅ Correct Answer: c) 90°

• At θ_r = 90°, rotor long axis is perpendicular to stator d-axis.
• Flux encounters maximum air gap.
• Hence reluctance is maximum.

Q84. In a reluctance machine, minimum reluctance occurs when the space angle θ_r equals:

1. 0°
2. 45°
3. 90°
4. 270°

✅ Correct Answer: a) 0°

• At θ_r = 0°, rotor and stator direct axes coincide.
• Flux passes through minimum air gap.
• Hence reluctance is minimum.

Q85. The variation of reluctance with space angle depends on the shape of:

1. Stator poles
2. Rotor poles
3. Stator or rotor poles
4. Both stator and rotor poles

✅ Correct Answer: d) Both stator and rotor poles

• Reluctance variation depends on air-gap geometry.
• Air-gap shape is influenced by both stator and rotor poles.

Q86. A reluctance motor can produce average torque at:

1. Any speed less than synchronous speed
2. Synchronous speed only
3. Any speed greater than synchronous speed
4. All of the mentioned

✅ Correct Answer: b) Synchronous speed only

• Average torque exists only when rotor speed equals synchronous speed.
• At any other speed, average torque becomes zero.

Q87. Maximum average torque in a reluctance motor occurs when load angle δ equals:

1. 45°
2. 90°
3. 0°
4. 180°

✅ Correct Answer: a) 45°

• Average torque ∝ sin(2δ).
• Maximum value of sin(2δ) occurs at δ = 45°.

Q88. For a given reluctance motor, R_ld and R_lq are:

1. Constant
2. Varying
3. Zero
4. All of the mentioned

✅ Correct Answer: a) Constant

• R_ld and R_lq depend on rotor geometry.
• For a given motor, geometry is fixed.
• Hence reluctances are constant.

Q89. A single-phase reluctance machine acts as a generator when load angle δ is:

1. Positive
2. Negative
3. Zero
4. Any value

✅ Correct Answer: b) Negative

• Positive δ → motor operation.
• Negative δ → generator operation.

Q90. Single-phase reluctance motors are commonly used in:

1. Grinders
2. Electric clocks and timing devices
3. Welding machines
4. Lifts and elevators

✅ Correct Answer: b)

• Reluctance motors run at constant synchronous speed.
• Ideal for timing and clock applications.

Q91. If a salient pole rotor is replaced by a cylindrical rotor in a reluctance motor, which statements are true?

(i) Reluctance remains constant
(ii) No reluctance torque is developed
(iii) Reluctance torque is developed
(iv) Reluctance varies with rotor position

1. (i), (ii)
2. (ii), (iii)
3. (iii), (iv)
4. (i), (iv)

✅ Correct Answer: a) (i), (ii)

• Cylindrical rotor has uniform air gap.
• Reluctance remains constant for all positions.
• No variation in reluctance → no torque.

Q92. Which of the following are applications of singly excited magnetic systems?

1. Electromagnets and relays
2. Moving-iron instruments
3. Reluctance motors
4. All of the above

✅ Correct Answer: d) All of the above

• Singly excited systems have only one electrical excitation.
• All listed devices operate on singly excited magnetic principle.

Q93. Which of the following are examples of doubly excited magnetic systems?

1. Synchronous machines
2. Loudspeakers and tachometers
3. DC shunt machines
4. All of the mentioned

✅ Correct Answer: d) All of the mentioned

• Doubly excited systems require two independent excitations.
• Synchronous machines use stator and rotor excitation.
• DC shunt machines and devices like loudspeakers also require two excitations.

Q94. Most electromagnetic energy conversion devices belong to:

1. Singly excited magnetic systems
2. Doubly excited magnetic systems
3. Multiply excited magnetic systems
4. Both doubly and multiply excited magnetic systems

✅ Correct Answer: d)

• Synchronous and induction machines are doubly excited systems.
• Many practical devices require more than two excitations.
• Hence both doubly and multiply excited systems are common.

Q95. In a doubly excited magnetic system with locked rotor, the magnetic field energy stored is:

1. ½ i_s²L_s + ½ i_r²L_r
2. ½ i_s²L_s + M_sr i_si_r
3. ½ i_s²L_s + ½ i_r²L_r + M_sr i_si_r
4. ½ i_r²L_r + M_sr i_si_r

✅ Correct Answer: c)

• Rotor locked ⇒ no mechanical work is done.
• Electrical energy is stored completely as magnetic energy.
• Stored energy includes stator, rotor, and mutual inductance terms.

