Power Electronics - Section 5

Multiphase choppers are used in

A. low power circuits
B. high power circuit
C. both low and high power circuits
D. medium power circuits

B

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

A. pie
B. a
C. pie - a
D. pie/2

A

In a parallel inverter

A. each thyristor is turned on twice during each cycle
B. each thyristor is turned on once during each cycle
C. each thyristor is turned on either once or twice in each load cycle
D. each thyristor may be turned on upto four times in each load cycle

A

A step up chopper can give an output voltage

A. higher than input voltage
B. lower than input voltage
C. both higher and lower input voltage
D. all of the above

A

The specified maximum junction temperature of thyristors is about

A. 50°C
B. 80°C
C. 120°C
D. 200°C

C

A bridge type single phase cycloconverter changes the frequency f to f/3. Then one half wave of output contains

A. 3 full waves of input
B. 3 half waves of input
C. 6 full waves of input
D. 6 half waves of input

B

A half bridge inverter

A. needs a 3 wire dc supply
B. may use a two wire or 3 wire dc supply
C. has higher output voltage if fed from 3 wire dc supply
D. has higher efficiency if fed from 3 wire dc supply

A

A 3 phase series inverter has

A. 12 thyristors
B. 6 thyristors
C. 3 thyristors
D. 3 or 6 thyristors

B

HVDC systems are preferable for only long distances.

A. True
B. False

A

Surge current rating of an SCR is

A. the maximum repetitive surge current in which the SCR can withstand
B. the maximum non-repetitive surge current in which the SCR can withstand
C. the maximum repetitive or non repetitive surge current in which the SCR can withstand
D. the maximum surge current in which the SCR can withstand if both surge current and steady current exist together

B

A dc chopper is feeding an RLE load. The condition for maximum ripple is

A. a = 1
B. a = 0.8
C. a = 0.5
D. a < 0.5

A

In a series inverter

A. the commutating elements L and C are in parallel
B. the commutating elements L and C are in series with load
C. the commutating L and C may be in series or parallel with the load
D. none of the above

B

When a power diode is conducting, the voltage drop across it is about

A. I V
B. 5 V
C. 20 V
D. 50 V

A

In thermal ohm's law, temperature and power loss are analogous to

A. V and I respectively of ohm's law
B. I and V respectively of ohm's law
C. R and V respectively of ohm's law
D. I and R respectively of ohm's law

B

A string of series connected thyristors needs equalising circuit to

A. ensure proper firing
B. ensure proper current division
C. ensure proper voltage division
D. none of the above

C

A surge voltage

A. has very high magnitude and very long duration
B. has very high magnitude and very small duration
C. has very high magnitude and very small duration and can be positive or negative
D. can be only positive

C

The minimum gate current which can turn on SCR is called :

A. trigger current
B. holding current
C. junction current
D. breakover current

A

The total number of leads in SUS, SBS and SCS respectively are

A. 3, 3, 4,
B. 3, 3, 3
C. 2, 3, 3
D. 3, 3, 5

A

Which of these devices can conduct in both directions?

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

C

Assertion (A): A half wave controlled rectifier has poorer efficiency and higher ripple factor than a full wave controlled rectifier.

Reason (R): The use of a freewheeling diode in half wave controlled rectifier circuit improves the waveform of load current and circuit power 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

B

A 3 phase dual converter consists of

A. two 3 phase semiconverters connected in parallel
B. two 3 phase semiconverters connected in antiparallel
C. two 3 phase full converters connected in antiparallel
D. two 3 phase full converters connected in parallel

C

Which inverter circuit uses coupled inductors?

A. Modified McMurray full bridge inverter
B. McMurray Bedford half bridge inverter
C. Modified McMurray Bedford half bridge inverter
D. Both (b) and (c)

D

When an SCR is in forward blocking mode, the applied voltage

A. appears across only one junction
B. is equally shared by two junctions
C. is equally shared by three junctions
D. may appear across one or more junctions

A

In sinusoidal PWM for inverters the width of different pulses are not the same.

A. True
B. False

A

A class D chopper

A. can operate in first quadrant only
B. can operate in second quadrant only
C. can operate in either first or fourth quadrant
D. can operate in all the quadrants

D

The efficiency of a chopper circuit is about

A. 80% or more
B. around 50%
C. around 20%
D. around 5%

A

If the frequency is less than the damped frequency in a series- inverter

A. the distortion of output voltage waveform is low
B. the distortion of output voltage waveform is high
C. the distortion of output voltage may be high or low depending on the load impedance
D. none of the above

B

The output frequency of a cycloconverter can be changed by changing the firing angle.

A. True
B. False

B

Mc Murray full bridge inverter uses

A. 4 thyristors and 2 diodes
B. 4 thyristors and 4 diodes
C. 8 thyristors and 4 diodes
D. 8 thyristors and 8 diodes

C

Assertion (A): A DIAC has four layers but only two terminals.

Reason (R): A DIAC 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

B

In a 3 phase full converter, the instantaneous output voltage can have negative part if

A. firing angle is less than 60°
B. firing angle is less than 90°
C. firing angle is more than 60°
D. firing angle is more than 90°

C

In a single phase full wave ac regulator using two thyristors only, the gate circuits of the two thyristors

A. must be isolated
B. may or may not be isolated
C. should be preferably, isolated
D. should not be isolated

A

A fully controlled bridge converter is feeding an RLE load. For some circuit conditions firing angle is 47°. If connections of battery are reversed, the firing angle is likely to be

A. 47°
B. less than 47°
C. more than 47°
D. either more or less than 47°

C

In a single phase half wave converter feeding resistive load and full wave converter (M - 2 connection) feeding a resistive load the circuit conditions are similar. The firing angle is a. Then the waveshapes of voltages across thyristors

A. are similar
B. are not similar
C. may be similar or dissimilar
D. are similar only for some values of a

A

In a 3 phase full converter feeding a highly inductive load, the average load current is 150 A. The peak current through thyristor is

A. 150 A
B. 75 A
C. 50 A
D. 300 A

A

A 3 phase bridge inverter is fed by 400 V battery. The load is star connected and has a resistance of 10 Ω per phase. The peak value of thyristor current is

A. 400/10
B. 400/15
C. 400/20
D. 400/30

B

The main utility of SCR is due to

A. gate triggering
B. high temperature triggering
C. DV/DT triggering
D. high breakdown voltage

A

In a single phase full wave ac regulator the firing angles during both half cycles are equal.

A. True
B. False

A

In a 3 phase full wave regulator feeding a star connected resistance load, firing angle is 75°. Then the number of thyristors conducting at one time is

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

B

A single phase half bridge inverter required to feed R-L loads needs

A. 2 thyristors
B. 4 thyristors
C. 2 thyristors and two diodes
D. 4 thyristors and 4 diodes

V

Typical values of DI/DT(S/U SEC) AND DV/DT 9V/U SEC) for an SCR are in the range of

A. 1 to 10
B. 25 to 500
C. 0.1 to 1
D. 1000 to 2000

B

When a dc chopper feeds an RLE load, the load current, during steady state operation

A. remains constant
B. varies between maximum and minimum values
C. may remain constant or vary
D. is constant if R is constant

B