MCQOPTIONS
Saved Bookmarks
MCQOPTIONS
This section includes 517 Mcqs, each offering curated multiple-choice questions to sharpen your Electrical Engineering knowledge and support exam preparation. Choose a topic below to get started.
| 301. |
When the excitation of an unloaded salient pole synchronous motor suddenly gets disconnected |
| A. | the motor stops |
| B. | it runs as a reluctance motor at the same speed |
| C. | it runs as a reluctance motor at a lower speed |
| D. | none of the above |
| Answer» B. it runs as a reluctance motor at the same speed | |
| 302. |
A synchronous motor can be started by |
| A. | pony motor |
| B. | D.C. compound motor |
| C. | providing damper winding |
| D. | any of the above |
| Answer» E. | |
| 303. |
If the field circuit of an unloaded salientpole synchronous motor gets suddenly open-circuited, then |
| A. | it runs at a slower speed |
| B. | the motor stops |
| C. | it continues to run at the same speed |
| D. | it runs at a very high speed |
| Answer» C. it continues to run at the same speed | |
| 304. |
The minimum armature current of the synchronous motor corresponds to operation at |
| A. | zero power factor leading |
| B. | unity power factor |
| C. | 0.707 power factor lagging |
| D. | 0.707 power factor leading |
| Answer» C. 0.707 power factor lagging | |
| 305. |
The operating speed of a synchronous motor can be changed to new fixed value by |
| A. | changing the load |
| B. | changing the supply voltage |
| C. | changing frequency |
| D. | using brakes |
| Answer» D. using brakes | |
| 306. |
If the field of a synchronous motor is underexcited, the power factor will be |
| A. | zero |
| B. | unity |
| C. | lagging |
| D. | leading |
| Answer» D. leading | |
| 307. |
The induced e.m.f. in a synchronous motor working on leading power factor will be |
| A. | more than the supply voltage |
| B. | less than the supply voltage |
| C. | equal to the supply voltage |
| D. | none of the above |
| Answer» B. less than the supply voltage | |
| 308. |
When the field of a synchronous motor is under-excited, the power factor will be |
| A. | leading |
| B. | lagging |
| C. | unity |
| D. | zero |
| Answer» C. unity | |
| 309. |
A synchronous motor can develop synchronous torque |
| A. | when under loaded |
| B. | while over-excited |
| C. | only at synchronous speed |
| D. | below or above synchronous speed |
| Answer» D. below or above synchronous speed | |
| 310. |
Change of D.C. excitation of a synchronous motor changes |
| A. | Applied voltage of the motor |
| B. | Motor speed |
| C. | Power factor of power drawn by the motor |
| D. | Any of the above |
| Answer» D. Any of the above | |
| 311. |
The maximum constant load torque under which a synchronous motor will pull into synchronism at rated rotor supply voltage and frequency is known as |
| A. | pull-up torque |
| B. | pull-in torque |
| C. | pull-out torque |
| D. | none of the above |
| Answer» C. pull-out torque | |
| 312. |
A synchronous motor connected to infinite bus-bars has at constant full load, 100% excitation and unity power factor. On changing the excitation only, the armature current will have |
| A. | no change of power factor |
| B. | lagging power factor with over-excitation |
| C. | leading power factor with under-excitation |
| D. | leading power factor with over-excitation |
| Answer» E. | |
| 313. |
A synchronous motor develops maximum power when load angle is |
| A. | 45° |
| B. | 60° |
| C. | 90° |
| D. | 120° |
| Answer» D. 120° | |
| 314. |
The working of a synchronous motor is similar to |
| A. | gear train arrangement |
| B. | transmission of mechancial power by shaft |
| C. | distribution transformer |
| D. | turbine |
| Answer» C. distribution transformer | |
| 315. |
When the rotor speed, in a synchronous machine, becomes more than the synchronous speed during hunting, the damper bars develop |
| A. | inductor motor torque |
| B. | induction generator torque |
| C. | synchronous motor torque |
| D. | d.c. motor toque |
| Answer» C. synchronous motor torque | |
| 316. |
Due to which of the following reasons a synchronous motor fails to pull into synchronism after applying D.C. field current? |
| A. | High field current |
| B. | Low short circuit ratio |
| C. | High core losses |
| D. | Low field current |
| Answer» E. | |
| 317. |
The efficiency of a properly designed synchronous motor will usually fall in range |
| A. | 60 to 70% |
| B. | 75 to 80% |
| C. | 85 to 95% |
| D. | 99 to 99.5% |
| Answer» D. 99 to 99.5% | |
| 318. |
The magnitude of field flux in a 3-phase synchronous motor |
