Thyristor Control of Electric Motors Interview Questions:

Following are the below Thyristor Control of Electric Motors Interview Questions and Important Questions which is very useful for interviews and exams

1. Why thyristor controlled drives employing both dc and ac motors find wide application in industry ? Explain.

2. Give the classification of static power converters.

3. What are the factors that govern the choice of current rating, voltage rating and switching frequency of the power electronic converter for electric motor drive ?

4. Explain working principle of single phase half controlled bridge converter fed separately excited dc motor and draw waveforms of motor terminal voltage, motor current and supply current.

5. Compare its merits and demerits with respect to when fed by fully controlled bridge converter.

6. Explain how the speed control of a separately excited dc motor can be achieved by a fully controlled single phase thyristor bridge converter.

7. Draw the complete drive circuit of a single phase fully controlled rectifier controlling a separately excited dc motor. The circuit should include the single phase supply, controlled rectifier (including the thyristors) and the motor. Draw typical waveforms of supply voltage, armature voltage and current at a fixed speed under continuous conduction. Derive an expression for motor speed in terms of supply voltage, firing angle and motor constants.

8. Compare and contrast a half-controlled converter with a fully controlled converter for speed control of a separately excited dc motor. In which case supply power factor is better and why ?

9. Compare the performance of a separately excited dc motor when fed from a I-phase fully controlled bridge converter with that of the same motor fed from a I-phase half controlled bridge. Draw the relevant waveforms in each case and sketch the variation of average output voltage vs firing angle.

10. Describe the operation of a single phase half controlled bridge converter fed separately excited dc motor and draw waveforms of motor terminal voltage, armature and supply currents assuming continuous current conduction. List its merits and demerits in comparison to that of with fully controlled bridge rectifier.

11. How do you change from motoring to braking operation in forward direction of a dc separately excited motor when two fully controlled thyristor converters are used in back-to-back connection? Explain in detail.

12. Draw the circuit arrangements for a dual converter and explain its operation.

13. Describe, with power circuit diagram, the dual converter. What is the relationship between firing angles of the two full converters of the dual converter under continuous current conduction mode ? Explain Why ?

14. Explain two modes in which a dual converter is used to control the dc motor speed. Which of the two methods is better and why ? Explain.

15. A three phase fully controlled bridge converter is feeding a separately excited dc motor. Assuming continuous load current, derive the equation relating speed and torque of a motor as a function of triggering delay ‘α’.

16. Enumerate the different methods of obtaining variable dc voltage from a fixed dc voltage source. Give their advantages and disadvantages.

17. What is a dc chopper ? Explain the principle of operation of a chopper giving its circuit diagram and output voltage waveforms.

18. Explain the time ratio control (TRC) and current limit control used for choppers.

19. What is the analogy between a chopper and transformer ? Describe the working and obtain an expression for average load voltage of step-up chopper.

20. Discuss in full, the chopper control of a dc separately excited motor.

21. Draw the block diagram for chopper fed dc motor drive.

22. Draw the power circuit of a single quadrant dc chopper and explain its working.

23. Draw the circuit for the chopper control of a separately excited dc motor and derive an expression for speed in terms of motor constants, supply voltage, duty ratio and load torque.

24. Regenerative braking is energy efficient braking, explain in view of chopper drives.

25. Discuss a chopper circuit for regenerative braking of a separately excited dc motor. Explain the operating principle.

26. State the advantages of chopper controlled dc motors.

27. How is the performance of a dc motor affected by thyristor power supply ? Explain.

28. What are the special problems associated with the operation of dc drives where speed is controlled by phase controlled converters and choppers ?

29. Explain the speed control of a dc series motor using thyristors.

30. Explain in brief, on the importance of supply harmonics, power factor and ripples in the motor current, in the performance of a converter fed dc motor drive.

31. Explain the chopper control techniques for each of a separately excited dc motor and dc series motor.

32. How ac motor speed control is achieved by using voltage source inverter ?

33. Explain, giving block diagram, the operation of a dc link voltage source inverter for speed control of 3-phase induction motors.

34. What is ‘pulse width modulation (PWM)’ ? Explain the purpose.

35. tate the advantages of ac voltage control by PWM technique. Explain the sinusoidal PWM technique together with relevant waveforms.

36. What is meant by VVVF control of induction motors ? Explain with a block diagram and with reference to both constant torque and constant power modes of operation.

37. Draw the power circuit of a 3-phase current source inverter and explain its working, State also its inherent advantages over voltage source inverter for speed control of ac motor.

38. Distinguish between voltage source and current source invertets, Compare the two inverters for speed control of 3-phase induction motor.

39. Discuss the advantages and disadvantages of cycloconverter for speed control of 3-phase induction motors.

40. What is a cycloconverter ? Enumerate some of its industrial applications.

41. What is meant by “soft start” ? State and explain the soft start schemes (including electronic means, if any) employed for induction motors.

42. Draw neat diagram for static rotor resistance control scheme using diode bridge and switch controlled resistor.

43. With neat circuit diagram, explain the speed control of a 3-phase slip-ring induction motor by static control of rotor resistance through chopper.

44. Discuss a static slip-power recovery scheme for speed control of slip-ring induction motor.

45. Explain the use of chopper for rotor resistance control in induction motors. Compare this scheme with slip-power recovery scheme, specially at lower speeds, stating the merits and demerits. Include circuits as needed.

46. Explain thyristor equivalent Kramer system of controlling speed of a slip-ring induction motor and regenerative operation.

47. Explain any one slip power recovery scheme for the control of three-phase induction motor drive. Can supersynchronous speeds be achieved through it ? Explain.

48. Explain the slip power recovery scheme of a three-phase induction motor control. What are its advantages and disadvantages ?

49. Explain working of static slip power recovery control scheme returning power to supply mains for speed control of three phase slip-ring induction motor. Why is supply power factor poor and how can it be improved ?

50. Discuss static Scherbius scheme for speed control of a slip-ring induction motor. Draw a neat circuit diagram of the complete scheme. Mention one main advantage of this scheme compared to rotor resistance control.

51. Discuss with the help of a suitable schematic diagram, the operation of a load commutated-fed synchronous motor drive. Why is this drive suitable for high speed and high power applications?

52. Explain the self controlled scheme for the synchronous motor control.

53. Describe self-controlled and load commutated inverter controlled synchronous motor drives in detail and compare them.

54. Explain static speed control of a single phase induction motor.

55. What are the similarities between a brushless dc motor and a conventional dc motor? Give the advantages of brushless dc motor over conventional dc motor.

56. Briefly explain the construction and principle of operation of a switched reluctance motor.

57. Describe the construction and principle of operation of a switched reluctance motor.

58. Describe working principle of switched reluctance motor. Discuss any one converter circuit used. What are its advantages over other ac motor drives.

59. Write short notes on the following :

• Half controlled converters.
• Full controlled converters.
• Dual converter drives.
• Chopper drives.
• Comparison of converter fed and chopper fed dc drives.
• Thyristor control of dc series motors.
• Control circuits for ac drives.
• Cycloconverters.
• Slip-power recovery scheme for speed control of slip-ring induction motor.
• Load commutated inverter drive for synchronous motor.
• Brushless dc motor.
• Switched reluctance motor.