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  • Question
  • 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°

  • Correct Answer
  • 90° 

    Explanation
    Vdc ? (1 + cos a). Maximum possible output voltage occurs when a = 0.

    Power Electronics problems


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    • 1. 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
    • Discuss
    • 2. 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
    • Discuss
    • 3. 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
    • Discuss
    • 4. 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)
    • Discuss
    • 5. 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
    • Discuss
    • 6. 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 = Vm 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
    • Discuss
    • 7. 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
    • Discuss
    • 8. 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
    • Discuss
    • 9. McMurray Bedford full bridge inverter uses

    • Options
    • A. natural commutation
    • B. auxiliary commutation
    • C. complementary commutation
    • D. any of the above
    • Discuss
    • 10. 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 + ?)
    • Discuss


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