Power Electronics & Drives Test - 4 - PDF Flipbook

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GATE
EEE

Power
Electronics&Drives

Test-04Solutions

POWER ELECTRONICS & DRIVES
1. Which is the correct sequence of the devices in the increasing

order of turn-off times?
a) MOSFET – IGBT – BJT – SCR
b) IGBT – MOSFET – BJT – SCR
c) SCR – BJT – MOSFET – IGBT
d) MOSFET – BJT – IGBT – SCR
Answer: (d)
2. For a single phase phase-controlled rectifier with a freewheeling
diode across the load
a) the instantaneous output voltage v0 is always positive
b) v0 may be positive zero or negative
c) v0 may be positive or zero
d) none of these
Answer: (b)
3. In a thyristor dc chopper which type of commutation results in
best performance?
a) Voltage commutation
b) Current commutation
c) Load commutation
d) Supply commutation
Answer: (b)
Solution:
Current rating of the thyristor is same as load current. But in
voltage commutated chopper it is more than load current.

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4. Which of the following devices does not have negative
resistance characteristics?
a) UJT
b) Tunnel diode
c) SCR
d) FET
Answer: (d)

5. Assertion (A): MOSFETs have larger power handling
capability in linear applications.
Reason (R): This can be attributed to their excellent thermal
stability due to their positive temperature co-efficient
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)
Solution:
MOSFET has positive temperature coefficient which avoid
problem of thermal runaway. Also MOSFET parallel operation
is easy.

6. Which one of the following expresses the thermal resistance
between the body of a power semiconductor device and the
ambient?
a) Voltage across the device divided by current through the
device.

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b) Temperature difference between body and ambient divided
by average Power dissipated in the device.

c) Average power dissipated in the device divided by the square
of the RMS current in the device.

d) Average power dissipated in the device divided by the
temperature difference between body and ambient.

Answer: (b)
Solution:

Tb – Temperature of body

Ta – Ambient temperature

Pav – Average power loss

Thermal resistance,

= −


7. A single-phase full-wave mid-point thyristor converter uses a

230/200V transformer with centre tap on the secondary side.

The P.I.V. per thyristor is

a) 100 V

b) 141.4 V

c) 200 V

d) 282.8 V

Answer: (d)

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Solution:

Pick inverse voltage or each thyristor = 2 × Vm
= 2 × 100 × √2
= 282.8 V

8. Consider the following statements.
Overlapping commutation in a phase controlled ac to dc
converter
1. increase the reverse bias duration of a thyristor.
2. reduces the displacement factor.
3. saves the reactive power.
4. is responsible for voltage regulation on the dc side of the
converter.
Which of these statements are correct?
a) 1, 2 and 3
b) 2, 3 and 4
c) 1, 3 and 4
d) 1, 2 and 4
Answer: (d)
Solution:
• Commutation interval increases which increase the reverse
bias duration of thyristor.

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• Output voltage reduces which leads to voltage regulation.
• It absorbs reactive power but does not save it.
9. When a line commutated converter operates in the inverter
mode
a) It draws both real and reactive power from the A.C supply
b) It delivers both real and reactive power to the A.C supply
c) It delivers real power to the A.C supply
d) It draws reactive power from the A.C supply
Answer: (b)
Solution:
Upto = 900 converter operates as a rectifier and beyond 900
the converter operates as a inverter i.e., the direction of voltage
changes finally power direction changes. The power passes from
load to source and it will support reactive power also.
10. The average on-state current for an SCR is 20A for a
conduction angle of 1200. Its average on-state current for 600
conduction angle would be
a) 20 A
b) 10 A
c) less than 20 A
d) 40 A
Answer: (c)

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11. A converter has been designed to operate on a 220V, 50 Hz
source. If the voltage falls, frequency and load current
remaining the same, there is a possibility for
a) improved regulation.
b) commutation failure in the rectifier region only of the
converter.
c) commutation failure in the inverting region only of the
converter.
d) commutation failure in general in the range of firing angles 0
≤ ∝ ≤ 1800
Answer: (c)
Solution:
In inverting region of converter, ac side facilitates the
commutation of current from are pair of thyristor to another. If
Vs decreases, commutation may suffer.

12. A single-phase voltage controller feeds an motor (A) and a
heater (B)
a) In both the loads, fundamental and harmonics are useful
b) In A only fundamental and in B only harmonics are useful
c) In A only fundamental and in B harmonics as well as
fundamental are useful
d) In A only harmonics and in B only fundamentals are useful
Answer: (c)

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13. The peak inverse voltage in ac to dc converter systems is
highest in
a) single-phase full-wave mid-point converter
b) single-phase full converter
c) 3-phase bridge converter.
d) 3-phase half-wave converter
Answer: (a)

14. In a self-controlled synchronous motor fed from a variable
frequency inverter
a) the rotor poles invariably have damper winding
b) there are stability problems
c) the speed of the rotor decides stator frequency
d) the frequency of the stator decides the rotor speed
Answer: (d)