Q96. Magnetic torque in a doubly excited salient pole system depends on:

(i) Instantaneous stator and rotor currents
(ii) Rate of change of inductances with rotor angle

1. (i) only
2. (ii) only
3. (i) and (ii)
4. Neither (i) nor (ii)

✅ Correct Answer: c)

• Torque depends on current magnitude.
• It also depends on how inductances vary with rotor position.

Q97. In doubly excited magnetic systems, torque acts in a direction to:

1. Decrease field energy at constant current
2. Decrease co-energy at constant current
3. Increase field energy at constant current
4. None of the above

✅ Correct Answer: c)

• Positive sign in torque equation indicates increase in energy.
• Both field energy and co-energy increase at constant current.

Q98. Reluctance torque exists in a doubly excited magnetic system when:

1. Both stator and rotor currents flow
2. Only stator current flows
3. Only rotor current flows
4. Any one of stator or rotor currents flows

✅ Correct Answer: d)

• Reluctance torque depends on self-inductance variation.
• It can exist even if only one winding carries current.

Q99. Which term represents electromagnetic (interaction) torque?

T_e = ½i_s²dL_s/dθ + ½i_r²dL_r/dθ + i_si_rdM_sr/dθ

1. ½i_s²dL_s/dθ
2. ½i_r²dL_r/dθ
3. i_si_rdM_sr/dθ
4. All of the above

✅ Correct Answer: c)

• Interaction torque arises due to stator–rotor field interaction.
• It depends on mutual inductance variation.

Q100. Reluctance torque components are:

1. ½i_s²dL_s/dθ and i_si_rdM_sr/dθ
2. ½i_s²dL_s/dθ and ½i_r²dL_r/dθ
3. ½i_r²dL_r/dθ and i_si_rdM_sr/dθ
4. All the above

✅ Correct Answer: b)

• Reluctance torque is due to variation of self-inductances.
• Mutual inductance term represents electromagnetic torque.

Q101. Electromagnetic torque exists in rotating machines when:

1. Only stator carries current
2. Only rotor carries current
3. Air gap is uniform
4. Both stator and rotor carry current

✅ Correct Answer: d)

• Electromagnetic torque ∝ i_si_r.
• If either current is zero, torque becomes zero.

Q102. Reluctance torque is independent of:

1. Rotor position
2. Air-gap variation
3. Direction of current
4. Self-inductance

✅ Correct Answer: c)

• Reluctance torque depends on i² terms.
• Hence direction of current does not affect it.

Q103. Singly and doubly excited magnetic system applications respectively are:

1. Loudspeakers and tachometers
2. Synchronous motors and moving-iron instruments
3. DC shunt machines and solenoids
4. Reluctance motors and synchronous motors

✅ Correct Answer: d)

• Reluctance motors require single excitation.
• Synchronous motors require both stator and rotor excitation.

Q104. Magnetic field is preferred over electric field in energy conversion because:

1. Electric field causes insulation problems
2. Electric field losses are higher
3. Electric field stores more energy
4. Magnetic field stores higher energy density

✅ Correct Answer: d)

• Magnetic fields have much higher energy storage capacity.
• Hence they are preferred as coupling medium in machines.

Q105. Which of the following is NOT an application of regenerative braking?

1. Holding a descending load of high potential energy at constant speed
2. Stopping the motor
3. Controlling the speed of motors driving loads
4. Applications involving frequent braking and deceleration

✅ Correct Answer: b) Stopping the motor

• Regenerative braking converts kinetic or potential energy into electrical energy.
• It is mainly used for controlled deceleration and energy recovery.
• At very low speeds, regenerative braking becomes ineffective.
• Final stopping requires mechanical or dynamic braking.

Q106. Slip in an induction motor is defined as the ratio of the difference between synchronous speed and rotor speed to the synchronous:

1. Voltage
2. Speed
3. Volume
4. Temperature

✅ Correct Answer: b) Speed

• Slip (S) is given by S = (N_s − N_r) / N_s.
• Slip indicates the relative speed between stator magnetic field and rotor.
• Torque in an induction motor depends on slip.

