When an induction motor runs at 1000 rpm with Ns = 1200 rpm, slip is

 Q51. When an induction motor is operating as a generator, the slip is:

A) 0
B) Positive
C) Negative
D) One
Answer: C
Explanation: When rotor speed > synchronous speed, slip becomes negative.

---

Q52. The starting torque of a wound rotor motor can be controlled by:
A) Supply voltage
B) Supply frequency
C) External rotor resistance
D) Stator reactance
Answer: C
Explanation: Inserting resistance in the rotor circuit increases starting torque.

---

Q53. The blocked rotor test of an induction motor is similar to:
A) Open-circuit test of transformer
B) Short-circuit test of transformer
C) Hopkinson’s test
D) Load test
Answer: B
Explanation: Blocked rotor test corresponds to transformer short-circuit test.

---

Q54. The core losses in an induction motor occur mainly in:
A) Stator
B) Rotor
C) Shaft
D) Bearings
Answer: A
Explanation: Alternating magnetic flux in stator causes hysteresis and eddy current losses.

---

Q55. In a double cage motor, the inner cage has:
A) High resistance and low reactance
B) Low resistance and high reactance
C) High resistance and high reactance
D) Low resistance and low reactance
Answer: B
Explanation: Inner cage provides good efficiency during normal running.

---

Q56. The most efficient speed control method is:
A) Voltage control
B) Frequency control
C) Rotor resistance control
D) Pole changing
Answer: B
Explanation: V/f control gives smooth and efficient speed variation.

---

Q57. The relationship between mechanical power developed and electromagnetic torque is:
A) $P = 2\pi N T$
B) $P = T/N$
C) $P = T \times N / 2\pi$
D) $P = V \times I$
Answer: A
Explanation: $P = 2\pi N T / 60$ (in watts if N in rpm).

---

Q58. The rotor current is maximum when:
A) Slip = 0
B) Slip = 1
C) Rotor resistance = 0
D) Speed = synchronous speed
Answer: B
Explanation: At standstill, slip = 1 → maximum induced emf and current.

---

Q59. The direction of torque in an induction motor is determined by:
A) Supply polarity
B) Lenz’s law
C) Direction of rotating magnetic field
D) Supply frequency
Answer: C
Explanation: Torque follows the direction of rotating magnetic field.

---

Q60. The main difference between slip ring and squirrel cage induction motor is:
A) Type of stator winding
B) Type of rotor construction
C) Type of bearings
D) Power factor
Answer: B
Explanation: Slip ring has wound rotor; squirrel cage has short-circuited bars.

---

Q61. The synchronous speed depends on:
A) Supply voltage
B) Supply frequency and number of poles
C) Supply current
D) Rotor resistance
Answer: B
Explanation: $N_s = 120f/P$.

---

Q62. The slip of an induction motor at full load is usually about:
A) 10%
B) 5%
C) 20%
D) 50%
Answer: B
Explanation: Typical slip range is 2–5% at full load.

---

Q63. A 6-pole, 50 Hz motor runs at 960 rpm. Slip = ?
A) 0.04
B) 0.05
C) 0.08
D) 0.1
Answer: B
Explanation: Ns = 1000 rpm; slip = (1000−960)/1000 = 0.04 (≈4%).

---

Q64. The maximum torque of an induction motor is:
A) Proportional to supply voltage
B) Proportional to square of supply voltage
C) Inversely proportional to voltage
D) Independent of voltage
Answer: B
Explanation: $T_{max} \propto V^2$.

---

Q65. The rotor bars of a squirrel cage motor are short-circuited by:
A) End rings
B) Slip rings
C) Brushes
D) Commutator
Answer: A
Explanation: End rings connect rotor bars to form a closed circuit.

---

Q66. The mechanical losses in an induction motor include:
A) Friction and windage losses
B) Copper losses
C) Core losses
D) Stray load losses
Answer: A
Explanation: Due to air friction and bearing friction.

---

Q67. If stator supply voltage is reduced to 50%, torque becomes:
A) 25%
B) 50%
C) 75%
D) 100%
Answer: A
Explanation: $T \propto V^2 \Rightarrow T = (0.5)^2 = 0.25 = 25$.

---

Q68. The air-gap flux in an induction motor depends upon:
A) Supply current
B) Supply voltage and frequency
C) Slip
D) Load
Answer: B
Explanation: Flux ∝ V/f.

---

Q69. The magnetizing current is drawn mainly to:
A) Overcome resistance
B) Produce rotating magnetic field
C) Generate voltage
D) Reduce slip
Answer: B
Explanation: It produces flux in the air gap.

---

Q70. The rotor iron losses are negligible because:
A) Flux is constant
B) Rotor frequency is low
C) Rotor current is small
D) Voltage is low
Answer: B
Explanation: Rotor frequency = s × f → very low at normal slip.

---

Q71. The torque developed at synchronous speed is:
A) Maximum
B) Minimum
C) Zero
D) Constant
Answer: C
Explanation: No relative motion → no induced current → zero torque.

---

Q72. The torque-slip characteristic is approximately linear:
A) Near standstill
B) Near synchronous speed
C) At maximum torque
D) None
Answer: B
Explanation: For small slip values, torque ∝ slip.

---

Q73. The line current of a star-connected induction motor is:
A) Equal to phase current
B) √3 × phase current
C) 1/√3 × phase current
D) 2 × phase current
Answer: A
Explanation: In star, line current = phase current.

---

Q74. The torque per unit rotor current is maximum when:
A) Slip = 1
B) sX₂ = R₂
C) Slip = 0
D) R₂ = 0
Answer: B
Explanation: Condition for maximum torque.

