An SCR is triggered at 40� in the positive half cycle only. The average anode cu

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

Correct Answer

less than 50 A but more than 25 A

Explanation

An SCR is triggered at 40� in the positive half cycle only. The average anode current is 50 A. If th

An SCR is triggered at 40� in the positive half cycle only. The average anode current is 50 A. If th .

Power Electronics problems

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1. In a single phase full wave converter (M-2 connection) feeding on R - L load, the input voltage is v = V_m sin ?t. The expression for dc output voltage is
Options
A.
B.
C.
D.

2. Assertion (A): Power transistor has lower switching time than SCR
Reason (R): Power transistor has greater switching time than SCR
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

3. A two winding transformer is feeding a single phase half wave rectifier circuit. The load is purely resistive. The rms value of transformer secondary current is I_s and rms value of load current in I_rms. Then
Options
A. I_s = I_rms irrespective of the value of firing angle
B. I_s = 0.5 I_rms irrespective of the value of firing angle
C. I_s = I_rms if firing angle is less than 30�
D. I_s = I_rms if firing angle is more than 30�

4. In a step down chopper using pulse width modulation, T_on = 3 x 10^-3 and T_off = 1 x 10^-3 s. The chopping frequency is
Options
A. 333.33
B. 250
C. 500
D. 1000

5. For a 12 pulse operation, the two 6 pulse units are fed by
Options
A. Y - Y and Y - Y transformers
B. ? - ? and ? - ? transformers
C. Y - Y and Y - ? transformers
D. Y - ? and Y - ? transformers

6. 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

7. 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)

8. 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

10. 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

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