The torque developed is zero at

 Q151. The slip at maximum torque is directly proportional to:

A) Rotor resistance
B) Rotor reactance
C) Supply voltage
D) Supply frequency
Answer: A
Explanation: $s_{max} = \frac{R_2}{X_2}$, hence proportional to rotor resistance.

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Q152. At maximum torque, the value of torque is proportional to:
A) Rotor resistance
B) Supply voltage squared
C) Supply voltage
D) Slip
Answer: B
Explanation: $T_{max} \propto V^2$.

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Q153. When rotor bars are skewed, it helps to:
A) Reduce magnetic noise
B) Reduce locking tendency
C) Improve starting torque
D) All of these
Answer: D
Explanation: Skewing rotor bars reduces cogging, noise, and improves smoothness.

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Q154. Cogging in induction motor occurs due to:
A) Magnetic locking between stator and rotor slots
B) Poor lubrication
C) High voltage
D) Bearing damage
Answer: A
Explanation: Cogging = magnetic locking between stator and rotor teeth.

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Q155. Crawling in induction motor occurs due to:
A) Harmonics in supply
B) High voltage
C) Rotor misalignment
D) Shaft load
Answer: A
Explanation: Caused by 7th harmonic producing a sub-synchronous torque.

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Q156. The slip frequency of a rotor current at 4% slip and 50 Hz supply is:
A) 2 Hz
B) 4 Hz
C) 50 Hz
D) 0.5 Hz
Answer: A
Explanation: $f_2 = s f = 0.04 \times 50 = 2 \text{ Hz}$.

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Q157. Rotor copper losses are proportional to:
A) Slip
B) Slip squared
C) Frequency
D) Voltage
Answer: A
Explanation: $P_{cu} = s \times P_{gap}$.

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Q158. The rotor speed of a 6-pole, 50 Hz induction motor with 3% slip is:
A) 950 rpm
B) 970 rpm
C) 960 rpm
D) 1000 rpm
Answer: B
Explanation: $N = 1000(1−0.03) = 970 \text{ rpm}$.

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Q159. Which of the following can run at synchronous speed?
A) Squirrel cage motor
B) Wound rotor induction motor
C) Synchronous motor
D) None
Answer: C
Explanation: Only synchronous motor runs at synchronous speed.

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Q160. The slip in an induction motor is zero when:
A) Rotor stationary
B) Rotor speed = synchronous speed
C) Rotor speed = zero
D) Supply off
Answer: B
Explanation: Slip $s = \frac{N_s - N}{N_s} = 0$ when $N = N_s$.

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Q161. Which loss in induction motor is negligible?
A) Stator copper loss
B) Rotor iron loss
C) Friction loss
D) Windage loss
Answer: B
Explanation: Rotor frequency is low → negligible rotor iron loss.

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Q162. The direction of rotation of a three-phase induction motor depends on:
A) Frequency
B) Voltage
C) Phase sequence
D) Load
Answer: C
Explanation: Interchanging two phases reverses the rotating field direction.

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Q163. Induction motor rotor speed is always:
A) Equal to synchronous speed
B) Less than synchronous speed
C) Greater than synchronous speed
D) Independent of synchronous speed
Answer: B
Explanation: Always less than synchronous speed in motoring mode.

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Q164. The speed-torque characteristic of an induction motor resembles that of:
A) DC shunt motor
B) DC series motor
C) Universal motor
D) Stepper motor
Answer: A
Explanation: Nearly constant speed for varying load → like DC shunt motor.

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Q165. If rotor copper loss is 200 W and slip is 4%, rotor input power is:
A) 5 kW
B) 4 kW
C) 2 kW
D) 3 kW
Answer: A
Explanation: $s = 0.04 \Rightarrow 0.04 P_{in} = 200 \Rightarrow P_{in} = 5000 \text{ W}$.

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Q166. For a given load, torque developed in an induction motor is proportional to:
A) Slip
B) 1/slip
C) Slip²
D) Voltage
Answer: A
Explanation: At low slip, torque ∝ slip.

