Electric Drives Interview Questions:

Following are the below Electric Drives Interview Questions and Important Questions which is very useful for interviews and exams

1. What is an electric drive ? What are its advantages ?

2. Discuss criteria for selection of electric motor for a particular duty.

3. Discuss various factors which are required to be considered for selecting a motor.

4. What are the disadvantages of dc drives due to which the three-phase induction motor drive is fastly replacing it ?

5. Enlist the classification of electric drives and explain.

6. Differentiate between group drive and individual drive and give their merits.

7. What is meant by “group drive” and “individual drive” ? Discuss their merits and demerits

8. Write advantages and disadvantages of group drives.

9. Explain the various types of loads. What are the components of the load torque ?

10. What is meant by nature of load torque ? Explain difference between “passive load” and “active load” and give two examples of each category.

11. Explain active load and passive load giving two examples for each.

12. How do you define passive load torques and active load torques ? What arc the differences between the two ?

13. What are the components of load torque in a drive ? How each component is dependent on speed ?

14. Discuss the motoring and braking operations of an electric drive in both directions, forward and reverse taking the hoist as an example.

15. Explain four quadrant operation of an electric drive.

16. Describe four quadrant operation of a motor with illustrative figures for each. Show clearly the direction of motor torque, load torque and speed for each quadrant.

17. A motor is driving a hoist load. Discuss four quadrant operation of the motor and show that in the speed-torque plane. Indicate the polarity of torque, speed and power in each quadrant.

18. Discuss factors which decide selection of electric drive for a particular industrial application.

19. Classify various mechanical loads on the basis of their offered torque-speed characteristics.

20. Give typical example for following types of loads encountered in a drive.

• Continuous constant load.
• Continuous variable load.
• Pulsating load.
• Impact load.
• Short time intermittent loads.

21. What are constant torque and constant power operations of an electric drive ?

22. What is meant by “constant torque drive” and “constant power drive” ? Explain them and mention one application of each such drive.

23. Derive equation of motion and explain under what conditions motor will be accelerating, decelerating or at steady state.

24. Draw the torque-speed characteristics, giving examples with justification of the following types of loads: (i) Constant torque (ii) Constant power (iii) Torque directly proportional to speed.

25. Give the characteristics of dc shunt motors. Why are such motors not suitable for traction purposes ?

26. Draw speed-torque characteristics of a dc series motor. What modifications are expected in above characteristics when armature reaction is considered ?

27. Explain that the characteristic of a dc series motor is suitable for traction application.

28. Define steady state stability of electric drive. Derive a condition for steady state stability. Using this condition determine steady state operation point on torque-slip characteristics of three phase induction motor supplying constant load torque.

29. Discuss, with the help of curves, the characteristics and applications of the following electric motors: (a) 3-phase, cage type induction motor (b) 3-phase wound rotor induction motor (c) single phase induction motor (d) synchronous motor (e) dc shunt motor.

30. What do you understand by “series” and “shunt” speed-torque characteristics of electric motors used in industrial drives ? Name (at least one of each type) ac and dc motor with such characteristics.

31. Name the characteristics and applications of the following type of motors :

• DC compound wound motor.
• Synchronous-induction motor.
• Double-squirrel cage induction motor
• Schrage motor.

32. Derive an expression for accelerating time of three-phase induction motor having pure inertia load, started from a constant voltage supply. Mention the factors which affect the accelerating time.

33. Explain how the accelerating time and energy losses are calculated during a starting of a three-phase induction motor. Also enlist the methods of reducing the energy loss during starting.

34. Explain the dynamics during starting of a three-phase induction motor. What are the methods of reducing energy loss during starting ? Explain.

35. Draw and explain the starting characteristics of various types of dc motors.

36. What is the essential difference in the principle of operation of a starter for slip-ring and squirrel cage motor ?

37. Describe with suitable diagrams the various methods of starting 3-phase induction motors. Give relative merits and demerits of each method.

38. Discuss, with circuit diagram, the star-delta starter for a squirrel cage induction motor. Explain factors which decide the starter-switch change-over from star-to-delta position.

39. What is the drawback of a star/delta starter for an induction motor ? How is it overcome ?

40. Explain why field (or flux) control is employed for obtaining speeds higher than rated value and armature voltage control for speeds lower than rated in the case of dc separately excited motor.

41. Explain which properties make series motors more suitable for heavy torque applications. Also discuss various methods for controlling speed of dc series motors.

42. Draw the speed-torque characteristics for dc series motor. Show the effect of increase in series resistance on above characteristics. What is base speed in dc motors? How these motors can be run above the base speed ?

43. Describe with the help of connection diagram the Ward-Leonard method of speed control and explain its working. What are the merits and demerits of this method ? State its field of application.

44. Name the different methods of speed control of a 3-phase induction motor. Describe in brief speed control of induction motor by pole changing.

