Q .
In a thyristor the gate current is increased, then
A. anode current will increase
B. anode current will decrease
C. anode current will remain constant
D. anode current may increase or decrease
E. NA
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Q . Assertion (A): If the duty cycle of chopper is low the current would not become discontinuous
Reason (R): If the time constant L/R of the load in chopper is low the current may become discontinuous.
A. Both A and R are correct and R is correct explanation of A
B. Both A and R correct but R is not correct explanation of A
C. A is correct but R is wrong
D. A is wrong but R is correct
E. NA
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Q .
When the core of a saturable reactor is saturated, the inductance is high.
A. True
B. False
C.
D.
E. NA
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Q .
A 3 phase semiconverter when supplying a certain load has a rectification efficiency of 0.6 and transformer utilisation factor of 0.35. When a 3 phase full converter is supplying the same load
A. rectification efficiency will be more than 0.6 and TUF will be more than 0.35
B. rectification efficiency will be more than 0.6 but TUF will be less than 0.35
C. rectification efficiency will be less than 0.6 and TUF will be less than 0.35
D. rectification efficiency will be less than 0.6 but TUF will be more than 0.35
E. NA
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Q . Assertion (A): A GTO circuit is more compact than SCR circuit
Reason (R): A GTO can be turned off by negative gate pulse.
A. Both A and R are correct and R is correct explanation of A
B. Both A and R correct but R is not correct explanation of A
C. A is correct but R is wrong
D. A is wrong but R is correct
E. NA
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Q .
A cycloconverter can be
A. step down
B. step up
C. step down or step up
D. none of the above
E. NA
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Q .
In the below figure the average load current is 15 A. The rms value of transformer secondary current is
A. 15 A
B. 10.61 A
C. 7.5 A
D. 14.14 A
E. NA
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Q .
In a multiphase chopper, the different choppers operate in
A. series and all simultaneously
B. parallel and one at a time
C. parallel and simultaneously
D. either (b) or (c)
E. NA
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Q .
In a parallel inverter
A. the commutating capacitor C is across the primary of transformer and the load is across the secondary
B. the commutating capacitor and load are connected across the secondary of transformer
C. the commutating capacitor C and load are connected in series and the combination is across the secondary of transformer
D. none of the above
E. NA
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Q . Assertion (A): A high dv/dt may cause triggering of a thyristor.
Reason (R): A high dv/dt results in charging current in the thyristor.
A. Both A and R are correct and R is correct explanation of A
B. Both A and R correct but R is not correct explanation of A
C. A is correct but R is wrong
D. A is wrong but R is correct
E. NA
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Q .
A single phase half wave rectifier is feeding a resistive load. Input voltage v = Vm sin ωt. The output dc voltage is Vdc and output rms voltage is Vrms . If firing angle is 180°, Vdc and Vrms respectively are
A. 0 and 0
B.
C. 0 and - Vm
D.
E. NA
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Q .
In a 3 phase fully controlled converter the firing frequency is
A. 3 times the line frequency
B. 6 times the line frequency
C. 9 times the line frequency
D. 12 times the line frequency
E. NA
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Q .
In a single phase full wave converter (M-2 connection) feeding on R - L load, the input voltage is v = Vm sin ωt. The expression for dc output voltage is
A.
B.
C.
D.
E. NA
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Q .
The terminals of a power BJT are called
A. emitter, base, collector
B. emitter, base, drain
C. source, base, drain
D. source, base, collector
E. NA
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Q .
Which of these commutation methods is used in AC-DC converters?
A. Class A
B. Class C
C. Class D
D. Class F
E. NA
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Q .
In a single phase full wave regulator, the firing angles in the positive and negative half cycles are generally
A. equal
B. different
C. equal or different
D. different but sometimes equal
E. NA
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Q .
On load tap changer using thyristors can have a maximum of two taps.
A. True
B. False
C.
D.
E. NA
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Q .
If Vdc1 is the dc output voltage in half wave controlled rectifier circuit feeding resistive load and Vdc2 is the dc output voltage in full wave controlled rectifier circuit (M - 2 connection) feeding resistive load, then
A. Vdc1 = 0.5 Vdc2 for all values of firing angle
B. Vdc1 = Vdc2 for all values of firing angle
C. Vdc1 ≤ 0.5 Vdc2 for all values of firing angle
D. Vdc1 ≥ 0.5 Vdc2 for all values of firing angle
E. NA
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Q .