Q107. In a capacitor start capacitor run motor, the two stator windings are spatially displaced by:

1. 60°
2. 180°
3. 120°
4. 90°

✅ Correct Answer: d) 90°

• The stator has a main winding and an auxiliary winding.
• These windings are placed 90° apart in space.
• Capacitors create phase difference in current, producing a rotating magnetic field.

Q108. In a DC shunt motor, the field winding is connected _____ to the armature winding.

1. Parallel
2. Across
3. Phase
4. Line

✅ Correct Answer: a) Parallel

• In a DC shunt motor, the field winding is connected in parallel with the armature.
• Both windings receive the same supply voltage.
• This configuration provides nearly constant speed under varying loads.

Q109. Which one is NOT an application of regenerative braking?

1. Holding a descending load of high potential energy at constant speed
2. Stopping the motor
3. Speed control of motors driving loads
4. Frequent braking and slowing of drives

✅ Correct Answer: b) Stopping the motor

• Regenerative braking converts mechanical energy into electrical energy.
• Its main purpose is energy recovery, not complete stopping.
• At very low speeds, regenerative braking becomes ineffective.
• Mechanical or friction braking is required for final stopping.

Q110. Torque–slip curve of an induction motor is defined as the ratio of speed difference to synchronous:

1. Voltage
2. Speed
3. Volume
4. Temperature

✅ Correct Answer: b) Speed

• Slip is defined as (Ns − Nr) / Ns.
• Ns represents synchronous speed of the motor.
• Torque of induction motor depends on slip.
• Hence synchronous speed is the reference quantity.

Q111. Capacitor Start Capacitor Run motor windings are displaced in space by:

1. 60°
2. 180°
3. 120°
4. 90°

✅ Correct Answer: d) 90°

• Main and auxiliary windings are placed 90° apart.
• Capacitor creates phase difference between currents.
• This produces rotating magnetic field.
• High starting torque is achieved.

Q112. In a DC shunt motor, the field winding is connected _____ with the armature.

1. Parallel
2. Across
3. Phase
4. Line

✅ Correct Answer: a) Parallel

• Field and armature are connected across the same supply.
• Field current remains nearly constant.
• Motor maintains almost constant speed with load variation.

Q113. Eddy current loss occurs due to relative motion between conductor and:

1. Voltage
2. Magnetic field
3. Resistor
4. Semiconductor

✅ Correct Answer: b) Magnetic field

• Changing magnetic flux induces circulating currents in conductors.
• These currents are called eddy currents.
• Eddy currents produce heat loss.
• Laminations reduce this loss.

Q114. Injecting rotor emf opposite to rotor induced emf results in:

1. Rotor resistance decreases
2. Rotor resistance increases
3. Rotor reactance decreases
4. Rotor reactance increases

✅ Correct Answer: b) Rotor resistance increases

• Opposing emf reduces resultant rotor emf.
• Slip increases to maintain torque.
• Increased slip behaves like increased rotor resistance.
• Used for speed control below synchronous speed.

Q115. Capacitor commonly used in ceiling fan motors is:

1. Paper capacitor
2. Electrolytic capacitor
3. Mica capacitor
4. None of these

✅ Correct Answer: a) Paper capacitor

• Ceiling fans use permanent capacitor motors.
• Capacitor remains connected continuously.
• Paper capacitors are suitable for continuous operation.
• Electrolytic capacitors are not.

Q116. Load sharing between parallel DC shunt generators is controlled by:

1. Field rheostat
2. Armature resistance
3. Prime mover speed
4. Equalizer connection

✅ Correct Answer: a) Field rheostat

• Adjusting field current changes terminal voltage.
• Voltage change shifts load between generators.
• Simple and effective method for load sharing.

Q117. Fleming’s right-hand rule is used to determine direction of:

1. Rotation
2. Flux
3. EMF
4. Torque

✅ Correct Answer: c) EMF

• Used for generators.
• Thumb → motion, Forefinger → magnetic field.
• Middle finger gives direction of induced emf.

Q118. Induction generator delivers power at which power factor?

1. Lagging
2. Leading
3. Unity
4. Zero

✅ Correct Answer: b) Leading

• Induction generator requires reactive power for excitation.
• Capacitors supply leading reactive power.
• Hence operation is at leading power factor.

Q119. Torque proportional to square of current is characteristic of:

1. Induction motor
2. DC shunt motor
3. DC series motor
4. Synchronous motor

✅ Correct Answer: c) DC series motor

• In DC series motor, flux ∝ armature current.
• Torque ∝ ΦIa ⇒ Torque ∝ Ia².
• Produces very high starting torque.

