5 March 2020

DC Chopper Interview Question Answer - 1


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.


2
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


3
Describe the advantages of DC choppers over motor – generator set.


Advantages of the DC choppers
  • High efficiency
  • Low maintenance
  • Fast response
  • Low cost
  • Small size


4
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
  • Load commutated
  • Impulse commutated


5
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


6
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.


7
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

8
Which is main factor responsible for control of the output voltage of the chopper?


Duty cycle

9
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


10
In which method of voltage control chopping frequency is kept constant in the chopper?


Constant frequency control ( Pulse width modulation control )

11
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.


12
Describe the disadvantage of current limit control in the DC chopper


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


13
Why higher value of inductor is preferred for step up chopper?


In order to reduce output voltage ripple

14
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.

15
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.

16
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.


17
Which chopper works in the second quadrant?


Type B chopper

18
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.


19
Which chopper configuration is used in the regenerative braking?


Type B chopper

20
Which chopper is parallel combination of type A and type B chopper?


Type C chopper




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