# Power Electronics Questions and Answers – Single Phase FW AC-DC-1

This set of Power Electronics Multiple Choice Questions & Answers (MCQs) focuses on “Single Phase FW AC-DC-1”.

1. The below shown single phase controlled rectifier configuration is a a) two pulse M-2 Connection
b) two pulse B-2 Connection
c) single pulse M-2 Connection
d) single pulse B-2 Connection

Explanation: It is a two pulse i.e full wave M-2 connection.

2. In the below shown circuit, considering the load to be purely R load, the thyristor T2 would conduct from ωt = a) 0 to π
b) α to π
c) π+α to 2π
d) π to 2π+α

Explanation: The bottom winding forward biases the SCR T2 at π, conduction starts after the pulse is given at α & stops when T2 is naturally commutated at 2π.

3. For a single pulse M-2 rectifier with RL load and firing angle α >0°, the value of the inductance L is such that the load current is continuous. Choose the correct statement.
a) The load current can never be positive
b) The load current can never be negative
c) The load current can never be zero
d) The load current is a constant DC

Explanation: As mentioned, L is such that the load current is continuous. Hence, it never falls below a predefined positive value.

4. For the circuit below, the transformer ratio is 1:1 and Vs = Vm sinωt. When T1 is forward biased and is triggered at α than T2 is a) subjected to a forward voltage of Vm sinα
b) subjected to a forward voltage of 2Vm sinα
c) subjected to a reverse voltage of Vm sinα
d) subjected to a reverse voltage of 2Vm sinα

Explanation: When T1 is gated, the other SCR T2 is reversed biased due to the windings so connected.

5. The circuit turn-off time of M-2 type connection is a) π/ω
b) α/ω
c) π-α/ω
d) π+α/ω

Explanation: When T1 is gated, SCR T2 is off from ωt = α to π
Thus t = π-α(offtime)/ω
Same can be said when T2 is gated and T2 is off.

6. Find the expression for the average value of the output voltage for the below given circuit. Consider the load current to be continuous, firing angle = α, transformer ration 1:1 and Vs = Vm sinωt.
a) (Vm/π)cosα
b) (Vm/π)(1+cosα)
c) (2Vm/π)cosα
d) (2Vm/π)(1+cosα)

Explanation: As the load current is continues, the voltage is positive from α to π and negative from π to π+α and so on. Thus,
Vavg = 1/Period ∫ Vm sin⁡ωt d(ωt)
Where period = π
And the integral runs from α to α+π.

7. A fully controlled converter uses
a) diodes only
b) thyristors only
c) both diodes and thyristors
d) none of the mentioned

Explanation: Fully controlled implys that all the elements are “fully controlled” hence, it uses SCRs only except the FD.

8. A single phase full-converter using R load is a _________ quadrant converter and that using an RL load without FD is a __________ quadrant converter
a) one, one
b) two, one
c) one, two
d) two, two

Explanation: In R load both V and I are positive, in RL load the voltage can be negative but current is always positive.

9. A single phase full controlled bridge converter (B-2) uses
a) 4 SCRs and 2 diodes
b) 4 SCRs
c) 6 SCRs
d) 4 SCRs and 2 diodes

Explanation: 4 SCR’s are connected in a bridge fashion.

10. In a B-2 type full controlled bridge converter
a) one SCR conducts at a time
b) two SCRs conduct at a time
c) three SCRs conduct at a time
d) four SCRs conduct at a time

Explanation: B-2 is the bridge type controller, in which 2 devices conduct at a time. One acting as the current supplying path and other acts as a return path.

Sanfoundry Global Education & Learning Series – Power Electronics.

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