301 MCQ Questions on Induction Motors

251. Imbalance in shaft of an induction motor occurs due to:

(a) Non uniform air gap
(b) Overheating of the winding
(c) Slip rings design
(d) Rugged construction

252. The induction motors which are provided with open slots have:

(a) More breakdown torque
(b) Less breakdown torque
(c) Leading power factor
(d) higher efficiency

253. If single phasing occurs when the motor is running, it should not be loaded beyond…. percent of the rated load. [UPRVUNL 2015]

(a) 5
 (b) 25
(c) 50
(d) 75

254. If an induction motor is to be run on unbalanced supply, then it should run at:

(a) Lower loads
(b) Higher loads
(c) Low speeds
(d) Higher speeds

255. The speed with which magnetic field of an induction motor rotates is called:

(a) Shaft speed
(b) Effective speed
(c) Synchronous speed
(d) Slip speed

256. A wound rotor induction motor runs with a slip of 0.05 when developing full load torque. Its rotor resistance is 0.45Ω per phase. If an external resistance of 0.50Ω per phase is connected across the slip rings, what is the slip for full torque?

(a) 0.03
(b) 0.06
(c) 0.09
(d) 0.1 [UPRVUNL 2015]

257. Which of the following methods could be used to start a three-phase cage-type induction motor?

1. Direct-on-line starting
2. Auto-transformer starting

III. Star-delta starting
(a) Only I and II
(b) Only II and III
(c) I, II and III
(d) Only I and III

258. What is the fuse rating of 10HP, 3-phase 415V squirrel cage inductor motor:

(a) 15A
(b) 35A
(c) 25A
(d) None of these

259. Starting torque of a three-phase squirrel cage induction motor at rated voltage is:

(a) 30% to 40% of the rated torque
(b) The rated torque
(c) 100% to 200% of the rated torque
(d) 5 to 7 times of the rated torque

260. A double squirrel cage induction motor has two

(a) Rotors moving in opposite direction
(b) Parallel winding on rotor
(c) Parallel windings on stator
(d) Series windings in stator

261. A three-phase induction motor has 2 poles and operates with a slip of 0.05 with 50Hz supply for a certain load. The speed of the rotor with respect to stator is:

(a) 300 rpm
(b) 3000 rpm
(c) 2850 rpm
(d) 0 rpm

262. In a three-phase induction motor, the rotor field runs at the following speed with respect to the stator structure:

(a) At synchronous speed in the direction of stator field
(b) At a slip speed in the direction of stator speed
(c) At synchronous speed in a direction opposite to that of stator field
(d) At zero speed

263. If a three-phase, 8-pole, 50 Hz induction motor runs at a speed of 735 r.p.m. then the slip is:

(a) 0.02
(b) 0.03
(c) 0.04
(d) 0.05

264. Out of the following which machine is also known as a generalized transformer:

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

265. If the stator winding of a three-phase slip ring induction motor is delta connected, the rotor winding

(a) Should be delta connected
(b) Should be star connected
(c) should not be delta connected
(d) May be delta or star connected

266. 3-phase, 50 Hz, 500 V induction motor develops 20 hp at slip of 5%. Mechanical losses are 1 hp, and stator loss is 1 kW. The efficiency will be:

(a) 95.3%
(b) 90.5%
(c) 91.3%
(d) 85.3%

267. Crawling in an induction motor is due to

(a) Space harmonics produced by winding currents
(b) Time harmonics in supply.
(c) Slip ring rotor
(d) Insufficient starting torque.

268. At low values of slip, the torque of an induction motor is

(a) Directly proportional to the square of the slip
(b) Inversely proportional to the square of the slip
(c) Directly proportional to the slip.
(d) Inversely proportional to the slip

269. If the supply frequency of a 3-phase induction motor is ‘f, then frequency of rotor emf at motor slip ‘s’ will be

(a) sf
(b) (1-s)f
(c) f/s
(d) None of these

270. A 3-4, 400/200 V, Y-Y connected wound rotor induction motor has 0.06 Ω   rotor resistance and 0.3 Ω   standstill reactance per phase. To make the starting torque equal to maximum torque of the motor the additional resistance required will be:

(a) 0.24 Ω 
(b) 0.45 Ω 
(c) 0.78 Ω 
(d) 0.666Ω   [MPJE – 2015]

271. A winding having number of slots is equal to number of poles, then this winding is called:

(a) Concentrated winding
(b) Short pitch winding
(c) Full pitch winding
(d) Distributed winding [MPJE 2015]

272. An induction motor, in general, is analogous to

(a) An auto-transformer
(b) Two-winding transformer with its secondary open circuited
(c) A two-winding transformer with its secondary short circuited
(d) Three winding transformer

273. What is the advantage of the double squirrel cage motor compared to round bar cage motor?

(a) The efficiency of the motor is higher
(b) The power factor of the motor is higher
(c) The slip of the motor is larger
(d) The starting current of the motor is lower

274. Changing the electrical connections to reverse the direction of rotation of a motor running at full speed is called

(a) Slugging
(b) Plugging
(c) Dynamic braking
(d) Brush shifting

275. The low power factor of induction motor is due to

(a) Rotor leakage reactance
(b) Stator reactance
(c) The reactive lagging magnetizing current necessary to generate the magnetic flux
(d) All of the above [UPSSSC – 2015]

276. Synchronous speed equation is

(a) N_S = 12f/P
(b) N_S = 12P/f
(c) N_S = Pf/120
(d) N_S = 120/Pf

276. During no-load test, an induction motor draws power

(a) For core loss and windage and friction loss
(b) For core loss and copper loss
(c) For copper loss and windage and friction loss
(d) Only for the very small copper loss

277. The torque of an induction motor in synchronous watt is equal to

(a) Rotor input
(b) Stator output
(c) Stator input
(d) Watts at rotor speeds [MPJE – 2016]

278. With the decreasing speed of the motor, what happens to the required capacitive KVAR? [MPJE – 2016]

(a) Increasing
(b) Increases then decreases
(c) Remains constant
(d) None of these

279. High starting torque can be obtained in

(a) Slip ring induction motor
(b) Squirrel cage induction motor
(c) Salient pole alternator
(d) Non Salient pole alternator

280. Which of the following does not increase/improve when load on Squirrel Cage induction motor is increased from No-load to Full-load?