| A. | remains constant at all loads |
| B. | varies with speed |
| C. | varies with the load |
| D. | varies with power factor |
| Answer» B. varies with speed | |
| 319. |
In a synchronous motor which loss varies with load ? |
| A. | Windage loss |
| B. | Bearing friction loss |
| C. | Copper loss |
| D. | Core loss |
| Answer» D. Core loss | |
| 320. |
A synchronous machine with low value of short-circuit ratio has |
| A. | lower stability limit |
| B. | high stability limit |
| C. | good speed regulation |
| D. | good voltage regulation |
| Answer» B. high stability limit | |
| 321. |
The duration of sudden short-circuit test on a synchronous motor is usually about |
| A. | one hour |
| B. | one minute |
| C. | one second |
| D. | none of the above |
| Answer» D. none of the above | |
| 322. |
Hunting in a synchronous motor cannot be due to |
| A. | windage friction |
| B. | variable load |
| C. | variable frequency |
| D. | variable supply voltage |
| Answer» B. variable load | |
| 323. |
By which of the following methods the constant speed of a synchronous motor can be changed to new fixed value ? |
| A. | By changing the supply frequency |
| B. | By interchanging any two phases |
| C. | By changing the applied voltage |
| D. | By changing the load |
| Answer» B. By interchanging any two phases | |
| 324. |
In a synchronous motor, the maximum power developed depends on all of the following except |
| A. | rotor excitation |
| B. | maximum value of coupling angle |
| C. | direction of rotation |
| D. | supply voltage |
| Answer» D. supply voltage | |
| 325. |
In case one phase of a three-phase synchronous motor is short-circuited the motor will |
| A. | not start |
| B. | run at 2/3 of synchronous speed |
| C. | run with excessive vibrations |
| D. | take less than the rated load |
| Answer» B. run at 2/3 of synchronous speed | |
| 326. |
An important advantage of a synchronous motor over wound round induction motor is that |
| A. | its power factor may be varied at will |
| B. | its speed is independent of supply frequency |
| C. | its speed may be controlled more easily |
| D. | none of the above |
| Answer» B. its speed is independent of supply frequency | |
| 327. |
The power factor of a synchronous motor is better than that of induction motor because |
| A. | stator supply is relieved of responsibility of producing magnetic field |
| B. | mechanical load on the motor can be adjusted |
| C. | synchronous motor runs at synchronous speed |
| D. | synchronous motor has large air gap |
| Answer» B. mechanical load on the motor can be adjusted | |
| 328. |
As the load is applied to a synchronous motor, the motor takes more armature current because |
| A. | The increased load has to take more current |
| B. | The rotor by shifting its phase backward causes motor to take more current |
| C. | The back e.m.f. decreases causing an increase in motor current |
| D. | The rotor strengthens the rotating field causing more motor current |
| Answer» C. The back e.m.f. decreases causing an increase in motor current | |
| 329. |
The size of a synchronous motor decreases with the increase in |
| A. | flux density |
| B. | horse power rating |
| C. | speed |
| D. | all of the above |
| Answer» B. horse power rating | |
| 330. |
For power factor correction, synchronous motors operate at |
| A. | no-load and greatly over-excited fields |
| B. | no-load and under-excited fields |
| C. | normal load with minimum excitation |
| D. | normal load with zero excitation |
| Answer» B. no-load and under-excited fields | |
| 331. |
The rotor copper losses, in a synchronous motor, are met by |
| A. | d.c. source |
| B. | armature input |
| C. | motor input |
| D. | supply lines |
| Answer» B. armature input | |
| 332. |
Riunting in a synchronous motor takes place |
| A. | When supply voltage fluctuates |
| B. | When load varies |
| C. | When power factor is unity |
| D. | Motor is under loaded |
| Answer» C. When power factor is unity | |
| 333. |
Stability of a synchronous machine |
| A. | decreases with increase in its excitation |
| B. | increases with increase in its excitation |
| C. | remains unaffected with increase in excitation |
| D. | any of the above |
| Answer» C. remains unaffected with increase in excitation | |
| 334. |
Which of the following losses, in a synchronous motor, does not vary with load? |
| A. | Windage loss |
| B. | Copper losses |
| C. | Any of the above |
| D. | None of the above |
| Answer» B. Copper losses | |
| 335. |
Which of the following motors is non-self starting ? |
| A. | D.C. series motor |
| B. | synchronous motor |
