McqMate
These multiple-choice questions (MCQs) are designed to enhance your knowledge and understanding in the following areas: Mechanical Engineering .
| 251. |
When three phase induction motor is started with star-delta starter, during starting stator is connected |
| A. | in star |
| B. | in delta |
| C. | in star or delta |
| D. | none of these |
| Answer» A. in star | |
| 252. |
When three phase induction motor is started with star-delta starter, during normal running condition, stator is connected |
| A. | in star |
| B. | in delta |
| C. | in star or delta |
| D. | none of these |
| Answer» B. in delta | |
| 253. |
The torque developed by three phase induction motor is approximately proportional to |
| A. | square of applied voltage |
| B. | applied voltage |
| C. | half of applied voltage |
| D. | double of appled voltage |
| Answer» A. square of applied voltage | |
| 254. |
The rotor of an induction motor can't run at synchronous speed, if it did so then |
| A. | rotor emf would be zero |
| B. | rotor current would be zero |
| C. | rotor torque would be zero |
| D. | all of these |
| Answer» D. all of these | |
| 255. |
A 3-phase , 50 Hz , 4-pole squirrel cage induction motor has its stator rewound for 6 poles without any alternations in the rotor. The motor would now run at a speed |
| A. | less than 1000 rpm |
| B. | less than 1500 rpm |
| C. | less than 3000 rpm |
| D. | zero rpm |
| Answer» A. less than 1000 rpm | |
| 256. |
If the input voltage and the frequency of supply is halved, to the 3-phase induction motor |
| A. | the maximum torque is halved |
| B. | the maximum torque remains the same |
| C. | the air-gap flux is halved |
| D. | none of these |
| Answer» A. the maximum torque is halved | |
| 257. |
The speed of the slip ring induction motor can be easily controlled by |
| A. | changing supply frequency |
| B. | changing number of poles |
| C. | changing the resistance of the rotor winding |
| D. | all of these |
| Answer» D. all of these | |
| 258. |
A 440V, 20HP, 3phase motor operates at full load, 88% efficiency and 0.65 power factor lagging, the current drawn by motor is |
| A. | 59.28a |
| B. | 0.0458a |
| C. | 34.22 a |
| D. | 26.50a |
| Answer» C. 34.22 a | |
| 259. |
A6 pole 3 phase 50Hz induction motor develops mechanical power of 40Hp and has mechanical losses of 1500W at a speed of 960rpm when connected to 500V. The output available at the shaft is (Take 1HP=746W) |
| A. | 27kw |
| B. | 31340w |
| C. | 29840w |
| D. | 28340w |
| Answer» D. 28340w | |
| 260. |
DOL starter is used for 3 phase I.M. motor having rating of |
| A. | ≤5 h.p. |
| B. | > 5 h.p. |
| C. | 5 to 10 h.p. |
| D. | >10h.p. |
| Answer» A. ≤5 h.p. | |
| 261. |
Thermal relays are used in DOL starter of Three Phase I.M. to |
| A. | protect the motor from high temperature |
| B. | protect the motor from overloading |
| C. | protect the motor from short circuit |
| D. | none of the above |
| Answer» B. protect the motor from overloading | |
| 262. |
No. of contacts used in a contactor of DOL starter |
| A. | 2 |
| B. | 3 |
| C. | 4 |
| D. | 1 |
| Answer» C. 4 | |
| 263. |
Q. In DOL starter NO(Normally open) contact is used as |
| A. | start push button |
| B. | stop push button |
| C. | a and b |
| D. | none of the above |
| Answer» A. start push button | |
| 264. |
In DOL starter NC(Normally closed) contact is used as |
| A. | start push button |
| B. | stop push button |
| C. | a and b |
| D. | none of the above |
| Answer» B. stop push button | |
| 265. |
The advantage of Autotransformer starter over stator resistance starter is |
| A. | produces more torque |
| B. | produces less losses |
| C. | smooth variation of speed |
| D. | none of the above |
| Answer» B. produces less losses | |
| 266. |
The rotor speed of a 6 pole 50Hz induction motor is 960 r.p.m. The percentage slip is ____. |
| A. | 0.03 |
| B. | 0.04 |
| C. | 0.05 |
| D. | 0.02 |
| Answer» B. 0.04 | |
| 267. |
