# Power Electronics Questions and Answers – Single Phase VSI-5

This set of Power Electronics assessment questions focuses on “Single Phase VSI-5”.

1. A single phase full bridge inverter has RLC load with R = 4 Ω, Xl = 11 Ω and Xc = 20.54 Ω. The dc input voltage is 230 V. Find the value of fundamental load power.
a) 1633 W
b) 1603 W
c) 1576 W
d) none of the mentioned

Explanation: Average value of fundamental output voltage = 4Vs/π = 292.85 V
Z = [R2 + (Xl – Xc)2]1/2
I = V/Z = 28.31
I(rms)(fundamental) = V/Z√2 = (292.85)/(1.414 x 10.345) = 20.02 A
Fundamental Load power = (20.02)2 x R = (20.02)2 x 4 = 1603.2 Watts.

2. A single phase full bridge inverter has RLC load with R = 4 Ω, L = 35 mH and C = 155 μF. The dc input voltage is 230 V and the output frequency is 50 Hz. Find the angle by which the third harmonic current will lead/lag the third harmonic output voltage.
a) 67.25°
b) 96.4°
c) 49.87°
d) 81.3°

Explanation:
XL = 2 x 3.14 x 50 x 0.035 = 10.99 Ω
XC = 1/(2 x 3.14 x 50 x 155 x 10-6) = 20.54 Ω
For the third harmonic component
XL(3rd harmonic) = 10.99 x 3 = 33 Ω (approx.)
XC(3rd harmonic) = 20.54/3 = 6.846 Ω
R = 4 Ω
P = tan-1 (XL – XC)/R = 81.3°.

3. A single phase half bridge inverter has RLC load. The dc input voltage (Vs/2) = 115 V and the output frequency is 50 Hz. The expression for the load voltage up to the fifth harmonic will be given by
a) 146 sin 314t + 48.81 sin 314t + 58.57 sin 318t + 28.31 sin 318t + 3.686 sin 318t
b) 146 sin 314t + 48.81 sin (3 x 314t) + 29.28 sin (5 x 318t)
c) 146 sin 314t + 48.81 sin (2 x 314t) + 58.57 sin (3 x 318t)
d) none of the mentioned

Explanation: In a single phase HALF bridge inverter only odd harmonics are present. i.e. 1,3,5 etc.
Vo = (2Vs/π) sin ωt + (2Vs/3π) sin 3ωt + (2Vs/5π) sin 5ωt . . .
(2Vs/π) = 146 V
ωt = 2 x f x π x t = 2 x 3.14 x 50 x t = 314t.

4. Let Vs be the amplitude of the output voltage and P be the output power for a single-phase half bridge inverter. Then the corresponding values for a full bridge inverter would be
a) 2Vs, 4P
b) 2Vs, 2P
c) Vs, P
d) 2Vs, P

Explanation: The voltage is doubled (4Vs/π) in full and (2Vs/π) in half bridge configuration. As the power is proportional to the square of the current which is proportional to the voltage it is 4 times that obtained by a half ridge configuration.

5. In VSI (voltage source inverters)
a) both voltage and current depend on the load impedance
b) only voltage depends on the load impedance
c) only current depends on the load impedance
d) none of the mentioned

Explanation: In VSIs the voltage is independent on the load impedance Z.
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6. The harmonic factor of nth harmonic is given by
a) Vn
b) V1/Vn
c) Vn/V1
d) None of the mentioned

Explanation: It is the ratio of rms value of the nth harmonic voltage component to the rms value of the fundamental voltage component (V1).

7. ____________ is the measure of the contribution of any individual harmonic to the inverter output voltage.
a) THD
b) Distortion Factor
c) Harmonic Factor
d) TUF

Explanation: The HF or Harmonic factor is the ratio of the nth harmonic voltage component to the fundamental voltage component. Hence it shows how much a particular harmonic is contributing in the total output of the circuit.

8. The Total Harmonic Distortion (THD) is the ratio of
a) rms value of all the harmonic components to the rms value of the fundamental component
b) average value of all the harmonic components to the rms value of the fundamental component
c) rms value of all the third harmonic component to the rms value of the fundamental component
d) rms value of all the fundamental component to the rms value of all the harmonic components

Explanation: THD = Vo/V1
V1 = Fundamental component
Vo = value of all the harmonic components expect the fundamental.

9. The HF (Harmonic factor of nth harmonic) is the ratio of
a) an average and a rms value
b) a rms and an average value
c) two volt-ampere values
d) two rms values

Explanation: HF is the ratio of rms value of the nth harmonic voltage component to the rms value of the fundamental voltage component.

10. The total harmonic distortion (THD) is the measure of
a) input vs output power factor
b) temperature sensitivity
c) waveform distortion
d) contribution of each harmonic to the total output

Explanation: Lower the value of THD, closer is the waveform to a sine-wave.

Sanfoundry Global Education & Learning Series – Power Electronics.

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