9294 + Mcqs in Electronics and Communication Engineering Page 139 McqOptions

6901.	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
Answer» D. 120

Discussion

6902.	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
Answer» C. a number of parallel inverters in parallel

Discussion

6903.	In an SCR, the anode current consists of
A.	only holes
B.	only electrons
C.	both electrons and holes
D.	either electrons or holes
Answer» D. either electrons or holes

Discussion

6904.	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
Answer» E.

Discussion

6905.	A silicon controlled switch has :
A.	2 leads
B.	3 leads
C.	4 leads
D.	5 leads
Answer» D. 5 leads

Discussion

6906.	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
Answer» C. about 51 mH

Discussion

6907.	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
Answer» B. 180 and 0 respectively

Discussion

6908.	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
Answer» B. Both A and R correct but R is not correct explanation of A

Discussion

6909.	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
Answer» B. are sometimes equal

Discussion

6910.	In a single phase full bridge converter (B - 2 connection), the input voltage is v = Vm sin t. The output dc voltage for resistive load
A.	<img src="/_files/images/electronics-and-communication-engineering/power-electronics/276-244-1.png">
B.	<img src="/_files/images/electronics-and-communication-engineering/power-electronics/276-244-2.png">
C.	<img src="/_files/images/electronics-and-communication-engineering/power-electronics/276-244-3.png">
D.	<img src="/_files/images/electronics-and-communication-engineering/power-electronics/276-244-4.png">
Answer» D. <img src="/_files/images/electronics-and-communication-engineering/power-electronics/276-244-4.png">

Discussion

6911.	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
Answer» D. either 6 pulse or 12 converters

Discussion

6912.	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)
Answer» C. about 360 rpm

Discussion

6913.	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
Answer» C. the time response to change in operation from one quadrant to other is the same in both the modes

Discussion

6914.	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
Answer» C. 6 times the line frequency

Discussion

6915.	In a thyristor, the forward breakover voltage
A.	is constant
B.	may be constant or may depend on gate current
C.	decreases as gate current is increased
D.	increases as gate current is increased
Answer» D. increases as gate current is increased

Discussion

6916.	In a 3 phase full converter, the firing angle is less than 60 . The instantaneous output voltage
A.	will have positive part only
B.	will have both positive and negative parts
C.	may have both positive and negative parts
D.	will have negative part if load is inductive
Answer» B. will have both positive and negative parts

Discussion

6917.	In a 3 phase bridge inverter with 180 mode of operation the numbers of thyristors conducting at one time are
A.	1
B.	2
C.	3
D.	4
Answer» D. 4

Discussion

6918.	A single phase full converter can operate in
A.	4 quadrants
B.	3 quadrants
C.	2 quadrants
D.	1 quadrant
Answer» D. 1 quadrant

Discussion

6919.	In the circuit of figure, the load current when thyristor is off
A.	is constant
B.	increases exponentially
C.	decreases exponentially
D.	zero
Answer» D. zero

Discussion

6920.	In a 3 phase semiconverter, the freewheeling diode comes into operation only if firing angle
A.	is zero
B.	is 60
C.	is more than 60
D.	is more than 90
Answer» D. is more than 90

Discussion

6921.	A 3 phase fully controlled bridge converter is fed by a 3 phase system having phase voltage v = Vm sin t. The firing angle is a. The dc output voltage is
A.	<img src="/_files/images/electronics-and-communication-engineering/power-electronics/278-277-1.png">
B.	<img src="/_files/images/electronics-and-communication-engineering/power-electronics/278-277-2.png">
C.	<img src="/_files/images/electronics-and-communication-engineering/power-electronics/278-277-3.png">
D.	<img src="/_files/images/electronics-and-communication-engineering/power-electronics/278-277-4.png">
Answer» B. <img src="/_files/images/electronics-and-communication-engineering/power-electronics/278-277-2.png">

Discussion

6922.	Which of these commutation methods uses an external pulse for commutation?
A.	Class A
B.	Class C
C.	Class E
D.	Class F
Answer» D. Class F

Discussion

6923.	A separately excited dc motor is run from a dc source through a chopper. At 600 rpm and rated torque, the duty cycle of chopper is 0.8. At 600 rpm and half the rated torque, the duty cycle is likely to be
A.	0.9
B.	0.8
C.	0.75
D.	0.6
Answer» D. 0.6

Discussion

6924.	A relaxation oscillator uses
A.	tunnel diode
B.	UJT
C.	both tunnel diode and UJT
D.	PIN diode
Answer» C. both tunnel diode and UJT

Discussion

6925.	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
Answer» C. almost equal to anode current

Discussion

6926.	In brushless excitation system of modern alternators, the main exciter is
A.	dc generator
B.	ac generator
C.	either dc or ac generator
D.	induction generator
Answer» C. either dc or ac generator

Discussion

6927.	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
Answer» C. on state voltage

Discussion

6928.	Assertion (A): Snubber circuit has an RC series circuit in parallel with SCRReason (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
Answer» B. Both A and R correct but R is not correct explanation of A

Discussion

6929.	In dielectric heating, the thyristor circuits consist of
A.	rectifier-chopper combination
B.	controlled rectifier
C.	ac regulator
D.	rectifier-inverter combination
Answer» E.

