## 05/03/2020

### DC Chopper Interview Question Answer - 1

Describe the function of the DC choppers.

Function of the DC choppers

The function of the DC choppers is to convert fixed direct voltage into variable direct voltage.

What are the different technologies to convert fixed direct voltage into variable direct voltage?

Fixed direct voltage in variable direct voltage

Line commutated converters by using controlled rectifier

AC link chopper – inverter and rectifier

DC Chopper

Describe the advantages of DC choppers over motor – generator set.

High efficiency

Low maintenance

Fast response

Low cost

Small size

Describe the various types of the DC chopper.

According to output voltage

Step up chopper and

Step down chopper

According to the direction of output voltage and current

Type A chopper

Type B chopper

Type C chopper

Type D chopper

Type E chopper

According to commutation method

Voltage commutated

Current commutated

Impulse commutated

Which type of semiconductor device is used as switch in the DC chopper circuit?

Semiconductor device as a switch

Silicon controlled rectifier

Power metal oxide field effect transistor

Insulated gate bipolar transistor

Gate turn off thyristor

MOS controlled thyristor

Explain the term : Step up chopper, Step down chopper

Step up chopper

It is a chopper in which the DC output voltage is more than that of the input DC voltage.

Step down chopper

It is a chopper in which the DC output voltage is less than that of the input DC voltage.

Define : Duty cycle

Duty cycle

It is defined as the ratio of the chopper on time to the chopper switching time.

K = TON / T

K = Duty cycle

TON  = Chopper turn on time

T = Chopper switching time

Which is main factor responsible for control of the output voltage of the chopper?

Duty cycle

Describe the various methods of output voltage control in the chopper

Method of chopper output voltage control

Constant frequency control ( Pulse width modulation control )

Variable frequency control ( Frequency modulation control )

Current limit control

In which method of voltage control chopping frequency is kept constant in the chopper?

Constant frequency control ( Pulse width modulation control )

Describe the disadvantages of the frequency modulation voltage control over the pulse width modulation voltage control.

Disadvantages of the frequency modulation voltage control over the pulse width modulation voltage control

There is a possibility of radio interference with nearby telephone lines and other signaling. Load becomes discontinuous if the chopper OFF time is larger than that of chopper ON time. Filter design is difficult due to wide frequency variations.

Describe the disadvantage of current limit control in the DC chopper.

If there is a small difference between maximum and minimum current, the output waveform contain ripple. Higher switching losses due to high chopping frequency

Why higher value of inductor is preferred for step up chopper?

In order to reduce output voltage ripple

Write the relation between input and output voltage in the step down chopper, step up chopper and step up / down chopper

Relation between output voltage and input voltage

Step down chopper

VO = KVIN   ( K < 1)

Step up chopper

VO = VIN / ( 1 – K )   ( 0 < K < 1 )

Step up / down chopper

VO = [ K / ( 1 – K ) ] × VIN

Where VO = Output voltage

VIN = Input voltage

K = Duty cycle

The chopper works as step down chopper if  0 < K < 0.5 . The chopper works as step up chopper if 0.5 < K < 1.

Why the operation of the chopper in the first quadrant is called as forward drive?

The output voltage and output current of the chopper is positive therefore the operation of the chopper in the first quadrant is called as forward drive.

Why the type A chopper is called as step down chopper?

Type A chopper ( Step down chopper )

As the output voltage is less than the input voltage in the type A chopper, it is also called as step down chopper.

Which chopper works in the second quadrant?

Type B chopper

Why the type B chopper is called as step up chopper?

Type B chopper ( Step up chopper )

As the power flow from load to source in type B chopper, it is also called as step up chopper.

Which chopper configuration is used in the regenerative braking?

Type B chopper

Which chopper is parallel combination of type A and type B chopper?

Type C chopper

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