Friday, October 24, 2025

The frequency of rotor emf when rotor speed = 0.96Ns is

 Q201. The torque of an induction motor under running condition is proportional to:

A) Slip
B) Slip / (R₂² + (sX₂)²)
C) Supply voltage
D) Power factor
Answer: B
Explanation: T=sE22R2R22+(sX2)2T = \frac{sE_2^2R_2}{R_2^2 + (sX_2)^2}, hence torque ∝ that fraction.

Q202. The air-gap power in an induction motor is:
A) Rotor input
B) Stator input
C) Mechanical power
D) Rotor output
Answer: A
Explanation: Air-gap power = power transferred from stator to rotor.

Q203. For maximum torque, rotor reactance = ?
A) Rotor resistance / slip
B) Slip × rotor resistance
C) Rotor resistance × slip
D) None
Answer: A
Explanation: Condition: R2/s=X2R_2 / s = X_2.

Q204. The starting torque of an induction motor increases with:
A) Increase in rotor resistance
B) Decrease in voltage
C) Increase in frequency
D) None
Answer: A
Explanation: Tstart∝R2R22+X22T_{start} \propto \frac{R_2}{R_2^2 + X_2^2}.

Q205. The mechanical losses of an induction motor are due to:
A) Friction and windage
B) Iron loss
C) Copper loss
D) Load
Answer: A
Explanation: Mechanical losses = friction + windage.

Q206. The synchronous wattage is equal to:
A) Torque × synchronous speed
B) Torque × rotor speed
C) Torque × slip speed
D) Torque × electrical frequency
Answer: A
Explanation: Synchronous watt = torque × ω_s.

Q207. In a slip-ring motor, external resistance is added to:
A) Stator
B) Rotor
C) Both
D) None
Answer: B
Explanation: Resistance added in rotor circuit to control speed and torque.

Q208. The main function of rotor resistance starter is to:
A) Reduce starting current
B) Increase starting torque
C) Control speed
D) All of these
Answer: D
Explanation: It reduces current, improves torque, and provides speed control.

Q209. The starting torque of an induction motor is maximum when:
A) R₂ = X₂
B) R₂ = sX₂
C) R₂/s = X₂
D) X₂ = sR₂
Answer: C
Explanation: Derived from torque equation for max torque condition.

Q210. The speed of induction motor decreases if:
A) Load increases
B) Load decreases
C) Voltage increases
D) Frequency increases
Answer: A
Explanation: More load → more torque → slip increases → speed drops.

Q211. The motor efficiency is maximum when:
A) Variable losses = Constant losses
B) Slip = 1
C) Slip = 0
D) Load = maximum
Answer: A
Explanation: True for all electrical machines.

Q212. The voltage ratio per phase of stator to rotor equals:
A) Turns ratio
B) Inverse of turns ratio
C) Square of turns ratio
D) Frequency ratio
Answer: A
Explanation: E2/E1=N2/N1E_2 / E_1 = N_2 / N_1.

Q213. The rotating magnetic field in a 3-phase induction motor rotates at:
A) Supply frequency
B) Rotor frequency
C) Synchronous speed
D) Slip speed
Answer: C
Explanation: Field speed = Ns = 120f/P.

Q214. The speed of the rotating magnetic field depends on:
A) Supply frequency and number of poles
B) Rotor resistance
C) Load
D) Voltage
Answer: A
Explanation: Ns=120f/PN_s = 120f/P.

Q215. The torque-slip curve of induction motor is nearly:
A) Parabolic
B) Linear
C) Hyperbolic
D) Exponential
Answer: A
Explanation: At low slips, T ∝ s; at high slips, T ∝ 1/s → parabolic.

Q216. Rotor emf frequency decreases with:
A) Increase in rotor speed
B) Decrease in rotor speed
C) Increase in stator frequency
D) None
Answer: A
Explanation: f2=sff_2 = s f, slip decreases as rotor speed increases.

Q217. Slip is defined as:
A) Ratio of speed loss to synchronous speed
B) Ratio of actual speed to synchronous speed
C) Ratio of torque to speed
D) Ratio of frequency to speed
Answer: A
Explanation: s=(Ns−N)/Nss = (N_s - N)/N_s.

Q218. Rotor e.m.f. per phase depends on:
A) Slip
B) Supply voltage
C) Turns ratio
D) All of these
Answer: D
Explanation: E2=sE20=s(N2/N1)VE_2 = s E_{20} = s (N_2/N_1) V.

Q219. Slip is zero when:
A) Rotor speed = synchronous speed
B) Rotor stationary
C) Motor starting
D) Rotor runs above synchronous speed
Answer: A
Explanation: At N = Ns, s = 0.

Q220. Induction motor cannot run above synchronous speed because:
A) Slip becomes negative
B) Torque reverses
C) Acts as generator
D) All of these
Answer: D
Explanation: Above Ns → generating region.

Q221. The frequency of rotor current is equal to supply frequency when:
A) Rotor stationary
B) Rotor runs at synchronous speed
C) Rotor runs at half synchronous speed
D) None
Answer: A
Explanation: s = 1 → f₂ = f.

Q222. When rotor speed increases, rotor frequency:
A) Increases
B) Decreases
C) Constant
D) Zero
Answer: B
Explanation: f2=sff_2 = s f, s decreases → f₂ decreases.

Q223. The torque in induction motor depends on:
A) Rotor current and rotor flux
B) Rotor speed
C) Supply frequency
D) Load
Answer: A
Explanation: Torque ∝ Φ × I₂ × cosφ₂.

Q224. The rotor induced emf is maximum when:
A) Rotor stationary
B) Rotor running
C) Slip = 0
D) Rotor speed = Ns
Answer: A
Explanation: Maximum relative speed → max emf.

