SESSION:20

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1)       What is AC voltage controller?

It is defined as the power electronics converter which converts the fixed AC voltage into a variable AC voltage without change in the frequency.

2)       What is inverter? What are the types of Inverter?

A device which converts dc power into ac power at desired output voltage and frequency is called as Inverter. Inverters are broadly classified into

·        Voltage Source Inverter (VSI)

·         Current Source Inverter (CSI)

3)       What is chopper?

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

Step Down Chopper:

In this circuit, the average output voltage [V_O = D . V_S] is less than the input supply voltage. It is also known as Buck converter.

Step Up Chopper:

It is also known as Boost converter. Here the average output voltage [V_O = V_S / 1 - D]  is more than the input supply voltage.

4)       What is meant by commutation?

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

5)       What are the types of commutation?

There are two types of commutation. They are:

·         Natural commutation

·         Forced commutation

6)       What is natural commutation?

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.

7)       What is forced commutation?

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

8)       What are the types of commutation with respect to commutation process?

With respect to commutation process, following are the types:

·         Voltage commutated chopper

·         Current commutated chopper

·         Load commutated chopper

9)       What is meant by cyclo-converter?

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

10)    What are the types of cyclo-converters?

There are two types of Cyclo-converter.

·         Step up cyclo-converter

·         Step down cyclo-converter.

11)    What is step down cyclo-converter?

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

12)    What is step up cyclo-converter?

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

13)    What is load commutation?

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

14)    What is meant by pulse triggered devices?

As the name indicates, to trigger these devices just a pulse is sufficient. Continuous gate voltage of entire on time is not required. The advantage of pulse triggering is that it will avoid the hard triggering.
Example, Thyristor, GTO.

15)    What are level-sensitive devices? Give some examples.

Some of the level sensitive devices are

·        MOSFET

·        IGBT

·        MCT

·        IGCT

In order to keep these devices in the ON state condition, we have to apply gate current/voltage continuously to these devices.