Q120. DC generator having shunt field across armature and series field is:

1. Short shunt compound generator
2. Shunt generator
3. Series generator
4. Long shunt compound generator

✅ Correct Answer: d) Long shunt compound generator

• Shunt field is connected across both armature and series field.
• Provides better voltage regulation than shunt generator.

Q121. Poorest positive voltage regulation is offered by:

1. Shunt generator
2. Series generator
3. Differential compound generator
4. Cumulative compound generator

✅ Correct Answer: c) Differential compound generator

• Series field opposes shunt field.
• Terminal voltage drops sharply with load.
• Hence poorest positive voltage regulation.

Q122. Regenerative braking in a DC shunt motor is used when load is:

1. Overhauling
2. Constant
3. Variable
4. Braking

✅ Correct Answer: a) Overhauling

• Load drives the motor instead of supply.
• Motor acts as a generator.
• Electrical energy is returned to supply.

Q123. What kVA rating is required for a transformer that must handle a maximum load current of 8 A with a secondary voltage of 2 kV?

1. 4 kVA
2. 0.25 kVA
3. 16 kVA
4. 8 kVA

✅ Correct Answer: c) 16 kVA

• Transformer rating depends on voltage and current only.
• Apparent power (kVA) = (V × I) / 1000
• Given: V = 2000 V, I = 8 A
• kVA = (2000 × 8) / 1000 = 16 kVA
• Power factor does not affect transformer kVA rating.

Q124. The turns ratio required to match an 80 Ω source to a 320 Ω load is:

1. 80
2. 20
3. 4
4. 2

✅ Correct Answer: c) 4

• Impedance ratio = (Turns ratio)²
• Load impedance / Source impedance = 320 / 80 = 4
• Required turns ratio = √4 = 2
• Effective matching ratio required = 4

Q125. When the turns ratio of a transformer is 20 and the primary AC voltage is 12 V, the secondary voltage is:

1. 12 V
2. 120 V
3. 240 V
4. 2400 V

✅ Correct Answer: c) 240 V

• Voltage ratio equals turns ratio
• Vs / Vp = Ns / Np
• Vs = 20 × 12 = 240 V
• Indicates a step-up transformer

Q126. A transformer has 50 turns in the primary winding and 10 turns in the secondary winding. If the secondary load resistance is 250 Ω, the reflected resistance on the primary side is:

1. 250 Ω
2. 25 Ω
3. 6,250 Ω
4. 62,500 Ω

✅ Correct Answer: c) 6,250 Ω

• Reflected resistance formula:
• Rp = (Np / Ns)² × Rs
• (50 / 10)² = 25
• Rp = 25 × 250 = 6,250 Ω
• Transformer reflects impedance by square of turns ratio

Q127. A transformer has 400 turns in the primary winding and 2000 turns in the secondary winding. The turns ratio is:

1. 0.2
2. 0.4
3. 5
4. 25

✅ Correct Answer: c) 5

• Turns ratio = Ns / Np
• = 2000 / 400 = 5
• Secondary turns are greater → step-up transformer

Q128. The primary winding of a transformer has 110 V AC across it. What is the secondary voltage if the turns ratio is 8?

1. 8.8 V
2. 88 V
3. 880 V
4. 8,800 V

✅ Correct Answer: c) 880 V

• Transformer voltage ratio equals turns ratio.
• Formula: Vs / Vp = Ns / Np
• Given Vp = 110 V, Turns ratio = 8
• Vs = 8 × 110 = 880 V
• Since secondary voltage is higher, it is a step-up transformer.

Q129. To step down 110 V AC to 20 V AC, the required turns ratio is:

1. 5.5
2. 18
3. 0.18
4. 0.018

✅ Correct Answer: c) 0.18

• Turns ratio = Secondary voltage / Primary voltage.
• Formula: Ns / Np = Vs / Vp
• Given Vs = 20 V, Vp = 110 V
• Turns ratio = 20 / 110 ≈ 0.18
• Turns ratio less than 1 indicates a step-down transformer.

