Power Electronics A. True B. False
It is proportional to frequency.
1. A cycloconverter can be
6. A semiconverter feeding an RLE load operates in
A. step down B. step up
A. first quadrant only
C. step down or step up D. none of the above
B. first and second quadrants
The output frequency can be more or less than input
frequency. C. first and fourth quadrants
2. In the below figure the average load current is 15 A. The D. first and third quadrants
rms value of transformer secondary current is
Semiconvertor operates only in first quadrant.
7. 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
A. 15 A B. 10.61 A
Source inductance delays turn off and hence causes
C. 7.5 A D. 14.14 A
overlap.
8. A thyristor has a maximum allowable junction
RMS secondary current = = 15/1.414 = 10.608 A. temperature of 120°C and the ambient temperature is
3. In a 3 phase fully controlled converter the firing 40°C. If thermal resistance is 1.6° C/W, the maximum
frequency is allowable internal power dissipation is
A. 3 times the line frequency A. 20 W B. 50 W
B. 6 times the line frequency C. 92 W D. 128 W
C. 9 times the line frequency
.
D. 12 times the line frequency
9. For a 12 pulse operation, the two 6 pulse units are fed
It is a 6 pulse converter. by
4. Two thyristor of same rating and same specifications A. Y - Y and Y - Y transformers
A. will have equal turn on and turn off periods B. Δ - Δ and Δ - Δ transformers
B. will have equal turn on but unequal turn off periods C. Y - Y and Y - Δ transformers
C. may have equal or unequal turn on and turn off D. Y - Δ and Y - Δ transformers
periods
30° phase difference is necessary.
D. will have unequal turn on and turn off periods
10. In a single phase full wave converter (M - 2
Turn on and turn off times of similar SCRs may not be connection) feeding a highly inductive load, the firing angle
same. for each thyristor is a in the respective half cycle. The
5. The amount of dielectric heating is inversely period of conduction of each thyristor is
proportional to frequency. A. pi - α B. pi
, C. pi + α D. pi - 2α B. Both A and R correct but R is not correct explanation of
A
Since conduction is from a to p + a, period of conduction
is p in each half cycle. C. A is correct but R is wrong
11. An SCR is triggered at 40° in the positive half cycle D. A is wrong but R is correct
only. The average anode current is 50 A. If the firing angle
Low turn off time enables fast switching.
is changed to 80°, the average anode current is likely to be
16. The value of capacitor C for dynamic equalising circuit
A. 50 A
of series connected thyristors is determined by
B. 25 A
A. forward characteristics of thyristors
C. less than 50 A but more than 25 A
B. reverse recovery characteristics of thyristors
D. less than 25 A
C. both forward and reverse recovery characteristics of
thyristors
D. none of the above
Value of C is proportional to maximum difference
between reverse charge of SCRs in series.
.
17. A chopper
12. The characteristics of a non-linear resistance is i =
kv^4. If i becomes 100 times, v becomes A. converts constant voltage dc into ac and then into
variable voltage dc
A. about 100 times B. about 10 times
B. converts constant voltage dc into variable voltage dc
C. about 3 times D. about twice directly
v ∝ (i)1/4 ∝ (100)1/4 ≈ 3. C. converts ac of one frequency into ac of another
13. In a step down chopper using pulse width modulation, frequency
Ton = 3 x 10-3 and Toff = 1 x 10-3 s. The chopping D. converts ac to dc
frequency is
A chopper is a dc-ac converter. It converts a fixed dc
A. 333.33 B. 250 voltage into a variable dc voltage
C. 500 D. 1000 18. 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.
14. Thyristors are suitable for dc circuit breakers but not
for ac circuit breakers. A. Both A and R are correct and R is correct explanation of
A
A. True B. False
B. Both A and R correct but R is not correct explanation of
They are suitable for both dc and ac circuit breakers.
A
15. Assertion (A): Inverters and choppers use fast
C. A is correct but R is wrong
switching thyristors
D. A is wrong but R is correct
Reason (R): Fast switching SCR has low turn off time.
If duty cycle is low, current may become discontinuous.
