MCQ

Synchronous Motors | Objective Type Question Answers

 

1. In a synchronous motor, damper winding is provided in order to

2. In a synchronous motor, the magnitude of stator back e.m.f. Eb depends on

3. An electric motor in which both the rotor and stator fields rotates with the same speed is called a/an ........motor.

4. While running, a synchronous motor is compelled to run at synchronous speed because of

5. The direction of rotation of a synchronous motor can be reversed by reversing

6. When running under no-load condition and with normal excitation, armature current Ia drawn by a synchronous motor

7. The angle between the synchronously-rotating stator flux and rotor poles of a synchronous motor is called........ angle.

8. If load angle of a 4-pole synchronous motor is 8º (elect), its value in mechanical degrees is

9. The maximum value of torque angle a in a synchronous motor is ........degrees electrical.

10. A synchronous motor running with normal excitation adjusts to load increases essentially by increase in its

11. When load on a synchronous motor running with normal excitation is increased, armature current drawn by it increases because

12. When load on a normally-excited synchronous motor is increased, its power factor tends to

13. The effect of increasing load on a synchronous motor running with normal excitation is to

14. Ignoring the effects of armature reaction, if excitation of a synchronous motor running with constant load is increased, its torque angle must

15. If the field of a synchronous motor is underexcited, the power factor will be

16. Ignoring the effects of armature reaction, if excitation of a synchronous motor running with constant load is decreased from its normal value, it leads to

17. A synchronous motor connected to infinite busbars has at constant full-load, 100% excitation and unity p.f. On changing the excitation only, the armature current will have

18. The V-curves of a synchronous motor show relationship between

19. When load on a synchronous motor is increased, its armature currents is increased provided it is

20. If main field current of a salient-pole synchronous motor fed from an infinite bus and running at no-load is reduced to zero, it would

21. In a synchronous machine when the rotor speed becomes more than the synchronous speed during hunting, the damping bars develop

22. In a synchronous motor, the rotor Cu losses are met by

23. A synchronous machine is called a doubly-excited machine because

24. Synchronous capacitor is


 

Alternators | Objective Type Question Answers

 

1. The power factor of an alternator is determined by its

2. For proper parallel operation, a.c. polyphase alternators must have the same

3. Of the following conditions, the one which does not have to be met by alternators working in parallel is

4. After wiring up two 3-φ alternators, you checked their frequency and voltage and found them to be equal. Before connecting them in parallel, you would

5. Zero power factor method of an alternator is used to find its

6. It is never advisable to connect a stationary alternator to live bus-bars because it

7. Two identical alternators are running in parallel and carry equal loads. If excitation of one alternator is increased without changing its steam supply, then

8. Keeping its excitation constant, if steam supply of an alternator running in parallel with another identical alternator is increased, then

9.The load sharing between two steam-driven alternators operating in parallel may be adjusted by varying the

10. Squirrel-cage bars placed in the rotor pole faces of an alternator help reduce hunting


 

Alternators | Objective Type Question Answers

 

1. The frequency of voltage generated by an alternator having 4-poles and rotating at 1800 r.p.m. is .......hertz.

2. A 50-Hz alternator will run at the greatest possible speed if it is wound for ....... poles.

3. The main disadvantage of using short-pitch winding in alterators is that it

4. Three-phase alternators are invariably Y-connected because

5. The winding of a 4-pole alternator having 36 slots and a coil span of 1 to 8 is short-pitched by ....... degrees.

6. If an alternator winding has a fractional pitch of 5/6, the coil span is ....... degrees.

7. The harmonic which would be totally eliminated from the alternator e.m.f. using a fractional pitch of 4/5 is

8. For eliminating 7th harmonic from the e.m.f. wave of an alternator, the fractional-pitch must be

9. If, in an alternator, chording angle for fundamental flux wave is α, its value for 5th harmonic is

10. Regarding distribution factor of an armature winding of an alternator which statement is false?

11. When speed of an alternator is changed from 3600 r.p.m. to 1800 r.p.m., the generated e.m.f./phases will become

12. The magnitude of the three voltage drops in an alternator due to armature resistance, leakage reactance and armature reaction is solely determined by

13. Armature reaction in an alternator primarily affects

14. Under no-load condition, power drawn by the prime mover of an alternator goes to

15. As load p.f. of an alternator becomes more leading, the value of generated voltage required to give rated terminal voltage

16 . With a load p.f. of unity, the effect of armature reaction on the main-field flux of an alternator is

17. At lagging loads, armature reaction in an alternator is

18. At leading p.f., the armature flux in an alternator ....... the rotor flux.

19. The voltage regulation of an alternator having 0.75 leading p.f. load, no-load induced e.m.f. of 2400 V and rated terminal voltage of 3000 V is ............... percent.

20. If, in a 3-φ alternator, a field current of 50 A produces a full-load armature current of 200 A on short-circuit and 1730 V on open circuit, then its synchronous impedance is ....... ohm.


