ELECTRICAL AND ELECTRONICS ENGINEERING : In an induction motor, mechanical losses include

Q351. In a 3-phase induction motor, the rotating magnetic field rotates at

A) Rotor speed
B) Synchronous speed
C) Half synchronous speed
D) Double rotor speed

Answer: B
Explanation: The stator produces a rotating magnetic field at synchronous speed $N_{s} = 120 f / P$ .

Q352. The torque of an induction motor is zero when
A) Slip = 1
B) Slip = 0
C) Slip = 0.5
D) Slip = 2

Answer: B
Explanation: At slip = 0, there is no relative motion → no induced emf → no torque.

Q353. For maximum torque, the slip is
A) Equal to rotor resistance/reactance ratio
B) Proportional to rotor resistance
C) Proportional to supply frequency
D) Equal to 1

Answer: A
Explanation: Condition for maximum torque is $s = R_{2} / X_{2}$ .

Q354. The speed of an induction motor can be varied by
A) Varying supply voltage
B) Varying frequency
C) Varying rotor resistance
D) All of these

Answer: D
Explanation: All methods can control speed but efficiency differs.

Q355. The efficiency of an induction motor at no-load is
A) 100%
B) Low
C) High
D) Zero

Answer: B
Explanation: At no-load, mechanical output is small; losses are significant.

Q356. In an induction motor, slip is directly proportional to
A) Load torque
B) Speed
C) Frequency
D) Voltage

Answer: A
Explanation: As load torque increases, slip increases to balance load.

Q357. If rotor resistance increases, the slip at maximum torque
A) Increases
B) Decreases
C) Remains constant
D) Becomes zero

Answer: A
Explanation: $s_{m a x} = R_{2} / X_{2}$ ; increasing R₂ increases slip at maximum torque.

Q358. The stator current in an induction motor is minimum when
A) At no-load
B) At full-load
C) At starting
D) At synchronous speed

Answer: A
Explanation: At no-load, only magnetizing and core loss currents flow.

Q359. The direction of rotation of a 3-phase induction motor can be reversed by
A) Changing any two rotor leads
B) Changing any two stator leads
C) Changing supply voltage
D) Replacing stator winding

Answer: B
Explanation: Interchanging any two stator phases reverses rotating magnetic field direction.

Q360. If an induction motor runs at 1440 rpm with 4 poles and 50 Hz, slip = ?
A) 0.02
B) 0.04
C) 0.06
D) 0.08

Answer: B
Explanation: Ns = 1500 rpm; $s = (1500 - 1440) / 1500 = 0.04$ .

Q361. A wound-rotor induction motor is preferred when
A) Low starting torque is required
B) High starting torque and speed control are needed
C) Fixed speed is required
D) None of these

Answer: B
Explanation: External resistance allows high starting torque and adjustable speed.

Q362. The equivalent rotor resistance referred to stator side is multiplied by
A) $K^{2}$
B) $1 / K^{2}$
C) K
D) 1/K

Answer: A
Explanation: Rotor resistance referred to stator = $R_{2}^{'} = R_{2} (N_{1} / N_{2})^{2} = R_{2} K^{2}$ .

Q363. The torque developed in an induction motor is zero when rotor speed equals
A) Supply speed
B) Synchronous speed
C) Half speed
D) None

Answer: B
Explanation: At synchronous speed, emf and current are zero → torque = 0.

Q364. The main difference between induction motor and synchronous motor is
A) Rotor construction
B) Speed of operation
C) Supply type
D) All of these

Answer: D
Explanation: Induction motor runs below synchronous speed; synchronous motor runs at constant synchronous speed.

Q365. In an induction motor, if supply frequency decreases, torque
A) Increases
B) Decreases
C) Remains same
D) Becomes zero

Answer: A
Explanation: $T \propto V^{2} / f^{2}$ ; decreasing frequency increases torque if voltage constant.

