Power Electronics - Section 6

In a single phase full wave ac regulator feeding R-L load, the gate pulses

A. are always of short duration
B. may be of short duration
C. should be rectangular with width (p - a)
D. cannot be rectangular with width (p - a)

C

In a semi converter feeding a resistive load,

A. the waveshapes of input voltage and input current are always similar
B. the waveshapes of input voltage and input current can never be similar
C. the waveshapes of input voltage and input current are similar only if firing angle is zero
D. the waveshapes of input voltage and input current are similar only if firing angle is high

C

In a single phase semi-converter the number of thyristors is

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

D

A dc chopper is feeding an RLE load. The current can become discontinuous if

A. R/L is high
B. a is low
C. E is high
D. none of the above

D

Which of these commutation methods uses two leads?

A. Class A
B. Class B
C. Class C
D. Class D

D

A single phase half wave rectifier is feeding a resistive load. As firing angle is increased

A. dc output voltage and rms output voltage decrease in the same proportion
B. dc output voltage and rms output voltage decrease
C. dc output voltage and rms output voltage increase
D. dc output voltage and rms output voltage increase in the same proportion

B

A cycloconverter for single phase operation has

A. one positive group and one negative group converters
B. two positive group and two negative group converters
C. three positive group and three negative group converters
D. any of the above

A

The difference between resonant commutation and self commutation is

A. both these methods are based on entirely different principles
B. in resonant commutation, the commutating elements carry load current but in self commutation, the commutating elements do not carry load current
C. resonant commutation uses L and C but self commutation uses only capacitor
D. none of the above

A

In a series inverter

A. the commutating elements carry full load current
B. the commutating elements carry upto 50% of load current
C. the commutating elements do not carry load current
D. any of the above

A

A NAND gate has 3 inputs and one output. The number of thyristors required for this gate are

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

C

n a single phase semiconverter feeding a highly inductive load, the load current is

A. sinusoidal
B. nearly constant
C. absolutely constant
D. rectangular

B

Two thyristors of the same rating and specifications

A. will have equal on state voltage drops
B. will have equal forward blocking voltage for the same current
C. will have different on state voltage drops
D. may have equal or different on state voltage drops and equal or different forward blocking voltage's for the same current

D

A 3 phase induction motor is controlled by voltage and frequency control such that the ratio V/f is constant. At 50 Hz, the starting torque is 100 N-m. At 25 Hz, the starting torque

A. will be less than 100 N-m
B. will be more than 100 N-m
C. may be more or less than 100 N-m depending on motor parameters
D. very small

B

In a three phase to single phase full wave bridge cycloconverter, the number of thyristors is

A. 36
B. 18
C. 12
D. 8

C

In a cycloconverter it is possible to vary

A. only output frequency
B. only output voltage
C. both output voltage and frequency but not simultaneously
D. both output voltage and output frequency simultaneously

D

A single phase full bridge inverter for R-L loads needs

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

B

In the basic series inverter circuit, the source current has

A. low harmonic content
B. high harmonic content
C. low or high harmonic content depending on firing angle
D. zero harmonic content

B

In a controlled rectifier a freewheeling diode is necessary if the load is

A. inductive
B. resistive
C. capacitive
D. any of the above

A

For an illumination control circuit it is advisable to use a matched TRIAC-DIAC pair.

A. True
B. False

A

Assertion (A): An ac regulator uses natural commutation

Reason (R): An ac regulator can be used for static on load tap changing of transformer.

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

When a UJT is used as a relaxation oscillator the current should be

A. less than Ip
B. more than Iv
C. more than Ip but less than Iv
D. negative

C

In auxiliary commutated chopper, the charging and discharging time of capacitor

A. are constant
B. depend on load current
C. may be constant or may depend on load current
D. none of the above

B

Which chopper circuit uses saturable reactor?

A. Auxiliary commutated
B. Jones chopper
C. Morgan chopper
D. Load commutated

C

A thyristor circuit is feeding an R-L load. The turn on time can be reduced by

A. decreasing R
B. decreasing L
C. increasing L
D. decreasing R and L together

B

A single phase semiconverter is supplying RLE load and has a freewheeling diode across the load. The firing angle is a. The freewheeling diode conducts for

A. a° in each half cycle
B. (p - a)° in each cycle
C. a° in one full cycle
D. (p + a)° in each half cycle

A

In a single phase full wave regulator feeding a resistive load, the average current over one full cycle

A. is always zero
B. cannot be zero
C. may be sometimes zero
D. is always positive

A

The total number of leads in MOS controlled thyristor is

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

B

Assertion (A): Chopper always requires forced commutation

Reason (R): Morgan chopper uses a saturable reactor.

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 power diode the reverse recovery time is the time from the instant the forward current is zero to the instant when reverse recovery current has decayed to

A. 50% of peak reverse current
B. 25% of peak reverse current
C. 10% of peak reverse current
D. zero value

A

Assertion (A): McMurray Bredford half bridge inverter uses complementary commutation

Reason (R): Three phase inverter can have two modes of operation.

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

Which of these devices have four layers?

A. UJT
B. SCR
C. TRIAC
D. SCR and TRIAC

B/D

When UJT is used for thyristor gate signal, the output of UJT is

A. sine wave
B. square wave
C. saw tooth wave
D. either (a) or (c)

C

The stud in a stud mounted SCR acts as

A. anode
B. cathode
C. gate
D. anode or cathode

A

In a single phase full wave regulator feeding R-L load, short gate pulse are not suitable.

A. True
B. False

A

Half bridge and full bridge inverters of one type use different commutation methods.

A. True
B. False

B

A UJT exhibits negative resistance characteristics when

A. Current is less than valley current
B. Current is less than peak current
C. Current is more than peak current but less than valley current
D. None of the above

C

If we connect two silicon unilateral switches in anti- parallel we get

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

B