Power Electronics and Drives
Exam Duration: 45 Mins Total Questions : 30
The frequency of ripple in the output voltage of a three-phase half controlled bridge rectifier depends on the
- (a)
firing angle
- (b)
lead inductance
- (c)
load resistance
- (d)
supply frequency
A single-phase half wave controlled rectifier has 400 sin 314t as the input voltage and R as the load. For the firing angle of 600 for the SCR, the average output voltage is
- (a)
\(\frac { 400 }{ \pi } \)
- (b)
\(\frac { 300 }{ \pi } \)
- (c)
\(\frac { 240 }{ \pi } \)
- (d)
\(\frac { 200 }{ \pi } \)
In a three-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
Chopper control for DC motor provides variation in
- (a)
Input voltage
- (b)
Frequency
- (c)
Both (a) and (b)
- (d)
None of these
The feedback diodes in a DC to AC thyristor inverter
- (a)
freewheel the load current
- (b)
provide reverse bias effectively to the thyristor for turn-off
- (c)
improve the switching properties o the inverter
- (d)
improve the harmonic distortion for the inverter output current
A single Phase bridge inverter delivers power to a series connected RLC load with R=2\(\Omega \), \(\omega L=8\Omega \) For this inverter load combination load commutation is possible in case the magnitude of \(1/\omega C\) in Ohm is
- (a)
10
- (b)
8
- (c)
6
- (d)
Zero
In single-phase modulation of PWM inverters pulse width is \(120^{ 0 }\) for an input voltage of 220 V D, the rms value of output voltage is
- (a)
179.63 V
- (b)
254.04 V
- (c)
127.02 V
- (d)
185.04 V
Which one of the following statements are correct in connection with inverters?
1.Both voltage source inverter require feedback diodes.
2.The current source inverter requires feedback diodes.
3.GTOs can be used only in the current source inverter
4.The voltage source inverter requires feedback diodes
- (a)
Only 1
- (b)
2 and 3
- (c)
3 and 4
- (d)
Only 4
In a series inverter supplying a load resistor R, the commutating elements L and C should be such that
- (a)
\(R^{ 2 }=\frac { 4L }{ C } \)
- (b)
\(R^{ 2 }<\frac { 4L }{ C } \)
- (c)
\(R^{ 2 }>\frac { 4L }{ C } \)
- (d)
\(R^{ 2 }<\frac { 2L }{ C } \)
In case of armature controlled sepearetly excited DC motor drive with closed-loop control an inner current loop is useful because it
- (a)
limits the speed o the motor to a sfe value
- (b)
helps in improving the drive energy efficiency
- (c)
limits the peak current o the motor to the permissible value
- (d)
reduces the steady state speed error.
In cycloconverter speed control of induction motor cycloconverter frequency
- (a)
is equal to that supply frequency o 50 Hz
- (b)
is higher than the supply frequency
- (c)
can be higher or lower than supply frequency
- (d)
is always less and about one third of supply frequency
The thermal resistance between the body o a power semiconductor device and the ambient is expressed as
- (a)
Voltge across the devices divided by current through the device
- (b)
average power dissipated in the device divided by the square of the rms current in device
- (c)
average power dissipated in the device divided by the temperature difference from body to ambient
- (d)
temperature difference from body to amkbient divided by average power disipated in the device
Which semiconductor power device out o the following is not a current triggered device?
