501 MCQ Questions on DC Machine

1. A DC cumulatively compounded motor delivers rated load torque at rated speed. If the series field is short-circuited, then the armature current and speed will

(a) both decrease.
(b) both increase.
(c) increase and decrease respectively.
(d) decrease and increase respectively.

2. A dc cumulatively compounded generator was operating satisfactorily and supplying power to an infinite bus when the mechanical power supply from the prime mover failed. The machine will then run as a

(a) differentially compounded motor with speed reversed.
(b) differentially compounded motor with the direction of speed the same as before.
(c) cumulatively compounded motor with the same direction of speed as before.
(d) cumulatively compounded motor with speed reversed.

3. A cumulatively compounded dc motor runs at 1000 rpm at no-load. On full load, the flux increases by 10%, whereas the full load drop in the combined resistance of the armature and series field is 5%. Neglecting magnetic saturation, the full load speed will be nearly

(a) 863 rpm.
(b) 909 rpm.
(c) 1000 rpm.
(d) 1050 rpm

4. A separately excited dc machine, having an armature resistance of 2 ohms was working on a 220 V supply and drawing 10 A armature current from the source when the supply voltage suddenly dropped to 200 V. Assuming that the field circuit source voltage remained unaffected how will the armature current of the machine react to the change?

(a) It will initially rise to 11A and then settle down to 10A.
(b) It will fall momentarily to 9.09A and then slowly attain 10A.
(c) It will reduce to zero first and then settle back to 10A.
(d) It will remain unaffected by the change and continue to be 10A.

5. A dc overcompound generator is supplying power to an infinite bus. If the prime mover is accidentally cut off, the dc machine will

(a) stop running.
(b) run as cumulatively compounded motor in reverse direction.
(c) run as differentially compounded motor in reverse direction.

6. A 6-pole lap-connected de generator with 480 conductors has an armature resistance of 0.06 ohm. If the conductors are reconnected to form a wave winding, other things remaining unchanged, the value of the armature resistance will be

(a) 0.01 ohm
(b) 0.06 ohm
(c) 0.36 ohm
 (d) 0.54 ohm

7. The introduction of interpoles in between the main poles improves the performance of a dc machine, because

(a) the interpoles produce additional flux to augment the developed torque.
(b) the flux waveform is improved with reduction in harmonics.
(c) the inequality of air-gap flux on the top and bottom halves of the armature is removed.
(d) a counter-emf is induced in the coil undergoing commutation.

8. In a dc machine, the armature mmf is

(a) rectangular and directed along the inter-polar axis.
(b) triangular and directed along the inter-polar axis.
(c) triangular and directed along the brush axis.
(d) rectangular and directed along the brush-axis.

9. A shunt motor is running at its rated speed on rated load. If the field circuit gets suddenly opened

(a) torque developed by the motor will be almost zero and the motor will stop without causing any damage to the motor.
(b) the motor will draw very high armature current but it will continue to drive the load at a reduced speed.
(c) the motor would tend to race to a high speed while driving the load.
(d) the motor would draw very high armature current and will stall with consequent damage to the armature.

10. When starting a differentially connected compound motor, it is desirable to short-circuit the series field winding to

(a) avoid very high starting time.
(b) avoid excessive starting speed.
(c) prevent the motor from starting in the reverse direction.
(d) avoid heavy in-rush of current.

11. A separately excited de motor is fed from a phase- controlled converter. The motor exhibits poor speed regulation, this is due to

(a) inductive voltage drop of the converter and possible discontinuous conduction.
(b) harmonics in the armature current.
(c) large inductance of the load current.
(d) poor power factor of the converter.

12. In a loaded de generator, if the brushes are given a shift from the inter polar axis in the direction of rotation, then the commutation will

(a) improve with fall of terminal voltage.
(b) deteriorate with fall of terminal voltage.
(c) improve with rise in terminal voltage.
(d) deteriorate with rise in terminal voltage.

13. A dc shunt motor is required to drive a constant power load at rated speed while drawing rated armature current. Neglecting saturation and all machine losses, if both the terminal voltage and the field current of the machine are halved, then

(a) the speed becomes 2 pu (per unit) but armature current remains at 1 pu.
(b) the speed remains at 1pu but armature current becomes 2 pu.
(c) both speed and armature current becomes 2 pu.
(d) both speed and armature current remain at 1 pu.

14. The series winding of a cumulatively compounded dc motor is short-circuited while driving a load at rated torque. This results in

(a) reduction in both the armature current and the motor speed.
(b) increase in the armature current and reduction in the motor speed.
(c) increase in both the armature current as well as the motor speed.
(d) reduction in the armature current and increase in the motor speed.

