Power Electronics & Drives Test - 1 - PDF Flipbook
Power Electronics & Drives Test - 1
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Electronics&Drives
Test-01Solutions
POWER ELECTRONICS & DRIVES
1. Turn-on and turn-off times of transistor depend on
a) static characteristic
b) function capacitances
c) current gain
d) None of the above
Answer: (b)
Solution:
Turn-on and turn-off times of transistor depend on junction
capacitance. Because of charging and discharging of junction
capacitance a transistor does not turn-on and turn off instantly
2. In the single phase diode bridge rectifier as shown in the below
figure, the load resistor R = 50 ohms. The source voltage V =
200 sinωt, where ω = 2π × 50 radians per second. What is the
power dissipated in the load resistor?
a) 3200 W
2
b) 400 W
c) 400 W
d) 800 W
Answer: (c)
1
3. Which one of the following is correct?
In a switched capacitor network for VAR compensation the
SCRs are commutated by
a) forced commutation
b) resonant commutation
c) natural commutation
d) delayed commutation
Answer: (b)
4. The thermal resistance between the body of a power
semiconductor device and the ambient is expressed as
a) Voltage across the device divided by current through the
device
b) Average power dissipated in the device divided by the square
of the RMS current in the device
c) Average power dissipated in the device divided by the
temperature difference from body to ambient
d) Temperature difference from body to ambient divided by
average power dissipated in the device
Answer: (d)
Solution:
= − ( ℎ )
� + + �
= −
Where, = + +
2
5. Which is the most suitable power device for high frequency
(>100 KHz) switching application?
a) Power MOSEFT
b) Bipolar junction transistor
c) Schotty diode
d) Microwave transistor
Answer: (a)
Solution:
Power MOSFET has lower switching losses. So, at high
frequency (up to 1 MHz) applications. Power MOSFET is the
obvious choice. But at lower operating frequencies (< 20 kHz),
BJT is superior.
6. Consider the following statements:
SCR can be turned on by
1. Applying anode voltage at a sufficiently fast rate
2. Applying sufficiently large anode voltage
3. Increasing the temperature of SCR to sufficiently large value
4. Applying sufficiently large gate current
Which of the above statements are correct?
a) 1, 2 and 3
b) 1, 3 and 4
c) 1, 3 and 4
d) 2, 3 and 4
Answer: (c)
3
Solution:
The SCR can be turned on by
• triggering, i.e. applying anode voltage at a sufficiently fast
rate.
• When anode voltage is greater than VBO i.e. forward break
over voltage SCR can be turned on.
• Applying sufficiently large gate current will turn on SCR.
• But increasing the temperature of SCR to sufficiently large
value will damage the device.
7. The triggering circuit of a thyristor is shown. The thyristor
requires a gate current of 10 mA, for guaranteed turn-on. What
is the value of R required for the thyristor to turn-on reliably
under all conditions of variation of Vb?
a) 10000 ohms
b) 1600 ohms
c) 1200 ohms
d) 800 ohms
Answer: (d)
4
Solution:
= 10
R should be such that for lowest value of Vb in range (12 ± 4),
current of 10 mA is ensured.
So limiting value of R is for Vb = 12 – 4 = 8V
= = 8 = 800
10×10−3
8. For the V-I characteristics of an SCR, which of the following
statements are correct?
1. It will trigger when the applied voltage is more than the
forward break over voltage.
2. Holding current is greater than latching current.
3. When reverse biased, a small value of leakage current will
flow.
4. It can be triggered without gate current.
a) 1, 2 and 3
b) 1, 3 and 4
c) 1, 2 and 4
d) 2, 3 and 4
Answer: (b)
Solution:
5
• It will trigger when the applied voltage is more than the
forward break over voltage (VBO).
• Holding current is lesser than latching current. Hence this
statement is wrong.
• When reverse biased, a small value of leakage current will
flow.
• It can be triggered without gate current.
9. A gate-turn-off (GTO) thyristor
a) requires a special turn-off circuit like a thyristor
b) can be turned off by removing the gate pulse
c) can be turned off by a negative current pulse at the gate
d) can be turned-off by a positive current pulse at the gate
Answer: (c)
Solution:
It can be turned-off by applying negative gate pulse. Negative
gate pulse current required is about 30% of anode current.
6
10. Statement (I): The ‘turn-on’ and ‘turn-off’ time of a MOSFET
is very small.
Statement (II): The MOSFET is a majority carrier device.
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 'turn on' and 'turn off' of a MOSFET is very small. Because
there are no minority carriers in the MOSFET. The minority
carriers in the device take more time to settle down during turn
off. So, here it is a majority carrier device the turn on and turn
off is very small for MOSFET.