15. A 3-phase full-bridge diode rectifier is fed from delta-star
transformer. The voltage fed to the rectifier has a maximum
value of V2 from line to neutral so that the voltage of phase ‘a’ is
Va = Vm sinωt. The phase sequence is a-b-c. At the instant the
phase 'a' voltage is passing through zero, what will be the
rectifier output voltage?
a) Zero
b)
c) √2
d) √3
Answer: (d)

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Solution:
For star side maximum phase voltage = Vmp
For a 3-ϕ diode full rectifier output voltage varies between
√3 and 1.5 .
At ωt = 0, Rectifier output voltage = √3
16. If the gate current of an SCR is increased, then forward
breakdown voltage VBR will
a) increase
b) decrease
c) not be affected
d) infinite
Answer: (b)
17. Surge current rating of an SCR specifies the maximum
a) repetitive current with sine wave
b) non-repetitive current with rectangular wave
c) non-repetitive current with sine wave
d) repetitive current with rectangular wave
Answer: (c)
18. Triangular PWM control, when applied to a BJT based three
phase voltage source inverter, introduces.
a) low order harmonic voltages on the dc side.
b) very high order harmonic voltage on the dc side
c) low order harmonic voltage on the ac side
d) none of these
Answer: (b)

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Solution:
By selecting suitable number of pulses, lower order harmonics
can be eliminated, and higher order harmonics will remain and
can be eliminated by additional filters.
19. The converter transformer interposed between the ac source
and midpoint (star point) converters have special construction
a) because the load currents are non-sinusoidal
b) in order to improve the converter performance
c) in order to strengthen them
d) in order to avoid dc magnetization of the core
Answer: (d)
Solution:
Due to the possibility of dc current and consequent
magnetization in secondary side of transformer, converter
transformer is specially designed.
20. A d.c chopper has an average value of load current of 10 A
while feeding a pure resistive load at a given time ratio and
frequency. If a large inductance is placed in series with the load
resistance and operated at the same switching frequency and the
same time ratio, then the average load current
a) remains constant at 10A
b) becomes less than 10A
c) becomes greater than 10A
d) variation cannot be estimated without knowing the value of

inductance

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Answer: (b)
21. The number of p-n junction in a thyristor is/are

a) 1
b) 2
c) 3
d) 4
Answer: (c)
22. A cycloconverter is a
a) frequency changer (fc) from higher to lower frequency with

one state conversion
b) fc from higher to lower frequency with two-stage conversion
c) fc from lower to high frequency with one-state conversion
d) either (a) or (c)
Answer: (d)
23. Single-phase fully controlled converter shown in the figure is
able to feedback load power to the source if the firing angle α is
varied from

a) 0 to 900 and the switches S1 and S2 are closed
b) 0 to 900 and the switches S3 and S4 are closed
c) 900 to 1800 and the switches S1 and S2 are closed

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d) 900 to 1800 and the switches S3 and S4 are closed
Answer: (c)
Solution:
For inverter mode, 900 < α < 1800 with connection as under

Now, E will feed power back to ac supply
24. The frequency of the ripple in the output voltage of a 3-phase

semi-converter depends upon
a) firing angle and load resistance
b) firing angle and load inductance
c) the load circuit parameters
d) firing angle and the supply frequency
Answer: (d)
25. In a 3-phase semi-converter the three SCRs are triggered at an
interval of
a) 600
b) 900
c) 1200
d) 1800
Answer: (c)

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26. A half-wave thyristor-converter supplies a purely inductive
load as shown in the below figure. If the triggering angle of the
thyristor is 1200, then what is its extinction angle?

a) 1200
b) 1800
c) 2000
d) 2400

Answer: (d)
Solution:

For an inductor,

=


= 1 ∫ sin ( )


= 1 (cos − cos )


For i = 0 ⇒ cos = cos

Firing angle, α = 1200 (Given)

cos = cos 1200 = − 1
2

= 1200, 2400

Excitation angle, = 2400

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27. Each diode of a 3-phase half-wave diode rectifier conducts for
a) 600
b) 1200
c) 1800
d) 900
Answer: (b)

28. “Six MOSFETS connected in a bridge configuration (having
no other power device) MUST be operated as Voltage Source
Inverter (VSI)”. This statement is
a) True, because being majority carrier devices. MOSFETS are
voltage driven
b) True, because MOSFETs have inherently anti - parallel
diodes
c) False, because it can be operated both as Current Source
Inverter (CSI) or a VSI
d) False, because MOSFETs can be operated as excellent
constant current sources in the saturation region
Answer: (b)

29. Which one of the following statements is correct?
In a transistor, the reverse saturation current Ico
a) doubles for every 100C rise in temperature
b) doubles for every 10C rise in temperature
c) increases linearly with temperature
d) decreases linearly with temperature
Answer: (a)

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30. On what factors does the ripple frequency of a 3-phase semi-
converter depend?
a) Firing angle and load resistance
b) Firing angle and load inductance
c) Firing angle and supply frequency
d) Load resistance and load inductance
Answer: (c)
Solution:
Ripple frequency depends upon supply frequency and firing
angle. As firing angle increases, ripple frequency decreases.

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