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EEE Interview Questions - V

1. What is Power Electronics?
    Power Electronics is the branch of electronics which deals with the control and conversion of electric signals. It uses switches, semiconductor devices for control and conversion.

2. What are the Semiconductor Devices?
  • Bipolor Junction Transistor(BJT)
  • Metal Oxide Semiconductor Field Effect Transistor(MOSFET)
  • Power Diode
  • Insulated-Gate Bipolor Transistor (IGBT)
  • Thyristor or Silicon Controlled Rectifier(SCR)
3. What are current controlled power electronic devices?
  • BJT
  • SCR
  • GTO
  • GTR
4. What are voltage controlled power electronics device?
  • MOSFET
  • IGBT
  • MCT
  • IGCT
  • SIT
5. How BJT is considered as a current control device?

    BJT is a current controlled device because its output characteristics are determined by its input characteristics as the base current controls the flow of current between the emitter and collector

6. How FET devices are considered as a voltage controlled device?

    FET is considered as a voltage controlled device because its output characteristics are determined by its field which depends on input voltage applied

7. How MOSFET controls the voltage?

     In MOSFET, the voltage applied to the gate controls the flow of current between the source and drain

8. Why IGBT are used for high voltage application than MOSFET?

     IGBT is used for high voltage application because MOSET is a  low voltage device with breakdown voltage less than 250 V where IGBT has breakdown voltage less than 1000 V

9. Why MOSFET are used for high frequency application than IGBT?

     MOSFET has less switching losses compared to the IGBT. IGBT has the frequency range upto 20 KHz where MOSFET has high frequency range upto 200KHz

10. What is chopper?
 
      Chopper is device that converts fixed DC input into variable DC output.

11.What are the types of chopper?
  • Class A (One-quadrant chopper)
  • Class B(One-quadrant chopper)
  • Class C(Two-quadrant chopper)
  • Class D(Two-quadrant chopper)
  • Class E(Four-quadrant chopper)
12. What is regenerative Braking?

      Regenerative braking occurs when the motor speed exceeds the synchronous speed. In case of the induction motor, motor would run as induction generator is converting the mechanical power into electrical power, which is delivered back to the electrical system. This method of braking is known as regenerative braking

13. What is mean by commutation?
      
      Commutation is the process of changing the current flow direction in a particular path. It is used to turn-off thyristor

14. What are the types of commutation?
  • Natural Commutation
  • Forced Commutation
15. What is natural commutation?

      The process of the current flowing through a natural zero of the thyristor and enables the thyristor to turn off is called as natural commutation

16. 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

17. What are the types of commutation process?
  • Voltage commutation
  • Current commutation
  • Load commutation
18. What is snubber circuit?

      The circuit used for dv/dt protection of SCR is called as snubber circuit. It consists of series combination of resistor and capacitor in parallel with SCR

19. What is the firing angle of SCR?

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

20.What is duty cycle of chopper?

     The ratio of ON time period to the total time period of chopper is called as duty cycle

                              D= Ton/[Ton+Toff]

21. What is first quadrant operation of chopper?

      In this, it is possible to apply the torque in the same vector polarity as the velocity i.e. the DC motor is rotating in the forward direction. Here motoring mode of action takes place

22. What is second quadrant operation of chopper?

       In this, it is possible to apply a negative torque, only when the velocity is negative i.e. the DC motor is accelerating in reverse (motoring) direction. Here reverse motoring mode of action takes place

23. What is Four quadrant operation of chopper?
    
       In this, it is possible to apply both torque to the motor. Negative torque can be applied to positive velocity to decelerate the motor also positive torque can be applied to negative velocity to accelerate the motor. Both Motoring and Braking takes place.

24. What are the action in four quadrant operation of chopper?
  •  First Quadrant - Forward motoring
  • Second Quadrant - Forward Braking
  • Third Quadrant -  Reverse Motoring
  • Fourth Quadrant - Reverse Braking


  




     

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