# Power Electronics Questions and Answers – PWM Inverters-2

This set of Power Electronics Multiple Choice Questions & Answers (MCQs) focuses on “PWM Inverters – 2”.

1. In case of a single-pulse width modulation with the pulse width = 2d, to eliminate the 5th harmonic from the output voltage waveform, the value of the pulse width (2d) must be equal to
a) 72°
b) 86°
c) 91°
d) 5°

Explanation: To eliminate the nth harmonic, nd = π.
Therefore, d = π/n = π/5 = 36°
Hence, 2d = 72°.

2. Several equidistant pulses per half cycle are used in ___________ type of modulation technique.
a) single-pulse
b) multiple-pulse
c) sine-pulse
d) equidistant-pulse

Explanation: In MPM, several equidistant pulses per half cycle are used.

3. In the multiple-pulse width modulation method, the Fourier coefficient an is
a) (Vs/π) [ cos(nπ/2) cos(nd) ].
b) 0
c) (4Vs/nπ) [sin(nπ/2) sin(nd)].
d) (2Vs/nπ) [sin(nπ/2) sin(nd)].

Explanation: As the positive and the negative half cycles are identical the coefficient an = 0.

4. In the multiple-pulse width modulation method with two pulses per half cycle of width = d each and ɣ as the distance between the first pulse and ωt=0, has the Fourier coefficient bn =
a) (8Vs/π) [ cos(ɣπ/2) cos(nd/2) ].
b) 0
c) (4Vs/nπ) [sin(nɣ) sin(nd/2)].
d) (8Vs/nπ) [sin(nɣ) sin(nd/2)].

Explanation: The Fourier analysis is as under:
bn = (4/π) ∫ Vs sin⁡ nωt .d(ωt) , Where the integration would run from (ɣ + d/2) to (ɣ – d/2)
bn = (8Vs/nπ) [sin(nɣ) sin(nd/2)].

5. The amplitude of the nth harmonic of the two-pulse MPM waveform is given by __________
Let d be the width of a single pulse and ɣ be the distance from 0 to the centre of the first pulse.
a) (8Vs/nπ) sin(nɣ) sin(nd/2)
b) (4Vs/nπ) sin(nɣ) sin(nd/2)
c) (8Vs/nπ) sin(nπ) sin(nd/2)
d) (8Vs/nπ) sin(nɣ/2) sin(nd/2)

Explanation: The Fourier series representation of such a wave will be given by bn = (8Vs/nπ) [sin(nɣ) sin(nd/2)] an = 0
Therefore,
Hence, the amplitude is the term excluding the sin nωt factor.
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6. In case of MPM with two pulses per half cycle of width = d each and ɣ as the distance between the first pulse and ωt=0, for eliminating the nth harmonic from the output voltage, which of the following condition must be satisfied?
a) d = 2π
b) d = π
c) d = n°
d) d = 2π/n

7. In case of MPM with two pulses per half cycle of width = d each and ɣ as the distance between the first pulse and ωt=0, for eliminating the nth harmonic from the output voltage, the value of gamma (ɣ) must be equal to
a) 0
b) π
c) π/n
d) d/n

8. Find the peak value of the fundamental component of voltage with MPM with two pulses having pulse width = 36° and ɣ = 54°. The Fourier representation of the waveform is as follows. a) 0.7484 x Vs
b) 1.414 x Vs
c) 0.637 x Vs
d) 2.54 x Vs

Explanation: For the fundamental component put n = 1.
n = 36° and ɣ = 54° . . . (given)
Vo1 = 8Vs/π x sin54 x sin18 = 0.637 Vs.

9. In the MPM method, the comparator is given _______ and _______ types of waveform at its input.
a) square, sine
b) square, quasi-square
c) sine, triangular
d) square, triangular

Explanation: To generate the modulated waves of equal width, square wave which is the reference signal is compared with the triangular wave which is the carrier signal wave.

10. In MPM, the square wave is the ________ signal whereas the triangular wave is the ________ signal.
a) reference, carrier
b) base, reference
c) carrier, reference
d) none of the mentioned

Explanation: To generate the modulated waves of equal width, square wave which is the reference signal is compared with the triangular wave which is the carrier signal wave.

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