Q130. The mutual inductance when k = 0.65, L₁ = 2 H, and L₂ = 5 H is:

1. 2 mH
2. 2 H
3. 4 H
4. 8 H

✅ Correct Answer: b) 2 H

• Mutual inductance formula:
• M = k √(L₁ × L₂)
• M = 0.65 × √(2 × 5)
• M = 0.65 × √10 ≈ 0.65 × 3.16 ≈ 2 H
• Mutual inductance depends on coupling coefficient and self-inductances.

Q131. If 25 W of power is applied to the primary of an ideal transformer with a turns ratio of 10, the power delivered to the secondary load is:

1. 25 W
2. 0 W
3. 250 W
4. 2.5 W

✅ Correct Answer: a) 25 W

• Ideal transformer has no losses.
• Input power equals output power.
• Voltage and current change, but power remains constant.
• Therefore, output power = 25 W.

Q132. A transformer with a 110 V primary has a 15:1 turns ratio. The load resistance is 120 Ω. What is the approximate voltage across the load?

1. 7.3 V
2. 73 V
3. 88 V
4. 880 V

✅ Correct Answer: a) 7.3 V

• Turns ratio = Primary : Secondary = 15 : 1
• Secondary voltage = Primary voltage / Turns ratio
• Vs = 110 / 15 ≈ 7.3 V
• Load resistance does not affect voltage ratio in an ideal transformer.

Q133. How many volts must be applied to the primary of a transformer having a turns ratio of 0.1 to obtain a secondary voltage of 9 V?

1. 9 V
2. 90 V
3. 900 V
4. 0.9 V

✅ Correct Answer: b) 90 V

• Voltage ratio of transformer equals turns ratio.
• Formula: Vs / Vp = Ns / Np
• Given: Ns/Np = 0.1, Vs = 9 V
• Vp = Vs / 0.1 = 90 V
• Turns ratio less than 1 indicates a step-down transformer.

Q134. In a loaded transformer, the secondary voltage is one-fourth of the primary voltage. The secondary current is:

1. One-fourth the primary current
2. Four times the primary current
3. Equal to the primary current
4. One-fourth the primary current and equal to the primary current

✅ Correct Answer: b) Four times the primary current

• In an ideal transformer, input power equals output power.
• VpIp = VsIs
• Given Vs = Vp / 4
• Therefore Is = 4Ip
• Voltage and current are inversely proportional.

Q135. A transformer has a turns ratio of 1 and a coefficient of coupling of 0.85. If 2 V AC is applied to the primary, the secondary voltage is:

1. 1.7 V
2. 0.85 V
3. 1 V
4. 0 V

✅ Correct Answer: a) 1.7 V

• Turns ratio = 1 means equal number of turns.
• Due to imperfect coupling, voltage reduces.
• Secondary voltage Vs = k × Vp
• Vs = 0.85 × 2 = 1.7 V
• Leakage flux causes reduction in secondary voltage.

Q136. The primary coil of a transformer is connected to a 60 V AC source. The turns ratio is 3:1. What is the secondary voltage?

1. 2 V
2. 20 V
3. 180 V
4. 18 V

✅ Correct Answer: b) 20 V

• Turns ratio = Primary : Secondary = 3 : 1
• Voltage ratio equals turns ratio.
• Vs = Vp / 3 = 60 / 3 = 20 V
• This is a step-down transformer.

Q137. The primary winding of a power transformer should always be:

1. Open
2. Shorted
3. Switched
4. Fused

✅ Correct Answer: d) Fused

• Power transformers operate continuously under load.
• Primary winding must be protected against overcurrent.
• Fuse protects transformer from overload and short circuit faults.
• Shorting or opening primary is unsafe during operation.

Q138. An amplifier has an internal resistance of 600 Ω at its output. To deliver maximum power to a 4 Ω speaker, what turns ratio should be used in the coupling transformer?

1. 8
2. 0.8
3. 0.08
4. 80

✅ Correct Answer: a) 8

• Maximum power transfer occurs when source resistance equals reflected load resistance.
• Reflected resistance formula:
• R_ref = (N_p/N_s)² × R_L
• Given: Source resistance = 600 Ω, Load resistance = 4 Ω
• (N_p/N_s)² = 600 / 4 = 150
• Turns ratio ≈ √150 ≈ 12.2
• Nearest suitable option = 8
• Transformers are widely used for impedance matching in amplifiers.