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Q167. The torque produced in an induction motor is due to:
A) Mutual flux between stator and rotor
B) Air-gap flux
C) Interaction of stator and rotor fields
D) Eddy currents
Answer: C
Explanation: Torque results from interaction between stator and rotor fields.

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Q168. Rotor bars of a squirrel cage induction motor are usually made of:
A) Aluminum or copper
B) Iron
C) Brass
D) Steel
Answer: A
Explanation: For good conductivity and castability.

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Q169. The number of slip rings in a three-phase wound rotor induction motor is:
A) 2
B) 3
C) 4
D) 6
Answer: B
Explanation: One for each phase of rotor winding → 3 rings.

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Q170. The torque-speed curve of an induction motor shifts right when:
A) Rotor resistance increases
B) Supply voltage increases
C) Frequency decreases
D) Load decreases
Answer: A
Explanation: Higher rotor resistance → torque peak shifts to higher slip.

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Q171. Rotor current frequency at standstill is:
A) Zero
B) 25 Hz
C) Equal to stator frequency
D) Depends on slip
Answer: C
Explanation: At standstill, slip = 1 → rotor frequency = stator frequency.

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Q172. Slip increases when:
A) Load increases
B) Load decreases
C) Voltage increases
D) Frequency increases
Answer: A
Explanation: More load → more torque → increased slip.

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Q173. The frequency of rotor emf when running at 96% of synchronous speed:
A) 1 Hz
B) 2 Hz
C) 3 Hz
D) 4 Hz
Answer: B
Explanation: Slip = 4% → $f_2 = 0.04 \times 50 = 2 \text{ Hz}$.

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Q174. If slip = 0.04 and Ns = 1500 rpm, then rotor speed = ?
A) 1450 rpm
B) 1470 rpm
C) 1425 rpm
D) 1480 rpm
Answer: B
Explanation: $N = 1500(1−0.04) = 1440 \text{ rpm}$.

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Q175. When slip = 1, motor torque is:
A) Zero
B) Maximum
C) Full load
D) Depends on resistance
Answer: B
Explanation: At starting (s=1), torque = starting torque, generally high.

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Q176. Stator current increases when:
A) Load increases
B) Slip decreases
C) Torque decreases
D) Speed increases
Answer: A
Explanation: More load requires more torque → higher current.

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Q177. Induction motor torque equation contains which term in denominator?
A) $R_2^2 + (sX_2)^2$
B) $R_2 + sX_2$
C) $sR_2 + X_2^2$
D) $s^2X_2^2$
Answer: A
Explanation: $T \propto \frac{sE_2^2R_2}{R_2^2 + (sX_2)^2}$.

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Q178. The magnetizing current of an induction motor is about:
A) 5%
B) 20–40%
C) 50%
D) 90% of full-load current
Answer: B
Explanation: Large magnetizing current needed for air-gap flux.

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Q179. The magnetizing component of no-load current is:
A) In phase with voltage
B) Lagging voltage by 90°
C) Leading voltage
D) Opposite to voltage
Answer: B
Explanation: Purely inductive → lags by 90°.

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Q180. The working principle of an induction motor is similar to:
A) DC motor
B) Transformer
C) Alternator
D) Stepper motor
Answer: B
Explanation: Based on electromagnetic induction like transformer.

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Q181. Rotor slots are skewed by:
A) 5°
B) 10°–15°
C) 15°–25°
D) 90°
Answer: C
Explanation: Typically one slot pitch skew reduces noise and cogging.

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Q182. The direction of rotation can be changed by:
A) Reversing any two supply leads
B) Reversing one lead
C) Reversing all three leads
D) Changing rotor connection
Answer: A
Explanation: Interchanging any two phases reverses rotating field.

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Q183. The power factor at no-load is:
A) High
B) Unity
C) Very low
D) Leading
Answer: C
Explanation: Magnetizing current dominates → very low PF (~0.2 lag).

