Q351. The torque developed by an induction motor is proportional to
A) Rotor resistance
B) Rotor reactance
C) Slip
D) Both rotor resistance and slip
Ans: D
Explanation: Torque ∝ (sR₂) / [(R₂)² + (sX₂)²]; thus both resistance and slip affect torque.
Q352. At starting, the slip of an induction motor is
A) 0
B) 0.5
C) 1
D) Infinite
Ans: C
Explanation: At starting, rotor speed is zero, hence slip = (Ns – 0)/Ns = 1.
Q353. In an induction motor, maximum torque occurs when
A) Slip = 1
B) sX₂ = R₂
C) sX₂ = 2R₂
D) sX₂ = 0
Ans: B
Explanation: The condition for maximum torque is R₂ = sX₂.
Q354. The frequency of rotor current at standstill equals
A) Supply frequency
B) Half supply frequency
C) Zero
D) Twice supply frequency
Ans: A
Explanation: At standstill, slip = 1, so rotor frequency = s × f = f.
Q355. In an induction motor, if rotor resistance increases, maximum torque
A) Decreases
B) Increases
C) Remains same
D) Becomes zero
Ans: C
Explanation: Maximum torque is independent of rotor resistance; only slip at which it occurs changes.
Q356. The power factor of induction motor at no load is
A) High
B) Unity
C) Low
D) Zero
Ans: C
Explanation: At no load, magnetizing current dominates, resulting in low power factor.
Q357. The direction of rotation of a three-phase induction motor can be reversed by interchanging
A) Any two supply terminals
B) Rotor terminals
C) Supply frequency
D) Supply voltage
Ans: A
Explanation: Reversing two phases changes phase sequence, reversing rotation.
Q358. The main flux in an induction motor is produced by
A) Rotor current
B) Stator current
C) Rotor resistance
D) Both stator and rotor
Ans: B
Explanation: Stator current creates the rotating magnetic field.
Q359. The speed of an induction motor is
A) Greater than synchronous speed
B) Equal to synchronous speed
C) Less than synchronous speed
D) Zero
Ans: C
Explanation: Slip always exists; rotor speed < synchronous speed.
Q360. The torque-slip curve of induction motor is
A) Linear throughout
B) Non-linear
C) Hyperbolic
D) Parabolic
Ans: B
Explanation: The torque-slip relation is non-linear, nearly linear for small slips.
Q361. The starting current of squirrel cage induction motor is about
A) 1–2 times full load current
B) 3–4 times full load current
C) 5–7 times full load current
D) 10–12 times full load current
Ans: C
Explanation: High inrush current due to low rotor impedance at start.
Q362. In slip ring induction motor, starting torque can be increased by
A) Decreasing rotor resistance
B) Increasing rotor resistance
C) Increasing supply voltage
D) Reducing stator frequency
Ans: B
Explanation: Adding resistance in rotor circuit increases starting torque.
Q363. Crawling occurs in induction motor due to
A) Harmonics in the supply
B) Rotor resistance
C) Unbalanced supply voltage
D) Overload
Ans: A
Explanation: 7th harmonic flux causes motor to crawl at about 1/7th of synchronous speed.
Q364. Cogging occurs when
A) Rotor teeth lock with stator teeth
B) Rotor resistance is high
C) Slip is small
D) Load is high
Ans: A
Explanation: Magnetic locking between rotor and stator teeth causes cogging (magnetic locking).
Q365. Slip of induction motor under normal running is
A) 0
B) 1
C) Between 0 and 1
D) More than 1
Ans: C
Explanation: Slip lies between 0 and 1 under running condition.
Q366. The efficiency of induction motor increases with
A) Load
B) Slip
C) Rotor resistance
D) Supply frequency
Ans: A
Explanation: Efficiency improves as load increases up to rated load.
Q367. The equivalent circuit of an induction motor is similar to that of
A) Transformer
B) DC shunt motor
C) Synchronous motor
D) DC series motor
Ans: A
Explanation: The induction motor can be represented as a transformer with a rotating secondary.
Q368. The core loss in an induction motor depends on
A) Supply frequency and voltage
B) Load
C) Rotor speed
D) Slip
Ans: A
Explanation: Core losses depend on supply voltage and frequency.
Q369. The starting torque of squirrel cage motor is
A) Low
B) High
C) Medium
D) Zero
Ans: A
Explanation: Because rotor resistance is low and current lags behind voltage.
Q370. The ratio of starting torque to full load torque for squirrel cage motor is about
A) 0.5
B) 1
C) 1.5
D) 2
Ans: A
Explanation: Starting torque ≈ 50% of full load torque in normal design.
Q371. If supply frequency increases, synchronous speed
A) Increases
B) Decreases
C) Remains same
D) Becomes zero
Ans: A
Explanation: Ns = 120f/P, so Ns increases with frequency.
Q372. In induction motor, rotor emf frequency is
A) f
B) s × f
C) (1–s) × f
D) Zero
Ans: B
Explanation: Rotor emf frequency = s × supply frequency.
Q373. The main cause of low power factor at light load is
A) High magnetizing current
B) High rotor current
C) Low slip
D) High load current
Ans: A
Explanation: Magnetizing current is constant and dominant at light loads.
Q374. Rotor copper losses are proportional to
A) Slip
B) (1–Slip)
C) Slip²
D) Supply voltage
Ans: A
Explanation: Rotor copper loss = s × rotor input power.
Q375. The torque developed in induction motor is zero when slip is
A) Zero
B) 1
C) 0.5
D) 0.2
Ans: A
Explanation: At slip = 0, no relative motion → no induced emf → no torque.
