# Power Electronics Questions and Answers – Harmonic Reduction

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

1. Which of the following is used as a harmonic reduction technique in inverters?
a) Amplitude modulation
b) Cycloconverter control
c) Transformer connection
d) Series connection of two inverters

Explanation: Two or more inverters are connected together by means of a transformer to get the net output voltage with reduced harmonic content.

2. For harmonic reduction by transformer connection, the output voltages from the two inverters must be
a) similar and in-phase with each other
b) dissimilar but in-phase with each other
c) similar but phase shifted from each other
d) dissimilar and phase shifted from each other

Explanation: The essential condition of this scheme is that the output voltages from the two inverters must be similar but phase shifted from each other.

3. The output voltage obtained by connecting two inverters through a transformer is a
a) square wave
b) sine wave
c) quasi-square wave
d) none of the mentioned

Explanation: The net output voltage is the addition of the two inverter voltages, as they are phase shifted from each other, the waveform obtained is a quasi-square wave.

4. Pulses of different widths and heights are superimposed in case of __________ harmonic reduction technique.
a) transformer connection
b) pulse width modulation
c) stepped-wave inverter
d) none of the mentioned

Explanation: In the steeped wave inverters pulses of different widths and heights are superimposed to produce a resultant stepped wave with reduced harmonic content.

5. In case of stepped wave inverters,
a) both the transformers have 1:1 turns ratio
b) both the transformers have 1:2 turns ratio
c) both the transformers have different transformer ratio
d) none of the mentioned

Explanation: The two transformers have different turns ratio, hence, different voltages levels are obtained at the secondary’s which are the added to get the net output voltage with reduced harmonic content.

6. In stepped wave inverters, one of the inverters are gated such as to obtain
a) one level modulation
b) two level modulation
c) zero level modulation
d) none of the mentioned

Explanation: In one-level modulation, during the first half cycle, the output voltage is either zero or positive. During the second half cycle the output voltage would be either zero or negative.

7. In three-level modulation
a) the output voltage is zero in the first half cycle.
b) the output voltage either zero or positive in the first half cycle.
c) the output voltage either zero or negative in the first half cycle.
d) the output voltage either zero, positive or negative in the first half cycle.

Explanation: In stepped wave inverters, one of the inverters is so gated as to obtain a three level modulation.

8. In single-phase modulation of PWM inverters, the lowest harmonic can be eliminated if the pulse width is made equal to __________
a) 30°
b) 0°
c) 120°
d) 60°

Explanation: In single-phase modulation of PWM inverters, the nth harmonic can be eliminated, if the pulse width (2d) is made equal to (2π/n).
Lowest harmonic is for n = 3.
2d = 2π/3 = 120°.

9. In single-phase modulation of PWM inverters, the 5th order harmonic can be eliminated if the pulse width is made equal to ___________
a) 30°
b) 36°
c) 72°
d) None of the mentioned

Explanation: In single-phase modulation of PWM inverters, the nth harmonic can be eliminated, if the pulse width (2d) is made equal to (2π/n).

10. The waveform obtained by __________ is more near to a sinusoidal wave
a) PWM inverters
b) Stepped wave inverters
c) parallel connected inverters
d) None of the mentioned

Explanation: The stepped wave output, is more nearer to a sine wave. More the number of steps, closer is the wave to a sinusoidal wave.

Sanfoundry Global Education & Learning Series – Power Electronics.

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