15. Which one of the following statements for a dc machine which is provided with interpole windings (IW) as well as compensating winding (CW) is correct?

(a) Both IW and CW are connected in series with the armature winding.
(b) Both IW and CW are connected in parallel with the armature winding.
(c) IW is connected in series but CW is connected in parallel with the armature winding.
(d) IW is connected in parallel but CW is connected in series with the armature winding.

16. Which one of the following is the correct statement? Field control of d.c. shunt motor gives

(a) Constant kW drive
(b) Constant torque drive
(c) Constant speed drive
(d) Variable load speed drive

17. To prevent the shifting of the magnetic neutral axis, caused by the armature reaction in a d.c machine, the most effective method, to neutralize the armature-flux is to

(a) shift the ‘brush-axis.’
(b) provide high-reluctance main pole tips.
(c) cut horizontal slots in the main poles.
(d) place compensating windings on the main-pole faces.

18. A separately excited d.c. motor takes a current I of a speed N. If it draws a current I/3 at a speed 3N, this condition is obtained with

(a) field control under constant torque.
(b) voltage control under constant torque.
(c) field control under constant output.
(d) voltage control under constant output.

19. Consider the following statements in respect of compensating windings in d.c. motors:

1. Compensating windings are connected in series with the armature.
2. Compensating windings improves commutation.
3. Compensating windings produce mmf in the same direction as that of armature mmf.

Which of these statements is/are correct?
(a) 2 and 3
(b) only 1
(c) 1 and 3
(d) 1 and 2

20. Match List-I (Test) with List-II (Object) and select the correct answer using the codes given below the lists:

List-I

1. Slip test
2. Open circuit and zero power factor test
3. Sumpner’s test
4. Swinburne’s test

List-II

1. Determination of constant losses of a d.c shunt machine.
2. Determination of efficiency and regulation of transformer.
3. Determination of synchronous Potier reactance of synchronous machine.
4. Determination of direct and quadrature axis synchronous reactances of salient pole synchronous machine.

(a) A-2, B-1, C-3, D-4
(b) A-4, B-3, C-2, D-1
(c) A-2, B-3, C-1, D-4
(d) A-4,B-1, C-3, D-2

21. The series and shunt field windings of a short shunt cumulatively compound d.c. motor get interchanged by mistake. On supplying rated voltage, the motor shall

(a) run normally.
(b) run in the reverse direction.
(c) run as differentially compounded.
(d) not run.

22. In a d.c. motor if the brushes are shifted opposite to its direction of rotation, then

(a) commutation is worsened and speed decreases.
(b) commutation is improved and speed decreases.
(c) commutation is worsened and speed increases.
(d) commutation is improved and speed increases.

23. A d.c. shunt motor with negligible armature resistance is required to drive a constant power load, under normal rated-load operating conditions when the terminal voltage V, 1.0 p.u., the speed N = 1.0 p.u. and the armature current I, = 1.0 p.u. and, with linear magnetizing characteristic the field flux = 1.0 p.u. If V1 = 0.5 p.u. and the flux 4 is kept constant at 1.0 p.u., then

(a) N = 1/2 p.u. and I = 2.0 p.u.
(b) N = 1.0 p.u. and I = 2.0 p.u.
(c) N = 2.0 p.u. and I = 1.0 p.u.
(d) N = 1/2 p.u. and I, = 1/2 p.u.

24. A dc shunt generator builds up to a voltage of 220 V at no-load while running at rated speed If the speed of the generator is raised by 25% keeping the circuit conditions unaltered, then the voltage to which the machine would build up will

(a) not change and remain at 220 V.
(b) increase to 1.25 times 220 V.
(c) increase to a value lying between 220 V and 1.25 times 220 V.
(d) increase to a value greater than 1.25 times 220 V.

25. In a dc machine, the demagnetizing effect of armature reaction is due to

(a) component of armature mmf along field axis.
(b) non-sinusoidal nature of armature mmf.
(c) magnetic saturation in half of the field pole.
(d) uneven air gap length.

26. Plugging of dc motors is normally done by

(a) connecting a resistance across the armature.
(b) reversing simultaneously the armature and field polarity.
(c) reversing the field polarity.
(d) reversing the armature polarity.

27. A dc shunt wound motor finds application in

(a) electric trains.
(b) tape recorders.
(c) mixer.
(d) steel rolling mills.

28. Consider the following statements: A dc shunt motor starter ensures that

1. Armature current is under limit during starting.
2. Field flux is maintained at the maximum value
3. Acceleration time is controlled.
4. Field failure is prevented.
5. Starting torque is always more than the load torque.