A thyristor has its gate current zero. To start conduction the forward voltage must exceed
A. about 0.5 V
B. forward breakover voltage
C. on state voltage
D. peak working off state forward voltage
E. NA
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Q .
A semi converter operates in two quadrants.
A. True
B. False
C.
D.
E. NA
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Q .
In a single phase full wave ac regulator, the firing angles in the two half cycles
A. are always equal
B. are sometimes equal
C. are never equal
D. may be equal or unequal
E. NA
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Q .
In single phase half wave regulator, the average current over one full cycle
A. is always positive
B. may be positive or negative
C. may be negative
D. is always negative
E. NA
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Q .
A 3 phase induction motor is controlled by voltage and frequency control such that the ratio V/f is constant. At 50 Hz, the speed is 1440 rpm. At 25 Hz, the speed is likely to be
A. about 1350 rpm
B. about 720 rpm
C. about 360 rpm
D. either (b) or (c)
E. NA
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Q .
The total number of leads in reverse conducting thyristor is
A. 2
B. 3
C. 1
D. 4
E. NA
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Q .
In an SCR, the anode current consists of
A. only holes
B. only electrons
C. both electrons and holes
D. either electrons or holes
E. NA
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Q .
A high frequency series inverter consists of
A. a number of series inverters in series
B. a number of series inverters in parallel
C. a number of parallel inverters in parallel
D. a number of parallel inverters in series
E. NA
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Q .
In a 3 phase semiconverter, the firing frequency is
A. equal to line frequency
B. 3 times the line frequency
C. 6 times the line frequency
D. 12 times the line frequency
E. NA
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Q .
In a single phase dual converter, a 1, and a 2 are firing angles of the two converters. Then
A. a 1 - a 2 =
B. a 1 + a 2 = p
C. a 1 - a 2 = p
D. a 1 + a 2 =
E. NA
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Q . Assertion (A): The surge current which an SCR can withstand is much higher than rms on state current.
Reason (R): The duration of surge current is very small.
A. Both A and R are correct and R is correct explanation of A
B. Both A and R correct but R is not correct explanation of A
C. A is correct but R is wrong
D. A is wrong but R is correct
E. NA
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Q .
A thyristor has maximum allowable junction temperature of 110° C and the ambient temperature is 40°C, If the maximum internal power dissipation is 50 W, the thermal resistance is
A. 2° C/W
B. 1.4° C/W
C. 0.7°C/W
D. 2.2° C/W
E. NA
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Q .
For a BJT a = 0.98, then β =
A. 0.02
B. 0.5
C. 4.9
D. 49
E. NA
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Q .
A single phase half wave regulator uses
A. 2 diodes and 2 thyristors
B. 2 thyristors
C. 1 diode and 1 thyristor
D. 2 diodes
E. NA
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Q .
A single phase half wave controlled rectifier circuit has a free wheeling diode. The load is a combination of R and L. The firing angle is a . The period of conduction of SCR and free wheeling diode respectively are
A. a < ωt < p and p < ωt < (2p + a )
B. a < ωt < p and p < ωt < 2p
C. a < ωt < (p + a ) and (p + a ) < ωt < 2p
D. a < ωt < p and p < ωt < (2p - a )
E. NA
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Q .
Present day HVDC converters are all
A. 3 pulse converters
B. 6 pulse converters
C. 12 pulse converters
D. either 6 pulse or 12 converters
E. NA
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Q .
In the below figure, the average value of load current is 15 A. The load is highly inductive. The peak value of thyristor current is
A. 15 A
B. 152 A
C.
D. 30 A
E. NA
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Q .
Between which points does UJT exhibit negative resistance characteristics
A. between peak and valley points
B. before peak point
C. after peak point
D. after valley point
E. NA
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Q .
A single phase fully controlled bridge rectifier is fed by a source having inductance. The output voltage during overlap period is
A. Vm
B. 0.5 Vm
C. zero
D. 0.5 Vm - voltage drop across inductance
E. NA
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Q .
The value of capacitor C for dynamic equalising circuit of series connected thyristors is determined by
A. forward characteristics of thyristors
B. reverse recovery characteristics of thyristors
C. both forward and reverse recovery characteristics of thyristors
D. none of the above
E. NA
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Q .
A single phase half wave converter is used to charge a battery of 50 V. The thyristor is continuously fired by dc signal. Input voltage is v = Vm sin ωt. The thyristor will conduct
A. from 0 to 180°
B. when v > 50 V
C. from 0 to 90°
D. When v < 50 V
E. NA
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Q .
