Power electronics Viva voce or interview questions part-2

 Q-26 What is meant by input power factor in controlled rectifier?

Ans The input power factor is defined as the ratio of the total mean input power to the total RMS input volt-amperes.

Q-27 What are the advantages of six pulse converter?

Ans a. Commutation is made simple.

b. Distortion on the ac side is reduced due to the reduction in lower order harmonics.

c. Inductance reduced in series is considerably reduced.

Q-28 What is meant by commutation?

Ans It is the process of changing the direction of current flow in a particular path of the circuit. This process is used in thyristors for turning it off.

Q-29 What are the types of commutation?

Ans a. Natural commutation

b. Forced commutation

Q-30 What is meant by natural commutation?

Ans Here the current flowing through the thyristor goes through a natural zero and enable the thyristor to turn off.

Q-31 What is meant by forced commutation?

Ans In this commutation, the current flowing through the thyristor is forced to become zero by external circuitry.

Q-32 What is meant by dc chopper?

Ans A dc chopper is a high speed static switch used to obtain variable dc voltage from a constant dc voltage.

Q-33 What are the applications of dc chopper?

Ans a. Battery operated vehicles

b. Traction motor control in electric traction

c. Trolly cars

d. Marine hoists

e. Mine haulers

f. Electric braking.

Q-34 What are the advantage of dc chopper?

Ans Chopper provides

a. High efficiency

b. Smooth acceleration

c. Fast dynamic response

d. Regeneration

Q-35 What is meant by step-up and step-down chopper?

Ans In a step- down chopper or Buck converter, the average output voltage is less

than the input voltage. In a step- up chopper or Boost converter, the average 

output voltage is more than the input voltage.

Q-36 What is meant by duty-cycle?

Ans Duty cycle is defined as the ratio of the on time of the chopper to the total time period of the chopper.

Q-37 What are the two types of control strategies?

Ans a. Time Ratio Control (TRC)

b. Current Limit Control method (CLC)

Q-38 What is meant by TRC?

Ans In TRC, the value of (Ton/T) is varied in order to change the average output voltage.

Q-39 What are the two types of TRC?

Ans a. Constant frequency control

b. Variable frequency control

Q-40 What is meant by FM control in a dc chopper?

Ans In frequency modulation control, the chopping frequency f (or the chopping period T) is varied. Here two controls are possible.

a. On-time Ton is kept constant

b. Off period Toff is kept constant.

Q-41 What is meant by PWM control in dc chopper?

Ans In this control method, the on time Ton is varied but chopping frequency is kept constant. The width of the pulse is varied and hence this type of control is known as Pulse Width Modulation (PWM).

Q-42 What are the different types of chopper with respect to commutation process?

Ans a. Voltage commutated chopper.

b. Current commutated chopper.

c. Load commutated chopper.

Q-43 What is meant by voltage commutation?

Ans In this process, a charged capacitor momentarily reverse biases the conducting thyristor and turn it off.

Q-44 What is meant by current commutation?

Ans In this process, a current pulse is made to flow in the reverse direction through the conducting thyristor and when the net thyristor current becomes zero, it is turned off.

Q-45 What is meant by load commutation?

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

Q-46 What are the advantages of current commutated chopper?

Ans a. The capacitor always remains charged with the correct polarity.

b. Commutation is reliable as load current is less than the peak commutation current ICP.

c. The auxiliary thyristor TA is naturally commutated as its current passes through zero value.

Q-47 What are the advantages of load commutated chopper?

Ans a. Commutating inductor is not required.

 b. It is capable of commutating any amount of load current.

 c. It can work at high frequencies in the order of kHz.

d. Filtering requirements are minimal.

Q-48 What are the disadvantages of load commutated chopper?

Ans a. For high power applications, efficiency becomes very low because of high switching losses at high operating frequencies.

b. Freewheeling diode is subjected to twice the supply voltage.

c. Peak load voltage is equal to twice the supply voltage.

d. The commutating capacitor has to carry full load current at a frequency of half chopping frequency.

e. One thyristor pair should be turned-on only when the other pair is commutated. This can be realized by sensing the capacitor current that is alternating.

Q-49 What is meant by inverter?

Ans A device that converts dc power into ac power at desired output voltage and frequency is called an inverter.

Q-50 What are the applications of an inverter?

Ans a. Adjustable speed drives

b. Induction heating

c. Stand-by aircraft power supplies

d. UPS

e. HVDC transmission