Of these statements:
(a) 1, 2, 3 and 5 are correct
(b) 1, 2, 3 and 4 are correct
(c) 1 and 4 are correct
(d) 1, 3 and 4 are correct

29. Fleming’s Right Hand Rule is used for the determination of direction of

(a) force acting on a current carrying conductor kept in a magnetic field.
(b) motion of a current carrying conductor kept in on electric field.
(c) current induced in a conductor due to its motion in a magnetic field
(d) deflection of a charged particle moving in a magnetic field.

30. A torque is developed in an electro mechanical energy conversion device, the value of which depends upon

(a) stator field strength and torque angle.
(b) stator field and rotor field strengths.
(c) stator field and rotor field strengths and the torque angle.
(d) stator field strength only.

31. If the field of a dc shunt motor gets opened while the motor is running, then the

(a) speed of the motor will be reduced.
(b) motor will attain dangerously high speed.
(c) armature current will drop.
(d) armature will oscillate about original speed as the mean speed.

32. The residual magnetism of a self-excited dc generator is lost. To build up its emf again

(a) the field winding must be replaced.
(b) the armature connection must be reversed.
(c) the field winding connection must be reversed.
(d) field winding must be excited by low voltage dc supply.

33. In a large DC series motor, why is shunt field winding provided?

(a) To avoid field distortion.
(b) To generate working flux.
(c) To improve commutation.
(d) To avoid runaway speeds at no-load.

34. A DC motor on switching was found to rotate in a direction opposite to the normal direction of rotation. What type of motor is?

(a) Cumulative compound
(b) Differential compound
(c) Series
(d) Shunt

35. In a regenerative braking, which of the following is generally true?

(a) Back e.m.f. in the motor exceeds the applied voltage.
(b) Back e.m.f is less than applied voltage.
(c) Kinetic energy of the motor is dissipated in a resistance.
(d) Kinetic energy of the motor is dissipated through free wheeling diode across the motor.

36. Wave winding is employed in a dc machine of

(a) high current and low voltage rating.
(b) low current and high voltage rating.
(c) high current and high voltage rating.
(d) low current and low voltage rating.

37. If a 230 V DC series motor is connected to a 230 V ac supply

(a) the motor will vibrate violently.
(b) the motor will run with less efficiency and more sparking.
(c) the motor will not run.
(d) the fuse will be blown.

38. The value of back emf (E) in a dc motor in maximum at

(a) no load.
(c) half full load.
(b) full load.
(d) none of the above.

39. The function of a commutator in a dc machine is

(a) to improve commutation.
(b) to change ac current to dc current.
(c) to change de voltage to ac voltage.
(d) to provide easy speed control.

40. As the speed of dc generator is increased, the generated emf

(a) increases.
(b) decreases.
(c) remains constant.
(d) first increases and then decreases.

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

(a) sinusoidal.
(b) trapezoidal.
(c) rectangular.
(d) alternating. [RRB SSE – 2015]

42. In a Brush Less DC (BLDC) motor, the construction of motor is similar to

(a) stepper motor.
(c) dc motor.
(b) universal motor.
(d) synchronous motor.

43. Which of the following results in an increase in the induced voltage?

(a) When we decrease the speed at which a conductor is moved through a magnetic field.
(b) When we increase the speed at which a conductor is moved through a magnetic field.
(c) When we first increase and then decrease the speed at which a conductor is moved through a magnetic field.
(d) None of these.

44. The direction of the magnetic field reverses when

(a) the current through the coil of an electromagnet increases beyond certain limit.
(b) the current through the coil of an electromagnet reverses.
(c) the voltage through the coil of an electromagnet halves.
(d) none of these.

45. What is the induced voltage across a coil with 220 turns located in a magnetic field that is changing at a rate of 10 Wb/s?

(a) 22V
(c) 2200 V
(b) 220V
(d) None of these

46. The flux density in a wire wound core can be increased by

(a) decreasing the current through the coil.
(b) increasing the current through the coil.
(c) increasing the ambient pressure.
(d) none of these.

47. Consider a 100-turn coil of wire with 0.5 A of current through it. What is the magnetomotive force?

(a) 50 AT
(b) 500 AT
(c) 5 AT
(d) None of these

48. What is the flux density of a magnetic field whose flux is 1000 μWb and cross-sectional area is 0.5 m²?

(a) 500 μT
(b) 5000 μT
(c) 2000 μT
(d) None of these

49. Which of the following does Faraday’s law deal with?

(a) A magnetic field and a conductor.
(b) A conductor in an extremely low temperature.
(c) A magnetic field of the planes.
(d) None of these.

50. A 250 V de motor has an armature resistance of 0.25Ω. It is drawing an armature current of 25A, driving a certain load. Calculate the induced emf in the motor under this condition. [UPPCL JE – 2014]

(a) 180.5 V
(b) 243.75 V
(c) 192.5 V
(d) 625 V