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Interview Question On Power Electronics Part 2

Q. Define circuit turn off time

A. It is defined as the time during which a reverse voltage is applied across the thyristor during its commutation process.

Q. Why the circuit turn off time should be greater than thyristor turn off time?

A. If the circuit turn off time is less than the thyristor turn off time the device may turn on at an undesired instant resulting in commutation failure.

Q. What is chopper?

A.  A dc Chopper is equivalent to the transformer in ac circuit. It is a static switch used to get the variable dc voltage from a constant dc voltage.

Q. What is meant by commutation?

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

Q. What are the types of commutation? (Or) TURN OFF methods?

A. Natural commutation
     Forced commutation

Q. What is natural commutation?

A. 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.

Q. What is forced commutation?

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

Q. What is step down chopper?

A. In step down chopper, the average output voltage is less than the input supply voltage. It is also known as Buck converter.

Q. What is step up chopper?

A. In step up chopper, the average output voltage is more than the input supply voltage. It is also known as Boost converter.

Q. What is voltage commutation?

A. The process of a charged capacitor momentarily reverse biases the conducting SCR and turns it off is called as voltage commutation.

Q. What is current commutation?

A. The process of a current pulse is made to flow in the reverse direction through the conducting SCR and thus made the net SCR current becomes zero, consequently turn off the SCR is called as current commutation.

Q. What are the advantages of current commuted chopper?

A. The advantages of current commutated chopper is;
1. Commutation is reliable as load current is less than the peak commutation current
2. The auxiliary SCR is naturally commutated as its current passes through zero value.
3. The capacitor always remains charged with the correct polarity.

Q. What is load commutation?

A. In load commutation, the load current flowing through the thyristor either becomes zero or is transferred to another device from the conducting SCR.

Q.  Name some of the current controlled (current driven) devices...

A. SCR, GTO, GTR

Q. Name some of the voltage driven (Voltage controlled) devices

A. IGBT, MCT, IGCT, SIT

Q. What is meant by pulse triggered devices?

A. To turn on these kind of devices single pulse of short duration is sufficient. Continuous gate voltage of entire on time is not required. It will avoid the hard triggering. Ex. Thyristor, GTO

Q. What is meant by level-sensitive devices?

A. In order to maintain these kind of devices in on-state, we need to apply continuous gate current/voltage. Some of the level sensitive devices are MOSFET, IGBT, MCT, IGCT.

Q. What are the advantages of IGBT over BJT, MOSFET?

A. IGBT has
Lower turn on and turn off times than BJT
Lower on state conduction losses than MOSFET
Excellent safe operating area


Q. What is a snubber circuit?

 A. A snubber circuit consists of a series combination of a resistor and a capacitor in parallel with the
thyristors. It is mainly used for dv / dt protection.

Q. Why circuit turn off time should be greater than the thyristor turn-off time?

 A. Circuit turn off time should be greater than the thyristor turn-off time for reliable turn-off, otherwise the

Q. . What is meant by phase controlled rectifier?

 It converts fixed ac voltage into variable dc voltage.

What is meant by delay angle?

 The delay angle is defined as the angle between the zero crossing of the input
voltage and the instant the thyristor is fired..

What is commutation angle or overlap angle?

 The commutation period when outgoing and incoming thyristors are conducting is
known as overlap period. The angular period, when both devices share conduction is
known as the commutation angle or overlap angle
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