---

Q75. For a constant load, if rotor resistance increases, the speed:
A) Increases
B) Decreases
C) Remains same
D) Becomes zero
Answer: B
Explanation: More resistance → more slip → less speed.

---

Q76. Induction motors are not self-starting in:
A) Single-phase motors
B) Three-phase motors
C) Both
D) None
Answer: A
Explanation: Single-phase motor lacks starting torque.

---

Q77. The torque in a 3-phase induction motor is produced by:
A) Interaction of stator and rotor fields
B) Rotor current only
C) Stator current only
D) Mechanical coupling
Answer: A
Explanation: Due to interaction between two rotating fields.

---

Q78. Which of the following methods gives smooth speed control?
A) Rotor resistance control
B) V/f control
C) Pole changing
D) Voltage control
Answer: B
Explanation: Variable frequency drive (VFD) gives smooth and efficient control.

---

Q79. Slip is directly proportional to:
A) Load torque
B) Voltage
C) Current
D) Frequency
Answer: A
Explanation: For small slip region, $T \propto s$, and torque ∝ load.

---

Q80. The power transferred from stator to rotor across the air gap is called:
A) Air-gap power
B) Mechanical power
C) Shaft power
D) Electrical power
Answer: A
Explanation: It is the electromagnetic power transferred to rotor circuit.

---

Q81. If rotor resistance is doubled, maximum torque:
A) Doubles
B) Halves
C) Remains constant
D) Becomes zero
Answer: C
Explanation: Maximum torque is independent of rotor resistance.

---

Q82. The main advantage of rotor resistance control is:
A) High efficiency
B) Wide speed control
C) Low loss
D) Simple circuit
Answer: B
Explanation: Adding resistance provides wide range of speed variation.

---

Q83. A 4-pole, 50 Hz motor runs at 1470 rpm. The slip is:
A) 1%
B) 2%
C) 3%
D) 5%
Answer: C
Explanation: Ns = 1500 rpm; slip = (1500−1470)/1500 = 0.02 (≈2%).

---

Q84. When rotor runs at synchronous speed, rotor current frequency is:
A) Zero
B) 50 Hz
C) 25 Hz
D) 100 Hz
Answer: A
Explanation: Slip = 0 → $f_r = s f = 0$.

---

Q85. The stator of a 3-phase induction motor is similar to:
A) DC armature winding
B) Transformer primary
C) DC field winding
D) Stepper motor
Answer: B
Explanation: Stator winding receives AC supply and induces emf like a transformer.

---

Q86. Torque developed in an induction motor is proportional to:
A) Rotor current only
B) Air-gap flux and rotor current
C) Stator current only
D) Supply voltage
Answer: B
Explanation: $T \propto \phi I_2 \sin \theta$.

---

Q87. The main function of the rotor circuit resistance in a wound-rotor motor is:
A) To improve starting torque
B) To reduce efficiency
C) To increase speed
D) To reduce slip
Answer: A
Explanation: External resistance increases torque at start.

---

Q88. The slip for maximum torque is:
A) Inversely proportional to rotor reactance
B) Directly proportional to rotor resistance
C) Directly proportional to voltage
D) Independent of rotor parameters
Answer: B
Explanation: $s_{max} = R_2 / X_2$.

---

Q89. The power factor of an induction motor improves with:
A) Decrease in load
B) Increase in load
C) Decrease in voltage
D) Increase in frequency
Answer: B
Explanation: More load → higher active current → improved power factor.

---

Q90. Slip ring induction motor is preferred where:
A) Constant speed required
B) High starting torque needed
C) High efficiency required
D) Small motors are used
Answer: B
Explanation: External resistance gives high starting torque.

---

Q91. At synchronous speed, the rotor EMF is:
A) Maximum
B) Minimum
C) Zero
D) Constant
Answer: C
Explanation: No relative motion → no induced emf.

---

Q92. Induction motor rotor never runs at synchronous speed because:
A) It will stop
B) Slip becomes zero → no torque
C) Speed decreases
D) Voltage reduces
Answer: B
Explanation: Torque depends on slip; zero slip means no torque.

---

Q93. When load on motor increases, the slip:
A) Increases
B) Decreases
C) Remains constant
D) Becomes zero
Answer: A
Explanation: More load → higher torque → more slip.

---

Q94. The equivalent circuit of an induction motor resembles that of a:
A) Transformer
B) DC motor
C) Alternator
D) Stepper motor
Answer: A
Explanation: Same energy transfer principle through magnetic coupling.

---

Q95. In slip ring induction motor, brushes are made of:
A) Copper
B) Carbon
C) Iron
D) Aluminum
Answer: B
Explanation: Carbon brushes provide good contact and low wear.

---

Q96. The rotor leakage reactance depends on:
A) Rotor speed
B) Rotor frequency
C) Supply voltage
D) Both A and B
Answer: D
Explanation: Reactance ∝ frequency and depends on speed (via slip).

---

Q97. Torque at standstill is called:
A) Pull-out torque
B) Starting torque
C) Breakdown torque
D) Rated torque
Answer: B
Explanation: Torque when rotor speed = 0 is starting torque.

---

Q98. In a 3-phase induction motor, rotor current is produced by:
A) Direct connection
B) Transformer action
C) Commutation
D) Brush excitation
Answer: B
Explanation: Induced emf in rotor due to rotating magnetic field.

---

Q99. For maximum torque, supply voltage should be:
A) Constant
B) Variable
C) Reduced
D) Increased
Answer: A
Explanation: Rated voltage ensures designed torque condition.

---

Q100. When an induction motor runs at 1000 rpm with Ns = 1200 rpm, slip is:
A) 0.05
B) 0.083
C) 0.1
D) 0.167
Answer: B
Explanation: $s = (1200−1000)/1200 = 0.1666 = 16.7\%$.