Power Electronics & Drives Test - 5 - PDF Flipbook
Power Electronics & Drives Test - 5
368 Views
83 Downloads
PDF 3,771,863 Bytes
GATE
EEE
Power
Electronics&Drives
Test-05Solutions
POWER ELECTRONICS & DRIVES
1. Turn-on time of an SCR can be reduced by using a
a) rectangular pulse of high amplitude and wide-width
b) rectangular pulse of low amplitude and hide-width
c) triangular pulse
d) trapezoidal pulse
Answer: (a)
2. The cycle converters require natural or commutation as under
a) natural commutation in both step-up and step-down
cycloconverter
b) forced commutation in both step-up and step-down
cycloconverter
c) forced commutation in step-up cyclo converters
d) forced commutation in step-down cycloconverter
Answer: (c)
3. A compared to oscillators, inverters provides
a) distortion less output
b) highly stabilized output
c) high frequency output
d) low frequency output
Answer: (d)
4. A chopper circuit is operating on TRC principle at a frequency
of 2 kHz on a 220 V dc supply. If the load voltage is 170 V, the
conduction and blocking period of the thyristor in each cycle
are, respectively
1
a) 3.86 μs and 1.14 μs
b) 38.6 ms and 11.4 ms
c) 3.86 ms and 1.14 ms
d) 0.386 ms and 0.114 ms
Answer: (d)
Solution:
For step-down chopper is operating on Time Ratio Control
(TRC) principle, 0 = = ∙
Given, chopping period,
= 1 = 1 = 0.5
2×103
Vs = 220V and V0 = 170V
Now, 170 = (220)( )
0.5×10−3
(0.5 × 10−3) �2127� =
= 0.386
= − = 0.5 − 0.386
= 0.114
5. A 3-phase voltage source Inverter is operated in 1800 conduction
mode. Which one of the following statements is true?
a) Both pole-voltage and line- voltage will have 3rd harmonic
components
b) Pole-voltage will have 3rd harmonic component but line-
voltage will be free from 3rd harmonic
c) Line-voltage will have 3rd harmonic component but pole-
voltage will be free from 3rd harmonic
2
d) Both pole-voltage and line-voltage will be free from 3rd
harmonic components
Answer: (b)
Solution:
= ∑∞ =1,3,5 4 cos sin � + 6 �
6
For triplent harmonic cos 3 = cos = 0
6 2
All triplen harmonic are absent from the line voltages.
Similarly,
Pole voltage = ∑∞ =1,3,5 2 sin
Pole voltage will have all odd harmonics
6. A single-phase full bridge inverter can operate in load
communication mode in case load consist of
a) RL
b) RLC under damped
c) RLC over damped
d) RLC critically damped
Answer: (b)
7. A dc chopper is fed from 100 V dc. Its load voltage consists of
rectangular pulses of duration in sec in an overall cycle time of 3
m sec. The average output voltage and ripple factor for this
chopper are respectively
a) 25 V, 1
b) 50 V, 1
c) 33.33 V, √2
3
d) 33.33 V, 1
Answer: (c)
Solution:
V0 = 1 × 100 = 33.33V
3
Rippled factor = � 1 − 1 = �11/3 − 1 = √3 − 1 = √2
8. Which of the following are the functions of feedback diodes in a
McMurray inverter?
1. To free-wheel the load current.
2. To provide required reverse bias across the outgoing
thyristor.
3. To provide the return path for the reactive current of load.
4. To provide a path for the excess of commutation current
above the load current.
Select the correct answer using the codes given below:
a) 2, 3 and 4
b) 2 and 4
c) 1, 2 and 3
d) 3 and 4
Answer: (a)
Solution:
Diode provide reverse bias voltage for thyristor and return path
for excess reactive power i.e. inductor current.
4
9. Which one of the following statements is correct?
For an SCR, dv/dt protection is achieved through at the use of
a) RL in series with SCR
b) RC across SCR
c) L in series with SCR
d) RC in series with SCR
Answer: (b)
10. When does a fully controlled line commutated converter
operate as an inverter?
a) In the range of firing angles 0 ≤ α ≤ 900
b) In the range of firing angles 900 ≤ α ≤ 1800
c) In the range of firing angles 900 ≤ α ≤ 1800 when there is a
suitable dc source in the load
d) When it supplies a back e.m.f load
Answer: (c)
Solution:
For 900 ≤ ≤ 1800 it operates as inverter. However, to supply
power to ac source, a dc source should be present in load
11. The advantages of SCR over SCS are
a) larger switching time and smaller VH
b) larger switching time and large VH
c) smaller switching time and smaller VH
d) smaller switching time and larger VH
Answer: (d)
5
12. A three-phase voltage source inverter with ideal devices
operating in 1800 conduction mode is feeding a balanced star-
connected resistive load. The DC voltage input is Vdc. The peak
of the fundamental component of the phase voltage is
a) Vdc
π
b) 2Vdc
π
c) 3Vdc
π
d) 4Vdc
π
Answer: (b)
Solution:
Peak value of fundamental component of phase voltage is
=6� 3 � 2
3
13. In a three phase half-wave rectifier, the peak inverse voltage
equals
a) √6E
b) √3E
c) 2E
d) �3/2E
where E is the R.M.S. voltage of secondary
Answer: (a)
6
14. In a current source inverter fed 3-phase induction motor, how
is air-gap flux maintained constant in the base speed region?
a) By measuring the air-gap flux.
b) By keeping v/f constant.
c) By varying the stator current as a function of slip frequency
using a function generator in the closed loop.
d) By varying the stator current as a function of stator frequency
using a function generator in the closed loop.
Answer: (b)
Solution:
Flux, ϕ ∝
By keep V/f constant, flux can be kept constant
15. Thermistor is used for measurement of power at
a) audio frequencies
b) high frequencies
c) very high frequencies
d) microwave frequencies
Answer: (d)
16. A bilateral p-n-p-n diode switch
a) consists of two p-n-p-n diodes in parallel but in opposite
order
b) is a 3-layer semiconductor device
c) is a unilateral device
d) is a bilateral device
Answer: (a)
7
17. A single-phase ac voltage controller feeds a pure resistive load.
At a firing angle α, thyristor conducts for an angle δ. If the load
is replaced by a pure inductance keeping the firing angle
constant, the conduction angle of the thyristor would be
a) 2δ
b) δ
c) δ/2
d) δ/3
Answer: (a)
Solution:
For firing angle, α ≤ ϕ (load angle), voltage controller will not
control the load voltage and current.
Here, α → constant
For inductor, ϕ =
So, 2
α >
2
For resistive load thyristor will conducts up to π i.e., when
voltage (Vs) goes to negative
δ = (π – α), α >
2
Thyristor will conduct after ‘π’ also as current in inductor
cannot change instantaneously.
8
18. A SCR's, break over voltages is 12 V and holding current IH =
5mA. What should be the input voltage V in the circuit for the
SCR to drop out if the voltage at drop out point is 0.7V?
a) 7.5 V
b) 11.3 V
c) 18.8 V
d) 8.2 V
Answer: (d)
9
19. In the single phase voltage controller circuit shown in the
figure, for what range of triggering angle (α), the output voltage
(V0) is not controllable?
a) 00 ˂ α ˂ 450
b) 450 ˂ α ˂ 1350
c) 900 ˂ α ˂ 1800
d) 1350 ˂ α ˂ 1800
Answer: (a)
Solution:
For controlling the load, the minimum value of firing angle α >
load phase angle
ϕ = tan−1 � �
= tan−1 �5500�
= 450
Firing angle for which output voltage is uncontrollable
=0
Data Loading...