# Power Electronics Questions and Answers – PWM Inverters-4

This set of Power Electronics Multiple Choice Questions & Answers (MCQs) focuses on “PWM Inverters-4”.

1. In pulse width modulated inverters, the output voltage is controlled by controlling the
a) input frequency
b) modulating index
c) amplification factor
d) none of the mentioned

Explanation: MI = Vr/Vc.
MI controls the output voltage waveform, as it is proportional to the fundamental component of the output voltage.

2. In case of sinusoidal pulse width modulation with MI < 1, if the number of pulses per half cycle (N) = 5, then
a) harmonics of order 5 and 7 become significant
b) harmonics of order 5 and 7 are eliminated
c) harmonics of order 9 and 11 become significant
d) harmonics of order 9 and 11 are eliminated

Explanation: For MI less than one, largest harmonic amplitudes in the output voltage are associated with harmonics of order 2N (+/-) 1. Thus, by increasing the number of pulses per half cycle (N), the order of the dominate harmonic can be raised, which can be filtered out easily.

3. In case of sinusoidal pulse width modulation with MI < 1, the order of the dominate harmonic can be raised by
a) increasing the number of pulses
b) reducing the number of pulses
c) lowering the input voltage frequency
d) raising the input voltage frequency

Explanation: For MI less than one, largest harmonic amplitudes in the output voltage are associated with harmonics of order 2N (+/-) 1. Thus, by increasing the number of pulses per half cycle (N), the order of the dominate harmonic can be raised.

4. In case of sinusoidal pulse width modulation with MI < 1, if the number of pulses per half cycle (N) = 6, then
a) harmonics of order 7 and 9 become significant
b) harmonics of order 7 and 9 are eliminated
c) harmonics of order 11 and 13 become significant
d) harmonics of order 11 and 13 are eliminated

Explanation: For MI less than one, largest harmonic amplitudes in the output voltage are associated with harmonics of order 2N (+/-) 1. Thus, by increasing the number of pulses per half cycle (N), the order of the dominate harmonic can be raised, which can be filtered out easily.

5. Increasing the number of pulses (N), ____________
a) reduces the output voltage amplitude
b) reduces the inverter efficiency
c) improves the inverter efficiency
d) none of the mentioned

Explanation: Increasing N, increasing the switching frequency of the SCRs. This amounts to more switching losses and therefore efficiency lowers down.
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6. In single-phase modulation of PWM inverters, the pulse width is 120°. For an input voltage of 220 V dc, the rms value of output voltage is
a) 185 V
b) 254 V
c) 127 V
d) 179 V

Explanation: Vo = (Vs) x (2d/π)1/2
Where, d = 2π/3 and Vs = 220 V.

7. In MPM the amplitudes of square wave and triangular wave are respectively 1 V and 2 V. For generating 5 pulses per half cycle, the pulse width should be ___________
a) 36°
b) 24°
c) 12°
d) 18°

Explanation: In multiple pulse width modulation, the pulse width is given by
[ 1 – (Vr/Vc) ] x (π/N) (in degrees)
Where, Vr = 1
Vc = 2
N = 5.
Hence, pulse width = 180/10 = 18°.

8. In an inverter, if the fundamental output frequency is 50 Hz, then the frequency of the lowest order harmonic will be
a) 50 Hz
b) 150 Hz
c) 250 Hz
d) 350 Hz

Explanation: The 3rd harmonic is the lowest order harmonic,
Hence, 50 x 3 = 150 Hz.

9. Calculate the pulse width in case of MPM, if the amplitudes of square wave and triangular wave are respectively 2 V and 3 V respectively. 16 pulses per cycle are generated.
a) 18°
b) 7.5°
c) 6.4°
d) 9°

Explanation: In multiple pulse width modulation, the pulse width is given by
[ 1 – (Vr/Vc) ] x (π/N) (in degrees)
Where, Vr = 2 V
Vc = 3 V
N = Pulses per half cycle = 16/2 = 8
Hence, pulse width = 180/(3×4) = 7.5°.

10. In a PWM inverter, if the frequency of the lowest harmonic is 180 Hz, then the frequency of the fundamental component would be ___________
a) 50 Hz
b) 60 Hz
c) 540 Hz
d) 90 Hz

Explanation: The 3rd harmonic is the lowest order harmonic,
Hence, 180/3 = 60 Hz.

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