A 3 phase induction motor connected from a 3 pjhase, 50Hz AC supply runs at 720rpm and has slip 4%. The number of poles in motor are _____. |
| A. | 4 |
| B. | 6 |
| C. | 8 |
| D. | 16 |
| Answer» C. 8 | |
| 268. |
The torque developed in an induction motor is nearly proportional to |
| A. | 1/v |
| B. | v |
| C. | v^2 |
| D. | none of these |
| Answer» C. v^2 | |
| 269. |
The air gap between the stator and rotor of a 3 phase induction motor ranges from |
| A. | 2 cm to 4 cm |
| B. | 0.4mm to 4mm |
| C. | 1cm to 2cm |
| D. | 4cm to 6cm |
| Answer» B. 0.4mm to 4mm | |
| 270. |
An induction motor under full load has slip of about____. |
| A. | 0.03 |
| B. | 0.1 |
| C. | 0.3 |
| D. | zero |
| Answer» A. 0.03 | |
| 271. |
A 4 pole, 440V, 50Hz induction motor is running at a slip of 4%. The speed of motor is ____. |
| A. | 1260 rpm |
| B. | 1440rpm |
| C. | 1500rpm |
| D. | 1560rpm |
| Answer» B. 1440rpm | |
| 272. |
A 4 pole, 50Hz induction motor runs at a speed of 1440rpm. The frequency of the rotor induced emf is _____. |
| A. | 3hz |
| B. | 2.5hz |
| C. | 2hz |
| D. | 1hz |
| Answer» C. 2hz | |
| 273. |
The value of slip is ___ at start. |
| A. | zero |
| B. | infinite |
| C. | 100 |
| D. | 1 |
| Answer» D. 1 | |
| 274. |
Slip rings are usually made up of ___. |
| A. | copper |
| B. | iron |
| C. | carbon |
| D. | phosphor-bronze |
| Answer» D. phosphor-bronze | |
| 275. |
The rotor standstill resistance and reactance of an induction motor are 0.5Ω and 2.5Ω hence the rotor impedance at 5% slip is____. |
| A. | 0.5Ω |
| B. | 0.025Ω |
| C. | 0.5154Ω |
| D. | 0.125Ω |
| Answer» C. 0.5154Ω | |
| 276. |
The torque of 3 phase induction motor depends on__. |
| A. | part of rotating magnetic field reacting with stator |
| B. | rotor current is running condition |
| C. | rotor power factor in running condition |
| D. | all of these |
| Answer» D. all of these | |
| 277. |
The constant 'k' in the torque equation of a 3 phase induction motor depends |
| A. | supply voltage |
| B. | rotor current in running condition |
| C. | synchronous speed |
| D. | all of these |
| Answer» C. synchronous speed | |
| 278. |
A 3 phase indution motor is running with slip of 4% hving synchronous speed of 1500rpm. If rotor induced emf per phase in standstill condition is 115.47V and rotor impedance in standstill condition is 0.1+j1 Ω then torque developed ia____. |
| A. | 118.54 |
| B. | 87.81 |
| C. | 225.45 |
| D. | 67.29 |
| Answer» B. 87.81 | |
| 279. |
A 3 phase induction motor running with full load slip of 1.2% and its rotor impedance per phase on standstill is 0.016+j0.265Ω then the ratio of full load torque to maximum torque is |
| A. | 0.382 |
| B. | 3.16 |
| C. | 0.1203 |
| D. | 0.556 |
| Answer» A. 0.382 | |
| 280. |
A 3 phase induction motor is running with full load slip of 1.2% and its rotor impedance per phase is 0.06+j0.265Ω then ratio of starting torque to maximum torque is |
| A. | 0.382 |
| B. | 3.16 |
| C. | 0.123 |
| D. | 0.556 |
| Answer» C. 0.123 | |
| 281. |
A 3 phase I.M. is running with full load slip of 1.2% and its rotor impedance per phase on standstill is 0.016+j0.265Ω then ratio of full load torque to starting torque is _____. |
| A. | 0.382 |
| B. | 3.16 |
| C. | 0.123 |
| D. | 0.556 |
| Answer» B. 3.16 | |
| 282. |
A 4 pole, 50Hz , 3 phase I M is running at 1425rpm on full load having rotor impedance per phase on standstill as 0.4+j2Ω. If voltage between the slip rings on open circuit is 520V then the torque developed on full load is ___ Nm. |
| A. | 202.52 |
| B. | 165.53 |
| C. | 430.36 |
| D. | 514.21 |
| Answer» A. 202.52 | |
| 283. |
A 4 pole, 50Hz , 3 phase I M is running at 1425rpm on full load having rotor impedance per phase on standstill as 0.4+j2Ω. If voltage between the slip rings on open circuit is 520V then the maximum torque developed is ___ Nm. |
| A. | 202.52 |
| B. | 165.53 |
| C. | 430.36 |
| D. | 514.21 |
| Answer» C. 430.36 | |
| 284. |
