Power electronics Viva voce or interview questions part-4

 Q-76 What is the advantage of ON-OFF control?

Ans Due to zero-voltage and zero current switching of thyristors, the harmonics generated by the switching action are reduced.

Q-77 What is the disadvantage of ON-OFF control?

Ans This type of control is applicable in systems that have high mechanical inertia and high thermal time constant.

Q-78 What is the duty cycle in ON-OFF control method?

Ans Duty cycle K = n/ (n + m), where n = no. of ON cycles, m = no. of OFF cycles.

Q-79 What is meant by unidirectional or half-wave ac voltage controller?

Ans Here the power flow is controlled only during the positive half-cycle of the input voltage.

Q-80 What are the disadvantages of unidirectional or half-wave ac voltage controller?

Ans a. Due to the presence of diode on the circuit, the control range is limited and the effective RMS output voltage can be varied between 70.7percent and 100percent.

 b. The input current and output voltage are asymmetrical and contain a dc component. If there is an input transformer, sdaturation problem will occur

 c. It is only used for low power resistive load.

Q-81 What is meant by bidirectional or half-wave ac voltage controller?

Ans Here the power flow is controlled during both cycles of the input voltage.

Q-82 What is the control range of firing angle inac voltage controller with RL

load?

Ans The control range is(α) = Φ to 180° , where Φ = load power factor angle.

Q-83 What type of gating signal is used in single phase ac voltage controller with RL load?

Ans High frequency carrier gating signal is used for single phase ac voltage controller with RL load.

Q-84 What are the disadvantages of continuous gating signal?

Ans a. More heating of the SCR gate.

b. Increases the size of pulse transformer.

Q-85 What is meant by high frequency carrier gating?

Ans Thyristor is turned on by using a train of pulses from α to π. This type of signal is called as high frequency carrier gating.

Q-86 What is meant by sequence control of ac voltage regulators?

Ans It means that the stages of voltage controllers in parallel triggered in a proper sequence one after the other so as to obtain a variable output with low harmonic content.

Q-87 What are the advantages of sequence control of ac voltage regulators?

Ans a. System power factor is improved.

b. Harmonics are reduced in the source current and the load voltage.

Q-88 What is meant by cyclo-converter?

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

Q-89 What are the two types of cyclo-converters?

Ans a. Step-up cyclo-converters

b. Step-down cyclo-converters

Q-90 What is meant by step-up cyclo-converters?

Ans In these converters, the output frequency is less than the supply frequency.

Q-91 What is meant by step-down cyclo-converters?

Ans In these converters, the output frequency is more than the supply frequency.

Q-92 What are the applications of cyclo-converter?

Ans a. Induction heating

b. Speed control of high power ac drives

c. Static VAR generation

d. Power supply in aircraft or ship boards

Q-93 What is meant by positive converter group in a cycloconverter?

Ans The part of the cycloconverter circuit that permits the flow of current during positive half cycle of output current is called positive converter group.

Q-94 What is meant by negative converter group in a cycloconverter?

Ans The part of the cycloconverter circuit that permits the flow of current during negative half cycle of output current is called negative converter group.

Q-95 What is a Thyristor?

Ans Thyristor is derived from the properties of a Thyratron tube and a Transistor. It is used as another name for SCR’S. They are power Semiconductor devices used for power control applications.

Q-96 What are SCR’s?

Ans SCR’s is Silicon controlled Rectifiers. They are basically used as Rectifier

Q-97 What is Forward break over voltage?

Ans The voltage Vak at which the SCR starts conducting is called as Forward Break over voltage Vbo. This happens when the junction J2 undergoes Avalanche breakdown due to high reverse bias on junction J2.

Q-98 What is Reverse break over voltage?

Ans If the reverse voltage is increased more than a critical value, avalanche Breakdown will occur at J1 and J3 increasing the current sharply. This is Reverse break over voltage VBO.

Q-99 Why is Vbo greater than Vbr?

Ans Region at junction J2 is higher during forward bias than that of J1 and J3 under reverse bias.

Q-100 What are modes of working of an SCR?

Ans Reverse blocking mode, Forward blocking mode and Forward conduction mode are the modes of working of an SCR.