Power Electronics & Drives Test - 2 - PDF Flipbook

Power Electronics & Drives Test - 2

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Power
Electronics&Drives

Test-02Solutions


POWER ELECTRONICS & DRIVES
1. A current source inverter is obtained by inserting a large

a) inductance in series with dc supply.
b) capacitance in parallel with dc supply.
c) inductance in parallel with dc supply.
d) capacitance in series with dc supply.
Answer: (a)
Solution:
In current source inverter the input current to almost ripple free,
an inductance in series is used with dc source.

2. An SCR is considered to be a semi – controlled device because
a) it can be turned OFF but not ON with a gate pulse
b) it conducts only during one half – cycle of an alternating
current wave
c) it can be turned ON but not OFF with a gate pulse
d) it can be turned ON only during one half – cycle of an
alternating voltage wave
Answer: (c)
Solution:
It is also known as half controlled switch.

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3. Statement (I): The output current of a current source inverter
remains constant irrespective of load.
Statement (II): The load voltage in CSI depends on the load
impedance.
a) Both Statement (I) and Statement (II) are individually true
and Statement (II) is the correct explanation of Statement (I).
b) Both Statement (I) and Statement (II) are individually true
but Statement (II) is not the correct explanation of Statement
(I).
c) Statement (I) is true but Statement (II) is false.
d) Statement (I) is false but Statement (II) is true.
Answer: (a)
Solution:
The amplitude of output current from CSI is independent of the
load. However, the magnitude of output voltage depends upon
the nature of load impedance.

4. The SCR circuit is shown in the given figure. Which one of the
following wave forms represents vT, when input voltage v is
sinusoidal?

2


Answer: (a)
Solution:
• For < 900, SCR is OFF, voltage appears across SCR.
• 900 ≤ < 1800 SCR is ON. Drop in SCR = 0V (ideal)
• 1800 ≤ ≤ 3600 SCR is reversed biased and OFF. Again

voltage appears across SCR.
5. In a McMurray inverter, diodes are connected in inverse parallel

to thyristors to
1. Protect the thyristor
2. make the turn off of the thyristor successful
3. Make the turn on of the thyristor successful
4. Provide path to the reactive component of the load current
Which of the above statements are correct?

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a) 1 and 3

b) 1 and 4

c) 2 and 4

d) 2 and 3

Answer: (c)

6. A single-phase fully controlled bridge is supplied at 120 V rms.

What will be the average load voltage for a delay angle of 900

assuming continuous load current?

a) 0 V

b) 70.4 V

c) 102 V

d) 170 V

Answer: (a)

Solution:

For single-phase fully controlled converter

Average load voltage,

0 = 2


Firing angle, = 900 (Given)

0 = 0
7. Consider the following statements.

Assertion (A): Power electronic converters are extensively used

in adjustable speed drives.

Reason (R): Power electronic converters do not produce

harmonic distortion.

Of these statements

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a) Both A and R are true and R is the correct explanation of A.
b) Both A and R are true but R is not the correct explanation of

A.
c) A is true but R is false.
d) A is false but R is true.
Answer: (c)
Solution:
Reason in not correct as power electronics has main drawback
of harmonic distortion.
8. Figure shows four electronic switches (i), (ii), (iii) and (iv).
Which of the switches can block voltages of either polarity
(applied between terminals 'a' and 'b') when the active device is
in the OFF state?

a) (i), (ii) and (iii)
b) (ii), (iii) and (iv)
c) (ii) and (iii)
d) (i) and (iv)
Answer: (c)

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Solution:
The switch need to block voltage of either polarity means
bipolar switch. From the given options (ii) and (iii) can block
both the polarities of Voltage. They will have steady state
operating points on V - I plane as shown below.