| C. | Squirrel cage induction motor |
| D. | Wound round induction motor |
| Answer» C. Squirrel cage induction motor | |
| 336. |
A synchronous motor working at leading power factor can be used as |
| A. | voltage booster |
| B. | phase advancer |
| C. | noise generator |
| D. | mechanical synchronizer |
| Answer» C. noise generator | |
| 337. |
Which of the following losses is not dissipated by the stator core surface in a synchronous motor ? |
| A. | Eddy current losses in the conductors |
| B. | Iron losses in the stator |
| C. | Copper losses in the slot portion of the conductors |
| D. | Windage losses |
| Answer» E. | |
| 338. |
While starting a synchronous motor by induction motor action, field winding is usually |
| A. | connected to D.C. supply |
| B. | short-circuited by low resistance |
| C. | kept open-circuited |
| D. | none of the above |
| Answer» C. kept open-circuited | |
| 339. |
An over-excited synchronous motor takes |
| A. | leading current |
| B. | lagging current |
| C. | both (A) and (B) |
| D. | none of the above |
| Answer» B. lagging current | |
| 340. |
Slip-rings in a synchronous motor carry |
| A. | direct current |
| B. | alternating current |
| C. | no current |
| D. | all of the above |
| Answer» B. alternating current | |
| 341. |
If excitation of a synchronous motor running with a constant load is decreased from its normal value, ignoring effects of armature reaction, it leads to |
| A. | increase in both armature current and power factor angle |
| B. | increase in back e.m.f. but decrease in armature current |
| C. | increase in both armature current and power factor which is lagging |
| D. | increase in torque angle but decrease in back e.m.f. |
| Answer» B. increase in back e.m.f. but decrease in armature current | |
| 342. |
In a synchronous motor, the magnitude of stator back e.m.f. Eb depends on |
| A. | load on the motor |
| B. | d.c. excitation only |
| C. | both the speed and rotor flux |
| D. | none of the above |
| Answer» C. both the speed and rotor flux | |
| 343. |
In a synchronous motor, the magnitude of stator back e.m.f. £& depends on |
| A. | d.c. excitation only |
| B. | speed of the motor |
| C. | load on the motor |
| D. | both the speed and rotor flux |
| Answer» B. speed of the motor | |
| 344. |
To limit the operating temperature an electrical machine should have proper |
| A. | voltage rating |
| B. | current rating |
| C. | power factor |
| D. | speed |
| Answer» C. power factor | |
| 345. |
The synchronous motor is not inherently self-starting because |
| A. | the force required to accelerate the rotor to the synchronous speed in an instant is absent |
| B. | the starting device to accelerate the rotor to near synchronous speed is absent |
| C. | a rotating magnetic field does not have enough poles |
| D. | the rotating magnetic field is produced by only 50 Hz frequency currents |
| Answer» B. the starting device to accelerate the rotor to near synchronous speed is absent | |
| 346. |
Synchronous motors are generally not self-starting because |
| A. | the direction of rotation is not fixed |
| B. | the direction of instantaneous torque reverses after half cycle |
| C. | startes cannot be used on these machines |
| D. | starting winding is not provided on the machines |
| Answer» C. startes cannot be used on these machines | |
| 347. |
If load (or torque) angle of a 4-pole synchronous motor is 6° electrical, its value in mechanical degrees is |
| A. | 2 |
| B. | 3 |
| C. | 4 |
| D. | 6 |
| Answer» C. 4 | |
| 348. |
The coupling angle or load angle of synchronous motor is defined as the angle between the |
| A. | rotor and stator teeth |
| B. | rotor and the stator poles of opposite polarity |
| C. | rotor and the stator poles of the same polarity |
| D. | none of the above |
| Answer» C. rotor and the stator poles of the same polarity | |
| 349. |
In a synchronous motor, the armature current has large values for |
| A. | high excitation only |
| B. | low excitation only |
| C. | both high and low excitation |
| D. | none of the above |
| Answer» D. none of the above | |
| 350. |
The synchronous motor is not inherently self-starting because%! |
| A. | the force required to accelerate the rotor to the synchronous speed in an instant is absent |
| B. | the starting device to accelerate the rotor to near synchronous speed is absent |
| C. | a rotating magnetic field does not have enough poles |
| D. | the rotating magnetic field is produced by only 50 Hz frequency currents |
| Answer» B. the starting device to accelerate the rotor to near synchronous speed is absent | |