An electric heater is controlled by thyristors through on-off control. If a = 0.

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Question
An electric heater is controlled by thyristors through on-off control. If a = 0.4, the heating is
Options
A. 16% maximum
B. 40% of maximum
C. 60% of maximum
D. 84% of maximum

Correct Answer

40% of maximum

Explanation

V_l = V a. Therefore V_l² - V² (a). Heating is proportional to a.

Power Electronics problems

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

3. A thyristor has a turn on time of 6 ?s. If the anode circuit is inductive, the turn on time will be
Options
A. 6 ?s
B. less than 6?s
C. more than 6 ?s
D. either 6 ?s or less

4. A single phase half wave controlled rectifier circuit has a free wheeling diode. The load is a combination of R and L. The firing angle is a. The period of conduction of SCR and free wheeling diode respectively are
Options
A. a < ?t < p and p < ?t < (2p + a)
B. a < ?t < p and p < ?t < 2p
C. a < ?t < (p + a) and (p + a) < ?t < 2p
D. a < ?t < p and p < ?t < (2p - a)

5. 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.
Options
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

6. 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
Options
A. 127 V
B. 120 V
C. 110 V
D. 100 V

8. The terminals of a power MOSFET are called
Options
A. emitter, base, collector
B. source, gate, drain
C. source, base, drain
D. emitter, gate, drain

9. In a 3 phase fully controlled converter the firing frequency is
Options
A. 3 times the line frequency
B. 6 times the line frequency
C. 9 times the line frequency
D. 12 times the line frequency

10. In a single phase full wave regulator, the firing angles in the positive and negative half cycles are generally
Options
A. equal
B. different
C. equal or different
D. different but sometimes equal

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