A single phase full wave regulator feeds R-L load. The best gating signal is
A. short duration pulses
B. long duration pulses
C. pulse train
D. either (a) or (b)
E. NA
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Q .
Figure shows a chopper feeding RLE load, The free wheeling diode conducts when
A. thyristor is on
B. thyristor is off
C. both when thyristor is on and thyristor is off
D. partly when thyristor is off and partly when thyristor is on
E. NA
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Q .
A single phase electric heater uses phase controlled thyristors. The firing angle is 90°. The degree of heating is
A. 25% of maximum
B. 50% of maximum
C. 75% of maximum
D. none of the above
E. NA
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Q . Assertion (A): Snubber circuit has an RC series circuit in parallel with SCR
Reason (R): Voltage across a capacitor can not change instantaneously.
A. Both A and R are correct and R is correct explanation of A
B. Both A and R correct but R is not correct explanation of A
C. A is correct but R is wrong
D. A is wrong but R is correct
E. NA
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Q .
The gate current required to turn on an SCR is
A. few amperes
B. a few mA
C. almost equal to anode current
D. about 50% of anode current
E. NA
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Q .
In a multiphase chopper, all choppers operate together.
A. True
B. False
C.
D.
E. NA
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Q .
Two thyristor of same rating and same specifications
A. will have equal turn on and turn off periods
B. will have equal turn on but unequal turn off periods
C. may have equal or unequal turn on and turn off periods
D. will have unequal turn on and turn off periods
E. NA
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Q . Assertion (A): An ideal rectifier should have unity harmonic factor
Reason (R): An ideal rectifier should have unity displacement factor.
A. Both A and R are correct and R is correct explanation of A
B. Both A and R correct but R is not correct explanation of A
C. A is correct but R is wrong
D. A is wrong but R is correct
E. NA
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Q .
A dc chopper V = 500 V feeds an electric fraction dc series motor. The chopping frequency is 400 Hz. The series inductance required to limit the maximum current swing to 10 A is
A. 20 mH
B. about 31 mH
C. about 51 mH
D. about 92 mH
E. NA
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Q . Assertion (A): A cycloconverter may be line commutated or forced commutated
Reason (R): A step up cycloconverter requires forced commutation.
A. Both A and R are correct and R is correct explanation of A
B. Both A and R correct but R is not correct explanation of A
C. A is correct but R is wrong
D. A is wrong but R is correct
E. NA
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Q .
In a dual converter both converters work as rectifiers.
A. True
B. False
C.
D.
E. NA
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Q .
A dc separately excited motor has constant field current. The armature is fed from a single phase supply through a full converter. When a = 0, speed is 500 rpm. If a = 45°, the speed is likely to be
A. about 350 rpm
B. about 250 rpm
C. about 175 rpm
D. about 125 rpm
E. NA
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Q .
For a 12 pulse operation, the two 6 pulse units are fed by
A. Y - Y and Y - Y transformers
B. Δ - Δ and Δ - Δ transformers
C. Y - Y and Y - Δ transformers
D. Y - Δ and Y - Δ transformers
E. NA
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Q .
In a fully controlled bridge converter feeding a resistive load the waveshapes of output voltage and output current
A. are always similar
B. can never be similar
C. may be similar or dissimilar
D. are similar only if a = 0
E. NA
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Q . Assertion (A): When an SCR is conducting, the voltage drop across is about 1 V.
Reason (R): When an SCR is conducting, the junction temperature is about 200°C.
A. Both A and R are correct and R is correct explanation of A
B. Both A and R correct but R is not correct explanation of A
C. A is correct but R is wrong
D. A is wrong but R is correct
E. NA
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Q .
In dielectric heating, the thyristor circuits consist of
A. rectifier-chopper combination
B. controlled rectifier
C. ac regulator
D. rectifier-inverter combination
E. NA
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Q .
In a semi converter feeding a resistive load, the firing angle is a . Each thyristor conducts for
A. p radians
B. (p - a ) radians
C. (p + a ) radians
D. (p - 0.5 a ) radians
E. NA
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Q .
A two winding transformer is feeding a single phase half wave rectifier circuit. The load is purely resistive. The rms value of transformer secondary current is Is and rms value of load current in Irms. Then
A. Is = Irms irrespective of the value of firing angle
B. Is = 0.5 Irms irrespective of the value of firing angle
C. Is = Irms if firing angle is less than 30°
D. Is = Irms if firing angle is more than 30°
E. NA
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Q .