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Q184. Torque developed in induction motor is proportional to:
A) Rotor current
B) Rotor emf × Rotor current × sin(φ)
C) Slip only
D) Supply frequency
Answer: B
Explanation: Torque ∝ product of rotor emf, current, and power factor.

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Q185. The slip of a 4-pole, 50 Hz motor running at 1470 rpm is:
A) 1%
B) 2%
C) 3%
D) 4%
Answer: C
Explanation: $s = (1500−1470)/1500 = 0.02 = 2\%$.

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Q186. At zero slip, rotor current frequency is:
A) Zero
B) 25 Hz
C) 50 Hz
D) Infinite
Answer: A
Explanation: $f_2 = s f$, at s=0 → 0 Hz.

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Q187. A 6-pole, 50 Hz motor runs at 960 rpm. Slip = ?
A) 2%
B) 4%
C) 6%
D) 8%
Answer: C
Explanation: Ns = 1000 rpm → s = (1000−960)/1000 = 0.04 = 4%.

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Q188. If rotor resistance is doubled, starting torque:
A) Increases
B) Decreases
C) Remains same
D) Becomes zero
Answer: A
Explanation: $T_{start} \propto R_2 / (R_2^2 + X_2^2)$, initially increases.

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Q189. The torque under running condition is maximum when:
A) Rotor reactance = rotor resistance/slip
B) Rotor resistance = s × reactance
C) R₂/s = X₂
D) R₂ = sX₂
Answer: C
Explanation: Condition for maximum torque.

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Q190. The efficiency of an induction motor is maximum when:
A) Rotor copper loss = Stator copper loss
B) Slip = 0
C) Input = Output
D) Load = Maximum
Answer: A
Explanation: Maximum efficiency when variable losses = constant losses.

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Q191. The starting torque of a squirrel cage induction motor is:
A) High
B) Low
C) Zero
D) Infinite
Answer: B
Explanation: Rotor resistance low → low starting torque.

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Q192. The rotor emf is induced due to:
A) Relative motion between stator field and rotor
B) Stator voltage
C) Rotor current
D) Supply current
Answer: A
Explanation: Relative motion induces emf in rotor conductors.

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Q193. For constant torque load, slip is:
A) Constant
B) Proportional to torque
C) Proportional to voltage
D) Zero
Answer: B
Explanation: Torque ∝ slip for small slips → linear relationship.

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Q194. A wound rotor motor allows:
A) Speed control
B) High starting torque
C) External resistance insertion
D) All of these
Answer: D
Explanation: Rotor resistance control gives high torque and adjustable speed.

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Q195. Which test is used to determine magnetizing current?
A) No-load test
B) Blocked-rotor test
C) Load test
D) Speed test
Answer: A
Explanation: No-load test provides magnetizing branch parameters.

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Q196. Which test is used to determine short-circuit current?
A) No-load test
B) Blocked-rotor test
C) Load test
D) Open-circuit test
Answer: B
Explanation: Rotor locked → measures equivalent impedance.

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Q197. If air-gap increases, magnetizing current:
A) Decreases
B) Increases
C) Constant
D) Zero
Answer: B
Explanation: Larger air-gap requires more MMF → higher magnetizing current.

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Q198. In an induction motor, stator copper loss varies as:
A) Load
B) Load²
C) Voltage²
D) Constant
Answer: B
Explanation: $P_{cu} \propto I^2 \propto \text{Load}^2$.

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Q199. The torque developed is zero at:
A) No-load
B) Slip = 0
C) Slip = 1
D) Full-load
Answer: B
Explanation: At synchronous speed (s = 0), no induced emf → no torque.

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Q200. The electromagnetic torque in an induction motor is produced by:
A) Interaction of stator and rotor fields
B) Rotor currents only
C) Stator current only
D) Mechanical rotation
Answer: A
Explanation: Interaction of rotating magnetic fields produces torque.