45. Describe the variable voltage, variable-frequency method of speed control of induction motors. Explain, with reasons, the voltage-frequency relationship for full-range speed control.

46. Derive the induction motor torque-speed characteristics under V/f control. Explain why E/f control is superior to V/f control.

47. What is electric braking ? What are the advantages and disadvantages of electric braking.

48. What do you understand by electric braking? Why is it necessary to have mechanical brakes in electric drives? What are the advantages of electric brakes over mechanical brakes? Describe in brief various methods of electric braking and mention their relative merits and demerits.

49. Explain various methods of electric braking. State the conditions to be fulfilled for each method of braking.

50. What are the advantages and disadvantages of electric braking ?

51. Explain the types of braking of dc motors. Which one is usually employed and why ?

52. Explain the various types of braking as used in a dc motor. What necessary modifications are required while braking a dc series motor ?

53. How can plugging be applied to a separately excited dc motor ?

54. Describe different methods of braking used for dc shunt and 3-phase squirrel cage induction motor.

55. Explain dc dynamic braking of 3-phase slip-ring induction motor and derive an expression for braking torque. How can the magnitude and the point of occurrence of the maximum braking torque be varied ?

56. Explain dc dynamic braking of 3-phase induction motor. Discuss effects of excitation and rotor resistance on the torque-slip characteristics of the motor.

57. Explain in 4-quadrant torque-speed plane the reversal of direction of a loaded dc shunt motor by plugging using a single resistor in the motor armature circuit.

58. Explain, with diagram, the Ward-Leonard dc drive using rotating machines. How is the regenerative braking achieved in this drive ?

59. What is regenerative braking? Why it cannot be accom­plished with a series wound dc motor without modifica­tions in the circuit diagram. Give its merits and demerits.

60. Specify the different types of enclosures of motors, highlighting the special feature of each type. Mention the area of application of each type.

61. Discuss the different types of drives required for transmission of power from the driving machines to the loads.

62. Discuss the following features pertaining to the selection of a motor: Types of enclosures, bearings, transmission of drive and noise.

63. What are the mechanical features to be considered in the selection of a motor for a particular service ?

64. What do you understand by the type of enclosure indicated as IP 44 ?

65. List the advantages of using roller or ball bearings for an induction motor.

66. State the factors that contribute to mechanical vibrations and noise.

67. Describe (i) flat belt drive (ii) V-belt drive (iii) rope drive (iv) chain drive (v) gear drive and (vi) direct drive.

68. From basic principles of heat transfer, develop an expression for temperature rise of an electric motor delivering fixed rated power at a fixed (rated) speed.

69. What is heating time constant ? Explain how the rating of a motor is affected by the temperature rise.

70. Explain what do you understand by ‘continuous’, ‘short time’, intermittent rating of an electric machine.

71. Define continuous duty, short time duty and periodic intermittent duty. Draw typical variations of output power losses, temperature rise with time in each case.

72. Derive an expression for the temperature rise of an electric motor. State the assumptions made.

73. A motor of smaller rating can be selected for a short time duty. why ?

74. Discuss different classes of motor duty cycles. Give industrial application for each class of duty cycle.

75. Discuss the equivalent power method for determining the rating of an electric motor which is subjected to intermittent periodic duty.

76. Draw the pattern of temperature rise characteristic under steady-state for (i) short-time duty (ii) intermittent duty.

77. What is the equivalent current method of estimating motor rating ? Explain. Can this method be used if motor speed varies during a load cycle ?

78. Explain the load diagram. Why is it important to know for selection of drive rating ?

79. Discuss ‘rms current’, ‘rrns torque’ and ‘rms power’ methods for evaluation and rating of motor for continuous duty, variable load conditions. Give their merits and limitations.

80. What are the problems faced in case a motor of wrong rating is chosen ?

81. What is load equalization ? How it is achieved ?

82. What are the reasons for load equalization in electric drive ?

83. Derive expression of torque equation in case of load increasing or flywheel releasing energy.

84. Derive an expression for the motor torque driving a rolling mill when equipped with flywheel.

85. What do you understand by load equalization ? Where do you find its applications ? Derive the expression for instantaneous motor torque, MOI of the flywheel, and the motor slip. State any assumptions made.

86. Explain the following statements.

• Pole changing method of speed control is used only in squirrel cage induction motors.
• Ward-Leonard method of speed control facilitates speed variation below the base speed, and is suited for constant torque drives.
• Motors used in domestic fans should not have low rotor resistance.
• By feeding energy to the rotor of an induction motor the speed variation can be achieved above the synchronous speed.

87. Write short notes on the following :

• Group and individual drives.
• Ward-Leonard method for speed control.
• Cascade connections of induction motors.
• Different methods of electric braking.
• Regenerative braking as applied to traction system.
• Types of enclosures for electrical machines.
• Rating of electric motors for different use with examples.
• Load equalization.