11. Satisfactory turn-off of a thyristor takes place if the reverse
bias period of the device, after its current has become zero, is
a) greater than its turn-off time
b) less than its turn-off time
c) greater than the circuit time-constant
d) less than the circuit time-constant
Answer: (a)
7
Solution:
Turn-off is complete when reverse voltage is maintained across
the thyristor for time more than thyristor turn-off time.
12. The circulating current inductor is required in a dual converter
to
a) Improve the p.f.
b) Smoothen the waveform of circulating current
c) Limit the circulating current
d) Increase the circulating current
Answer: (c)
Solution:
Though average output voltages of the two converters in dual
converter are equal, yet their instantaneous voltages V01 and V02
are out of phase which results in a voltage difference and as a
consequence, a large circulating current flow between the two
converters but not through the load. This circulating current is
limited by inserting an inductor between the two converters.
13. Which of the following does not cause permanent damage of
an SCR?
a) High current
b) High rate of rise of current
c) High temperature rise
d) High rate of rise of voltage
Answer: (d)
8
Solution:
High rate of rise of voltage may turn on SCR without giving
trigger pulse. It cannot damage SCR.
14. Consider the following statements:
Phase controlled converters at small values of output voltage
have
1. Large harmonics in utility system
2. Poor power factor
3. High efficiency
4. Notches in line voltage waveform
Which of the above statements are correct?
a) 1 and 2 only
b) 1, 2 and 4
c) 2, 3 and 4
d) 1 and 4 only
Answer: (b)
Solution:
Only statement 3 is wrong, because small value of output
voltage is obtained at large firing angles.
The performance of the converter is degraded at the higher
range of firing angles, that means efficiency is very low.
15. The input power factor of the converter circuit may be defined
as the ratio of
a) total mean input power to the total rms input volt amperes
b) total rms input volt amperes to the total mean input power
9
c) total peak input volt amperes to the total rms input volt
amperes
d) total input rms volt amperes to the total input peak volt
amperes
Answer: (a)
Solution:
Input power-factor (pf):
This is defined as the ratio of the total mean input power to the
total rms apparent power input to the converter. Since only the
fundamental component contributes to the mean input power,
the pf may be defined as
= 1 1 cos ∅1
= � 1 � cos ∅1
Where, 1 = = phase voltage
1 = Fundamental component of supply current
∅1 = Input displacement angle
= Supply rms current
16. The main reason for connecting a pulse transformer at a output
stage of a thyristor triggering circuit is to
a) amplify the power of the triggering pulse.
b) provide electrical isolation.
c) reduce the turn-on time of the thyristor.
d) avoid spurious triggering of the thyristor due to noise.
Answer: (b)
10
Solution:
Function of pulse transformer is to isolate low voltage gate
cathode circuit from high voltage anode-cathode circuit.
17. A dc to dc transistor chopper supplied from a fixed voltage dc
source feeds a fixed resistive inductive load and a freewheeling
diode. The chopper operates at 1 kHz and 50% duty cycle.
Without changing the value of the average dc current through
the load if it is desired to reduce the ripple content of the load
current, the control action needed will be to
a) Increase the chopper frequency keeping its duty cycle
constant
b) Increase the chopper frequency and duty cycle in equal ratio
c) Decrease only the chopper frequency
d) Decrease only the duty cycle
Answer: (a)
Solution:
For the given average output current ripple current can be
reduced by increasing the chopper frequency and keeping duty
ratio constant.
11
∆ ∝
For constant output current discontents
18. Match List-I (Waveforms) with List-II (Descriptions) and
select the correct answer:
List-I
List-II
1. Single phase fully controlled ac to dc converter.
2. Voltage commutated dc to dc chopper with input dc voltage
E.
3. Phase voltage of a three phase inverter with 1800 conduction
and input dc voltage E.
4. Line voltage of a three phase inverter with 1200 conduction
and input dc voltage E.
5. Three-phase diode bridge rectifier.
12
Codes:
ABC D
a) 3 4 1 5
b) 5 1 4 2
c) 3 1 4 5
d) 5 4 1 2
Answer: (a)
Solution:
In 3-ϕ, 1800 modes each switch conducts for 1800 while in 3-ϕ,
1200 mode, each switch conducts for 1200. But in both modes
gating signals are applied and removal at 600 interval of the
output voltage waveform.
19. Why is a resistor connected across the gate and cathode of an
SCR?
a) To protect against high di/dt
b) To bypass noise signals
c) To protect against over voltages
d) To protect against high temperature rises
Answer: (b)
Solution:
13
1 and zener diode are used against over current and voltage
problem of Gate 2 and C used to bypass the noise signal.