A 4 pole, 50Hz , 3 phase I M is running at 1425rpm on full load having rotor impedance per phase on standstill as 0.4+j2Ω. The ratio of full load torque to maximum torque is ___. |
| A. | 0.817 |
| B. | 1.223 |
| C. | 0.384 |
| D. | 0.4705 |
| Answer» D. 0.4705 | |
| 285. |
A 4 pole, 50Hz , 3 phase I M is running at 1425rpm on full load having rotor impedance per phase on standstill as 0.4+j2Ω. The external resistance required in the rotor per phase to get maximum torque at start is ___. |
| A. | 0.2 |
| B. | 1.6 |
| C. | 0.8 |
| D. | 0.4 |
| Answer» B. 1.6 | |
| 286. |
A 3 phase I.M. has rotor impedance on standstill as 0.8+j3.8Ω. Then the maximum torque will occur at slip of ____% |
| A. | 21.05 |
| B. | 20 |
| C. | 30 |
| D. | 35.26 |
| Answer» A. 21.05 | |
| 287. |
A 50Hz, 8 pole, 3 phase I.M. has a full load slip of 4%. Its rotor impedance per phase on standstill is 0.01+j0.005 ohms. Then the speed at maximum torque is ____. |
| A. | 200 |
| B. | 300 |
| C. | 750 |
| D. | 600 |
| Answer» D. 600 | |
| 288. |
A 50Hz 20 poles 3 phase I.M. has a full load slip 1.33%. Its rotor impedance per phase on standstill is 0.02+j0.3ohms. Then the ratio of starting torque to the maximum torque is____. |
| A. | 0.384 |
| B. | 0.527 |
| C. | 0.132 |
| D. | 0.854 |
| Answer» C. 0.132 | |
| 289. |
The external resistance added in the rotor circuit at start _____. |
| A. | increase the starting torque |
| B. | acts as starter |
| C. | prevents inrush current |
| D. | all of these |
| Answer» D. all of these | |
| 290. |
A 3 phase induction motor is running with 4% slip. Its rotor input is 10kW then rotor copper losses are |
| A. | 1kw |
| B. | 4kw |
| C. | 400w |
| D. | 0.04kw |
| Answer» C. 400w | |
| 291. |
A 3 phase I.M. is running with slip 5%. Its air gap power is 12kW then total mechanical power developed is____. |
| A. | 11.4kw |
| B. | 12kw |
| C. | 11.25kw |
| D. | 16.6kw |
| Answer» A. 11.4kw | |
| 292. |
A 3 phase I.M. running with slip 5%. Its rotor input is 20kW then its power output is ____ if frictional losses are 1kW. |
| A. | 19.75kw |
| B. | 1.8kw |
| C. | 18kw |
| D. | 21kw |
| Answer» C. 18kw | |
| 293. |
A 3 phase I.M. is running with 5% slip. Its rotor input is 20kW and its synchronous speed is 1500rpm then its shaft torque is ___ if frictional losses are 1kW. |
| A. | 226.89nm |
| B. | 120.62nm |
| C. | 135.48nm |
| D. | 214.7nm |
| Answer» B. 120.62nm | |
| 294. |
A 3 phase I.M. is running with 5% slip. Its rotor input is 20kW then its rotor efficiency is ____. |
| A. | 0.95 |
| B. | 0.65 |
| C. | 0.75 |
| D. | insufficient data |
| Answer» A. 0.95 | |
| 295. |
A 25kW, 3 phase I.M. is running with 6% slip. Its rotor copper losses are ____ if frictional losses are 850W. |
| A. | 1700 |
| B. | 1650 |
| C. | 1550 |
| D. | 1400 |
| Answer» B. 1650 | |
| 296. |
A 25kW, 3 phase I.M. is running with 6% slip. Its rotor resistance per phase is____ if frictional losses are 850W and rotor current per phase is 65A. |
| A. | 0.39 |
| B. | 0.165 |
| C. | 0.13 |
| D. | 0.215 |
| Answer» C. 0.13 | |
| 297. |
A 4 pole, 3 phase I.M. is running with a slip of 4% producing shaft torque of 300Nm then its output power is ____ kW. |
| A. | 46.89 |
| B. | 28.54 |
| C. | 52.12 |
| D. | 45.23 |
| Answer» D. 45.23 | |
| 298. |
For a 3 phase I.M. running at 1440 rpm the torque lost in friction is 50Nm then its frictional losses are____. |
| A. | 7.54 |
| B. | 4.23 |
| C. | 6.25 |
| D. | 8.45 |
| Answer» A. 7.54 | |
| 299. |
A 3 phase I.M. is running with speed 1455 rpm on full load then its synchronous speed must be ____ rpm. |
| A. | 1600 |
| B. | 1500 |
| C. | 2000 |
| D. | 1000 |
| Answer» B. 1500 | |
| 300. |
An induction motor is |
| A. | self starting with zero torque |
| B. | self starting with high torque |
| C. | self starting with small torque as compared to rated torque |
| D. | assisted starting with zero torque |
| Answer» C. self starting with small torque as compared to rated torque | |
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