# Power Electronics Lab Viva Questions

## Power Electronics Lab Viva Questions and Answers

1. What you understand about the Safe Operating Area (SOA) of a power device?

The boundaries of voltage and current in which the power device can be operated without destructive failures can be determined by the SOA. The SOA of a bipolar power transistor is given below:

2. What are cycloconverters? List the different types of cycloconverters?

The main use of a cycloconverter is to convert a constant frequency, constant voltage AC signal to another frequency AC waveform. By synthesizing the output waveforms from the AC supply segments, this can be achieved.

There are mainly two types of cycloconverters

(a) Step down cycloconverter

By using forced communication, a step down cycloconverter gives the output with frequency lower than the input frequency.

(b) Step up cycloconverter

By using line communication, a step up cycloconverter gives an output with frequency greater than the input frequency.

3. What you know about the operational amplifiers (Op Amps)?

Basically op-amp is a integrated circuit (IC), which can be used to work as a voltage amplifier. An op-amp has two inputs with different polarity hence it is fed with a differential input. So op-amp has two different polarity inputs and single output. An op-amp circuit gets high gain i.e., the output signal is very high as compared to input signal. There are mainly two types of op-amps.

(a) Inverting op-amps:

A inverting amplifier possesses negative feedback. The input is given to the inverting terminal (-ve terminal) of the op-amp and the non-inverting terminal is grounded. R1 is the input resistance and R2 is the feedback resistance.

(b) Non-inverting amplifier

The non-inverting amplifier gives amplified output signal and the output is in phase with the input signal.

4. Write the equations for gains of inverting and non inverting amplifiers?

Gain of non inverting op-amp is

Gain of inverting amplifier is

5. What you understand about forced communication?

By the effect of external circuitry, the current flowing through the thyristor is forced to become zero. This process is termed as forced communication.

6. List down the different types of voltage controlled (voltage driven) devices?

The different types of voltage controlled devices are:

IGBT, MCT, IGCT ,SIT etc.

7. In AC mode, what does the voltmeter shows?

In AC mode, the multimeter shows  the Root Mean Square (RMS) value of voltage or current. In DC mode, the multimeter shows only the RMS value.

8. What you understand about the  holding current of a SCR?

The minimum mandatory current for holding the SCR in forward conduction state is known as the holding current. The SCR switches from the state of forward conduction to forward blocking state, if the forward current is less than the holding current.

9. What you understand about universal motors?

Universal motors can be used for working with single phase AC source as well as DC source. It is often known as single phase series motor and is a commutation type motor. The motor runs in the same direction, when we reverse the polarity of the line terminals of a DC series motor. We cannot run a DC series motor as a universal motor, even though both are built in similar manner. Universal motors are widely used for speed control and high speed application purposes.  They can be used in wash machines, power drills, kitchen applications and blowers.

10. What you know about the hard switching of a thyristor?

The SCR can be said to be hard fired, if the gate current is very-very higher than the required gate current. This will enhances the di/dt capability and reduces the turn ON time.

11. MOSFET or IGBT is used for high voltage applications?

IGBT is mostly preferred for high voltage applications. For breakdown voltage up to 1000 V, IGBT is commonly used.

 Fig : IGBT

MOSFET are commonly used for breakdown voltage less than 250V.

12. What is the process of commutation?

The direction of current flow is changed in a particular path of the circuit for turn off the SCR. This process is called commutation.

13. Discuss about the control strategies of chopper?

The different control strategies of chopper are:

Frequency Modulation (constant TON or TOFF , variable frequency)

Current Limit Control (CLC)

Pulse Width Modulation PWM (Variable TON, constant frequency)

14. What you understand about the latching current of SCR?

The minimum current required to turn on (latch) the SCR from forward blocking state to forward conduction state is termed as the latching current.

15. For isolating the power circuits, power semiconductor from the low-power circuits which components are used?

Transformers and Opto-Couplers

16. What you know about natural commutation?

The process of enabling the thyristor to turn off and the current flowing through the thyristor goes through a natural zero is called natural commutation. Natural commutation occurs in AC circuits.

17. For DC applications, is it possible for using fuses with an AC voltage rating?

Fuses can be rated for using voltage AC or DC.  Normally the DC voltage rating of fuses is half of the maximum AC voltage rating.

18. For Turn ON the SCR, what methods you know?

For turn on the SCR, different voltage triggering methods are used. They are:

(a) Light Triggering

(b) Gate Triggering

(c) dv/dt Triggering

(d) Temperature Triggering

19. List the different types of current driven (current controlled) devices?

The different types of current controlled devices are:

SCR

GTO

GTR

20. Discuss about the characteristics of an ideal op amp?

The different characteristics of an ideal op amp are:

(a) Infinite input impedance Rin, and so zero input current

(b) Infinite open-loop gain G = Vout / Vin

(c) Infinite bandwidth with zero phase shift and infinite slew rate

(d) Zero output impedance Rout

(e) Zero input offset voltage

(f) Infinite power supply rejection ratio.

(g) Zero noise

(h) Infinite common-mode rejection ratio (CMRR)

(i) Infinite output voltage range.

21. Relating to the commutation process, what are the different types of commutations you know?

(a) Voltage commutated chopper

(c) Current commutated chopper

22. Give some examples of power electronics applications?

(a) Speed control of motors

(b) Uninterruptible Power Supply (UPS)

(c) Switch Mode Power Supply (SMPS)

(d) ICU

23. What you know about a snubber circuit?

The snubber circuit is a series combination of a resistor and a capacitor in parallel with the SCR. The circuit is mainly used for the dv/dt protection of SCR.

24. IGBT or MOSFET is most commonly used for high frequency applications? Justify your answer?

Since MOSFET has low switching losses than IGBT, MOSFET is commonly used for high frequency applications. For the applications of frequency range upto 20 KHz, IGBT are used and for frequency range greater than 200 KHz, MOSFET are used.

 Fig : MOSFET

25. What you know about the duty cycle of a chopper?

The duty cycle of a chopper can be defined as the ratio of ON time of the chopper to the total time period of the chopper.

26. List the advantages of freewheeling diode in rectifier circuits?

The advantages of freewheeling diodes are:

(a) Input power factor is improved

(b) The load current waveform is improved

(c) Prevents the output voltage from becoming negative

27. What you know about the firing angle (delay angle) of an SCR?

The firing angle also known as delay angle can be defined as the angle between the zero crossing of the input voltage and the instant the SCR is fired. When the zero crossing of supply voltage takes place, one pair of thyristor is forward biased. These SCRs are triggered after the delay angle (α).

28. What are the uses of special machines?

The limitations of general purpose motors (Induction motors, synchronous motors) are they are neither precision position nor precision speed motors. For the applications of high precise position and high precise speed, special purpose motors (Stepper motors, PMDC motors etc) are commonly used.

29. What you know about a Permanent Magnetic DC motor (PMDC)?

A PMDC is same as that of an ordinary dc shunt motor. In PMDC, the field is provided by permanent magnets structure instead of salient- pole wound field structure.

The major advantages of PMDC are:

(a) Low noise

(b) Small size

(c) Low manufacturing cost

(d) High efficiency

There are mainly three types of permanent magnets used.

(i) Rare-Earth magnets

(ii) Ceramic magnets

(iii) Alnico magnets