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Power Electronics
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A dc chopper is feeding an RLEload.
The condition for maximum ripple is
a = 1
a < 0.5
a = 0.8
a = 0.5
A static excitation system uses
transformer and thyristors
dc exciter and ac exciter
transformer and dc exciter
transformer and ac exciter
Assertion
(A): A dual converter may be circulating current type or non circulating currenttype.
Reason (R): In circulating current dual converter only one converter can be operated at one time
A is correct but R is wrong
Both A and R are correct and R is correct explanation of A
Both A and R correct but R is not correct explanation of A
A is wrong but R is correct
The value of capacitor C for dynamic equalising circuit of series connected thyristors is determined by
reverse recovery characteristics of thyristors
None of the above
forward characteristics of thyristors
both forward and reverse recovery characteristics of thyristors
A thyristor has a turn on time of 6 μs. If the anode circuit is inductive, the turn on time will be
6 μs
more than 6 μs
either 6 μs or less
less than 6μs
In integral cycle control of ac regulator, the input power factor is equal to
a2
1/3
duty cycle a
A
In a full bridge single phase inverter feeding RLC load
two thyristors or two diodes conduct simultaneously
one thyristor and one diode conduct simultaneously
four thyristors conduct simultaneously
two thyristors conduct simultaneously
In a single phase cycloconverter with centre tapped transformer
the durations of currents in all thyristors may or may not be equal
all thyristors carry currents for equal duration
the durations of currents in all thyristors cannot be equal
the duration of currents in all thyristors must be equal for satisfactory operation
In a single phase semiconverter feeding a highly inductive load, the load current is
nearly constant
sinusoidal
absolutely constant
Rectangular
The factor η for a UJT is about
0.9
0.1
0.6
0.2
Which of these commutation methods uses an external pulse for commutation
Class E
Class F
Class C
Class A
A single phase semiconverter has a freewheelingdiode.
If the firing angle is a and the load is purely resistive, the periods of conduction and freewheeling respectively are
p - a and 0
p + a and a
p - a and a
p + a and 0
In a semi converter feeding a resistive load, the firing angle is
a. Each thyristor conducts for
(p - 0.5 a) radians
(p - a) radians
p radians
(p + a) radians
The specified maximum junction temperature of thyristors is about
200°C
80°C
120°C
50°C
The average load current supplied by a thyristor depends on
firing angle
All of the above
magnitude of gate current
firing frequency
The number of leads in a power semiconductor diode are
2
3
2 or 3
4
A cycloconverter is
dc-ac converter
dc-dc converter
ac-dc converter
ac-ac converter
Two single phase semi-converters are connected in series to form a seriesconverter.
The input is v = Vm sin ωt and a1, a2 are the firingangles.
If a1 = 0, the dc output voltage is
(1 + cos a2)
(4 + cos a2)
(3 + cos a2)
(2 + cos a2)
In a single phase full wave regulator, the firing angles in the positive and negative half cycles are generally
Different
Equal
different but sometimes equal
equal or different
A single phase half bridge inverter required to feed R-L loads needs
2 thyristors
4 thyristors
2 thyristors and two diodes
4 thyristors and 4 diodes
Which of these commutation methods uses two leads
Class B
Class D
Class C
Class A
A dc circuit breaker must use
forced commutation
natural commutation
both natural and forced commutation
either natural or forced commutation
In a single phase converter using a centre tapped transformer and with a highly inductive load, the firing angles is
a. Each thyristor conducts for a period
P
p - a
A
A single phase electric heater uses phase controlledthyristors.
The firing angle is 90°. The degree of heating is
75% of maximum
25% of maximum
50% of maximum
None of the above
In a 3 phase full converter the average load current is 150
A. The average thyristor current is
75A
150 A
25 A
50 A
A class E chopper
can operate in second quadrant only
can operate in all the four quadrants
can operate in first or third quadrant
can operate in either second or third quadrant
For a 12 pulse operation, the two 6 pulse units are fed by
Y - Δ and Y - Δ transformers
Δ - Δ and Δ - Δ transformers
Y - Y and Y - Y transformers
Y - Y and Y - Δ transformers
An electric heater is controlled by thyristors through on-offcontrol.
If a = 0.4, the heating is
84% of maximum
60% of maximum
40% of maximum
16% maximum
A 3 phase induction motor is controlled by voltage and frequency control such that the ratio V/f isconstant.
At 50 Hz, the starting torque is 100 N-m. At 25 Hz, the starting torque
may be more or less than 100 N-m depending on motor parameters
will be less than 100 N-m
very small
will be more than 100 N-m
A NAND gate has 3 inputs and oneoutput.
The number of thyristors required for this gate are
2
1
3 or 4
3
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