 

DC Motors | Objective Type Question Answers

 

1. The counter e.m.f. of a d.c. motor

2. The normal value of the armature resistance of a d.c. motor is

3. The mechanical power developed by the armature of a d.c. motor is equal to

4. The induced e.m.f. in the armature conductors of a d.c. motor is

5. A d.c. motor can be looked upon as d.c. generator with the power flow

6. In a d.c. motor, the mechanical output power actually comes from

7. The maximum torque of d.c. motors is limited by

8. Which of the following quantity maintains the same direction whether a d.c. machine runs as a generator or as a motor ?

9. Under constant load conditions, the speed of a d.c. motor is affected by

10. It is possible to increase the field flux and, at the same time, increase the speed of a d.c. motor provided its .......... is held constant.

11. The current drawn by a 120 - V d.c. motor of armature resistance 0.5 Ω and back e.m.f. 110 V is .......... ampere.

12. The shaft torque of a d.c. motor is less than its armature torque because of .......... losses.

13. A d.c. motor develops a torque of 200 N-m at 25 rps. At 20 rps it will develop a torque of .......... N-m.

14. Neglecting saturation, if current taken by a series motor is increased from 10 A to 12 A, the percentage increase in its torque is ........ percent.

15. If load on a d.c. shunt motor is increased, its speed is decreased due primarily to

16. If the load current and flux of a d.c. motor are held constant and voltage applied across its armature is increased by 10 per cent, its speed will

17. If the pole flux of a d.c. motor approaches zero, its speed will

18. If the field circuit of a loaded shunt motor is suddenly opened

19. Which of the following d.c. motor would be suitable for drives requiring high starting torque but only fairly constant speed such as crushers ?

20. A d.c. shunt motor is found suitable to drive fans because they require

21. As the load is increased, the speed of a d.c. shunt motor

22. Between no-load and full-load, .......... motor develops the least torque

23. As compared to shunt and compound motors, series motor has the highest torque because of its comparatively .......... at the start.

24. When load is removed, .......... motor will run at the highest speed.

25. A series motor is best suited for driving

26. A 220 V shunt motor develops a torque of 54 N-m at armature current of 10 A. The torque produced when the armature current is 20 A, is

27. The d.c. series motor should never be switched on at no load because

28. A shunt d.c. motor works on a.c. mains

29. A 200 V, 10 A motor could be rewound for 100 V, 20 A by using .......... as many turns per coil of wire, having .......... the cross-sectional area.


 

DC Motors-2| Objective Type Question Answers

 

1. The speed of a d.c. motor can be controlled by varying

2. The most efficient method of increasing the speed of a 3.75 kW d.c. shunt motor would be the ...........method.

3. Regarding Ward-Leonard system of speed control which statement is false ?

4. In the rheostatic method of speed control for a d.c. shunt motor, use of armature divertor makes the method

5. The chief advantage of Ward-Leonard system of d.c. motor speed control is that it

6. In practice, regenerative braking is used when

7. Ward-Leonard system of speed control is NOT recommended for

8. Thyristor chopper circuits are employed for

9. An invertor circuit is employed to convert

10. The phase-control rectifiers used for speed of d.c. motors convert fixed a.c. supply voltage into

11. If some of the switching devices in a convertor are controlled devices and some are diodes, the convertor is called

12. A solid-state chopper converts a fixed-voltage d.c. supply into a

13. The d.c. motor terminal voltage supplied by a solid-state chopper for speed control purposes varies...........with the duty ratio of the chopper

14. One of the main advantages of Swinburne’s test is that it

15. The main disadvantage of Hopkinson’s test for finding efficiency of shunt d.c. motors is that it

16. The most economical method of finding no-load losses of a large d.c. shunt motor is .....test.

17. Retardation test on a d.c. shunt motor is used for finding ........ losses.

18. The main thing common between Hopkinson’s test and Field’s test is that both


 

Transformer | Objective Type Question Answers

 

1. A transformer transforms

2. Which of the following is not a basic element of a transformer ?

3. In an ideal transformer,

4. The main purpose of using core in a transformer is to

5. Transformer cores are laminated in order to

6. A transformer having 1000 primary turns is connected to a 250-V a.c. supply. For a secondary voltage of 400 V, the number of secondary turns should be

7. The primary and secondary induced e.m.fs. E1 and E2 in a two-winding transformer are always

8. A step-up transformer increases

9. The primary and secondary windings of an ordinary 2-winding transformer always have

10. In a transformer, the leakage flux of each winding is proportional to the current in that winding because

11. In a two-winding transformer, the e.m.f. per turn in secondary winding is always.......the induced e.m.f. per turn in primary.

12. In relation to a transformer, the ratio 20 : 1 indicates that

13. In performing the short circuit test of a transformer

14. The equivalent resistance of the primary of a transformer having K = 5 and R1 = 0.1 ohm when referred to secondary becomes.......ohm.