A thyristor has a turn on time of 6 μs. If the anode circuit is inductive, the turn on time will be
A. 6 μs
B. less than 6μs
C. more than 6 μs
D. either 6 μs or less
E. NA
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Q .
A single phase semiconverter is feeding a highly inductive load and has freewheeling diode across the load. The waveshapes of output voltage and output current
A. are similar
B. are not similar
C. may be similar or dissimilar
D. are similar only if firing angle is zero
E. NA
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Q .
An SCR has a turn on time of 4 μs. The gate pulse should have a duration of about
A. 2 μs
B. 5 μs
C. 20 μs
D. 50 μs
E. NA
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Q .
When thyristors are connected in series and parallel, it may be necessary to have :
A. current derating
B. voltage derating
C. both current and voltage derating
D. none of the above
E. NA
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Q . Assertion (A): A TRIAC is a bidirectional SCR.
Reason (R): A TRIAC is a four layers 3 terminal device and can conduct in both directions.
A. Both A and R are correct and R is correct explanation of A
B. Both A and R correct but R is not correct explanation of A
C. A is correct but R is wrong
D. A is wrong but R is correct
E. NA
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Q .
A 3 phase induction motor is fed by a 3 phase ac regulator to change the stator voltage. The variation in speed will be more if
A. load torque is constant
B. load torque increases with increase in speed
C. load torque decreases with increase in speed
D. either (a) or (b)
E. NA
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Q .
A chopper
A. converts constant voltage dc into ac and then into variable voltage dc
B. converts constant voltage dc into variable voltage dc directly
C. converts ac of one frequency into ac of another frequency
D. converts ac to dc
E. NA
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Q .
Thyristors are not suitable for logic circuits.
A. True
B. False
C.
D.
E. NA
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Q .
The dynamic equalising circuit consists of a series combination of capacitor C and resistor RC across each thyristor. This resistance RC (along with parallel connected diode)
A. limits charging current
B. limits discharging current
C. limits both charging and discharging currents
D. none of the above
E. NA
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Q .
A semiconverter feeding an RLE load operates in
A. first quadrant only
B. first and second quadrants
C. first and fourth quadrants
D. first and third quadrants
E. NA
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Q .
In the below figure the rms voltage across each half of secondary is 200 V. The peak inverse voltage across each of thyristor is
A. 200 V
B. 2 (200) V
C. 2 (400) V
D. 800 V
E. NA
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Q .
As regards single phase dual converter
A. the change in operation from one quadrant to the other is faster in non-circulating current mode
B. the change in operation from one quadrant to the other is faster in circulating current mode
C. the time response to change in operation from one quadrant to other is the same in both the modes
D. the time response to change in operation from one quadrant to the other may be faster in any of the two modes
E. NA
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Q .
In a single phase full wave converter (M - 2 connection) feeding a highly inductive load, the firing angle for each thyristor is a in the respective half cycle. The period of conduction of each thyristor is
A. p - a
B. p
C. p + a
D. p - 2a
E. NA
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Q .
The symbol in figure is for
A. DIAC
B. P-N junction diode
C. Schottky diode
D. None of the above
E. NA
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Q .
Current derating may be necessary when thyristors are operating in
A. series
B. parallel
C. series or parallel
D. none of the above
E. NA
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Q .
The number of leads in an SCR are :
A. 2
B. 3
C. 4
D. 5
E. NA
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Q .
In a 3 phase semiconverter, the dc output voltage for a certain firing angle is 50% of the maximum possible value. The firing angle is
A. 30°
B. 60°
C. 90°
D. 120°
E. NA
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Q .
The terminals of a power MOSFET are called
A. emitter, base, collector
B. source, gate, drain
C. source, base, drain
D. emitter, gate, drain
E. NA
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Q .
In controlled heating circuits, thyristors act as
A. rectifier
B. inverter
C. ac regulator
D. chopper
E. NA
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Q .
In a step down chopper using pulse width modulation, Ton = 3 x 10-3 and Toff = 1 x 10-3 s. The chopping frequency is
A. 333.33
B. 250
C. 500
D. 1000
E. NA
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Q .
A cycloconverter cannot give a frequency higher than input frequency.
A. True
B. False
C.
D.
E. NA
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Q . Assertion (A): Inverters and choppers use fast switching thyristors
Reason (R): Fast switching SCR has low turn off time.