- (a)
Thyristor
- (b)
GTO
- (c)
Triac
- (d)
MOSFET
If a diode is connected in antiparallel with a thyristor then
- (a)
Both turn-o power loss and turn-off time decrease
- (b)
Turn-off power loss decreases but turn-off time increases
- (c)
Turn-off power loss increases but turn-off time decreases
- (d)
None of the above
The circuit in figure shows three-phase half wave rectifier The source is a symmetrical three phase four wire system ?The line to line voltage o the source is 100 V.The supply frequency is 400 Hz The ripple frequency at the output is
- (a)
400 Hz
- (b)
800 Hz
- (c)
1200 Hz
- (d)
2400 Hz
When a line commutated converter operates in the inverter mode
- (a)
it draws both real and reactive powers from the AC supply
- (b)
it delivers both real and reactive power to the AC supply
- (c)
it delivers real power to the AC supply
- (d)
it draws reactive power from the AC supply
AC to DC circulating current dual converters are operated with then following relationship between their triggering angles (\(\alpha _{ 1 }\) and \(\alpha _{ 2 }\))
- (a)
\(\alpha _{ 1 }+\alpha _{ 2 }=180^{ 0 }\)
- (b)
\(\alpha _{ 1 }+\alpha _{ 2 }=360^{ 0 }\)
- (c)
\(\alpha _{ 1 }-\alpha _{ 2 }=180^{ 0 }\)
- (d)
\(\alpha _{ 1 }+\alpha _{ 2 }=90^{ 0 }\)
A fully controlled natural commulated three phase bridge rectifier is operating with a firing angle \(\alpha =30^{ 0 }\) The peak to peak voltage ripple expressed as a ratio of the peak output DC voltage at the output of the converter bridge is
- (a)
0.5
- (b)
\(\frac { \sqrt { 3 } }{ 2 } \)
- (c)
\(\left( 1-\frac { \sqrt { 3 } }{ 2 } \right) \)
- (d)
\(\sqrt { 3 } \)-1
The output voltage waveform of a three-phase square wave inverter contains
- (a)
only even harmonics
- (b)
both odd and even harmonics
- (c)
only odd harmonicw with no triplen
- (d)
only triplen harmonics
In a thyristor DC chopper which type of commutation results ijn best performance?
- (a)
Voltage commutation
- (b)
current commutation
- (c)
load commutation
- (d)
supply commutation
Figure below shows two thyristors each rated 500 A (continuous) sharing a load current through thyristor y is 120 A The current through thyristor a will be nearly
- (a)
120 A
- (b)
100 A
- (c)
90 A
- (d)
110 A
When the firing angle \(\alpha \) of a single-phase fully controlled rectifier feeding constant DC current into a load is \(30^{ 0 }\) the displacement power factor o the rectifier is
- (a)
1
- (b)
0.5
- (c)
\(\frac { 1 }{ \sqrt { 3 } } \)
- (d)
\(\frac { \sqrt { 3 } }{ 2 } \)
An SCR having a turn-on time of 5 \(\mu s\) latching current of 50 mA and holding current of 40 mA is triggered by a short duration pulse and is used in the circuit shown in figure .The minimum pulse width required to turn the SCR On will be
- (a)
251 \(\mu s\)
- (b)
150 \(\mu s\)
- (c)
100 \(\mu s\)
- (d)
5 \(\mu s\)
Consider a phase controlled converter shown in figure The thyritor is fired at an angle \(\alpha \) in every positive half cycle of the input voltage If the peak value of the instaneous output equals 230 V.The firing angle is close to
- (a)
\(45^{ 0 }\)
- (b)
\(135^{ 0 }\)
- (c)
\(90^{ 0 }\)
- (d)
\(83.6^{ 0 }\)
The duty cycle will be
- (a)
0.1
- (b)
0.2
- (c)
0.5
- (d)
0.667
The chopping frequency will be
- (a)
25 Hz
- (b)
50 Hz
- (c)
25 KHz
- (d)
50 KHz
The value of the output voltage will be
- (a)
12 V
- (b)
24 V
- (c)
36 V
- (d)
60 V
A three-phase fully controlled converter bridge converter is feeding a load drawing constant and ripple free load current of 10 A at a firing angle of \(30^{ 0 }\) The approximate Total Harmonic Distortion (THD) and the rms value of fundamental component of the input current will respectively be
- (a)
31% and 6.8 A
- (b)
31% and 7.8 A
- (c)
66% and 6.8 A
- (d)
66% and 7.8 A
Figure shows a step-down chopper switched at 1 KHz with a duty ratio D=0.5 The peak to peak ripple in the load current is close to
- (a)
10 A
- (b)
0.5 A
- (c)
0.125 A
- (d)
0.25 A
The minimum approximate volt-second rating of the pulse transformer suitable for trigeering the SCR should be (volt second rating is the maximum of product of the voltage and the width of the pulse that may be applied)
- (a)
2000 \(\mu V-s\)
- (b)
200 \(\mu V-s\)
- (c)
20 \(\mu V-s\)
- (d)
2.0 \(\mu V-s\)
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