# Power Electronics Questions and Answers – Single Phase Semi-Converters-1

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

1. In the below shown semi-converter circuit, T1 and T2 are fired at an angle α, then from ωt = α to π a) T1 and T2 conduct
b) T1 and D1 conduct
c) D1 and D2 conduct
d) FD conducts

Explanation: At ωt = α T1 is gated and current starts to flow from T1-R-L-E-D1-Vs-T1.

2. In the below shown semi-converter circuit T1 & T2 are fired at an angle α, then from ωt = π to α+π a) T1 is conducting
b) T2 is conducting
c) D1 is conducting
d) FD conducting

Explanation: None of the SCRs are gated, FD is forward biased and starts to conduct charging the L.

3. In the below shown semi-converter circuit T1 & T2 are fired at an angle α, the output voltage is zero when a) π<ωt<α
b) 0<ωt<α+π
c) π<ωt<π+α
d) π<ωt<2π

Explanation: FD is conducting as none of the SCRs are gated which S.C’s the load circuit hence zero voltage.

4. In a semi-converter with RLE load during the freewheeling period, the energy is
a) fed back to the source
b) fed to the inductor(L) and absorbed by E
c) absorbed by the L & E and dissipated at R
d) fed to the L & E and dissipated at R

Explanation: The energy is fed back fed to the L & E and dissipated at R.

5. A semi-converter circuit gives the following voltage waveform. Find the expression for the average output voltage with Vs = Vm sinωt and firing angle α a) (Vm/π) cosα
b) (Vm/π) (1+cosα)
c) (2Vm/π) cosα
d) (2Vm/π) (1+cosα)

Explanation: Vo = 1/π ∫ Vm sinωt d(ωt), Where the integration runs from α to π.
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6. For the below given circuit configuration, the SCR is fired at angle α. For continuous current mode from ωt = π to 2π a) FD conducts
b) SCR and D1 conduct
c) D3 and D2 conducts
d) None of the mentioned

Explanation: From ωt = π to 2π, the negative cycle is active and the SCR is reversed biased. D3 and D2 are ative and are supplying power to the load.

7. For the below given circuit configuration, the SCR is fired at angle α. For continuous current mode, find the expression for the average output voltage a) (Vm/2π) (1+ cosα)
b) (Vm/2π) (2+ cosα)
c) (Vm/2π) (3+ cosα)
d) (3Vm/π)

Explanation: The output voltage will have two parts
V1 = [ ∫ Vm sinωt d(ωt) ] where the integration would run from α to π
V2 = [ ∫ Vm sinωt d(ωt) ] where the integration would run from π to 2π
Vo = 1/2π x V1+V2.

8. A semi-converter circuit gives the following voltage waveform on R load. Find the expression for the average output current with Vs = Vm sinωt and firing angle α a) (Vm/Rπ) cosα
b) (Vm/Rπ) (1+cosα)
c) (2Vm/Rπ) cosα
d) (2Vm/Rπ) (1+cosα)

Explanation: Vo = 1/π ∫ Vm sinωt d(ωt) Where the integration runs from α to π
Io = Vo/R.

9. A semi-converter with RLE load and a freewheeling diode has discontinuous load current with firing angle α and extinction angle β. The freewheeling period is
a) π>ωt<β
b) π>ωt<β
c) π<ωt>β
d) π>ωt>β

Explanation: The freewheeling period is from π to the extinction angle.

10. A semi-converter with RLE load and a freewheeling diode has discontinuous load current with firing angle α and extinction angle β. If β < π and Vm sinβ < E then
a) the converter operates as a line commutated inverter
b) the conduction period is absent
c) the freewheeling period is absent
d) none of the mentioned

Explanation: As the extinction angle is less than 90 degrees, the freewheeling period is absent.

Sanfoundry Global Education & Learning Series – Power Electronics.

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