A single phase half wave converter is used to charge a battery of 50 V. The thyr

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Question
A single phase half wave converter is used to charge a battery of 50 V. The thyristor is continuously fired by dc signal. Input voltage is v = V_m sin ?t. The thyristor will conduct
Options
A. from 0 to 180�
B. when v > 50 V
C. from 0 to 90�
D. When v < 50 V

Correct Answer

when v > 50 V

Explanation

Thyristor can conduct only when it is forward biased. This occurs only when v > 50 V.

Power Electronics problems

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1. In a 3 phase semiconverter, the dc output voltage for a certain firing angle is 50% of the maximum possible value. The firing angle is
Options
A. 30�
B. 60�
C. 90�
D. 120�

2. A thyristor has internal power dissipation of 40W and is operated at an ambient temperature of 20�C. If thermal resistance is 1.6� C/W, the junction temperature is
Options
A. 114�C
B. 164�C
C. 94� C
D. 84� C

3. A single phase half wave regulator uses
Options
A. 2 diodes and 2 thyristors
B. 2 thyristors
C. 1 diode and 1 thyristor
D. 2 diodes

4. To protect an SCR against high di/dt we connect
Options
A. a capacitance in series with SCR
B. a capacitance in parallel with SCR
C. an inductance in parallel with SCR
D. an inductance in series with SCR

5. 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)

6. Assertion (A): A GTO circuit is more compact than SCR circuit
Reason (R): A GTO can be turned off by negative gate pulse.
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. In a stud mounted SCR, the stud is
Options
A. in thermal contact with heat sink
B. insulated from heat sink
C. in electrical and thermal contact with heat sink
D. in either electrical or thermal contact with heat sink

9. In a single phase full wave regulator feeding an R-L load, the power factor angle of load is ?. The range of control of firing angle a is
Options
A. 0 ? a ? p
B. ? ? a ? p
C. 0 ? a ? (p - ?)
D. ? ? a ? (p + ?)

10. If a step down chopper is fed by a dc voltage V with T_on and T_off are on and off periods then is
Options
A. average output voltage
B. rms output voltage
C. average or rms output voltage depending on load
D. neither of the above

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