# Power Electronics Questions and Answers – Sequence Controller

This set of Power Electronics Multiple Choice Questions & Answers (MCQs) focuses on “Sequence Controller”.

1. Sequence control of ac voltage controllers is employed for the improvement of _________
a) output frequency
b) input frequency
c) commutation
d) system power factor

Explanation: It is used to improve the power factor at both the input and output side.

2. A two stage sequence control is
a) two SCRs in anti parallel
b) two voltage controllers in parallel
c) two voltage controllers in series
d) a voltage controller having two voltage level

Explanation: Sequence control of ac voltage controller means the use of two or more stages of voltage controllers in parallel for the regulation of output voltage.

3. From the below given statements regarding sequence control of ac voltage, which of them are true?
i) It improves system power factor
ii) It reduces the harmonic content at the output
iii) Wider control of output voltage is possible
a) only (i)
b) only (i) and (ii)
c) only (i) and (iii)
d) all of them are true

Explanation: All of the above are the advantages of using two or more stages of voltage controllers in parallel. Harmonic content is reduced at two common parameters get cancelled out.

4. The below given circuit is that of a a) four stage sequence controller
b) two stage sequence controller
c) full wave ac voltage controller
d) none of the mentioned

Explanation: Sequence control of ac voltage controller means the use of two or more stages of voltage controllers in parallel for the regulation of output voltage.

5. If T1, T2 are kept off and T3, T4 are having a firing angle of 180° each, then the output voltage is ____ if the turns ratio is 1:1 a) Vs
b) √2Vs
c) 0
d) 2Vs

Explanation: If T3 and T4 are fired at an angle of 180°, they are equivalent to an off switch, as they are fired and naturally commutated at the same time, hence they are never on.
Hence, as all the four SCRs are always off, the output voltage is zero.
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6. For obtaining the output voltage control from V to 2V, the firing angle must be a) always 0 for T3, T4
b) always 180 for T3, T4
c) 0 to 180 from T3, T4
d) none of the mentioned

Explanation: For obtaining the voltage control from V to 2V, the lower controller i.e. T3, T4 pair must always be on hence the firing angle from them must always be 0° ideally.

7. For obtaining the output voltage control from V to 2V, the firing angle must be a) always 0 for T1, T2
b) always 180 for T1, T2
c) 0 to 180 from T1, T2
d) none of the mentioned

Explanation: For obtaining the voltage control from V to 2V, the upper controller must be controlled by varying its firing angle from 0 to 180° whereas the T3, T4 pair must always be on hence the firing angle from them must always be 0° ideally.

8. A single-phase two stage sequence controller is designed to work on 230 V supply, and upper and lower current ratings must be 20 A and 21 A respectively. Find the transformer rating.
a) 230 VA
b) 4600 VA
c) 9430 VA
d) 9200 VA

Explanation: Transformer rating is Vs(I1 + I2).

9. In a N-stage sequence controller, each secondary is rated for __________
a) n x Vs
b) Vs
c) Vs/n
d) Vs x (n-1)

Explanation: In a N-stage sequence controller, n voltage controllers are used each having to anti parallel SCR pairs and each secondary is rated for Vs/n.

10. A single-phase sinusoidal voltage controller has
a) one primary and n secondary windings
b) one primary and (n-1) secondary windings
c) n primary and n secondary windings
d) (n-1) primary and n secondary windings

Explanation: A sinusoidal voltage controller is used to obtain continuous voltage control over wide range with low harmonic content.

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