Thyristor Control of Electric Motors Interview Questions and Answers:

1. Why the thyristor control is preferred over Ward Leonard system of speed control ?

Ans. The thyristor control has some distinct advantages over Ward Leonard system of speed control such as low installation cost, ease of maintenance, higher accuracy, greater reliability, quick response, higher efficiency of operation, lesser floor space requirements, operation at a wide range of temperature etc.

2. What are freewheeling diodes ?

Ans. Freewheeling diode is a bypass or commutating diode. It is connected across motor terminals to allow for dissipation of energy stored in motor inductance and to provide for continuity of motor current when the thyristors are blocked. It also provides protection against transient overvoltages.

3. Why half-wave converter is not used for supply to the field circuit of a dc motor ?

Ans. If half-wave converter is used for supply to the field circuit of a dc motor, the high ripple content in the field circuit would cause increase in iron losses of the machine.

4. What is the relationship of Vdc and Vac in terms of firing angle α for half-controlled and full-controlled converters?


For half-controlled converter,

half-controlled converter

For full-controlled converter,

full-controlled converter

5. What are the drawbacks of half-wave rectifier circuits ?

Ans. The output voltage wave of a half-wave rectifier is not a perfect dc. It is a discontinuous current and contains various harmonics. So half-wave converter drives have poor performance and their use is limited to small drives (rating up to 500W).

6. What is the necessity of dc choke coil and freewheeling diode in a converter circuit ?

Ans. Freewheeling diode allows for dissipation of energy stored in the motor inductance and dc choke coil which reduces the rate of decay of current during the freewheeling operation when thyristors are blocked and thus motor current is made continuous. Discontinuous armature current results in deterioration of motor performance.

7. Why three-phase half-wave drives are generally not employed in industrial applications ?

Ans. Because of presence of dc component in supply currents.

8. What is a chopper ?

Ans. A chopper is a static switch to provide variable dc voltage from a source of constant dc voltage source.

9. Give the applications of dc choppers.

Ans. DC choppers are employed in trolley cars, marine hoists, forklift trucks, mine haulers and electric traction.

10. What is meant by duty ratio of a dc chopper ?

Ans. The ratio of on-period to the time period is known as duty ratio, i.e.,

duty ratio of a dc chopper

11. What are the advantages of chopper fed drive over converter fed drive ?

Ans. The advantages of chopper fed drive over converter fed drive are:

  1. The ripple in the output of chopper is much less than that in the output of a converter. Lesser ripple means lesser losses.
  2. The chances of discontinuous operation in chopper drive are lesser than that in converter drive. So speed regulation and transient response of chopper drive are better than that in converter drive.

12. Enumerate the control strategies used in dc choppers.

Ans. There are two types of control strategies for operating the switches that can be used in dc choppers. These control strategies are time ratio control (TRC) and current limit control.

13. What is TRC ? In how many ways can it be affected ?

Ans. TRC (time-ratio control) is a control strategy in which the value of Ton/T is varied. This is affected in two ways namely variable frequency control (FM control) and constant frequency control (PWM).

14. What quantity is varied in PWM and frequency modulation ?

Ans. In pulse width modulation (PWM), the chopping period T is kept constant but the pulse width Ton is varied to control the output.

In frequency modulation the chopping frequency is varied and for this purpose either Ton or Toff is varied.

15. What is a step-up chopper ?

Ans. A step-up chopper, like a step-up transformer, raises the level of input dc voltage.

16. On what principle does a step-up chopper operate ?

Ans. A step-up chopper operates on the principle of storing energy in an inductance to provide output voltage higher than source voltage.

17. What is steady-state ripple ?

Ans. When chopper feeds R-L or R-L-Eb load, the current varies between Imax and Imin. The difference of Imax and Imin is known as steady-state ripple.

18. When is steady-state ripple maximum.

Ans. When α = 0.5, steady-state ripple is maximum.

19. What factors can make the current discontinuous in a chopper feeding an R-L-Eb load ?

Ans. Higher L/R ratio, low duty cycle and higher back emf can make the current discontinuous.

20. The commutators of the thyristor controlled dc motors are made larger than those for ordinary dc motors. Why ?

Ans. The commutators of the thyristor controlled dc motors are made larger in order to provide extra insulation to withstand larger and rapid voltage fluctuations.

21. Semiconverter system is preferred for dc series motor drive. Why ?

Ans. The semiconverter system, because of its freewheeling action helps in maintaining continuous current and thus provides better performance to dc series motors in comparison to full converter system.

22. Why are ac regulators preferred over conventional methods of providing ac variable voltage of constant frequency for ac motor drives ?

Ans. AC regulators using thyristors are becoming popular because of high efficiency, fast control and compact size.

23. What is difference between on-off control and phase control in the case of ac voltage controller ?

Ans. In on-off control, also known as integral cycle control, the thyristors are used to connect the motor to the supply source for a certain number of cycles and then to disconnect it for another certain number of cycles; each of the on and off times consist of an integral number of cycles. On the other hand in phase control, the thyristors are employed as switches for connecting the motor to the supply for a certain portion of each cycle of the supply voltage.

24. What is the significance of maintaining the ratio of supply voltage to frequency constant in variable frequency drives of induction motors.

Ans. If only frequency is changed and stator voltage is kept constant, the stator flux will not be at its rated value. The operation with flux below or above the rated value is not desirable.

The noteworthy point is that electrical machine is designed corresponding to near saturation point on the magnetization curve (or B-H curve). This is done from the point of view of full utilization of the core. If the stator frequency is reduced, keeping the stator voltage constant, motor will operate in the saturation region and therefore, the motor will draw large magnetizing current causing increase in stator core and copper losses and therefore, decrease in motor efficiency. However, if only supply frequency is increased keeping the stator voltage constant, the motor will operate at low flux density and thus motor capacity will be under utilized.

For constant flux operation, it is necessary that the induced emf varies linearly with applied frequency. At higher voltages and at high frequency operation stator voltage drops are very small and constant flux operation may be obtained by keeping V/f ratio constant.

25. State two advantages of current source inverter.

Ans. Current source inverter has the following two main advantages:

  1. It can handle reactive or regenerative loads without freewheeling diodes.
  2. Misfiring of devices and short circuits, because of constant input current, do not pose any problem.

26. Mention any two drawbacks of cycloconvcrters.

Ans. The two drawbacks of cycloconverters are :

  1. It needs more power semiconductors than an inverter.
  2. They can produce only a sub-frequency output.

27. Give some applications of cycloconverter.

Ans. Cycloconverter drives are normally used for large sized motors. They have also been employed in gearless cement mill or ball mill drives.

28. What is meant by GTO ?

Ans. Gate-turn-off switch is a special thyristor which can be turned on by a positive gate signal and can be turned-off by a negative gate signal.