Power Electronics - Section 8

In a 3 phase bridge inverter, the gating signals for the three phases have a phase difference of

A. 120°
B. 60°
C. 240°
D. any of the above

A

A single phase semiconverter is feeding a highly inductive load and has a freewheeling diode across the load. The waveshape of input current is

A. sinusoidal
B. rectangular
C. sinusoidal or rectangular
D. triangular

B

A crowbar circuit is generally needed for protection under fault conditions.

A. True
B. False

A

Forward DV/DT rating of an SCR is

A. the maximum rate of rise of anode voltage which will trigger the SCR
B. the maximum rate of rise of anode voltage which will not trigger the SCR if gate signal is applied
C. the maximum rate of rise of anode voltage which will not trigger the SCR if gate signal is not applied
D. either (b) or (c)

C

The method of triggering SCS is

A. application of positive pulse at cathode gate or negative pulse at anode gate
B. application of pulse at anode gate
C. application of pulse at cathode gate
D. application of low voltage at anode

A

The number of thyristors in a single phase fully controlled bridge rectifier is

A. 16
B. 8
C. 4
D. 2

C

In a bridge inverter

A. an output transformer is essential
B. an output transformer is not essential
C. an output transformer cannot be connected
D. an output transformer, if used, can improve the efficiency of the circuit

B

An ideal rectifier should have

A. 100% efficiency, zero ripple factor, zero harmonic factor and unity displacement factor
B. 100% efficiency, zero tipple factor, zero harmonic factor and zero displacement factor
C. 100% efficiency, zero ripple factor, unity harmonic factor and unity displacement factor
D. 100% efficiency, unity ripple factor, zero harmonic factor and unity displacement factor

A

When thyristors are connected in parallel, the current distribution may become non-uniform due to

A. inductive effect of current carrying conductors
B. capacitive effect of current carrying conductors
C. both inductive and capacitive effects
D. none of the above

A

In a single phase half wave ac regulator feeding a resistive load, the rms load current

A. is always positive
B. may be positive or negative
C. is always negative
D. can be even zero

A

In a bridge inverter with single pulse width modulation

A. the output frequency is equal to frequency of reference signal
B. the output frequency is equal to frequency of control signal
C. either (a) or (b)
D. neither (a) nor (b)

A

In a modified series inverter with two inductors and two capacitors, the load current

A. has 2 parallel paths
B. has only one parallel path
C. can have three parallel paths
D. has four parallel paths

A

The number of layers in a Shockley diode are

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

C

In a modified series inverter

A. the maximum output frequency is less than frequency of damped oscillations
B. the output frequency can be even higher than the frequency of damped oscillations
C. the output frequency cannot exceed the frequency of damped oscillations
D. any of the above

B

Assertion (A): Light triggering is very suitable for HVDC transmission.

Reason (R): Light triggering has the advantage of complete electrical isolation of gate circuit.

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

A

Class A and class B chopper

A. operate in first quadrant
B. operate in second quadrant
C. class A chopper operates in first quadrant while class B operates in second quadrant
D. may operate in either first or second quadrant

C

To turn off an SCR, it is necessary to reduce its current to less than :

A. trigger current
B. holding current
C. breakover current
D. none of the above

B

When a single phase dual converter is operating in circulating current mode

A. both converters are working as rectifiers simultaneously
B. one converter is always working as rectifier and the other as inverter
C. only one converter is working at one time
D. both converters are either working as rectifiers or as inverters simultaneously

B

The load circuit of a single phase full bridge inverter

A. must be underdamped
B. must be overdamped
C. may be underdamped or overdamped
D. may be any type of load

D

When an SCR is reverse biased

A. two junctions are reverse biased and one junction is forward biased.
B. all the three junctions are reverse biased.
C. one junction is reverse biased and two junctions are forward biased.
D. any of the above depending on the magnitude of reverse bias.

A

A dc series motor is coupled to a load whose torque is proportional to speed. It is fed by a single phase ac supply through a full converter. When speed is 500 rpm, firing angle is 45°. It is desired to raise the speed. It will be necessary to

A. decrease the firing angle
B. increase the firing angle
C. either decrease or increase the firing angle depending on the required speed
D. decrease the load torque

A

In Jone chopper, the commutating element

A. is a capacitor
B. is a capacitor and an inductor
C. is a capacitor and an auto transformer
D. none of the above

C

A LASCR has

A. 4 semiconductor layers and 3 junctions
B. 3 semiconductor layers and 2 junctions
C. 2 semiconductor layers and 2 junctions
D. any of these depending on the rating

A

A single phase cycloconverter with centre tapped input transformer requires

A. 2 thyristors
B. 4 thyristors
C. 8 thyristors
D. either 2 or 4 thyristors

B

In a 3 phase bridge inverter with 120° mode of operation the. number of thyristors conducting at one time are

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

B

Assertion (A): A class C chopper can operate in first and second quadrants

Reason (R): A class B chopper operates only in second quadrant.

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 single phase dual converter consists of

A. two fully controlled converters connected in parallel
B. two fully controlled converters connected in antiparallel
C. two fully controlled semiconverters connected in antiparallel
D. two fully controlled semiconverters connected in parallel

B

The snubber circuit consists of

A. a capacitor connected in series with thyristor
B. a capacitor connected in parallel with thyristor
C. a resistor and capacitor in series connected across thyristor
D. none of the above

C

The effect of source inductance in a 3 phase fully controlled bridge converter is to

A. decrease the output voltage
B. increase the output voltage
C. decrease the output voltage and increase the output current
D. increase the output voltage and decrease the output current

A

In disc mounting of SCR

A. SCR is clamped on one side of disc shaped heat sink
B. SCR is clamped between two disc shaped heat sinks
C. The SCR has low current rating
D. None of the above

B

As compared to a p-n junction diode

A. a schottky diode has lower cut in voltage and lower reverse saturation current
B. a schottky diode has higher cut in voltage and higher reverse saturation current
C. a Schottky diode has lower cut in voltage and higher reverse saturation current
D. a schottky diode has higher cut in voltage and lower reverse saturation current

C

From which instant is the turn off time of a thyristor defined

A. when anode voltage becomes zero
B. when anode current becomes zero
C. when both anode voltage and current become zero
D. none of the above

B

In a single phase cycloconverter with centre tapped transformer

A. all thyristors carry currents for equal duration
B. the durations of currents in all thyristors may or may not be equal
C. the durations of currents in all thyristors cannot be equal
D. the duration of currents in all thyristors must be equal for satisfactory operation

B

Thyristors can be used for control of

A. dc separately excited motor
B. dc shunt motor
C. dc series motor
D. all of the above

D

Assertion (A): Thyristors can be used in controlled heating, excitation systems of alternators, speed control of motors and HVDC

Reason (R): A static var system using thyristors is very commonly used in high voltage ac systems.

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 dual converter operated in circulating current mode, the circulating current flows

A. when load current is zero
B. when load current is non-zero
C. even when load current is zero
D. none of the above

C

In a full bridge circuit supplying an RLC under damped circuit

A. the waveshapes of output voltage and load current are square
B. the waveshapes of output voltage and load current are sinusoidal
C. the waveshapes of output voltage is square but the waveshape of load current is nearly sinusoidal
D. none of the above

C