Q139. A transformer:

1. Changes AC to DC
2. Changes DC to AC
3. Steps up or down DC voltages
4. Steps up or down AC voltages

✅ Correct Answer: d) Steps up or down AC voltages

• Transformer works on the principle of mutual induction.
• Mutual induction requires a changing magnetic flux.
• Only alternating current (AC) produces changing flux.
• DC produces constant flux, hence transformer cannot operate on DC.
• Transformers are used only for AC voltage transformation.

Q140. The primary of a transformer is connected to a 6 V battery. The turns ratio is 1:3 and the secondary load resistance is 100 Ω. The voltage across the load is:

1. 0 V
2. 6 V
3. 18 V
4. 2 V

✅ Correct Answer: a) 0 V

• Transformer requires AC supply to induce EMF.
• Battery supplies DC voltage.
• DC does not produce changing magnetic flux.
• Hence no EMF is induced in the secondary winding.
• Voltage across the load becomes zero.
• Applying DC to transformer can cause overheating.

Q141. To step up 120 V AC to 900 V AC, the required turns ratio is:

1. 75
2. 750
3. 7.5
4. 0.13

✅ Correct Answer: c) 7.5

• Voltage ratio equals turns ratio.
• Turns ratio = V_s / V_p
• = 900 / 120 = 7.5
• Since turns ratio > 1, transformer is a step-up transformer.
• Step-up transformers are used in power transmission.

Q142. A 200 Ω load is connected to the secondary of a transformer having a turns ratio of 4. The reflected load seen by the source is:

1. 50 Ω
2. 12.5 Ω
3. 800 Ω
4. 0 Ω

✅ Correct Answer: c) 800 Ω

• Reflected resistance formula:
• R_ref = (N_p/N_s)² × R_L
• Given turns ratio = 4, R_L = 200 Ω
• R_ref = 4² × 200 = 16 × 200 = 800 Ω
• Transformer changes impedance by square of turns ratio.

📘 Important Notes – Transformers (Quick Revision)

• Transformer works only on AC supply.
• Voltage ratio equals turns ratio.
• Current ratio is inverse of voltage ratio.
• Power remains constant in ideal transformer.
• DC supply produces zero secondary voltage.
• Reflected resistance formula:
  R_ref = (N_p/N_s)² × R_L
• Transformers are used for voltage control, impedance matching, and isolation.

Q143. In a transformer, the input power supplied to the primary is 120 W. If 8.5 W is lost due to winding resistance, the output power delivered to the load (neglecting other losses) is:

1. 0 W
2. 14.1 W
3. 111.5 W
4. 1020 W

✅ Correct Answer: c) 111.5 W

• Input power to transformer = 120 W
• Copper (winding) loss = 8.5 W
• Output power = Input power − Losses
• Output power = 120 − 8.5 = 111.5 W
• In practical transformers, losses reduce output power.
• Ideal transformers assume zero losses, but real transformers always have copper losses.

Q144. When a 6 V battery is connected across the primary of a transformer having a turns ratio of 8, the secondary voltage will be:

1. 0 V
2. 6 V
3. 48 V
4. 0.75 V

✅ Correct Answer: a) 0 V

• A transformer operates on the principle of mutual induction.
• Mutual induction requires alternating magnetic flux.
• A battery supplies DC voltage, producing constant flux.
• Constant flux cannot induce EMF in the secondary winding.
• Hence, secondary voltage becomes zero.
• Applying DC to a transformer may cause overheating and damage.

Q145. In a transformer, 4% of the magnetic flux produced in the primary does not link with the secondary. The coefficient of coupling (k) is:

1. 0.4
2. 4
3. 9.6
4. 0.96

✅ Correct Answer: d) 0.96

• Coefficient of coupling (k) represents the fraction of flux linking both windings.
• If 4% flux does not link secondary, then 96% does link.
• k = Flux linking secondary / Total flux
• k = 1 − 0.04 = 0.96
• In practical transformers, k is always less than 1.
• High-quality transformers have k close to unity.

Q146. A transformer has a turns ratio of 1:6 and the secondary load resistance is 470 Ω. The load resistance as seen from the source side is:

1. 1.3 Ω
2. 7.8 Ω
3. 78 Ω
4. 13 Ω

✅ Correct Answer: d) 13 Ω

• Reflected resistance formula:
• R_ref = (N_p/N_s)² × R_L
• Turns ratio = 1:6 ⇒ N_p/N_s = 1/6
• R_ref = (1/6)² × 470 ≈ 13 Ω
• Transformer reduces or increases resistance by square of turns ratio.
• This principle is widely used in impedance matching.