(a) Stator power factor
(b) Torque developed by the motor (up to the magnitude of applied torque)
(c) Slip
(d) Magnetising component of stator exciting current

281. The major disadvantage of DOL starting of Induction motors is that its application is restricted to

(a) Low horse power motors
(b) High speed motors
(c) High horse power motors
(d) Variable speed motors [UPPCL – 2016]

282. Slip ring motor is preferred over Squirrel Cage motor when

1. High starting torque is required
2. Load torque is heavy
3. Heavy pull out torque is required

(a) Only 3
(b) Only 1
(c) Only 1 and 2
(d) Only 2 and 3

283. Sometimes, in a 3-phase induction motor, in order to improve starting torque

(a) Piece of wood are placed deep inside the rotor
(b) Copper bars are removed from the rotor
(c) Cast Iron bearings are used to support rotor
(d) Copper bars are placed deep inside the rotor [UPPCL – 2016]

284. The voltage limit of slip ring induction motor is

(a) 1 kV
(b) 1.5 kV
(c) 250 V
(d) 11 kV [UPPCL – 2016]

285. The rotor reactance per phase, in an induction motor is

(a) Directly proportional to the square of slip
(b) Directly proportional to slip
(c) Inversely proportional to the square of slip
(d) Inversely proportional to slip [UPPCL – 2016]

286. An induction motor at a supply frequency of 60 Hz runs at 1200 rpm. The number of poles in the motor will be

 
(a) 6
(b) 7
(c) 4
(d) 12

287. There are ………. slip rings in a Squirrel Cage induction motor.

(a) 3
(b) 0
(c) 4
(d) 2

288. A 3-phase induction motor is running at slip ‘s’. If its two supply leads are interchanged, then the operating slip at that instant will be

(a) 2s
(b) (1-s)
(c) (2-s)
(d) Zero [DMRC – 2016]

289. The fractional slip of an induction motor is the ratio of

(a) Rotor copper loss/stator copper loss
(b) Stator copper losses/stator input
(c) Rotor copper losses/rotor input
(d) Rotor copper loss/rotor output

290. The main reason for low power factor is the installation of

(a) Synchronous motors
(b) Single phase motor
(c) Induction motors
(d) Rectifiers

291. In induction motor, which of the following supports the rotor shaft? [UPPCL – 2016]

(a) Bush bearings
(b) Tapered roller bearings
(c) Ball bearings
(d) Taper roller bearings

292. It is given that the starting current of a 3-φ induction motor is 3 times the rated current, while the rated slip is 6%. What would be the ratio of starting torque to full-load torque? [UPRVUNL (AE) – 2014]

(a) 0.6
(b) 1.8
(c) 0.54
(d) 1

293. Slip of an induction machine is 0.04 and the stator supply frequency is 150 Hz. What will be the frequency of the rotor induced emf? [UPPCL (AE) – 2016]

(a) 0.06 Hz
(b) 6 Hz
(c) 16 Hz
(d) 4 Hz

294. If rotor speed of an induction motor is 480 rpm and rotor gross output is 2000W, the magnitude of shaft torque developed is:

(a) 31.14 N-m
(b) 39.80 N-m
(c) 36.67 N-m
(d) 41.89 N-m [UPPCL (AE) 2016]

295. For a 3-phase induction motor,

Rotor Input – Rotor Cu loss – Rotor Core loss = ?

(a) Shaft Power
(b) Stator Cu Loss
(c) Mechanical Power developed
(d) Rotational loss [UPPCL (AE) – 2016]

296. A 4-pole, 3-phase induction motor which has a rotor current of frequency 3 Hz gets supply from a 6-pole 3-phase alternator running at 600 rpm. At what speed does the motor operated? [UPPCL (AE) – 2016]

(a) 810 rpm
(b) 750 rpm
(c) 720 rpm
(d) 1060 rpm

297. Slip of a 3-phase induction motor is 0.1. If the line frequency is 60 Hz, the rotor frequency will be…..

(a) 59.4 Hz
(b) 54 Hz
(c) 6 Hz
(d) 5.4 Hz [UPPCL (AE) 2016]

298. Motor operating at maximum power output has an efficiency of approximately:

(a) 26.50%
(b) 50%
(c) 43%
(d) 71% [UPPCL (AE) – 2016]

299. A 3-phase cage rotor induction motor has its stator wound for 8 poles. The rotor of this motor can be used in any other induction motor of the same size and having

(a) 4 poles
(b) 2 poles
(c) 8 poles
(d) any number of poles

300. In capacitor start-capacitor run motor, the starting and running capacitors respectively are:

(a) Oil capacitor, electrolytic
(b) Electrolytic, oil capacitor
(c) Electrolytic, mica capacitor
(d) Mica capacitor, oil capacitor