The harmonic distortion is least in

 1. In a single-phase fully controlled converter with resistive load, the average output voltage is given by

A) $V_{avg} = \frac{Vm}{\pi}(1 + \cos \alpha)$
B) $V_{avg} = \frac{Vm}{\pi}\cos \alpha$
C) $V_{avg} = \frac{2Vm}{\pi}\cos \alpha$
D) $V_{avg} = Vm\cos \alpha$
Answer: B) $V_{avg} = \frac{Vm}{\pi}\cos \alpha$

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2. In a 3-phase full converter, the output voltage is controlled by

A) Delay angle α
B) Firing frequency
C) Commutation circuit
D) Source impedance
Answer: A) Delay angle α

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3. The input power factor of a controlled rectifier decreases with

A) Decreasing α
B) Increasing α
C) Increasing load
D) Reducing load current
Answer: B) Increasing α

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4. In current commutation, the SCR is turned off by

A) Applying reverse voltage
B) Forcing current to zero
C) Applying forward bias
D) Reducing load current
Answer: B) Forcing current to zero

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5. The main disadvantage of line commutation is

A) Requires complex circuits
B) Not suitable for DC supply
C) Requires high-frequency AC source
D) High cost
Answer: B) Not suitable for DC supply

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6. The function of a snubber circuit is to

A) Increase voltage
B) Limit rate of change of voltage (dv/dt)
C) Control current
D) Increase frequency
Answer: B) Limit rate of change of voltage (dv/dt)

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7. The device combining the advantages of MOSFET and BJT is

A) GTO
B) TRIAC
C) IGBT
D) UJT
Answer: C) IGBT

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8. A step-up chopper increases

A) Current
B) Voltage
C) Frequency
D) Resistance
Answer: B) Voltage

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9. In a step-down chopper, the average output voltage is

A) Directly proportional to duty ratio
B) Inversely proportional to duty ratio
C) Independent of duty ratio
D) Equal to input voltage
Answer: A) Directly proportional to duty ratio

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10. For a 3-phase inverter, the number of pulses per cycle in the output voltage is

A) 2
B) 3
C) 6
D) 12
Answer: C) 6

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11. The harmonic order of a 6-pulse converter is given by

A) 6n ± 1
B) 3n ± 1
C) n
D) 12n ± 1
Answer: A) 6n ± 1

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12. In PWM inverters, harmonics can be minimized by

A) Increasing switching frequency
B) Decreasing switching frequency
C) Using line commutation
D) Reducing DC voltage
Answer: A) Increasing switching frequency

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13. The turn-on time of an SCR depends on

A) Junction temperature
B) Gate current magnitude
C) Load current
D) All of the above
Answer: D) All of the above

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14. In a DC chopper, the inductor is used to

A) Smooth current
B) Block voltage
C) Control frequency
D) Reduce harmonics
Answer: A) Smooth current

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15. In a buck-boost converter, the output voltage polarity is

A) Same as input
B) Opposite to input
C) Zero
D) Alternating
Answer: B) Opposite to input

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16. A forced commutation circuit is used in

A) AC circuits
B) DC circuits
C) Line commutated converters
D) All circuits
Answer: B) DC circuits

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17. The current tailing problem occurs in

A) MOSFET
B) BJT
C) IGBT
D) GTO
Answer: C) IGBT

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18. The on-state voltage drop of a GTO is

A) Higher than SCR
B) Lower than SCR
C) Same as SCR
D) Negligible
Answer: A) Higher than SCR

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19. The advantage of MOSFET over BJT in high-frequency operation is due to

A) Low input impedance
B) Voltage control
C) High switching speed
D) Low cost
Answer: C) High switching speed

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20. Cycloconverter converts

A) Fixed DC to variable DC
B) Fixed AC to variable AC of lower frequency
C) Fixed DC to AC
D) AC to DC
Answer: B) Fixed AC to variable AC of lower frequency

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21. The commutation method used in inverters is mostly

A) Line commutation
B) Load commutation
C) Current commutation
D) Forced commutation
Answer: D) Forced commutation

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22. For a single-phase inverter with resistive load, the RMS output voltage is

A) $\frac{Vm}{\sqrt{2}}$
B) $\frac{Vm}{2}$
C) $\frac{Vm}{\pi}$
D) $Vm$
Answer: A) $\frac{Vm}{\sqrt{2}}$

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23. The conduction angle in a controlled rectifier is

A) 90°
B) (π - α)
C) (π + α)
D) α
Answer: B) (π - α)

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24. A dual converter consists of

A) Two fully controlled converters in parallel
B) Two fully controlled converters in antiparallel
C) Two half converters in parallel
D) One converter and inverter
Answer: B) Two fully controlled converters in antiparallel

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25. In regenerative braking, power flows

A) From source to load
B) From load to source
C) No power flow
D) Alternates both ways
Answer: B) From load to source

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26. The typical switching frequency of PWM inverter is

A) 50 Hz
B) 500 Hz
C) 5 kHz – 20 kHz
D) 100 kHz
Answer: C) 5 kHz – 20 kHz

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27. In a current source inverter, the output current waveform is

A) Square
B) Sinusoidal
C) Triangular
D) Constant
Answer: A) Square

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28. The commutation circuit in a GTO is required for

A) Turn-on
B) Turn-off
C) Both
D) None
Answer: B) Turn-off

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29. The voltage control in inverters can be achieved using

A) Pulse width modulation
B) Phase control
C) Line commutation
D) Frequency modulation
Answer: A) Pulse width modulation

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30. The harmonic distortion is least in

A) Single-pulse inverter
B) Multiple-pulse inverter
C) PWM inverter
D) Square-wave inverter
Answer: C) PWM inverter