Q366. The leakage reactance of stator and rotor depend on
A) Frequency only
B) Flux leakage paths
C) Voltage
D) Speed

Answer: B
Explanation: Leakage reactance is due to flux leakage paths around conductors.

Q367. Induction motor torque is independent of
A) Rotor voltage
B) Shaft load
C) Shaft speed
D) Supply phase sequence

Answer: B
Explanation: Torque depends on slip, not directly on load torque; load adjusts slip.

Q368. The rotating magnetic field speed depends on
A) Supply voltage
B) Supply frequency and poles
C) Rotor speed
D) Core losses

Answer: B
Explanation: $N_{s} = 120 f / P$ .

Q369. The core of an induction motor is laminated to reduce
A) Copper losses
B) Eddy current losses
C) Hysteresis losses
D) Mechanical losses

Answer: B
Explanation: Laminations minimize eddy currents by increasing resistance to induced currents.

Q370. In an induction motor, the mechanical power developed is maximum when
A) Slip = 1
B) Slip = s_max
C) Slip = 0.5
D) Rotor copper loss = Mechanical loss

Answer: B
Explanation: Maximum torque occurs at slip = $R_{2} / X_{2}$ .

Q371. The torque under running condition varies as
A) 1/s
B) s
C) s/(R₂² + (sX₂)²)
D) (R₂²)/(sX₂)

Answer: C
Explanation: Torque ∝ $s E_{2}^{2} R_{2} / (R_{2}^{2} + (s X_{2})^{2})$ .

Q372. If supply voltage is reduced by 20%, torque reduces by
A) 20%
B) 36%
C) 40%
D) 64%

Answer: D
Explanation: $T \propto V^{2}$ ; so 0.8² = 0.64 → 64% torque.

Q373. The efficiency of induction motor increases with
A) Increase in load
B) Decrease in speed
C) Increase in slip
D) Decrease in load

Answer: A
Explanation: Losses are nearly constant, so efficiency increases as load increases.

Q374. Induction motor is called a rotating transformer because
A) Energy is transferred inductively
B) Rotor current is induced
C) Secondary is movable
D) All of these

Answer: D
Explanation: It works on transformer principle but with rotating secondary.

Q375. The synchronous speed of 6-pole, 60 Hz motor is
A) 1000 rpm
B) 1200 rpm
C) 1500 rpm
D) 1800 rpm

Answer: B
Explanation: $N_{s} = 120 f / P = 120 \times 60 / 6 = 1200 rpm$ .

Q376. The frequency of rotor current depends on
A) Slip and supply frequency
B) Voltage
C) Load
D) Power factor

Answer: A
Explanation: $f_{r} = s \times f$ .

Q377. The air-gap power equals
A) Rotor input
B) Rotor output
C) Stator copper loss
D) Shaft power

Answer: A
Explanation: Power crossing air-gap = rotor input power.

Q378. A 4-pole, 60 Hz motor runs at 1740 rpm. Find slip.
A) 0.02
B) 0.04
C) 0.05
D) 0.1

Answer: B
Explanation: Ns = 1800; s = (1800–1740)/1800 = 0.033 ≈ 0.04.

Q379. If slip is 4% and supply frequency 50 Hz, rotor frequency = ?
A) 2 Hz
B) 4 Hz
C) 5 Hz
D) 10 Hz

Answer: A
Explanation: $f_{r} = s \times f = 0.04 \times 50 = 2 Hz$ .

Q380. The magnetic field produced by stator is
A) Stationary
B) Alternating
C) Rotating
D) None

Answer: C
Explanation: 3-phase currents produce a rotating magnetic field.

Q381. If number of poles increases, synchronous speed
A) Increases
B) Decreases
C) Remains constant
D) Doubles

Answer: B
Explanation: $N_{s} = 120 f / P$ ; increasing poles decreases speed.

Q382. In a 3-phase induction motor, unbalanced voltage causes
A) Unbalanced currents
B) Heating
C) Reduced torque
D) All of these

Answer: D
Explanation: Voltage unbalance → current unbalance → heating & torque pulsations.