Discussion

6930.	On load tap changer using thyristors can have a maximum of two taps.
A.	True
B.	False
Answer» C.

Discussion

6931.	Assertion (A): A cycloconverter may be line commutated or forced commutatedReason (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
Answer» B. Both A and R correct but R is not correct explanation of A

Discussion

6932.	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
Answer» B. rectification efficiency will be more than 0.6 but TUF will be less than 0.35

Discussion

6933.	Which of these commutation methods is used in AC-DC converters?
A.	Class A
B.	Class C
C.	Class D
D.	Class F
Answer» E.

Discussion

6934.	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
Answer» D. 2 diodes

Discussion

6935.	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
Answer» C. 75% of maximum

Discussion

6936.	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
Answer» D. none of the above

Discussion

6937.	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
Answer» C. in place of UJT

Discussion

6938.	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)
Answer» C. parallel and simultaneously

Discussion

6939.	Assertion (A): The gate current of an SCR is always in the form of pulses. Reason (R): The gate losses, in pulse triggering, are low.
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
Answer» B. Both A and R correct but R is not correct explanation of A

Discussion

6940.	A single phase half wave controlled rectifier circuit is feeding on RL load. The circuit has a freewheeling diode. The input voltage is v = Vm sin t. If I0 is the load current during the conduction of freewheeling diode, the KVL equation during the conduction of free wheeling diode is
A.	<img src="/_files/images/electronics-and-communication-engineering/power-electronics/276-239-1.png">
B.	<img src="/_files/images/electronics-and-communication-engineering/power-electronics/276-239-2.png">
C.	<img src="/_files/images/electronics-and-communication-engineering/power-electronics/276-239-3.png">
D.	<img src="/_files/images/electronics-and-communication-engineering/power-electronics/276-239-4.png">
Answer» C. <img src="/_files/images/electronics-and-communication-engineering/power-electronics/276-239-3.png">

Discussion

6941.	Thyristors connected in series need :
A.	static equalizing circuit
B.	dynamic equalizing circuit
C.	both static and dynamic equalizing circuits
D.	none of the above
Answer» D. none of the above

Discussion

6942.	The symbol in figure is for
A.	SUS
B.	SBS
C.	SCS
D.	None of the above
Answer» D. None of the above

Discussion

6943.	Induction heating systems require low frequency inverters.
A.	True
B.	False
Answer» C.

Discussion

6944.	The characteristics of which device has peak point and valley point
A.	SCR
B.	PUT
C.	SCS
D.	Shockley diode
Answer» C. SCS

Discussion

6945.	In a step down chopper using time ratio control, the input voltage is V and duty cycle is a. The load is purely resistive having resistance R. The average and rms load currents are
A.	<img src="/_files/images/electronics-and-communication-engineering/power-electronics/280-321-1.png">
B.	<img src="/_files/images/electronics-and-communication-engineering/power-electronics/280-321-2.png">
C.	<img src="/_files/images/electronics-and-communication-engineering/power-electronics/280-321-3.png">
D.	<img src="/_files/images/electronics-and-communication-engineering/power-electronics/280-321-4.png">
Answer» C. <img src="/_files/images/electronics-and-communication-engineering/power-electronics/280-321-3.png">

Discussion

6946.	In a stud mounted SCR, the stud is
A.	in thermal contact with heat sink
B.	insulated from heat sink
C.	in electrical and thermal contact with heat sink
D.	in either electrical or thermal contact with heat sink
Answer» D. in either electrical or thermal contact with heat sink

Discussion

6947.	In a full bridge inverter, the waveshape of load current depends on
A.	relative values of load parameters R, L and C
B.	relative values of load parameters R and L
C.	relative values of load parameters L and C
D.	gating frequency
Answer» B. relative values of load parameters R and L

Discussion

6948.	In a dual converter both converters work as rectifiers.
A.	True
B.	False
Answer» C.

Discussion

6949.	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)
Answer» D. either (a) or (b)

Discussion

6950.	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)
Answer» C. load torque decreases with increase in speed

Discussion

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Which of these commutation methods uses an external pulse for commutation?

A separately excited dc motor is run from a dc source through a chopper. At 600 rpm and rated torque, the duty cycle of chopper is 0.8. At 600 rpm and half the rated torque, the duty cycle is likely to be

A relaxation oscillator uses

The gate current required to turn on an SCR is

In brushless excitation system of modern alternators, the main exciter is

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In dielectric heating, the thyristor circuits consist of

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A single phase half wave regulator uses

A single phase electric heater uses phase controlled thyristors. The firing angle is 90 . The degree of heating is

When thyristors are connected in series and parallel, it may be necessary to have :

A Shockley diode can be used

In a multiphase chopper, the different choppers operate in

Assertion (A): The gate current of an SCR is always in the form of pulses. Reason (R): The gate losses, in pulse triggering, are low.

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Thyristors connected in series need :

The symbol in figure is for

Induction heating systems require low frequency inverters.

The characteristics of which device has peak point and valley point

In a step down chopper using time ratio control, the input voltage is V and duty cycle is a. The load is purely resistive having resistance R. The average and rms load currents are

In a stud mounted SCR, the stud is

In a full bridge inverter, the waveshape of load current depends on

In a dual converter both converters work as rectifiers.

A single phase full wave regulator feeds R-L load. The best gating signal is

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