Q376. The type of rotor used in slip ring induction motor is
A) Squirrel cage
B) Wound rotor
C) Permanent magnet rotor
D) Hollow rotor
Ans: B
Explanation: Slip ring motor uses wound rotor for adding resistance.
Q377. Slip at maximum torque is directly proportional to
A) Rotor resistance
B) Supply voltage
C) Rotor reactance
D) Frequency
Ans: A
Explanation: sāāā = R₂ / X₂.
Q378. The torque at synchronous speed is
A) Maximum
B) Zero
C) Infinite
D) Constant
Ans: B
Explanation: Slip = 0 ⇒ no induced emf ⇒ zero torque.
Q379. Induction motor behaves as a generator when
A) Slip is positive
B) Slip is negative
C) Slip is 1
D) Slip is zero
Ans: B
Explanation: Negative slip (speed > Ns) → generates power back to supply.
Q380. In an induction motor, stator losses consist of
A) Copper and iron losses
B) Copper loss only
C) Iron loss only
D) Mechanical loss only
Ans: A
Explanation: Stator losses = I²R (copper) + core (iron) losses.
Q381. The magnetizing reactance of induction motor is
A) Low
B) High
C) Constant
D) Variable
Ans: B
Explanation: High magnetizing reactance limits magnetizing current.
Q382. If the load torque increases, rotor speed
A) Increases
B) Decreases
C) Constant
D) Becomes zero
Ans: B
Explanation: To produce more torque, slip increases → speed decreases.
Q383. For same rating, slip ring induction motor is costlier because
A) Complicated construction
B) Rotor windings and slip rings
C) Higher copper usage
D) All of the above
Ans: D
Explanation: Extra rotor winding, slip rings, and maintenance increase cost.
Q384. The motor used in cranes and hoists is
A) Squirrel cage motor
B) Slip ring motor
C) Stepper motor
D) Universal motor
Ans: B
Explanation: Slip ring motor gives high starting torque and speed control.
Q385. If supply voltage is reduced to half, torque becomes
A) Half
B) One-fourth
C) Double
D) Unchanged
Ans: B
Explanation: Torque ∝ V², so torque becomes (½)² = ¼.
Q386. Slip of induction motor under braking condition is
A) Positive
B) Negative
C) Zero
D) Infinite
Ans: B
Explanation: During braking, rotor speed > Ns ⇒ slip negative.
Q387. Efficiency of an induction motor is maximum when
A) Mechanical losses are zero
B) Stator and rotor copper losses are equal
C) Slip is unity
D) Power factor is one
Ans: B
Explanation: Condition for maximum efficiency: stator Cu loss = rotor Cu loss.
Q388. Which of the following methods is used to start small induction motors?
A) Rotor resistance
B) Auto-transformer
C) DOL starter
D) Star-delta starter
Ans: C
Explanation: Direct On Line starter is used for small motors (<5 HP).
Q389. The air-gap in induction motor is kept
A) Very small
B) Very large
C) Moderate
D) Variable
Ans: A
Explanation: Small air-gap reduces magnetizing current and leakage reactance.
Q390. The rotor slots are skewed to
A) Reduce magnetic noise
B) Reduce cogging
C) Improve torque
D) All of the above
Ans: D
Explanation: Skewing reduces noise, vibration, and ensures smooth torque.
Q391. When rotor speed equals synchronous speed, emf induced in rotor is
A) Maximum
B) Zero
C) Half of supply
D) Double
Ans: B
Explanation: No relative motion ⇒ no induced emf.
Q392. The synchronous watt is
A) Torque developed per synchronous speed
B) Power developed per slip
C) Constant
D) Speed constant
Ans: A
Explanation: 1 synchronous watt = 1 W per synchronous speed.
Q393. The main function of end rings in squirrel cage rotor is
A) Provide mechanical strength
B) Short-circuit rotor bars
C) Act as cooling fins
D) Both A and B
Ans: D
Explanation: End rings short-circuit rotor bars and provide rigidity.
Q394. In a 4-pole, 50 Hz motor, rotor speed is 1450 rpm. Slip = ?
A) 2%
B) 3%
C) 4%
D) 5%
Ans: D
Explanation: Ns = 120f/P = 1500 rpm; slip = (1500–1450)/1500 = 0.033 ≈ 5%.
Q395. The no-load speed of induction motor is nearly
A) 0
B) 50% of Ns
C) 95–99% of Ns
D) 120% of Ns
Ans: C
Explanation: At no load, slip is very small → near synchronous speed.
Q396. The air-gap power in induction motor equals
A) Rotor input
B) Rotor output
C) Mechanical loss
D) Supply input
Ans: A
Explanation: Air-gap power = total power transferred to rotor.
Q397. For constant load torque, if supply voltage increases, motor speed
A) Increases
B) Decreases
C) Remains constant
D) Becomes zero
Ans: A
Explanation: Higher voltage → more torque → less slip → higher speed.
Q398. A 3-phase induction motor cannot run at synchronous speed because
A) Slip would be zero
B) No emf induced in rotor
C) No torque developed
D) All of the above
Ans: D
Explanation: At Ns, slip = 0 ⇒ no emf ⇒ no torque.
Q399. For high starting torque, rotor resistance should be
A) Low
B) High
C) Zero
D) None
Ans: B
Explanation: Higher rotor resistance increases starting torque.
Q400. The percentage slip at full load is usually
A) 0.1–0.3%
B) 0.5–1%
C) 2–6%
D) 10–20%
Ans: C
Explanation: Typical slip at full load ranges from 2% to 6%.
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