Q101. The starting current of a three-phase induction motor is:
A) Equal to full-load current
B) 2 times full-load current
C) 5–7 times full-load current
D) 10–12 times full-load current
Answer: C
Explanation: At starting, rotor impedance is low and slip = 1, so current is high (5–7 times).
Q102. The power factor at starting is:
A) High
B) Low
C) Unity
D) Zero
Answer: B
Explanation: Due to high inductive reactance, the current lags voltage considerably.
Q103. In an induction motor, the rotor copper loss at full load is:
A) s × air-gap power
B) (1−s) × air-gap power
C) Equal to stator copper loss
D) Equal to mechanical loss
Answer: A
Explanation: Rotor copper loss = s × air-gap power.
Q104. The efficiency of an induction motor increases when:
A) Slip decreases
B) Slip increases
C) Load decreases
D) Rotor resistance increases
Answer: A
Explanation: Lower slip means smaller rotor losses and higher efficiency.
Q105. The slip of an induction motor running at no-load is approximately:
A) 10%
B) 1%
C) 0.1%
D) 5%
Answer: C
Explanation: Very small load → speed nearly synchronous → slip ≈ 0.1%.
Q106. The speed of a 4-pole, 60 Hz induction motor is 1750 rpm. Slip = ?
A) 0.02
B) 0.04
C) 0.05
D) 0.06
Answer: C
Explanation: Ns = 1800 rpm → s = (1800−1750)/1800 = 0.027 ≈ 3%.
Q107. The maximum torque of an induction motor is independent of:
A) Supply voltage
B) Rotor resistance
C) Rotor reactance
D) Supply frequency
Answer: B
Explanation: Torque ∝ V² / (2X₂) and is independent of rotor resistance.
Q108. If the supply frequency is halved, synchronous speed:
A) Halves
B) Doubles
C) Remains same
D) Becomes zero
Answer: A
Explanation: Ns = 120f/P → proportional to frequency.
Q109. Induction motor acts as an induction generator when:
A) Slip is positive
B) Slip is zero
C) Slip is negative
D) Slip is one
Answer: C
Explanation: When rotor speed > Ns, slip becomes negative → generator action.
Q110. The magnetic field in an induction motor rotates at:
A) Rotor speed
B) Stator speed
C) Synchronous speed
D) Slip speed
Answer: C
Explanation: Rotating magnetic field always rotates at synchronous speed.
Q111. In an induction motor, speed decreases as:
A) Load increases
B) Load decreases
C) Slip decreases
D) Rotor resistance decreases
Answer: A
Explanation: Load increase → more torque required → increased slip → lower speed.
Q112. The supply current of an induction motor at no-load is about:
A) 20–30% of full-load current
B) 40–50% of full-load current
C) 60–70% of full-load current
D) 90–100% of full-load current
Answer: A
Explanation: No-load current consists mainly of magnetizing component (20–30%).
Q113. The efficiency of a small induction motor (few HP) is about:
A) 50%
B) 60–70%
C) 80–85%
D) 90–95%
Answer: B
Explanation: Smaller machines have higher losses → lower efficiency.
Q114. The torque developed by an induction motor is zero when:
A) Slip = 0
B) Slip = 1
C) Voltage = 0
D) Load = 0
Answer: A
Explanation: At synchronous speed (s = 0), no induced emf → no torque.
Q115. The motor speed regulation is given by:
A) (No-load speed − Full-load speed)/Full-load speed
B) Full-load speed/No-load speed
C) (Slip × Ns)/N
D) (Ns − N)/Ns
Answer: A
Explanation: Regulation = % change in speed between no-load and full-load.
Q116. The air-gap power is equal to:
A) Stator input − Stator losses
B) Rotor output
C) Rotor losses
D) Mechanical losses
Answer: A
Explanation: Power transferred from stator to rotor = Stator input − stator Cu & core losses.
Q117. The ratio of rotor copper loss to rotor input power = ?
A) s
B) (1−s)
C) (1/s)
D) s/(1−s)
Answer: A
Explanation: .
Q118. The ratio of mechanical power to rotor input = ?
A) s
B) (1−s)
C) (1/s)
D) s/(1−s)
Answer: B
Explanation: .
Q119. The torque-speed curve of an induction motor is similar to that of:
A) DC shunt motor
B) DC series motor
C) DC compound motor
D) Stepper motor
Answer: A
Explanation: Nearly constant speed under varying load.
Q120. In an induction motor, torque is maximum when:
A) Slip = R₂/X₂
B) Slip = 0
C) Slip = 1
D) Slip = 2
Answer: A
Explanation: Condition for maximum torque.
Q121. The core of an induction motor is laminated to:
A) Reduce copper losses
B) Reduce hysteresis and eddy current losses
C) Reduce weight
D) Increase strength
Answer: B
Explanation: Laminations minimize eddy current loss.
Q122. The material of the rotor core is usually:
A) Cast iron
B) Silicon steel
C) Aluminum
D) Brass
Answer: B
Explanation: Silicon steel minimizes core losses.
Q123. Which test gives total losses of an induction motor?
A) No-load test
B) Blocked rotor test
C) Load test
D) Hopkinson’s test
Answer: C
Explanation: Direct load test measures total input and output → total losses.
Q124. The per-phase equivalent circuit of an induction motor is similar to that of:
A) Transformer
B) Alternator
C) DC generator
D) Synchronous motor
Answer: A
Explanation: Energy transfer by electromagnetic induction, like transformer.
