Power Electronics - Section 1

A cycloconverter can be

A. step down
B. step up
C. step down or step up
D. none of the above

C

In a 3 phase fully controlled converter the firing frequency is

A. 3 times the line frequency
B. 6 times the line frequency
C. 9 times the line frequency
D. 12 times the line frequency

B

Two thyristor of same rating and same specifications

A. will have equal turn on and turn off periods
B. will have equal turn on but unequal turn off periods
C. may have equal or unequal turn on and turn off periods
D. will have unequal turn on and turn off periods

C

The amount of dielectric heating is inversely proportional to frequency.

A. True
B. False

B

A semiconverter feeding an RLE load operates in

A. first quadrant only
B. first and second quadrants
C. first and fourth quadrants
D. first and third quadrants

A

The phenomenon of overlap in converter operation due to

A. source resistance
B. source inductance
C. both source resistance and source inductance
D. high value of firing angle

B

For a 12 pulse operation, the two 6 pulse units are fed by

A. Y - Y and Y - Y transformers
B. Δ - Δ and Δ - Δ transformers
C. Y - Y and Y - Δ transformers
D. Y - Δ and Y - Δ transformers

C

In a single phase full wave converter (M - 2 connection) feeding a highly inductive load, the firing angle for each thyristor is a in the respective half cycle. The period of conduction of each thyristor is

A. pie - alpha
B. pie
C. pie + alpha
D. pie - 2alpha

B

Since conduction is from a to pie + a, period of conduction is pie in each half cycle.

An SCR is triggered at 40° in the positive half cycle only. The average anode current is 50 A. If the firing angle is changed to 80°, the average anode current is likely to be

A. 50 A
B. 25 A
C. less than 50 A but more than 25 A
D. less than 25 A

C

The characteristics of a non-linear resistance is i = kv^4. If i becomes 100 times, v becomes

A. about 100 times
B. about 10 times
C. about 3 times
D. about twice

C

In a step down chopper using pulse width modulation, Ton = 3 x 10-3 and Toff = 1 x 10-3 s. The chopping frequency is

A. 333.33
B. 250
C. 500
D. 1000

B

.
Thyristors are suitable for dc circuit breakers but not for ac circuit breakers.

A. True
B. False

B

Assertion (A): Inverters and choppers use fast switching thyristors

Reason (R): Fast switching SCR has low turn off time.

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

The value of capacitor C for dynamic equalising circuit of series connected thyristors is determined by

A. forward characteristics of thyristors
B. reverse recovery characteristics of thyristors
C. both forward and reverse recovery characteristics of thyristors
D. none of the above

B

Value of C is proportional to maximum difference between reverse charge of SCRs in series.

A chopper

A. converts constant voltage dc into ac and then into variable voltage dc
B. converts constant voltage dc into variable voltage dc directly
C. converts ac of one frequency into ac of another frequency
D. converts ac to dc

B

Assertion (A): If the duty cycle of chopper is low the current would not become discontinuous

Reason (R): If the time constant L/R of the load in chopper is low the current may become discontinuous.

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

C

If duty cycle is low, current may become discontinuous.

In a single phase full wave regulator, the firing angles in the positive and negative half cycles are generally

A. equal
B. different
C. equal or different
D. different but sometimes equal

A

An electric heater is controlled by thyristors through on-off control. If a = 0.4, the heating is

A. 16% maximum
B. 40% of maximum
C. 60% of maximum
D. 84% of maximum

B

In single phase half wave regulator, the average current over one full cycle

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

D

The input current during positive half cycle is less than that during negative half cycle.

The dynamic equalising circuit consists of a series combination of capacitor C and resistor RC across each thyristor. This resistance RC (along with parallel connected diode)

A. limits charging current
B. limits discharging current
C. limits both charging and discharging currents
D. none of the above

B

Discharge current flows through Re .

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

A. are similar
B. are not similar
C. may be similar or dissimilar
D. are similar only if firing angle is zero

B

When freewheeling diode conducts output current exists but output voltage is zero.

Current derating may be necessary when thyristors are operating in

A. series
B. parallel
C. series or parallel
D. none of the above

B

Since current does not divide equally, current derating is necessary.

Assertion (A): Power transistor has lower switching time than SCR

Reason (R): Power transistor has greater switching time than SCR

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

D

Power transistor does not have hight dv/dt rating.

In a circuit using a full wave converter (M - 2 connection) the PIV of each thyristor is 400 V. For the same output voltage and fully controlled bridge converter, PIV will be

A. 400 V
B. 800 V
C. 200 V
D. 200sqrt2 V

C

In bridge connection PIV is half of that in M-2 connection.

In a multiphase chopper, all choppers operate together.

A. True
B. False

B

They operate in a sequence.

Assertion (A): The surge current which an SCR can withstand is much higher than rms on state current.

Reason (R): The duration of surge current is very small.

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

Since duration of surge current is small, the associated energy is small.

Assertion (A): A transistor requires a continuous base signal for conduction but a thyristor requires a gate pulse.

Reason (R): Transistor find widespread application in power electronic circuits.

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

C

Transistors are used but not very widely.

In a thyristor the gate current is increased, then

A. anode current will increase
B. anode current will decrease
C. anode current will remain constant
D. anode current may increase or decrease

C

Gate current does not affect the magnitude of anode current.

Thyristors are not suitable for logic circuits.

A. True
B. False

B

The number of leads in an SCR are :

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

B

A thyristor has a turn on time of 6 μs. If the anode circuit is inductive, the turn on time will be

A. 6 μs
B. less than 6μs
C. more than 6 μs
D. either 6 μs or less

C

Inductive load increases the turn on time.

The total number of leads in reverse conducting thyristor is

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

B

Assertion (A): A TRIAC is a bidirectional SCR.

Reason (R): A TRIAC is a four layers 3 terminal device and 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

A

Since a TRIAC can conduct in both directions it is bi directional SCR.

If Vdc1 is the dc output voltage in half wave controlled rectifier circuit feeding resistive load and Vdc2 is the dc output voltage in full wave controlled rectifier circuit (M - 2 connection) feeding resistive load, then

A. Vdc1 = 0.5 Vdc2 for all values of firing angle
B. Vdc1 = Vdc2 for all values of firing angle
C. Vdc1 ≤ 0.5 Vdc2 for all values of firing angle
D. Vdc1 ≥ 0.5 Vdc2 for all values of firing angle

A

A semi converter operates in two quadrants.

A. True
B. False

B

It operates only in quadrant.

Assertion (A): An ideal rectifier should have unity harmonic factor

Reason (R): An ideal rectifier should have unity displacement 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

D

Ideal rectifier should have zero harmonic factor and unity displacement factor.

A single phase half wave rectifier is feeding a resistive load. When a = 0, dc output voltage is 127 V. When a = 30°, the dc output voltage will be about

A. 127 V
B. 120 V
C. 110 V
D. 100 V

B