9. Match List-I with List-II and select the correct answer using the
code given below the lists:
List-I
A. Voltage source inverter
B. Current source inverter
C. Phase controlled a c. to d.c converter
D. Cyclo converter
List-II
1. Larger source inductance
2. Poor power factor
3. Inverter limit
4. Small source inductance

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Codes:
ABC D

a) 4 2 3 1
b) 3 1 4 2
c) 4 1 3 2
d) 3 2 4 1
Answer: (c)
Solution:
A voltage source inverter is one in which the dc source has
small impedance. A current source inverter is fed with
adjustable current from a dc source of high impedance. The
input power factor of cyclo-converter is worse if the fabricated
output voltage is decreased.
10. Assertion (A): A half-controlled converter has improved
power factor on the line side.
Reason (R): During free-wheeling of the converter, a part of the
energy stored in the inductance is utilized as active power in the
load.
a) Both A and R are true, and R is the correct explanation of A.
b) Both A and R are true, but R is not a correct explanation of

A.
c) A is true, but R is false.
d) A is false, but R is true.
Answer: (a)

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Solution:

For single-phase half controlled converter

Input, . = √2 (1 + cos )
� ( − )

For single-phase full controlled converter

Input, . = 2√2 (cos )


Hence, half-controlled converter has better power factor on the

line side.

11. What is the power factor of a single phase a.c regulator feeding

a resistive load?

a) (Per unit power)2

b) (Per unit power)1/2

c) (Per unit power)2
√2

d) (Per unit power)1/2
√2

Answer: (b)

Solution:

Average power = = 2 = 2 = 2 �( − ) + 1 sin 2 �
2 2

Maximum power = 2
2

Per unit power = = 1 �( − ) + 1 sin 2 �
2

. = � 1 �( − ) + 1 sin 2 ��1/2
2

= ( )1/2

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12. Which of the following are the advantages of freewheeling of
load current in phase-controlled converters?
1. It reduces reactive power demand of the converter.
2. It improves the displacement factor.
3. It improves distortion factor.
4. It requires larger inductance to provide continuous
conduction.
Select the correct answer using the codes given below:
a) 1, 2, 3 and 4
b) 1, 2 and 3
c) 1 and 4
d) 2, 3 and 4
Answer: (a)
Solution:
1. It reduces the ripple which improves distortion factor.
2. Takes less reactive power from system.
3. Improves displacement factor and input power factor
4. It requires large inductor to keep conduction continuous.

13. What are the advantages of switching power supplies over
linear power supplies?
1. The devices operate in linear/active region.
2. The devices operate as switches.
3. Power losses are less.
Select the correct answer using the code given below:

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a) 1 and 3
b) 2 and 3
c) 1 and 2
d) 1, 2 and 3
Answer: (b)
Solution:
Device operates as switch so not in active region only in
saturation and cut-off region.
14. In the ac regulator of figure-1, the supply voltage and gate
currents waveforms are as in figure-2 what is the load voltage
waveform tor R = 0?

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Answer: (a)
Solution:
Due to pure inductive load, firing angle α, must be greater than
900 and we cannot fire T2 until T1 conducts.
15. The triac circuit shown in the below figure controls the ac
output power to the resistive load. What is the peak power
dissipation in the load?

a) 10580 W
b) 7935 W
c) 5290 W
d) 3968 W
Answer: (a)

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Solution:

Peak power dissipation,

= 2


= 230√2 ,

= 10 ( )

So, = �230√2�2 = 10580
10

16. A single-phase ac regulator fed from 50 Hz supply feeds a load

having 4 Ω resistance and 12.73 mH inductance. The control

range of firing angle will be

a) 00 to 1800

b) 450 to 1800

c) 900 to 1800

d) 00 to 450

Answer: (b)

Solution:

The minimum value of firing angle = Load phase angle

= tan−1


= tan−1 �2 ×12.734×10−3×50�

= tan−1(1)

= 450

∴ 450 ≤ ≤ 1800

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17. A single phase diode bridge rectifier supplies a highly
inductive load. The load current can be assumed to be ripple
free. The ac supply side current waveforms will be
a) Sinusoidal
b) Constant dc
c) Square
d) Triangular
Answer: (c)
Solution:
Ripple free means, the output current is constant. So the shape
of the supply current is square wave form.

