301 MCQ Questions on Induction Motors

201. When single phasing occurs in a three phase induction motor carrying K times rated load the motor may operate satisfactorily

(a) K = 1
(b) K = 0.5
(c) K = 2
(d) None of the other options

202. A 3 phase, 4 pole squirrel cage induction motor has 36 stator and 28 rotor slots. The number of phase in the rotor is

(a) 3
(b) 9
(c) 7
(d) 8 [LMRC AE – 2016]

203. A 4 pole induction motor, supplied by a slightly unbalanced three phase 50 Hz source is rotating at 1440 rpm. What is the electrical frequency is Hz of the induced negative sequence current in the rotor? [LMRC AE 2016]

(a) 100 Hz
(b) 98 Hz
(c) 52 Hz
 (d) 48 Hz

204. An induction motor at 50 Hz with 1000 rpm speed; will have:

(a) 8 poles
(b) 6 poles
(c) 4 poles
(d) 2 poles

205. For a single phase capacitor start induction motor which of the following is valid?

(a) Rotation can be changed by reversing main winding terminals
(b) Direction of rotation cannot be changed.
(c) Rotation can be changed by interchanging supply terminals
(d) None of these

206. When the equivalent circuit diagram of doubled squirrel-cage induction motor is constructed, the two cages can be considered?

(a) In series
(b) In parallel
(c) In series-parallel
(d) In parallel with stator

207. In a three-phase induction motor, the number of poles in the rotor winding is always:

(a) Zero
(b) More than the number of poles in stator
(c) Less than the number of poles in stator
(d) Equal to the number of poles in stator

208. The speed of the squirrel-cage induction motor can be controlled by all of the following except:

(a) Changing supply frequency
(b) Changing number of poles
(c) Changing winding resistance
(d) Reducing supply voltage

209. The ‘cogging’ of an induction motor can be avoided by:

(a) Proper ventilation
(b) Using DOL starter
(c) Auto-transformer starter
(d) Having number of rotor slots more or less than the number of stator slots (not equal)

210. If an induction motor with certain ratio of rotor to stator slots, runs at 1/7 of the normal speed, the phenomenon will be termed as:

(a) Humming
(b) Hunting
(c) Crawling
(d) Cogging

211. Short-circuit test on an induction motor cannot be used to determine:

(a) Windage losses
(b) Copper losses
(c) Transformation ratio
(d) Power scale of circle diagram

212. In 3 phase induction motor sometimes copper bars are placed deep in the rotor to:

(a) Improve starting torque
(b) Reduce copper losses
(c) Improve efficiency
(d) Improve power factor

213. A 500 kW, 3-phase, 440 volts, 50 Hz, AC induction motor has a speed of 960 rpm, on full load. The machine has of 6 poles. The slip of the machine will be:

(a) 0.01
(b) 0.02
(c) 0.03
(d) 0.04

214. For starting a Scharge motor, 3-phase supply is connected to:

(a) Rotor via slip rings
(b) Regulating windings
(c) Secondary winding via brushes
(d) Startor as in an ordinary induction motor [ESIC JE – 2016]

215. The frame of an induction motor is made of

(a) Carbon
(b) Closed grained cast iron
(c) Aluminium
(d) Stainless steel

216. When a squirrel cage induction motor fails to start which of the following could not be the reason for the same?

(a) Uneven air gap
(b) Blown fuses
(c) Overload
(d) One or two phase open

217. A 3-phase delta connected motor is supplied at a line voltage of 400 V, the voltage across each winding of the motor will be:

(a) 400 V
(b) 200 V
(c) 100 V
(d) 300 V [FCI – 2015]

218. No load test on a three-phase squirrel cage induction motor at rated voltage is performed to obtain:

(a) Stator and rotor resistance and reactance
(b) Series branch parameters of equivalent circuit
(c) Copper and core losses
(d) Shunt branch parameters of equivalent circuit

219. At low values of slip ‘s’, the torque of a 3-phase induction motor is-

(a) T
(b) Tα s
(c) T α s^-1
(d) T α s²

220. In a three phase induction motor, the ratio of starting current to full load current is 12.56 and the ratio of starting torque to full load torque is 1.6. Percentage slip at full load is

(a) 0.1%
(b) 12.64%
(c) 1%
(d) None of these

221. In a 3-phase induction motor working at full load, the torque ‘T’ is related with slip ‘s’ and supply voltage V_o as follows

(a) T α V_o/s
(b) T_s α V_o²
(c) T α V_o
(d) T α sV_o²

222. If stator impedance is neglected, the maximum torque in an induction motor occurs at a rotor resistance of

(a) (1+s)X₂
(b) (1-s)X2
(c) sX₂
(d) X₂

223. A single-phase induction motor is

(a) Inherently self-starting with high torque
(b) Inherently self-starting with low torque
(c) Inherently non-self starting with low torque
(d) Inherently non-self starting with high torque

224. Maximum torque in 3-phase induction motor varies as

(a) f
(b) l/f
(c) 1/f²
(d) 1/f³ [Uttrakhand AE 2013]