A. Both A and R are correct and R is correct explanation of
A
It is proportional to frequency.
1. A cycloconverter can be
6. A semiconverter feeding an RLE load operates in
A. step down B. step up
A. first quadrant only
C. step down or step up D. none of the above
B. first and second quadrants
The output frequency can be more or less than input
frequency. C. first and fourth quadrants
2. In the below figure the average load current is 15 A. The D. first and third quadrants
rms value of transformer secondary current is
Semiconvertor operates only in first quadrant.
7. 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
A. 15 A B. 10.61 A
Source inductance delays turn off and hence causes
C. 7.5 A D. 14.14 A
overlap.
8. A thyristor has a maximum allowable junction
RMS secondary current = = 15/1.414 = 10.608 A. temperature of 120°C and the ambient temperature is
3. In a 3 phase fully controlled converter the firing 40°C. If thermal resistance is 1.6° C/W, the maximum
frequency is allowable internal power dissipation is
A. 3 times the line frequency A. 20 W B. 50 W
B. 6 times the line frequency C. 92 W D. 128 W
C. 9 times the line frequency
.
D. 12 times the line frequency
9. For a 12 pulse operation, the two 6 pulse units are fed
It is a 6 pulse converter. by
4. Two thyristor of same rating and same specifications A. Y - Y and Y - Y transformers
A. will have equal turn on and turn off periods B. Δ - Δ and Δ - Δ transformers
B. will have equal turn on but unequal turn off periods C. Y - Y and Y - Δ transformers
C. may have equal or unequal turn on and turn off D. Y - Δ and Y - Δ transformers
periods
30° phase difference is necessary.
D. will have unequal turn on and turn off periods
10. In a single phase full wave converter (M - 2
Turn on and turn off times of similar SCRs may not be connection) feeding a highly inductive load, the firing angle
same. for each thyristor is a in the respective half cycle. The
5. The amount of dielectric heating is inversely period of conduction of each thyristor is
proportional to frequency. A. pi - α B. pi
, C. pi + α D. pi - 2α B. Both A and R correct but R is not correct explanation of
A
Since conduction is from a to p + a, period of conduction
is p in each half cycle. C. A is correct but R is wrong
11. An SCR is triggered at 40° in the positive half cycle D. A is wrong but R is correct
only. The average anode current is 50 A. If the firing angle
Low turn off time enables fast switching.
is changed to 80°, the average anode current is likely to be
16. The value of capacitor C for dynamic equalising circuit
A. 50 A
of series connected thyristors is determined by
B. 25 A
A. forward characteristics of thyristors
C. less than 50 A but more than 25 A
B. reverse recovery characteristics of thyristors
D. less than 25 A
C. both forward and reverse recovery characteristics of
thyristors
D. none of the above
Value of C is proportional to maximum difference
between reverse charge of SCRs in series.
.
17. A chopper
12. The characteristics of a non-linear resistance is i =
kv^4. If i becomes 100 times, v becomes A. converts constant voltage dc into ac and then into
variable voltage dc
A. about 100 times B. about 10 times
B. converts constant voltage dc into variable voltage dc
C. about 3 times D. about twice directly
v ∝ (i)1/4 ∝ (100)1/4 ≈ 3. C. converts ac of one frequency into ac of another
13. In a step down chopper using pulse width modulation, frequency
Ton = 3 x 10-3 and Toff = 1 x 10-3 s. The chopping D. converts ac to dc
frequency is
A chopper is a dc-ac converter. It converts a fixed dc
A. 333.33 B. 250 voltage into a variable dc voltage
C. 500 D. 1000 18. 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.
14. Thyristors are suitable for dc circuit breakers but not
for ac circuit breakers. A. Both A and R are correct and R is correct explanation of
A
A. True B. False
B. Both A and R correct but R is not correct explanation of
They are suitable for both dc and ac circuit breakers.
A
15. Assertion (A): Inverters and choppers use fast
C. A is correct but R is wrong
switching thyristors
D. A is wrong but R is correct
Reason (R): Fast switching SCR has low turn off time.
If duty cycle is low, current may become discontinuous.
A. Both A and R are correct and R is correct explanation of
A
