Power electronics Viva voce or interview questions part-11

 Q-251 What is variable DC linked inverter?

Ans An inverter is called variable DC linked inverter if the input voltage is

controllable.

Q-252 What is inverter gain?

Ans The inverter gain may be defined as the ratio of the AC output to DC input

voltage.

Q-253 What are the advantages and disadvantages of variable DC linked inverter?

Ans Advantages:

 1. Harmonic content does not change with output voltage.

 2. Control circuit of an inverter is simple.

Disadvantages:

 1. Additional chopper or control rectifier is required.

 2. Efficiency of a circuit is reduced due to double conversion.

 3. Transistors have to handle variable input voltages.

Q-254 Compare between Voltage source and Current source inverters

Ans Voltage source inverters

1. Input is constant voltage.

2. Short circuit can damage the circuit.

3. Peak current of power-device depends on load.

4. Current wave forms depend on load.

5. Free wheeling diodes are required in case of inducted load.

Current source inverters

1. Input is constant current.

2. Short circuit cannot damage the circuit.

3. Peak current of power-device is limited.

4. Voltage wave forms depend on load.

5. Free wheeling diodes are not required.

Q-255 Explain the principle of variable DC linked inverter?

Ans Harmonic content of the signal also changes if pulse width is varied. This

problem is taken care by DC link inverter. Instead of varying the pulses of

inverter, an input DC voltage is varied. Therefore rms value of output voltage is

varied.

Q-256 What is the commutation technique used in the parallel inverter?

Ans Complementary commutation technique.

Q-257 What is the role of the diodes D1 and D2?

Ans Diodes D1 and D2 act as free wheeling diodes, they conduct when both SCR’s

turn off. They also provide a path for conduction.

Q-258 Why is the inductor used?

Ans The inductor does not allow drastic changes in current and hence provide di/dt

protection.

Q-259 From where does the inverter derive its dc power input?

Ans It derives the dc power input from the inverter specific external VRPS.

Q-260 Why the output voltage of an inverter is to be controlled?

Ans The output voltage of the inverter is to be varied as per the load requirement.

Whenever the input DC varies the output voltage can change. Hence, these

variations need to be compensated. The output voltage and frequency of an

inverter is adjusted to keep voltage and frequency constant. Thus, the output

voltage of an inverter is to be controlled.

Q-261 What are series inverters?

Ans Inverters in which the commutating elements are permanently connected in

series with the load resistance.

Q-262 What is the condition for selecting commutating element?

Ans They are selected in such a way that the current flow through series connected

elements R, L, C is under damped

Q-263 What are the drawbacks of a basic series inverter?

Ans *If the inverter frequency exceeds the circuit ringing frequency the dc source

will be short-circuited.

*For output frequencies much smaller than the circuit ringing frequency, the

load voltage is di started.

*The source current flows only during the period when the Thyristor T1 is

conducting. This results in large ripple in the source current and peak current

rating of the source inverters.

Q-264 What are the applications of series inverters?

Ans *Induction heating

*Fluorescent lighting

*Variable speed ac motor drives

*Aircraft power supplies

*UPS

*High voltage dc transmission lines

Q-265 Why are the inductors L1, L2 and why are two capacitors needed?

Ans *The resonant frequency, which is, if it is nearby inverter output frequency,

commutation failure will take place. Hence it should be ensured that the

capacitor and inductor are so chosen that it be not near to resonant frequency.

*Equal values of L1, L1’ or C1, C1’ to be chosen so that the uniform inverter

output is maintained.

Q-266 What are the waveforms (output) obtained in inverter?

Ans The output voltage waveforms of ideal inverters are sinusoidal. But for practical

inverters they are non-sinusoidal and contain harmonics due to which the

waveforms may be square wave or quassi square wave.

Q-267 Why can’t we see current waveforms on CRO?

Ans The resistance of CRO is very high. Therefore the current measurement is

incorrect. An attempt to reduce the resistance of CRO reduces the input

impedance, which draws heavy current from the source.