Power Electronics Lab Viva Questions and Answers

Question No. 01: What is meant by SOA?

Answer: SOA – Safe Operating Area determines the voltage and current boundary within which the Power Device can be operated without destructive failure.

Question No. 02: What is the op amp?

Answer: An operational amplifier (or an op-amp) is an integrated circuit (IC) that operates as a voltage amplifier. An op-amp has a differential input. That is, it has two inputs of opposite polarity. An op-amp has a single output and a very high gain, which means that the output signal is much higher than input signal.

Question No. 03: What is forced commutation?

Answer: The process of the current flowing through the thyristor is forced to become zero by external circuitry is called as forced commutation.

Question No. 04: What are the types of cycloconverters?

Answer: The two types of cycloconverters are:

1. Step up cycloconverter
2. Step down cycloconverter.

Question No. 05: What does the Voltmeter in AC mode show? Is it RMS value or peak value?

Answer: Multimeter in AC mode shows RMS value of the voltage or current. Also when it is DC mode it will show the RMS value only.

Question No. 06: What is Universal Motor?

Answer: It is defined as a motor which can be operated either on DC or single-phase AC supply at approximately the same speed and output.

The universal motor is built exactly like a series DC motor. But a series DC motor cannot be run as a universal motor, even though both motors look the same internally and externally. We cannot use these motors in the industrial applications due to the low efficiency (25% 35%). It has high starting torque and a variable speed characteristic. It runs at dangerously high-speed on no load.

Question No. 07: What is holding current in SCR?

Answer: It is the minimum current required to hold the SCR in forward conduction state.

When the forward current becomes less than holding current, SCR turns from forward conduction state to forward blocking state.

Question No. 08: What is hard switching of the thyristor?

Answer: When gate current is several times higher than the required gate current, the SCR is said to be hard fired. It reduces the turn ON time and enhances the di/dt capability.

Question No. 09: Name some of the voltage driven (Voltage controlled) devices

Answer: IGBT, MCT, IGCT, SIT

Question No. 10: For High voltage applications will you prefer MOSFET or IGBT?

Answer: For High voltage applications we have to use IGBT.

Because MOSFETs are low voltage devices. i.e., Their voltage rating is lesser than IGBT.

General rule is MOSFETs are suitable for applications which have breakdown voltage less than 250 V.

The IGBTs are suitable for applications which have breakdown voltage upto 1000V.

Question No. 11: What is meant by commutation?

Answer: The process of changing the direction of current flow in a particular path of the circuit. It is used to turn off the SCR.

Question No. 12: What is meant by cycloconverter?

Answer: A cycloconverter (CCV) or a cycloinverter converts a constant voltage, constant frequency AC waveform to another AC waveform of a lower frequency by synthesizing the output waveform from segments of the AC supply without an intermediate DC link.

(or, It is also known as frequency changer. It converts input power at one frequency to output power at another frequency with one stage conversion.)

Question No. 13: What are the control strategies of chopper?

Answer: The control strategies of chopper are:

1. Pulse width modulation PWM (Variable TON, Constant frequency)
2. Frequency modulation (Constant TON or TOFF, Variable frequency)
3. Current Limit Control (CLC)

Question No. 14: What is meant by PMDC?

Answer: PMDC stands for Permanent Magnet DC Motor. A Permanent Magnet DC Motor is similar to an ordinary dc shunt motor except that its field is provided by permanent magnets instead of salient-pole wound field structure.

There are three types of permanent magnets used for such motors namely;

1. Alnico Magnets
2. Ceramic magnets
3. Rare-earth magnets

The major advantages are low noise, small size, high-efficiency, low manufacturing cost.

Question No. 15: What is latching current in SCR?

Answer: It is the minimum current required to latch (turn on) the SCR from forward blocking state to forward conduction state.

Question No. 16: What are the main components used for isolating the Power Circuits, Power Semiconductor from the low-power circuit?

Answer: OptoCouplers, Transformers

Question No. 17: Can fuses with an AC voltage rating be used in DC applications?

Answer: Fuses must be rated for the voltage AC or DC in which they will be used.

Generally, fuses have a DC voltage rating that is half of the maximum AC voltage rating.

Question No. 18: What is natural commutation?

Answer: The process of the current flowing through the thyristor goes through a natural zero and enable the thyristor to turn off is called as natural commutation.

Question No. 19: What is step up cycloconverter?

Answer: It is the converter whose output frequency is more than the input frequency.

Question No. 20: What is delay angle or what is firing angle of phase controlled rectifier?

Answer: The delay angle is the angle at which Thyristors are triggered after zero crossing. After zero crossing of supply voltage, one pair of Thyristors is forward biased. After delay angle (α) these SCRs are triggered.

Question No. 21: What a turn on methods of SCR?

Answer: Here the different Forward voltage triggering

1. Gate triggering
2. dv/dt triggering
3. Temperature triggering
4. Light triggering

Question No. 22: Name some of the current controlled (current driven) devices

Answer: SCR, GTO, GTR

Question No. 23: What are the characteristics of an ideal op amp? or, What is an ideal amplifier?

Answer: An ideal op-amp is usually considered to have the following characteristics:

1. Infinite open-loop gain G = v_out / v_in
2. Infinite input impedance R_in, and so zero input current
3. Zero input offset voltage
4. Infinite output voltage range
5. Infinite bandwidth with zero phase shift and infinite slew rate
6. Zero output impedance R_out
7. Zero noise
8. Infinite common-mode rejection ratio (CMRR)
9. Infinite power supply rejection ratio.

Question No. 24: What are the types of commutation with respect to commutation process?

Answer: The types of commutation are:

1. Voltage commutated chopper
2. Current commutated chopper
3. Load commutated chopper

Question No. 25: Give some examples of power electronics applications in the day-to-day life?

Answer: We can list a huge number of power electronics applications. Few of the applications which we can see in our daily life are:

1. UPS – Uninterruptible Power Supply
2. SMPS – Switch Mode Power Supply
3. Speed Control of Motors
4. ICU

Question No. 26: What is snubber circuit?

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

Question No. 27: For High frequency applications will you prefer MOSFET or IGBT? Why?

Answer: For High frequency applications, MOSFET is the right choice of the device.

Because MOSFET has low switching losses compare to that of IGBT.

General rule of thumb is for low-frequency applications having frequency range upto 20 kHz, we have to use IGBT.

For high frequency applications having frequency range of more than 200 kHz, we have to use MOSFET.

Question No. 28: What is step down cycloconverter?

Answer: It is the converter whose output frequency is less than the input frequency.

Question No. 29: What is duty cycle?

Answer: It is the ratio of the ON time of the chopper to total time period of the chopper.

D = T_on / [T_on + T_off]

Question No. 30: What is firing angle?

Answer: The angle between the zero crossing of the input voltage and the instant the SCR is fired is called as delay angle or firing angle.

Question No. 31: What are the advantages of freewheeling diode in rectifier circuit?

Answer: The input power factor is improved. It prevents the output voltage from becoming negative. Load current waveform is improved.

Question No. 32: What is the necessity to use the special machines?

Answer: General purpose motors (Induction motors, synchronous motors) are neither precision speed nor precision position motors. For many automated systems require high precise speed and high precise positioning motors. In such cases special purpose motors like stepper motors, PMDC motors etc. are used.