225. At s = 0, the torque of an induction motor is:

(a) Zero
(b) Equal to full-load torque
(c) Very high
(d) Very low [Uttrakhand AE 2013]

226. A 3-phase induction motor under de dynamic braking work as:

(a) Induction generator
(b) DC generator
(c) DC motor
(d) Synchronous generator

227. Wide speed control range of three phase squirrel cage induction motor is obtained by:

(a) Variable voltage constant frequency control method
(b) Slip power recovery method
(c) Pole changing method
(d) Constant voltage to frequency ratio (V/f)

228. The type of single phase induction motor having the highest power factor at full load is:

(a) Capacitor run type
(b) Capacitor start type
(c) Shaded pole type
(d) Split phase type

229. A 3-phase, 60 Hz induction motor runs at 1150 rpm, its synchronous speed is:

(a) 1300 rpm
(b) 1500 rpm
(c) 1200 rpm
(d) 1000 rpm

230. The change in speed of a 3 φ induction motor from no load to full load is about:

 (a) 4%
(b) 8%
(c) 15%
(d) 50%

231. In order to run an induction motor at synchronous speed

(a) Load must be reduced
(b) Friction at bearing must be reduced
(c) e.m.f. must be injected in the rotor in a phase with rotor e.m.f.
(d) e.m.f. must be injected in the rotor in phase opposite to the rotor e.m.f.

232. “Cogging” in induction motor occurs when ….

(a) Number of stator teeth – number of rotor teeth = odd number
(b) Number of stator teeth – number of rotor teeth = zero
(c) Number of stator teeth – number of rotor teeth = even number
(d) Number of stator teeth – number of rotor teeth = negative number

233. In a single phase induction motor, how are the windings placed?

(a) Both main and auxiliary windings are on stator
(b) Both windings are on rotor
(c) Main winding on rotor
(d) Auxiliary on stator and main on rotor

234. A 3-phase 50Hz, 6 pole squirrel cage induction motor will run at a synchronous speed of….

(a) 960 rpm
(b) 1000 rpm
(c) 1500 rpm
(d) 1600 rpm

235. Standard Motor Starter supplies ……… initial voltage for proper starting of the motor.

(a) Higher
(b) Constant
(c) Normal
(d) Lower

236. The motor normally used for crane travel is

(a) Synchronous motor
(b) D.C. differentially compounded motor
(c) Ward Leonard controlled d.c. shunt motor
(d) A.C. slip ring motor

237. The speed of a motor generator set consisting of 6-pole induction motor and a 4-pole dc generator fed from a 3 phase 50 Hz supply is ……….. rpm.

(a) 1000
(b) 600
(c) 1500
(d) 3000

238. Induction motor rotor always rotate at a speed …… than synchronous speed.

(a) Same
 (b) Greater
(c) Less
(d) Infinite

239. The effect of increasing the length of air gap in an induction motor will be to increase the

(a) Power factor
(b) Speed
(c) Magnetising current
(d) Air-gap flux

240. How many windings does a simple resolver have?

(a) One stator and two rotor winding
(b) One rotor and two stator windings
(c) Two stator and two rotor windings
(d) One stator and one rotor windings

241. An induction motor has a starting torque 300 Nm when started by direct switching. If a star delta starter is used for starting, the starting torque will be

(a) 300 Nm
(b) 100 Nm
(c) 173.2 Nm
(d) 900 Nm

242. The speed of an 3 phase cage-rotor induction motor depends on

(a) Frequency of the supply
(b) Number of poles only
(c) Number of poles and frequency of supply
(d) Input voltage

243. In a 3 phase induction motor, the relative speed of stator Field with respect to ………….. is zero.

(a) Stator winding
(b) Rotor
(c) Rotor flux
(d) Space

244. Almost 90% of the induction motors are squirrel cage type because

(a) They have better starting torque
(b) Their speed can be controlled very smoothly over a wide range
(c) They can run both at lagging and leading power factors
(d) They have the simplest and most rugged construction

245. A three phase Induction motor always

(a) Works at lagging power factor
(b) Works at leading power factor
(c) Works at maximum (i.e. unity) power factor
(d) The power factor keeps changing with the load

246. In which method of measurement of slip percentage in slip is given by = 100n/n_sn_r

(a) Synchronous watt
(b) Electromechanical counter
(c) Mechanical differential counter
(d) Stroboscopic Method

247. A direct on-line starter is used for starting motors up to:

(a) 5 hp
(b) 10 hp
(c) 20 hp
(d) 40 hp

248. Which frame of motor is preferred in moist surroundings?

(a) Drip proof
(b) Splash proof
(c) Totally enclosed fan cooled
(d) Open type

249. Compensating winding is used in:

(a) Capacitor start motor
(b) Shaded pole motor
(c) AC series motor
(d) Capacitor run motor

250. Which of the following is correct with respect to the speed of cage rotor induction motor?

The speed of 3-phase cage rotor induction motor depends upon :

1. Number of poles
2. Frequency of supply
3. Input voltage

(a) Only A
(b) Only B
(c) Only B and C
(d) Only A and B