Power Electronics - Section 7

Assertion (A): SUS is a PUT and avalanche diode connected between gate and cathode

Reason (R): SBS has two SUS inbuilt together and connected in anti-parallel

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

B

In a single phase full wave controlled rectifier using centre tap transformer, the voltage across each half of secondary is Vm sin ωt. The peak inverse voltage is

A. 2 Vm
B. Vm
C. 0.5 Vm
D. 0.25 Vm

A

In a three phase series inverter, the firing frequency is

A. three times output frequency
B. six times output frequency
C. twice the output frequency
D. equal to output frequency

B

A 3 phase fully controlled converter is a

A. 3 pulse converter
B. 6 pulse converter
C. 2 pulse converter
D. 12 pulse converter

B

In a half bridge inverter the gate pulses of the two thyristors have a phase displacement of

A. 60°
B. 90°
C. 120°
D. 180°

D

A fully controlled bridge converter can operate in quadrants

A. 1
B. 1 and 2
C. 1 and 3
D. 1 and 4

D

A cycloconverter uses

A. natural commutation
B. forced commutation
C. either natural or forced commutation
D. both natural and forced commutation together

C

Assertion (A): A fully controlled bridge converter can operate in first and fourth quadrant.

Reason (R): A semi converter is cheaper than a full converter.

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

B

The commutation method in an inverter, is

A. line commutation
B. forced commutation
C. either (a) or (b)
D. none of the above

C

Inverters find applications in

A. HVDC transmission
B. UPS
C. variable speed ac drives
D. all of the above

D

When a thyristor is conducting, the voltage drop across it is about

A. 1 V
B. 10 V
C. 100 V
D. 0.1 V

A

The use of free wheeling diode in controlled rectifier improves the waveshape of load current.

A. True
B. False

A

As the firing of a thyristor is delayed, the output voltage of a controlled rectifier decreases.

A. True
B. False

A

Assertion (A): A GTO can be turned on and turned off by gate pulses

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

B

In a full bridge single phase inverter feeding RLC load

A. one thyristor and one diode conduct simultaneously
B. two thyristors conduct simultaneously
C. two thyristors or two diodes conduct simultaneously
D. four thyristors conduct simultaneously

C

It is possible to use a freewheeling diode in

A. single phase half wave controlled rectifier as well as in single phase full wave controlled rectifier (M - 2 connection)
B. single phase half wave controlled rectifier but not in single phase full wave controlled rectifier (M - 2 connection)
C. single phase full wave controlled rectifier (M - 2 connection) but not in single phase half wave controlled rectifier
D. all controlled rectifier circuits

D

Assertion (A): When the anode current of an SCR starts increasing during turn on process, the anode cathode voltage drop starts decreasing.

Reason (R): During turn on the reverse biased junction in SCR breaksdown.

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

S

In a certain inverter circuit each thyristor carries 26.67 A for 60°, 13.33 A for 120° and zero A for 180° of the cycle. The rms current of thyristor is

A. 20 A
B. 13.33 A
C. 6.67 A
D. 3.33 A

B

The factor η for a UJT is about

A. 0.1
B. 0.2
C. 0.6
D. 0.9

C

In a single phase full converter fed by a source having inductance, the number of thyristors conducting during overlap is

A. 1
B. 2
C. 3
D. 4

D

An SCR is triggered at 80° in the positive half cycle, only. The rms anode current is 25 A. If the firing angle is changed to 160°, the rms current is likely to be

A. 25 A
B. 12.5 A
C. less than 25 A
D. less than 12.5 A

D

The output wave of a cycloconverter consists of a number of segments derived from the input wave.

A. True
B. False

A

A TRIAC can be turned on :

A. by applying a positive signal to gate
B. by applying a negative signal to gate
C. by applying either positive or negative signal to gate
D. none of the above

C

Which of the following devices has two gates?

A. SCR
B. SCS
C. SUS
D. SBS

B

In auxiliary commutated chopper, the peak value of thyristor current is

A. equal to peak value of load current
B. equal to peak value of capacitor current
C. sum of peak value of load current and capacitor current
D. none of the above

C

The static equalisation circuit for thyristors connected in series is

A. one resistor in series with each thyristor
B. one resistor in parallel with each thyristor
C. one resistor and one capacitor in parallel with each thyristor
D. none of the above

B

A cycloconverter is a group of controlled rectifiers.

A. True
B. False

A

A single phase full wave regulator feeds a resistive load. The firing angle of thyristors is 90°. The input power factor is

A. 1
B. 0.707 lagging
C. 0.5 lagging
D. 0.2 lagging

B

The leads in a TRIAC are denoted by

A. anode cathode, gate
B. T1 T2, gate
C. T1, T2, anode
D. T1, T2, T3

B

A step down cycloconverter has an input frequency f. If it uses natural commutation, the output frequency

A. must be a sub-multiple of input frequency
B. can be any frequency
C. must be an even sub-multiple of input frequency
D. must be an odd sub-multiple of input frequency

A

Lead mounting for SCR is used for

A. rating less than mA
B. rating about 1 A or less
C. rating of 20 A or less than 20 A
D. rating 100 A or less

B

A load commutated chopper circuit has

A. one thyristor
B. two thyristors
C. four thyristors
D. six thyristors

C

Assertion (A): The output voltage of an inverter can be controlled by PWM technique

Reason (R): In a current source inverter the input current is constant.

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

B

A TRIAC is equivalent to

A. two thyristors in series
B. two thyristors in parallel
C. one thyristor and one diode
D. one thyristor and one transistor

B

Which of the following commutation methods is the overall circuit under damped?

A. Class A
B. Class B
C. Class C
D. Both class A and Class B

D

In a half bridge inverter, the freewheeling diodes are needed

A. even when load is resistive
B. only when load is inductive
C. only when load is inductive or capacitive
D. for all types of load

C

When a thyristor is conducting, the voltage drop across it

A. is absolutely constant
B. decreases with increase in load current
C. increases slightly with increase in load current
D. none of the above

C

The advantage of modified series inverter with two inductors and two capacitors is

A. the intermittent operation of battery is avoided
B. efficiency of circuit is more
C. the output frequency is low
D. all of the above

A

An SUS is

A. exactly similar to SCR but with higher rating
B. similar to UJT
C. PUT and avalanche diode connected in anti parallel
D. SCR and avalanche diode connected in parallel

C

Which chopper circuit operates in quadrants 1 and 4?

A. A
B. B
C. D
D. E

C