Q225. The starting torque of a wound rotor induction motor can be increased by:
A) Inserting external resistance
B) Reducing supply voltage
C) Increasing frequency
D) Decreasing pole number
Answer: A
Explanation: Resistance insertion improves torque by better matching R₂/X₂.

Q226. In blocked-rotor test, slip is:
A) Zero
B) Unity
C) Fractional
D) Negative
Answer: B
Explanation: Rotor is stationary → s = 1.

Q227. At no-load, slip of induction motor is:
A) Very small
B) Unity
C) Zero
D) Negative
Answer: A
Explanation: Only friction and windage losses → small slip (~0.5%).

Q228. Induction motor speed is inversely proportional to:
A) Slip
B) Number of poles
C) Voltage
D) Load
Answer: B
Explanation: Ns=120f/PN_s = 120f/P.

Q229. The power factor of induction motor improves when:
A) Load increases
B) Load decreases
C) Voltage increases
D) Speed increases
Answer: A
Explanation: With load, active component increases → better PF.

Q230. The magnetic field of the rotor rotates at:
A) Rotor speed
B) Slip speed
C) Synchronous speed
D) Zero speed
Answer: C
Explanation: Both rotor and stator fields rotate at synchronous speed in same direction.

Q231. The supply current at no-load is mostly:
A) Active current
B) Magnetizing current
C) Resistive current
D) None
Answer: B
Explanation: Only magnetizing current required to create flux.

Q232. The starting torque of an induction motor varies as:
A) V²
B) V
C) 1/V
D) V³
Answer: A
Explanation: Tstart∝V2T_{start} ∝ V^2.

Q233. In a slip-ring induction motor, external resistance is inserted during:
A) Running
B) Starting
C) Stopping
D) All
Answer: B
Explanation: Added only during starting for high torque.

Q234. The short-circuit current of an induction motor is proportional to:
A) Supply voltage
B) Supply frequency
C) Both
D) None
Answer: A
Explanation: Isc∝VI_{sc} ∝ V at constant impedance.

Q235. The torque of an induction motor becomes zero when:
A) s = 0
B) s = 1
C) N = 0
D) R₂ = 0
Answer: A
Explanation: Slip zero → no induced emf → no torque.

Q236. Slip of an induction motor operating as generator is:
A) Positive
B) Negative
C) Zero
D) One
Answer: B
Explanation: Rotor speed > Ns → negative slip.

Q237. The rotor emf at standstill is proportional to:
A) Frequency
B) Slip
C) Supply voltage
D) Both A and C
Answer: D
Explanation: E2∝f∝VE_2 ∝ f ∝ V.

Q238. If rotor frequency is 2 Hz and supply is 50 Hz, slip = ?
A) 2%
B) 4%
C) 5%
D) 10%
Answer: C
Explanation: s=f2/f=2/50=0.04=4%s = f_2 / f = 2/50 = 0.04 = 4\%.

Q239. The torque of induction motor is independent of:
A) Rotor resistance at max torque
B) Slip
C) Supply frequency
D) None
Answer: A
Explanation: Max torque ∝ V²/X₂ → independent of R₂.

Q240. The number of rotor slots is usually:
A) Less than stator slots
B) Equal to stator slots
C) More than stator slots
D) Double
Answer: A
Explanation: To avoid magnetic locking, rotor slots < stator slots.

Q241. The slip is 0.02 and Ns = 3000 rpm. The rotor speed = ?
A) 2940 rpm
B) 2960 rpm
C) 2900 rpm
D) 2980 rpm
Answer: B
Explanation: N=(1−0.02)×3000=2940 rpmN = (1−0.02)×3000 = 2940 \text{ rpm}.

Q242. For constant torque, power input is proportional to:
A) Slip
B) Speed
C) Voltage
D) Torque
Answer: A
Explanation: Slip ∝ rotor losses → power drawn increases with slip.

Q243. The torque at synchronous speed is:
A) Maximum
B) Zero
C) Minimum
D) Infinite
Answer: B
Explanation: At s=0 → zero torque.

Q244. The rotating field produced in stator is due to:
A) DC current
B) Three-phase AC
C) Single-phase AC
D) DC excitation
Answer: B
Explanation: 3-phase currents displaced by 120° produce rotating field.

Q245. The frequency of rotor emf when rotor speed = 0.96Ns is:
A) 2 Hz
B) 4 Hz
C) 3 Hz
D) 5 Hz
Answer: A
Explanation: Slip = 0.04 → f2=0.04×50=2 Hzf_2 = 0.04×50 = 2 \text{ Hz}.

Q246. The maximum torque is directly proportional to:
A) Square of supply voltage
B) Rotor resistance
C) Supply frequency
D) Slip
Answer: A
Explanation: Tmax∝V2/(2X2)T_{max} ∝ V^2 / (2X_2).

Q247. The starting current of induction motor is:
A) Very high
B) Low
C) Same as full-load
D) Zero
Answer: A
Explanation: Rotor emf high at start → large inrush current.

Q248. At no-load, the power factor is:
A) High lagging
B) Leading
C) Low lagging
D) Unity
Answer: C
Explanation: Magnetizing current causes low lagging PF.

Q249. The induction motor draws current at no-load mainly to:
A) Overcome friction
B) Produce air-gap flux
C) Produce torque
D) Supply rotor losses
Answer: B
Explanation: No-load → current for flux establishment.

Q250. The torque developed by an induction motor under blocked rotor condition is called:
A) Starting torque
B) Pull-out torque
C) Breakdown torque
D) Full-load torque
Answer: A
Explanation: At standstill (s=1), torque = starting torque.

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