A. Both A and R are correct and R is correct explanation of A
B. Both A and R correct but R is not correct explanation of A
C. A is correct but R is wrong
D. A is wrong but R is correct
E. NA
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Q .
The number of gates in an SCR is
A. 1
B. 3
C. 4
D. 6
E. NA
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Q .
A 3 kV circuit uses SCR of 800 V rating. If derating is 25%, the number of SCRs in series is
A. 4
B. 5
C. 6
D. 8
E. NA
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Q .
Thyristors are suitable for dc circuit breakers but not for ac circuit breakers.
A. True
B. False
C.
D.
E. NA
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Q .
The amount of dielectric heating is inversely proportional to frequency.
A. True
B. False
C.
D.
E. NA
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Q .
An electric heater is controlled by thyristors through on-off control. If a = 0.4, the heating is
A. 16% maximum
B. 40% of maximum
C. 60% of maximum
D. 84% of maximum
E. NA
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Q . Assertion (A): A transistor requires a continuous base signal for conduction but a thyristor requires a gate pulse.
Reason (R): Transistor find widespread application in power electronic circuits.
A. Both A and R are correct and R is correct explanation of A
B. Both A and R correct but R is not correct explanation of A
C. A is correct but R is wrong
D. A is wrong but R is correct
E. NA
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Q .
To protect an SCR against high di/dt we connect
A. a capacitance in series with SCR
B. a capacitance in parallel with SCR
C. an inductance in parallel with SCR
D. an inductance in series with SCR
E. NA
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Q .
A single phase full converter gives maximum and minimum output voltage at firing angles of
A. 0 and 180° respectively
B. 180° and 0° respectively
C. 0 and 90° respectively
D. 90° and 0° respectively
E. NA
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Q .
In a single phase full bridge converter (B - 2 connection), the input voltage is v = Vm sin ωt. The output dc voltage for resistive load
A.
B.
C.
D.
E. NA
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Q .
If duty cycle is a and the load is a resistance R, the rms value of output voltage is
A. a V
B. a V
C. a 2V
D.
E. NA
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Q .
A thyristor has a maximum allowable junction temperature of 120°C and the ambient temperature is 40°C. If thermal resistance is 1.6° C/W, the maximum allowable internal power dissipation is
A. 20 W
B. 50 W
C. 92 W
D. 128 W
E. NA
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Q .
A thyristor has internal power dissipation of 40W and is operated at an ambient temperature of 20°C. If thermal resistance is 1.6° C/W, the junction temperature is
A. 114°C
B. 164°C
C. 94° C
D. 84° C
E. NA
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Q .
The waveshape of output voltage of half bridge inverter is
A. sinusoidal
B. square
C. triangular
D. either (a) or (b)
E. NA
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Q .
A Shockley diode can be used
A. in place of SCR
B. as a trigger switch for SCR
C. in place of UJT
D. both in place of SCR and as a trigger switch for SCR
E. NA
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Q .
A silicon controlled switch has :
A. 2 leads
B. 3 leads
C. 4 leads
D. 5 leads
E. NA
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Q . Assertion (A): Power transistor has lower switching time than SCR
Reason (R): Power transistor has greater switching time than SCR
A. Both A and R are correct and R is correct explanation of A
B. Both A and R correct but R is not correct explanation of A
C. A is correct but R is wrong
D. A is wrong but R is correct
E. NA
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Q .
In a circuit using a full wave converter (M - 2 connection) the PIV of each thyristor is 400 V. For the same output voltage and fully controlled bridge converter, PIV will be
A. 400 V
B. 800 V
C. 200 V
D. 2002 V
E. NA
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Q .
The characteristics of a non-linear resistance is i = kv4. If i becomes 100 times, v becomes
A. about 100 times
B. about 10 times
C. about 3 times
D. about twice
E. NA
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Q .
A single phase half wave rectifier is feeding a resistive load. When a = 0, dc output voltage is 127 V. When a = 30°, the dc output voltage will be about
A. 127 V
B. 120 V
C. 110 V
D. 100 V
E. NA
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Q .
The phenomenon of overlap in converter operation due to
A. source resistance
B. source inductance
C. both source resistance and source inductance
D. high value of firing angle
E. NA
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Q .
An SCR is triggered at 40° in the positive half cycle only. The average anode current is 50 A. If the firing angle is changed to 80°, the average anode current is likely to be
A. 50 A
B. 25 A
C. less than 50 A but more than 25 A
D. less than 25 A
E. NA