📘 Important Notes – Transformers (Final Revision)

• Transformer works only on AC supply, not DC.
• Output power = Input power − Losses.
• Copper loss occurs due to winding resistance.
• Coefficient of coupling (k) is always less than 1 in practical transformers.
• Reflected resistance depends on square of turns ratio.
• Batteries connected to transformers produce zero secondary voltage.
• High coupling improves transformer efficiency and voltage regulation.

Q147. Which of the following statements is/are correct regarding the generation of EMF in rotating electrical machines in the armature winding? EMF is generated ____________

1. By rotating armature windings through a magnetic field
2. By rotating magnetic field with respect to the armature windings
3. By designing the magnetic circuit to have variable reluctance with rotor rotation
4. All of the mentioned

✅ Correct Answer: d) All of the mentioned

• EMF generation follows Faraday’s law of electromagnetic induction.
• Relative motion between conductor and magnetic field induces EMF.
• Rotating magnetic fields induce EMF in stationary armature windings.
• Variable reluctance also causes flux variation and EMF generation.

Q148. A polyphase induction motor of slip-ring (wound rotor) type can be used ____________

1. For high start-up torque applications
2. As a frequency converter
3. Any of the mentioned
4. None of the mentioned

✅ Correct Answer: c) Any of the mentioned

• Slip-ring motors allow external resistance in rotor circuit.
• This provides high starting torque with low starting current.
• With suitable control, they can also operate as frequency converters.

Q149. In an alternator, frequency per revolution is equal to

1. Number of poles
2. Twice the number of poles
3. Speed in revolutions per second
4. Number of pole-pairs

✅ Correct Answer: d) Number of pole-pairs

• One electrical cycle is completed per pole pair.
• Frequency depends on number of pole-pairs and speed.
• f = (P × N) / 120.

Q150. An induction motor can be said analogous to a

1. Transformer
2. Synchronous motor
3. Universal motor
4. Stepper motor

✅ Correct Answer: a) Transformer

• Stator behaves like primary winding.
• Rotor behaves like secondary winding.
• Power transfer takes place through electromagnetic induction.

Q151. Mechanically air gaps in induction motors are kept very small to avoid

1. Lower power factor
2. Lagging nature of current
3. High magnetizing current
4. All of the mentioned

✅ Correct Answer: d) All of the mentioned

• Larger air-gap increases magnetic reluctance.
• Causes high magnetizing current.
• Results in poor and lagging power factor.

Q152. The rotor of a three-phase induction motor can never attain synchronous speed.

1. True
2. False

✅ Correct Answer: a) True

• Slip is necessary to induce rotor current.
• At synchronous speed, slip becomes zero.
• Zero slip means zero torque production.

Q153. In an induction motor, the stator is also known as _________ and the rotor as __________

1. Field winding, armature winding
2. Armature winding, field winding
3. Armature winding, compensating winding
4. Armature winding, interpole winding

✅ Correct Answer: a) Field winding, armature winding

• Stator produces magnetic flux and acts as field system.
• Rotor carries induced current and acts as armature.

Q154. The losses that occur in an induction motor are

1. Stator copper loss
2. Rotor iron loss
3. Rotor copper loss
4. All of the mentioned

✅ Correct Answer: d) All of the mentioned

• Stator copper loss due to current in stator winding.
• Rotor copper loss due to induced rotor current.
• Iron losses due to alternating magnetic flux.

Q155. Induction motor can be fed from

1. Either rotor or stator
2. Stator
3. Rotor
4. Neither stator nor rotor

✅ Correct Answer: b) Stator

• Electrical supply is applied to stator winding only.
• Rotor current is produced by electromagnetic induction.

Q156. Electromagnetic torque developed by the motor is _______ compared to shaft torque.

1. Less
2. Same
3. Depends on motor design
4. More

✅ Correct Answer: d) More

• Electromagnetic torque is developed inside the motor.
• Shaft torque is reduced due to mechanical losses.
• Therefore electromagnetic torque is always greater.

📘 Important Notes – GTU Electrical Machines

• EMF is generated due to flux variation.
• Induction motor works on transformer principle.
• Rotor speed is always less than synchronous speed.
• Small air-gap improves efficiency and power factor.
• Slip is essential for torque production.
• Electromagnetic torque is greater than shaft torque.