Power Electronics Viva
Power Electronics Viva
Power Electronics Viva
ii) Power converter converts input power in the form suitable for load.
iii) All power devices are solid state. Hence they are called power electronic devices.
Answer
Q6. Differentiate between SCR and BJT.
SCR:
i) Four layer PNPN device.
ii) Turns on by regeneration.
iii) Gate has no control once SCR is turned on.
iv) External circuits are required to turn off the SCR.
v) Switching frequencies are low.
BJT:
i) Three layer (PNP or NPN) device.
ii) No regeneration exists.
iii) Base has full control over the operation of BJT.
iv) No external circuits are required. BJT turns off if base drive is removed.
v) Switching frequencies are high.
Answer
Q7. What are the differences between BJT and MOSFET?
BJT:
i) This is bipolar device.
ii) Controlled by base.
iii) Current controlled device.
iv) Negative temp coefficients.
v) Losses are low.
MOSFET:
i) This is majority carrier device.
ii) Controlled by gate.
iii) Voltage controlled devices.
iv) Positive temp coefficients.
v) Losses are high than BJTs.
Answer
Q8. What are the different ways to Turn-on and Turn-off the Thyristor.
Turn-on:
i) Gate drive.
Analog Communiaction Viva-Voce Question With Answers for 5th Semester Students
Analog Communiaction Viva-Voce Question With Answers for 5th Semester Students
Power Electronics (PE) Notes by Arunkumar for 7th semester Electronics (EC) VTU
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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 (), the SCRs are triggered.
Answer
Q6. Why is power factor of semiconverter better than full converter?
Answer
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.
Answer
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.
Answer
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.
Answer
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 stepup operation.
Answer
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.
Answer
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 doesnt need
freewheeling diodes.
Answer
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.
Answer
Q14. What is cycloconverter?
Cycloconverter is AC to DC converter. It has 1 or 3 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.
Answer
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.