# Power Electronics Questions and Answers – AC Voltage Controllers-1

This set of Power Electronics Multiple Choice Questions & Answers (MCQs) focuses on “AC Voltage Controllers-1”.

1. AC voltage controllers convert
a) fixed ac to fixed dc
b) variable ac to variable dc
c) fixed ac to variable ac
d) variable ac to fixed ac

Explanation: Voltage controllers convert the fixed ac voltage to variable ac by changing the values of the firing angle.

2. In AC voltage controllers the
a) variable ac with fixed frequency is obtained
b) variable ac with variable frequency is obtained
c) variable dc with fixed frequency is obtained
d) variable dc with variable frequency is obtained

Explanation: Voltage controllers convert the fixed ac voltage to variable ac by changing the values of the firing angle. The available ac obtained has the same fixed frequency as the input ac.

3. Earlier then the semiconductor technology, ___________ devices were used for voltage control applications.
a) cycloconverters
b) vacuum tubes
c) tap changing transformer
d) induction machine

Explanation: A tap changing transformer can give variable ac from fixed ac without a change in frequency.

4. The AC voltage controllers are used in __________ applications.
a) power generation
b) electric heating
c) conveyor belt motion
d) power transmission

Explanation: In electric heating, variable ac supply is needed. The devices are fired appropriately to apply enough temperature.

5. In the principle of phase control
a) the load is on for some cycles and off for some cycles
b) control is achieved by adjusting the firing angle of the devices
c) control is achieved by adjusting the number of on off cycles
d) control cannot be achieved

Explanation: Switching devices is so operated that the load gets connected to ac source for a part of each half cycle.
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6. A single-phase half wave voltage controller consists of
a) one SCR is parallel with one diode
b) one SCR is anti parallel with one diode
c) two SCRs in parallel
d) two SCRs in anti parallel

Explanation: As it is half wave, it consists of one SCR ( the control element) in anti parallel with one diode.

7. In the below given voltage controller circuit

a) the positive half cycle at the load is same as the supply Vs
b) the negative half cycle at the load is same as the supply Vs
c) the positive and negative half cycles at the load are identical to the supply
d) none of the mentioned

Explanation: As T1 is triggered at an angle α, the conduction in the positive have cycle will start at α. In the negative half cycle, the diode is forward biased and load is connected as it is to the supply.

8. The below shown controller circuit is a

a) half wave controller
b) full wave controller
c) none of the mentioned
d) will depend upon the firing angle

Explanation: As it consists one diode and one SCR only, the control is only in one cycle (the positive half in this case), hence it is a half wave controller.

9. The below given controller circuit is a

a) half wave controller
b) full wave controller
c) none of the mentioned
d) will depend upon the firing angle

Explanation: As it consists of two SCRs, it is a full wave controller.

10. In the below given voltage controller circuit

a) only the negative cycle can be controlled
b) only the positive cycle can be controlled
c) both the cycles can be controlled
d) none of the mentioned

Explanation: As it consists of two SCRs, it is a full wave controller and both the half cycles can be controlled by varying their respective firing angles.

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