Q125. The maximum torque occurs at slip:
A) s = R₂/X₂
B) s = X₂/R₂
C) s = 1
D) s = 0
Answer: A
Explanation: Derived from torque equation .
Q126. When voltage is reduced to 80%, torque becomes:
A) 80%
B) 64%
C) 50%
D) 20%
Answer: B
Explanation: .
Q127. In a 3-phase induction motor, torque at starting is directly proportional to:
A) Voltage
B) Voltage²
C) Frequency
D) Current
Answer: B
Explanation: .
Q128. The rotor reactance at standstill depends on:
A) Frequency
B) Slip
C) Both
D) None
Answer: A
Explanation: , and at standstill, f = supply frequency.
Q129. The relation between torque and slip in stable region:
A) Torque ∝ 1/slip
B) Torque ∝ slip
C) Torque ∝ slip²
D) Torque ∝ 1/slip²
Answer: B
Explanation: At low slip, torque is approximately proportional to slip.
Q130. Induction motor rotor emf frequency is given by:
A) f
B) s × f
C) (1−s)f
D) f/s
Answer: B
Explanation: Rotor emf frequency = slip × supply frequency.
Q131. If supply voltage is doubled, torque will become:
A) 2 times
B) 4 times
C) 8 times
D) ½ times
Answer: B
Explanation: .
Q132. The stator copper loss varies with:
A) Supply voltage
B) Load current²
C) Slip
D) Torque
Answer: B
Explanation: .
Q133. The main reason of humming noise in induction motor is:
A) Mechanical fault
B) Magnetic pulsations
C) Bearing friction
D) Loose windings
Answer: B
Explanation: Magnetic flux pulsations produce audible hum.
Q134. In an induction motor, torque under running conditions varies as:
A) s/(R₂² + s²X₂²)
B) s/(R₂ + sX₂)
C) s²/(R₂² + s²X₂²)
D) R₂/s
Answer: A
Explanation: Derived from torque equation.
Q135. The magnetic field produced in stator rotates because of:
A) Single-phase supply
B) Three-phase currents
C) DC supply
D) Alternating flux
Answer: B
Explanation: Three-phase currents are displaced by 120° creating rotating field.
Q136. The speed control by voltage variation causes:
A) High losses
B) Poor torque regulation
C) High efficiency
D) Both A and B
Answer: D
Explanation: Voltage variation reduces torque capability and efficiency.
Q137. The term “pull-out torque” refers to:
A) Maximum torque motor can develop
B) Minimum torque
C) Starting torque
D) Rated torque
Answer: A
Explanation: Maximum torque before stalling = pull-out torque.
Q138. Induction motor will run in reverse direction if:
A) Supply reversed
B) Any two phases are interchanged
C) Frequency doubled
D) Voltage reduced
Answer: B
Explanation: Interchanging two supply leads reverses rotating field.
Q139. Slip power is the power:
A) Transferred to rotor
B) Lost in rotor copper
C) Converted to mechanical
D) Returned to supply
Answer: A
Explanation: Power transferred across air gap = slip power.
Q140. The braking of an induction motor can be done by:
A) Plugging
B) Regenerative braking
C) Dynamic braking
D) All of these
Answer: D
Explanation: All three methods are used for braking.
Q141. Plugging in induction motor means:
A) Reversing supply phase
B) Applying mechanical brake
C) Reducing frequency
D) Using DC injection
Answer: A
Explanation: Two-phase reversal → opposite torque → rapid braking.
Q142. In dynamic braking, the motor runs as:
A) Generator feeding power to supply
B) Generator dissipating energy in resistance
C) Motor with reduced speed
D) None
Answer: B
Explanation: Kinetic energy dissipated in external resistor.
Q143. Induction motor power factor improves with:
A) Decrease in speed
B) Increase in load
C) Increase in voltage
D) Increase in slip
Answer: B
Explanation: Load current increases, reducing magnetizing current dominance.
Q144. The core loss in the rotor is negligible because:
A) Frequency of rotor current is low
B) Rotor is laminated
C) Rotor resistance is high
D) Magnetic flux is weak
Answer: A
Explanation: Rotor frequency = s×f → small → negligible core loss.
Q145. The equivalent resistance of the rotor circuit referred to stator is:
A) R₂
B) R₂/s
C) sR₂
D) R₂(1−s)
Answer: B
Explanation: Referred resistance = R₂/s in per-phase equivalent circuit.
Q146. The per-phase rotor current referred to stator is given by:
A) I₂′ = E₂ / (R₂/s + jX₂)
B) I₂′ = E₂ / (R₂ + jX₂)
C) I₂′ = sE₂ / (R₂ + jX₂)
D) I₂′ = E₂ / (R₂ + sX₂)
Answer: A
Explanation: Rotor current referred to stator includes term R₂/s.
Q147. The mechanical losses in induction motor are independent of:
A) Speed
B) Load
C) Voltage
D) Current
Answer: B
Explanation: Friction and windage losses remain almost constant irrespective of load.
Q148. Induction motor’s torque under steady load increases with:
A) Decrease in slip
B) Increase in slip
C) Decrease in supply
D) Increase in voltage
Answer: B
Explanation: Higher slip → higher torque (in linear region).
Q149. The air-gap length of an induction motor is kept:
A) Small
B) Large
C) Moderate
D) Variable
Answer: A
Explanation: Smaller air gap reduces magnetizing current and improves power factor.
Q150. The maximum torque of an induction motor depends upon:
A) Supply voltage and rotor reactance
B) Rotor resistance only
C) Slip only
D) None
Answer: A
Explanation: .
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