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Friday, February 7, 2020

Power Electronics and Drives February 07, 2020

POWER ELECTRONICS AND DRIVES PART 24

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°

Answer: c

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

Answer: b

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)].

Answer: b

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)].

Answer: d

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)
Answer:a

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

Answer: d

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

Answer: c

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

Answer: c
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

Answer: d

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

Answer: a

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Power Electronics and Drives February 07, 2020

POWER ELECTRONICS AND DRIVES PART 23

1. In the single-pulse width modulation method, the output voltage waveform is symmetrical about __________
a) π b) 2π c) π/2 d) π/4

Answer: c
2. In the single-pulse width modulation method, the output voltage waveform is symmetrical about ____________ in the negative half cycle.
a) 2π b) 3π/2 c) π/2 d) 3π/4

Answer: b

3. The shape of the output voltage waveform in a single PWM is
a) square wave
b) triangular wave
c) quasi-square wave
d) sine wave

Answer: c

4. In the single-pulse width modulation method, the Fourier coefficient bn is given by
a) (Vs/π) [ sin(nπ/2) sin(nd) ].
b) 0
c) (4Vs/nπ) [sin(nπ/2) sin(nd)].
d) (2Vs/nπ) [sin(nπ/2) sin(nd)].

Answer: c

5. In the single-pulse width modulation method, the Fourier coefficient an is given by
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)].

Answer: b

6. In the single-pulse width modulation method, when the pulse width of 2d is equal to its maximum value of π radians, then the fundamental component of output voltage is given by
a) Vs
b) 4Vs/π
c) 0
d) 2Vs/π

Answer: b

7. In case of a single-pulse width modulation with the pulse width = 2d, the peak value of the fundamental component of voltage is given by the expression
a) 4Vs/π
b) Vs
c) (4Vs/π) sin 2d
d) (4Vs/π) sin d

Answer: d
8. In case of a single-pulse width modulation with the pulse width = 2d, to eliminate the nth harmonic from the output voltage
a) d = π
b) 2d = π
c) nd = π
d) nd = 2π

Answer: c
9. Find the peak value of the fundamental component of voltage with a pulse width of 2d = 90 and Vs = 240 V for single-pulse modulation in a full wave bridge inverter.
a) 305 V
b) 216 V
c) 0 V
d) 610 V

Answer: b

10. In case of a single-pulse width modulation with the pulse width = 2d, to eliminate the 3rd harmonic from the output voltage waveform, the value of the pulse width (2d) must be
a) 0°
b) 60°
c) 120°
d) 180°

Answer: c

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Power Electronics and Drives February 07, 2020

POWER ELECTRONICS AND DRIVES PART 22

1. In the figure shown below, the capacitor C is used to

a) maintain the load voltage constant
b) protect the load
c) protect the chopper switch (SW)
d) maintain the load current constant
Answer:a

2. In the below shown step-up/step-down chopper circuit, when the switch (SW) is on

a) current through inductor rises and the load voltage is zero
b) current through inductor rises and the load voltage is Vs
c) current through inductor decays and the load voltage is zero
d) current through inductor decays and the load voltage is Vs

Answer: b

3. For a step-up/step-down chopper, if α (duty cycle) < 0.5 then
a) Vo = Vs
b) Vo < Vs
c) Vo > Vs
d) none of the mentioned

Answer: b

4. In the below shown step-up/step-down chopper circuit, when the chopper switch (SW) if first on and then off, then the current flows from path

a) Vs-CH-L-Vs
b) L-Load-D-L
c) L-D-Load-L
d) no current flows

Answer: b

5. For a step-up/step-down chopper, if current increases from I1 to I2 linearly during Ton, then find the energy stored in the inductor during Ton
a) zero
b) Vs x (I1 + I2)
c) Vs x [ (I1 + I2)/2 ] x Ton
d) Vs x [ (I1 + I2)/2 ] x T

Answer: c

6. A step-down chopper is also called as a
a) first-quadrant chopper
b) second-quadrant chopper
c) third-quadrant chopper
d) fourth-quadrant chopper

Answer: a

7. For the type-B chopper shown below

a) during Ton, Vo = 0 and L stores energy
b) during Ton, Vo is not 0 and L stores energy
c) during Ton, Vo = 0 and L releases energy
d) during Ton, Vo is not 0 and L releases energy

Answer: a

8. For the type-B chopper shown below

a) during Toff, Vo = (E + Ldi/dt) and D is conducting
b) during Toff, Vo = (E + Ldi/dt) and D is not-conducting
c) during Toff, Vo = 0
d) during Toff, Vo = Vs

Answer: a

9. The below given chopper is that of a

a) step-up chopper
b) step-down chopper
c) step-up/step-down chopper
d) depends on the duty cycle

