ELECTRICAL AND ELECTRONICS ENGINEERING

Friday, October 24, 2025

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).

n a 4-pole, 50 Hz motor, rotor speed is 1450 rpm. Slip = ?

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: sₘₐₓ = 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%.

ELECTRICAL AND ELECTRONICS ENGINEERING

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Friday, October 24, 2025

In an induction motor, mechanical losses include

n a 4-pole, 50 Hz motor, rotor speed is 1450 rpm. Slip = ?

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