Q383. The main flux in an induction motor is produced by
A) Rotor current
B) Stator current
C) Both
D) Supply voltage only

Answer: B
Explanation: Stator current produces the main flux in the air gap.

Q384. The most suitable method for speed control of induction motor is
A) Rotor resistance control
B) Voltage control
C) Frequency control (V/f)
D) Pole changing

Answer: C
Explanation: Variable frequency drive (VFD) gives efficient, smooth speed control.

Q385. Rotor copper loss is proportional to
A) Slip
B) Slip × air-gap power
C) (1–Slip)
D) Speed

Answer: A
Explanation: $P_{c u} = s \times P_{g}$ .

Q386. The starting torque of a slip-ring motor can be adjusted by
A) External resistance
B) Voltage control
C) Frequency control
D) Pole changing

Answer: A
Explanation: Adding rotor resistance increases starting torque.

Q387. A double-cage motor gives
A) Low starting torque
B) High starting torque
C) High running slip
D) None

Answer: B
Explanation: Outer cage gives high resistance → high starting torque.

Q388. Induction motor operating with negative slip is
A) Motoring
B) Generating
C) Plugging
D) Starting

Answer: B
Explanation: Negative slip indicates regenerative braking (generator mode).

Q389. For constant torque, flux should
A) Increase
B) Decrease
C) Remain constant
D) Vary linearly with speed

Answer: C
Explanation: Constant torque requires constant flux.

Q390. The magnetizing current depends on
A) Supply voltage
B) Frequency
C) Core design
D) All of these

Answer: D
Explanation: All three affect magnetizing current magnitude.

Q391. Induction motor is self-starting because
A) Rotor has short circuit
B) Rotating magnetic field
C) Stator design
D) High resistance

Answer: B
Explanation: The rotating magnetic field induces torque automatically.

Q392. The slip for maximum torque is independent of
A) Supply voltage
B) Rotor resistance
C) Rotor reactance
D) Rotor current

Answer: A
Explanation: Slip for max torque = R₂/X₂, independent of voltage.

Q393. The direction of rotation depends on
A) Rotor resistance
B) Stator phase sequence
C) Supply frequency
D) Shaft load

Answer: B
Explanation: Changing stator phase sequence reverses direction.

Q394. If stator voltage drops by 10%, torque reduces by
A) 10%
B) 20%
C) 19%
D) 36%

Answer: D
Explanation: $T \propto V^{2} \to (0.9)^{2} = 0.81$ , so 19% drop.

Q395. The starting torque in squirrel cage motor can be improved by
A) Using deep-bar rotor
B) Using capacitor
C) Reducing voltage
D) Increasing frequency

Answer: A
Explanation: Deep-bar rotor increases resistance during start.

Q396. The line current in star-delta starting is
A) 1/√3 of DOL current
B) Same as DOL
C) 3 times
D) 1/3

Answer: A
Explanation: Star reduces phase voltage → line current reduced by 1/√3.

Q397. The torque of induction motor under running condition is directly proportional to
A) Rotor copper loss
B) Rotor input power
C) Air-gap power × (1 – slip)
D) Supply voltage²

Answer: D
Explanation: Torque ∝ V² for constant slip.

Q398. The frequency of stator voltage is
A) Proportional to slip
B) Constant
C) Equal to rotor frequency
D) Zero at full load

Answer: B
Explanation: Stator frequency = supply frequency, always constant.

Q399. In an induction motor, mechanical losses include
A) Windage
B) Friction
C) Bearing losses
D) All of these

Answer: D
Explanation: All are mechanical losses that reduce shaft output.

Q400. The power transferred across air gap is known as
A) Air-gap power
B) Rotor input power
C) Both A and B
D) Stator loss

Answer: C
Explanation: Air-gap power = power transferred from stator to rotor (same as rotor input).

ELECTRICAL AND ELECTRONICS ENGINEERING

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Friday, October 24, 2025

In an induction motor, mechanical losses include

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