Which of these commutation methods is used in AC-DC converters? Class A Class C

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Question
Which of these commutation methods is used in AC-DC converters?
Options
A. Class A
B. Class C
C. Class D
D. Class F

Correct Answer

Class F

Explanation

Class F is natural commutation AC = DC converter use natural (or line) commutation.

Power Electronics problems

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1. Assertion (A): Snubber circuit has an RC series circuit in parallel with SCR
Reason (R): Voltage across a capacitor can not change instantaneously.
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

2. In a fully controlled bridge converter feeding a resistive load the waveshapes of output voltage and output current
Options
A. are always similar
B. can never be similar
C. may be similar or dissimilar
D. are similar only if a = 0

3. In a parallel inverter
Options
A. the commutating capacitor C is across the primary of transformer and the load is across the secondary
B. the commutating capacitor and load are connected across the secondary of transformer
C. the commutating capacitor C and load are connected in series and the combination is across the secondary of transformer
D. none of the above

4. As regards single phase dual converter
Options
A. the change in operation from one quadrant to the other is faster in non-circulating current mode
B. the change in operation from one quadrant to the other is faster in circulating current mode
C. the time response to change in operation from one quadrant to other is the same in both the modes
D. the time response to change in operation from one quadrant to the other may be faster in any of the two modes

5. A Shockley diode can be used
Options
A. in place of SCR
B. as a trigger switch for SCR
C. in place of UJT
D. both in place of SCR and as a trigger switch for SCR

8. Assertion (A): A high dv/dt may cause triggering of a thyristor.
Reason (R): A high dv/dt results in charging current in the thyristor.
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

9. A thyristor has maximum allowable junction temperature of 110� C and the ambient temperature is 40�C, If the maximum internal power dissipation is 50 W, the thermal resistance is
Options
A. 2� C/W
B. 1.4� C/W
C. 0.7�C/W
D. 2.2� C/W

10. A 3 phase induction motor is controlled by voltage and frequency control such that the ratio V/f is constant. At 50 Hz, the speed is 1440 rpm. At 25 Hz, the speed is likely to be
Options
A. about 1350 rpm
B. about 720 rpm
C. about 360 rpm
D. either (b) or (c)

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