15. A transformer has negative voltage regulation when its load power factor is

16. The primary reason why open-circuit test is performed on the low-voltage winding of the transformer is that it

17. No-load test on a transformer is carried out to determine

18. The main purpose of performing open-circuit test on a transformer is to measure its

19. During short-circuit test, the iron loss of a transformer is negligible because

20. The all-day efficiency of a transformer depends primarily on

21. In operating a 400 Hz transformer at 50 Hz

22. The marked increase in kVA capacity produced by connecting a 2 winding transformer as an auto-transformer is due to

23. Transformers are rated in kVA instead of kW because

24. When a 400-Hz transformer is operated at 50 Hz its kVA rating is

25. At relatively light loads, transformer efficiency is low because

26. A 200 kVA transformer has an iron loss of 1 kW and full-load Cu loss of 2kW. Its load kVA corresponding to maximum efficiency is ....... kVA.

27. If Cu loss of a transformer at 7/8th full load is 4900 W, then its full-load Cu loss would be .......watt.

28. The ordinary efficiency of a given transformer is maximum when

29. The output current corresponding to maximum efficiency for a transformer having core loss of 100 W and equivalent resistance referred to secondary of 0.25 Ω is ....... ampere.

30. The maximum efficiency of a 100-kVA transformer having iron loss of 900 kW and F.L. Cu loss of 1600 W occurs at ....... kVA.

31. Which of the following connections is best suited for 3-phase, 4-wire service ?

33. In a three-phase Y-Y transformer connection, neutral is fundamental to the

34. Of the following statements concerning parallel operation of transformers, the one which is not correct is

35. Instrument transformers are used on a.c. circuits for extending the range of

36. Before removing the ammeter from a current transformer, its secondary must be short-circuited in order to avoid

37. The kVA rating of an ordinary 2-winding transformer is increased when connected as an autotransformer because

38. The saving in Cu achieved by converting a 2-winding transformer into an autotransformer is determined by

39. An autotransformer having a transformation ratio of 0.8 supplies a load of 3 kW. The power transferred conductively from primary to secondary is.......kW.

40. The essential condition for parallel operation of two 1-φ transformers is that they should have the same

41. If the impedance triangles of two transformers operating in parallel are not identical in shape and size, the two transformers will

42. Two transformers A and B having equal outputs and voltage ratios but unequal percentage impedances of 4 and 2 are operating in parallel. Transformer A will be running over-load by ....... percent.


 
 

AC Motors | Objective Type Question Answers

 

1. In a squirrel cage induction motor, torque with autostarter is ....... times the torque with direct-switching. Where K is the transformation ratio of the autotransformer.

2. If stator voltage of a squirrel cage induction motor is reduced to 50 per cent of its rated value, torque developed is reduced by ....... per cent of its full-load value.

3. For the purpose of starting an induction motor, a Y-Δ switch is equivalent to an auto-starter of ratio.......per cent.

4. A double squirrel-cage motor (DSCM) scores over SCIM in the matter of

5. In a DSCM, outer cage is made of high resistance metal bars primarily for the purpose of increasing its

6. A SCIM with 36-slot stator has two separate windings : one with 3 coil groups/ phase/pole and the other with 2 coil groups/phase/pole. The obtainable two motor speeds would be in the ratio of

7. A 6-pole 3-φ induction motor taking 25 kW from a 50-Hz supply is cumulatively-cascaded to a 4-pole motor. Neglecting all losses, speed of the 4-pole motor would be ....... r.p.m.

8. Two separate induction motors, having 6 poles and 5 poles respectively and their cascade combination from 60 Hz, 3-phase supply can give the following synchronous speeds in rpm

9. When a stationary 3-phase induction motor is switched on with one phase disconnected

10. If single-phasing of a 3-phase induction motor occurs under running conditions, it

11. One of the characteristics of a single- phase motor is that it

12. After the starting winding of a single- phase induction motor is disconnected from supply, it continues to run only on ............winding.

13. The direction of rotation of a single-phase motor can be reversed by

14. If a single-phase induction motor runs slower than normal, the more likely defect is

15. The capacitor in a capacitor-start induction- run ac motor is connected in series with ...... winding.

16. The starting torque of a capacitor-start induction-run motor is directly related to the angle α between its two winding currents by the relation

17. In a two-value capacitor motor, the capacitor used for running purposes is a/an

18. If the centrifugal switch of a two-value capacitor motor using two capacitors fails to open, then

19. Each of the following statements regarding a shaded-pole motor is true except

20. Compensating winding is employed in an ac series motor in order to

21. A universal motor is one which

22. In a single-phase series motor the main purpose of inductively-wound compensating winding is to reduce the

23. A repulsion motor is equipped with

24. A repulsion-start induction-run single- phase motor runs as an induction motor only when

25. If a dc series motor is operated on ac supply, it will

26. An outstanding feature of a universal motor is its

27. The direction of rotation of a hysteresis motor is determined by the

28. Speed of the universal motor is

29. In the shaded pole squirrel cage induction motor the flux in the shaded part always

30 . Which of the following motor is an interesting example of beneficially utilizing a phenomenon that is often considered undesirable ?


 

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