Answer: a

10. The type-C chopper or two quadrant type-A chopper has
a) type-A and type-B choppers in series
b) type-A and type-B choppers in parallel
c) two type-A choppers in series
d) two type-A choppers in parallel

Answer: b

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Power Electronics and Drives February 07, 2020

POWER ELECTRONICS AND DRIVES PART 21

1. For a step-down chopper shown in the figure below, dc source voltage = 230 V and the voltage drop across the chopper = 2 V. Find the value of average output voltage for a duty cycle of 0.4.

a) 230 V
b) 92 V
c) 90 V
d) 91.2 V

Answer: d

2. For the step-down chopper shown in the figure below, dc source voltage = 230 V and the voltage drop across the chopper = 2 V. Find the value of rms output voltage for a duty cycle of 0.4.

a) 91.2 V
b) 230 V
c) 144.2 V
d) 145 V

Answer: c

3. For the step-down chopper shown in the figure below, dc source voltage = 230 V. Find the power delivered to the load of R = 10 Ω. Duty cycle = 40 %. Take voltage drop at the switch to be 2 V.

a) 1 kW
b) 2.08 kW
c) 569 W
d) 207 W

Answer: b

4. For the step-down chopper shown in the figure below, dc source voltage = 230 V. Find the power input to the chopper If load of R = 10 Ω is connected. Duty cycle = 40 %.

a) 2300 W
b) 2097 W
c) 2560 W
d) 5290 W

Answer: b

5. The below given chopper circuit is that of a

a) step-up chopper
b) step-down chopper
c) step-up/step-down chopper
d) none of the mentioned

Answer: c

6. The expression for a step-up/step-down chopper with α as the duty cycle and Vs as the dc input voltage is
a) Vs/1 – α
b) α x Vs
c) Vs (α/1-α)
d) Vs (α/1+α)

Answer: c

7. For a step-up/step-down chopper, if the duty cycle > 0.5 then
a) Vo = Vs
b) Vo < Vs
c) Vo > Vs
d) None of the mentioned

Answer: c

8. A step down chopper has Vs = 230 V and R = 10 Ω. For a duty cycle of 0.4, the power taken by the chopper is 2097 Watts. Find the chopper efficiency. Take the voltage drop across the chopper switch as 2 V.
a) 98 %
b) 89.96 %
c) 99.14 %
d) 96.54 %

Answer: c

9. A step down chopper has input dc voltage of 220 V and R = 10 Ω in series with L = 65 mH. If the load current varies linearly between 11 A and 17 A, then find the duty cycle α.
a) 1
b) 0.4
c) 0.6
d) 0.7

Answer: d

10. For a step-up/step-down chopper, if α (duty cycle) = 0.5 then
a) Vo = Vs
b) Vo < Vs
c) Vo > Vs
d) none of the mentioned

Answer: a

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Power Electronics and Drives February 07, 2020

POWER ELECTRONICS AND DRIVES PART 20

1. If a step up chopper’s switch is always kept off then (ideally)
a) Vo = 0
b) Vo = ∞
c) Vo = Vs
d) Vo > Vs

Answer: c

2. If a step up chopper’s switch is always kept open then (ideally)
a) Vo = 0
b) Vo = ∞
c) Vo = Vs
d) Vo > Vs

Answer: b

3. Find the average value of output voltage of a basic step-down chopper, with duty cycle = α and load = R Ω.

a) I = Vs x α
b) I = (Vs x α)/R
c) I = 0
d) I = Vs/R

Answer: b

4. For the below given circuit, find the output current at the instant of commutation.

a) Vo/R
b) Vs/R
c) 0
d) α/2

Answer: b

5. For a step-down chopper, find the rms value of output voltage. Let α be the duty cycle and Vs be the input voltage.
a) α x Vs
b) Vs/α
c) √α x Vs
d) Vs/2

Answer: c

6. A step down chopper is operated at 240V at duty cycle of 75%. Find the value of RMS switch (IGBT/MOSFET) current. Take R = 10 Ω.
a) 2.07 A
b) 200 mA
c) 1.58 A
d) 2.4 A

Answer: a

7. Find the expression for effective input resistance of a step down chopper. With R load and duty cycle = α.
a) R x α
b) R/2
c) 0
d) R/α

Answer: d

8. A step-up chopper has input voltage of 220 V and output voltage of 660 V. If the conducting time of the IGBT based chopper is 100 μs, compute Toff width of the output voltage pulse.
a) 100 μs
b) 150 μs
c) 50 μs
d) Insufficient data

Answer: c

9. For a step-up chopper, when the duty cycle is increased the average value of the output voltage
a) increases
b) decreases
c) remains the same
d) none of the mentioned

Answer: a

10. For a step-down chopper, when the duty cycle is increased the average value of the output voltage
a) increases
b) decreases
c) remains the same
d) none of the mentioned

Answer: a

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