Power electronics Viva voce or interview questions part-3

 Q-51 What are the main classification of inverter?

Ans a. Voltage Source Inverter

 b. Current Source Inverter

Q-52 Why thyristors are not preferred for inverters?

Ans Thyristors require extra commutation circuits for turn off which results inuncreased complexity of the circuit. For these reasons thyristors are not preferred for inverters.

Q-53 How output frequency is varied in case of a thyristor?

Ans The output frequency is varied by varying the turn off time of the thyristors in the inverter circuit, i.e. the delay angle of the thyristors is varied.

Q-54 Give two advantages of CSI.

Ans a. CSI does not require any feedback diodes.

b. Commutation circuit is simple as it involves only thyristors.

Q-55 What is the main drawback of a single phase half bridge inverter?

Ans It require a 3-wire dc supply.

Q-56 Why diodes should be connected in antiparallel with the thyristors in inverter circuits?

Ans For RL loads, load current will not be in phase with load voltage and the diodes connected in antiparallel will allow the current to flow when the main thyristors are turned off. These diodes are called feedback diodes.

Q-57 What types of inverters require feedback diodes?

Ans VSI with RL load.

Q-58 What is meant a series inverter?

Ans An inverter in which the commutating elements are connected in series with the load is called a series inverter.

Q-59 What is meant a parallel inverter?

Ans An inverter in which the commutating elements are connected in parallel with the load is called a parallel inverter.

Q-60 What are the applications of a series inverter?

Ans The thyristorised series inverter produces an approximately sinusoidal waveform at a high output frequency, ranging from 200 Hz to 100kHz. It is commonly used for fixed output applications such as

 a. Ultrasonic generator.

 b. Induction heating.

 c. Sonar Transmitter

 d. Fluorescent lighting.

Q-61 How is the inverter circuit classified based on commutation circuitry?

Ans a. Line commutated inverters.

 b. Load commutated inverters.

 c. Self-commutated inverters.

 d. Forced commutated inverters.

Q-62 What is meant by McMurray inverter?

Ans It is an impulse commutated inverter which relies on LC circuit and an auxiliary thyristor for commutation in the load circuit.

Q-63 What are the applications of a CSI?

Ans a. Induction heating

 b. Lagging VAR compensation

 c. Speed control of ac motors

 d. Synchronous motor starting.

Q-64 What is meant by PWM control?

Ans In this method, a fixed dc input voltage is given to the inverter and a controlled ac output voltage is obtained by adjusting the on and off periods of the inverter components. This is the most popular method of controlling the output voltage and this method is termed as PWM control.

Q-65 What are the advantages of PWM control?

Ans a. The output voltage can be obtained without any additional components.

 b. Lower order harmonics can be eliminated or minimized along with its output voltage control. As the higher order harmonics can be filtered easily, the filtering requirements are minimized.

Q-66 What are the disadvantages of the harmonics present in the inverter system?

Ans a. Harmonic currents will lead to excessive heating in the induction motors. This will reduce the load carrying capacity of the motor.

b. If the control and the regulating circuits are not properly shielded, harmonics from power ride can affect their operation and malfunctioning can result.

 c. Harmonic currents cause losses in the ac system and can even some time produce resonance in the system. Under resonant conditions, the instrumentation and metering can be affected.

 d. On critical loads, torque pulsation produced by the harmonic current can be useful.

Q-67 What are the methods of reduction of harmonic content?

Ans a. Transformer connections

 b. Sinusoidal PWM

 c. Multiple commutation in each cycle

 d. Stepped wave inverters

Q-68 Compare CSI and VSI.

Ans VSI

1.Input voltage is maintained constant

2. The output voltage does not depend on the load

3. The magnitude of the output current and its waveform depends on the nature

of the load impedance

4. It requires feedback diodes

5. Commutation circuit is complicated i.e. it contains capacitors and inductors.

CSI

1. Input current is constant but adjustable

2. The output current does not depend on the load

3. The magnitude of the output voltage and its waveform depends on the nature of the load impedance

4. It does not requires feedback diodes

5. Commutation circuit is simple i.e. it contains only capacitors.

Q-69 What are the disadvantages of PWM control?

Ans SCRs are expensive as they must possess low turn-on and turn-off times.

Q-70 What does ac voltage controller mean?

Ans It is device which converts fixed alternating voltage into a variable voltage without change in frequency.

Q-71 What are the applications of ac voltage controllers?

Ans a. Domestic and industrial heating

b. Lighting control

c. Speed control of single phase and three phase ac motors

d. Transformer tap changing

Q-72 What are the advantages of ac voltage controllers?

Ans a. High efficiency

b. Flexibility in control

c. Less maintenance

Q-73 What are the disadvantages of ac voltage controllers?

Ans The main drawback is the introduction of harmonics in the supply current and the load voltage waveforms particularly at low output voltages.

Q-74 What are the two methods of control in ac voltage controllers?

Ans a. ON-OFF control

b. Phase control

Q-75 What is the difference between ON-OFF control and phase control?

Ans ON-OFF control: In this method, the thyristors are employed as switches to connect the load circuit to the source for a few cycles of the load voltage and disconnect it for another few cycles. Phase control: In this method, thyristor switches connect the load to the ac source for a portion of each half cycle of input voltage.