18. In a forward converter, a tertiary winding is used,
What is the reason?
a) To provide di/dt protection to the switching device.
b) To provide dv/dt protection to the switching device.
c) To provide electrical isolation between the input and output.
d) To demagnetize the core before the application of the next
switching pulse
Answer: (d)
Solution:
In forward converter a tertiary winding is added to the
transformer so that the magnetizing current can return to the
input source VS when the transistor turns off. The transformer
magnetizing current must be reset to zero at the end of each

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cycle. Otherwise, the transformer can be driven into saturation,
which can cause damage to the switching device.
19. Statement (I): A switched-mode dc power supply has high
efficiency and light weight.
Statement (II): A switched-mode dc power supply uses a dc to
dc switching converter switched at high frequency.
a) Both Statement (I) and Statement (II) are individually true

and Statement (II) is the correct explanation of Statement (I).
b) Both Statement(I) and Statement (II) are individually true but

Statement (II) is not the correct explanation of Statement (I).
c) Statement (I) is true but Statement (II) is false.
d) Statement (I) is false but Statement (II) is true.
Answer: (a)
Solution:
SMPS is based on the chopper principle, with very high
switching speed. Hence the advantage of SMPS is for the same
power rating. SMPS is of smaller size, lighter in weight and
possesses higher efficiency because of its high frequency
operation.
20. When a thyristor gets turned on, the gate drive
a) should not be removed as it will turn-off the SCR
b) maybe may not be removed
c) should be removed
d) should be removed in order to avoid increased losses and

higher junction temperature

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Answer: (d)

21. For a step-down chopper, VS is the source voltage, ‘R’ is the

load resistance and ‘α’ is the duty cycle. RMS and average

values of thyristor currents for this chopper are, respectively

a) √


b) √ √


c)


d) √


Answer: (d)

Solution:

For step-down chopper,

= √


=


22. If the rms source voltage is volts, the minimum and maximum

values of firing angles for a single phase, half-wave controlled

rectifier, supplying a load with a back emf of 40 volts are

a) 00 1800

b) = sin−1 40 1800
√2

c) = sin−1 40 � − sin−1 √420 �
√2

d) 00 � − sin−1 √420 �

Answer: (c)

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Solution:

Minimum firing angle, = sin−1 = sin−1 40
√2

Maximum value of firing angle = − = − sin−1 40
√2

23. An IGBT has three terminals called

a) collector, emitter and base

b) drain, source and base

c) collector, emitter and gate

d) none of these

Answer: (d)

24. In a 3 - phase controlled bridge rectifier, with an increase of

overlap angle the output dc voltage

a) Decreases

b) Increases

c) Does not change

d) Depends upon load inductance

Answer: (a)

25. A register connected across the gate and cathode of an SCR

increases its

a) dv/dt rating

b) holding current

c) noise immunity

d) all of these

Answer: (d)

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26. Which one of the following is the correct statement?
In a dc chopper the freewheeling diode across the load
a) improves the efficiency of power conversion of the chopper.
b) causes high voltage across the main thyristor.
c) causes high voltage across the auxiliary thyristor.
d) avoids high voltage when the main thyristor is turned-off.
Answer: (d)
Solution:

When chopper is OFF, the energy stored in inductor is
discharged in R through FD. If FD is not present, then energy
stored in inductor appears as high voltage across chopper.
27. The on-state voltage of a GTO is
a) 0.7 V
b) 1.2 V
c) 2.3 V
d) > 3 V
Answer: (d)

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28. The di/dt rating of an SCR is specified for its

a) decaying anode current

b) decaying gate current

c) rising gate current

d) rising anode current

Answer: (d)

29. In a chopper circuit, given that Edc = 50 V, R = 80 Ohm and

duty cycle = 30%. What is the average value of the thyristor

current?

a) 0.09375 A

b) 0.1875 A

c) 0.3750 A

d) 0.625 A

Answer: (b)

Solution:

Given, R = 80 Ω

Assume it is step-down chopper,

0 = = .


Duty cycle = = 0.3


= = 50

0 = (50)(0.3) = 15

0 = 0


= 15 = 0.1875
80

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30. In phase controlled rectifiers the power factor
a) is unaffected by phase control
b) worsens if the delay angle is small
c) worsens for increasing delay angle
d) worsens for large output voltage
Answer: (c)

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