20. In a commutation circuit employed to turn off an SCR,
Satisfactory turn-off is obtained when.
a) Circuit turn-off time
< device turn-off time
b) Circuit turn-off time
> device turn-off time
c) Circuit time constant
> device turn-off time
d) Circuit time constant
< device turn-off time
Answer: (b)
21. In a self-controlled synchronous motor fed from a variable
frequency inverter
a) The rotor poles invariably have damper windings
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)
Solution:
Rotor rotates at synchronous speed = 120
So the rotor speed is decided by the frequency of the stator.
14
22. For a single-phase, full-bridge inverter supplying power to a
highly inductive load as shown below, the correct sequence of
operation of switches and diodes is
a) S1S4 – S3S2 – S1S4 – S3S2
b) S1S2 – D1D2 – S3S4 – D3D4
c) S1D3 – S1S4 – S4D2 – D2D3
d) S2D4 – D4D1 – D1S3 – S3S2
Answer: (a)
Solution:
For highly inductive load, waveform of voltage across load and
current through the load is shown below:
15
23. The thermal resistances for the thyristor-sink combination are
0.16 and 0.090C. If the junction temperature is 1250C and the
heat sink temperature is 750C, what is the average power
dissipation in the thyristor-sink combination?
a) 150 W
b) 175 W
c) 200 W
d) 225 W
Answer: (c)
Solution:
= � − � = 125−75
0.16+0.09 0.25
= 50 = 200
0.25
24. In a three phase voltage source inverter operating in square
wave mode the output line voltage is free from
a) 3rd harmonic
b) 7th harmonic
c) 11th harmonic
d) 13th harmonic
Answer: (a)
16
Solution:
Output line voltage equation of 3-ϕ 1800 mode VSI
= ∑ =1,3,5 4 cos sin � − 6 �
6
For n = 3, cos 3 = 0.
6
Thus all triplen harmonics are absent from the line voltages.
25. Natural commutation of a thyristor takes place when
a) anode current becomes zero
b) gate current becomes zero
c) voltage across the device becomes zero
d) voltage across the device becomes negative
Answer: (a)
Solution:
When anode current < holding current, which is nearly zero,
natural commutation of a thyristor takes place.
26. Consider the following statements, with respect to the power
transistors used in inverters:
1. Maximum collector-emitter voltage VCEO.
2. Maximum collector current.
3. Maximum Power dissipation
4. Maximum current gain at minimum load current.
5. Maximum current gain at maximum load current.
Which of these statements is/are correct?
a) 1 only
b) 1, 2, 3 and 5
17
c) 2 and 3 only
d) 2, 3 and 4
Answer: (b)
27. A bipolar junction transistor (BJT) is used as power control
switch by biasing it in cut-off region (OFF state) or in the
saturation region (ON state). In the ON state, for the BJT
a) both the base - emitter and base - collector junctions are
reverse biased.
b) the base-emitter junction is reverse biased, and the base-
collector junction is forward biased.
c) the base - emitter junction is forward biased, and the base -
collector junction is reverse biased.
d) both the base - emitter and base - collector junctions are
forward biased.
Answer: (d)
Solution:
Cut off region � → Reverse bias
Active region BE → F. Bias
BC → R. Bias
Saturation region BE → F.B
BC → F.B
Inverse Active BE → Reverse Bias
Region BC → Forward Bias
B.E = Base Emitter junction
18
B.C = Base collector junction
28. A constant current source inverter supplies 20 A to a load
resistance of 1 Ω to a load resistance change to 5 Ω, then the
load current
a) remains same at 20 A and the load voltage changes to 100V.
b) changes to 4 A from 20 A and the load voltage changes to 20
V.
c) changes to 4 A from 20 A and the load voltage changes to 80
V.
d) and load voltage stay at 20 A and 20 V respectively.
Answer: (a)
Solution:
As it is a current source inverter, it supply constant current
whatever may be the load in according to load resistance the
load voltage changes.
29. Assertion (A): The L and C components of the
communication circuit in McMurray inverter are chosen such,
that the peak value of resonant current pulse during
communication is sufficiently greater than the load current.
Reason (R): A thyristor will successfully turn off if the current
is maintained below holding value for a time greater than the
turn off time of the device.
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.
19
c) A is true but R is false.
d) A is false but R is true.
Answer: (a)
Solution:
Note: McMurray inverters are force-cumulated inverters. In
such inverters, the function of capacitor is to provide the energy
required for cumulating the main thyristors inductance L is for
limiting di/dt to a safe value in main and auxiliary thyristors.
30. A single phase diode bridge rectifier with finite value of source
inductance feeds on R-L load. The load current can be assumed
to be ripple free. What is the source current wave form?
a) Sinusoidal
b) Triangular
c) Square wave
d) Nearly trapezoidal
Answer: (c)
Solution:
20
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