Power Electronics(PE) Lab Viva Question-Answers

Q1. What are the difference between transistor and SCR?

1. SCR is four layer PNPN device, whereas transistor is three layer (PNP or NPN) device.
2. SCR turns on by regeneration but there is no regeneration in BJT.
3. Gate has no control over the conduction once SCR turns on. BJT can be turned off by base drive.
4. BJT’s have higher switching frequencies than SCR’s.

Q2. Explain latching current and holding current of the thyristor?

Latching current: It’s the minimum forward current that flows through the thyristor to keep it in forward conduction mode at the time of triggering. If the forward current is less than latching current, thyristor doesn’t turn on.
Holding Current: It is the minimum forward current that flows through the thyristor to keep it in forward conduction mode. When forward current reduces below holding current, thyristor turn off.

Q3. What are the advantages of MOSFET’s over BJT’s?

i) MOSFET’s have simpler gate drive circuits.
ii) They have high switching frequencies.
iii) MOSFET’s are majority carrier devices.
iv) MOSFET’s have positive temperature coefficients.

Q4. Why pulsed gate drive is used for SCR?

Once SCR is turned on, gate drive is no more required. Hence if pulsed gate drive is used, SCR turns on at particular pulse. The remaining pulses are not used. But pulsed drive reduces losses in gate cathode of the SCR.

Q5. Define the delay angle of phase-controlled rectifier.

It is the angle at which thyristors are triggered after zero crossing. After zero crossing of supply voltage, one pair of thyristor is forward biased. After delay angle (α), the SCR’s are triggered.

Q6. Why is power factor of semiconverter better than full converter?

The output voltage of semiconverter doesn’t go negative. Hence it’s reactive power is reduced. Where as output voltage of full converter goes negative. Hence it’s reactive power is more. Therefore power factor of semiconverter is better than full converter.

Q7. What are the differences between freewheeling diode and feedback diode?

Freewheeling diode is used to circulate the load energy in the load itself. Feedback diode is used to transfer the load energy to the source. Feedback diode is normally used in inverters. Freewheeling diode is normally used in controlled rectifiers.

Q8. What is inverting operation of the converter?

Converters are basically AC to DC controlled rectifiers. The power flows from AC supply to DC load. In inverting operation, the power flows from DC load to AC supply. Under such situation, the DC load is normally a battery. Thus the power of DC battery load is given to AC supply through controlled rectifier. For this operation, firing angle is greater than 90˚.

Q9. What are control strategies of chopper?

Choppers have following control strategies:
1. Fixed frequency operation: in this type of control, the frequency of chopper remains fixed , but on and off periods of chopper change as per the duty cycle.
2. Variable frequency operation: in this type of control, the on or off period remains fixed. Hence frequency of the chopper changes when duty cycle is changed.
3. Current limit control (CLC) : in this type of control, the output is sensed, when output current falls below reference level, chopper is turned on. And when output current rises above reference level, chopper is turned off.

Q10. Explain the use of step-up chopper.

Step-up chopper is normally used for energy transfer. The load energy is transferred to source by step-up operation.

Q11. What is four quadrant chopper?

Four quadrant chopper operates in all the four quadrants of Vo – Io plane. It can perform forward and reverse motoring as well as forward and reverse regeneration.

Q12. What is the main difference between voltage source inverter and current source inverter?

1. In the voltage source inverter, input voltage remains fixed and current varies according to load variations. But in current source inverter, input current remains fixed and voltage varies according to the load.
2. In voltage source inverters, short circuit of the devices can damage them. In current source inverter short circuit cannot damage the circuit.
3. Voltage source inverters need freewheeling diodes. But current source inverters doesn’t need freewheeling diodes.

Q13. What are PWM inverters?

PWM inverters are basically pulse width modulated inverters. Voltage control in such inverters is achieved by varying width of the pulse. In one half cycle there can be single or multiple pulses. Number of pulses determine lowest order harmonic present in inverter output voltage. Thus PWM inverters provide voltage control as well as harmonic reduction simultaneously.

Q14. What is cycloconverter?

Cycloconverter is AC to AC converter. It has 1ф or 3 ф AC input. Output is variable voltage and variable frequency. Cycloconverters are used for induction motor drives. They have an advantages of variable frequency output over other AC voltage.

Q15. What are the advantages and disadvantages of AC voltage controller?

Advantages:
1. They use line communication, hence no extra communication circuits are required.
2. They have high efficiencies since device losses are reduced.
Disadvantages:
1. Large ripple and harmonics are present in the output.
2. Output waveforms are not sinusoidal.

Note: More question-answers from the same subject and other subjects will be posted very soon. so, please keep update yourself with this website via our facebook like buuton.
Thanks.