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Interview Question On Power Electronics

Q. Definition of power electronics?

A. Power electronics refers to control and conversion of electrical power by power semiconductor devices wherein these devices operate as switches.

Q. What is the main purpose of power electronics?

A. The main task of power electronics is to control and convert electrical power from one form to another.
AC to DC conversion: Rectifier is used for converting an AC voltage to a DC voltage.
Rectifier applications: Variable speed dc drives, Battery chargers, DC power supplies and Power supply for a specific application like electroplating.
DC to AC conversion: Inverter circuit is used to convert DC voltage to an alternating voltage.
Inverter applications: Emergency lighting systems, AC variable speed drives, Un-interrupted power supplies and Frequency converters.
DC to DC conversion: A dc-to-dc converter circuit was called a chopper.
Chopper applications:  DC drive, Battery charger and DC power supply.
AC to AC conversion: A cycloconverter converts an AC voltage to another AC voltage.
Cycloconverter applications: It is rarely used.  Can be used for controlling the speed of an AC traction motor

Q. What are the different operation regions of the thyristor (SCR)?

 A. SCR or thyristor will have three regions of operations based on the mode in which the device is connected in the circuit.
Reverse blocking region: When the cathode of the thyristor is made positive with respect to the anode and no gate signal is applied. In this region SCR exhibits the reverse blocking characteristics similar to diode.
Forward blocking region: In this region the anode of the thyristor is made positive with respect to the cathode and no gate signal is applied to the thyristor. A small leakage current flow in this mode of operation of the thyristor.
Forward conduction region: when the forward voltage applied between the anode and cathode increases at particular break over voltage avalanche breakdown takes place and thyristor starts conducting current in forward direction. By this type of triggering the device damages the scr. Hence a gate signal is applied before the forward break over voltage to trigger the scr.

Q. What is holding current in SCR?

A. Holding current is the minimum current required to hold the SCR in forward conduction state. When the forward current becomes less than holding current (If< Ih ), SCR changes from forward conduction state to forward blocking state.

Q. What is latching current in SCR?

A. Latching current is the minimum current required to latch (turn on) the SCR from forward blocking state to forward conduction state.

Q. What are the losses that occur in a thyristor during working conditions?

A. Forward conduction losses
     Loss due to leakage current during forward and reverse blocking
     Switching losses at turn on turn off
     Gate triggering loss

Q. What are the different TURN ON methods of SCR?

A. Forward voltage triggering
     Gate Triggering
     dv/dt triggering
     Thermal triggering (Temperature Triggering)
      Light triggering (Radiation triggering)

Q. What is firing angle?

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

Q. What are the advantages of freewheeling diode in rectifier circuit?

A. The input power factor is improved. It prevents the output voltage from becoming negative. Load current waveform is also improved.

Q. Explain the function of cyclo-converter?

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

Q. What are the types of cyclo-converters?

A. Step up cyclo-converter: In this output frequency is less than the input frequency.
     Step down cyclo-converter: In this output frequency is more than the input frequency.

Q. What is AC voltage controller?

A. It is the one which converts fixed alternating voltage to a variable voltage without change in frequency.

Q. What is an inverter?

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

Q. What are the types of inverter?

A. Voltage Source Inverter
     Current Source Inverter

Q. What do you mean by duty cycle?

A. It is the ratio of the on time of the chopper to total time period of the chopper.
D = Ton / [Ton + T off]
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