This set of Electric Drives Multiple Choice Questions & Answers (MCQs) focuses on “State Switching Circuits – Single Phase, Half-Wave, AC/DC Conversion for Resistive Loads”.
1. In Half-wave controlled rectifier calculate the average value of the voltage if the supply is 13sin(25t) and firing angle is 13°.
a) 4.08 V
b) 4.15 V
c) 3.46 V
d) 5.48 V
View Answer
Explanation: In Half-wave controlled rectifier, the average value of the voltage is Vm(1+cos(∝))÷2π=13(1+cos(13°))÷6.28=4.08 V. The thyristor will conduct from ∝ to π.
2. Calculate the extinction angle in purely inductive load if the firing angle is 13°.
a) 328°
b) 347°
c) 349°
d) 315°
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Explanation: The extinction angle in the purely inductive load is 2π-∝=360°-13°=347°. The extinction angle is the angle at which current in the circuit becomes zero. The average value of the voltage in a purely inductive load is zero.
3. Calculate the conduction angle in purely inductive load if the firing angle is 165°.
a) 78°
b) 55°
c) 30°
d) 19°
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Explanation: The conduction angle in the purely inductive load is β-α=2(π-∝)=2(180°-165°)=30°. The conduction angle is the angle for which the current exists in the circuit. The average value of the voltage in a purely inductive load is zero.
4. R-L-C underdamped loads are generally lagging power factor loads.
a) True
b) False
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Explanation: R-L-C underdamped loads are generally leading power factor loads. They do not require forced commutation. Anti-Parallel diodes help in the commutation process.
5. In Half-wave uncontrolled rectifier calculate the average value of the voltage if the supply is 3sin(5t).
a) .95 V
b) .92 V
c) .93 V
d) .94 V
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Explanation: In Half-wave uncontrolled rectifier, the average value of the voltage is Vm÷π=3÷π=.95 V. The diode will conduct only for the positive half cycle. The conduction period of the diode is π.
6. In Half-wave uncontrolled rectifier calculate the r.m.s value of the voltage if the supply is 89sin(41t).
a) 91.5 V
b) 44.5 V
c) 25.1 V
d) 15.1 V
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Explanation: In Half-wave uncontrolled rectifier, the r.m.s value of the voltage is Vm÷2=89÷2=44.5 V. The diode will conduct only for the positive half cycle. The conduction period of the diode is π.
7. In Half-wave uncontrolled rectifier calculate the power dissipation across the 8 Ω resistor if the supply is 29sin(22t).
a) 26.2 W
b) 24.2 W
c) 26.1 W
d) 29.1 W
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Explanation: In Half-wave uncontrolled rectifier, the r.m.s value of the voltage is Vm÷2=29÷2=14.5 V. The diode will conduct only for the positive half cycle. Power dissipation across the resistor is V2r.m.s÷R=14.52÷8=26.2 W.
8. The conduction period of diode in Half-wave uncontrolled rectifier for resistive load is ______________
a) π
b) 2π
c) 3π
d) 4π
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Explanation: The conduction period of the diode in Half-wave uncontrolled rectifier for the resistive load is π. For the negative A.C supply diode will be reverse biased.
9. In Half-wave uncontrolled rectifier calculate the average value of the current for 3 Ω resistive load if the supply is 34sin(11t).
a) 3.6 A
b) 2.6 A
c) 2.5 A
d) 3.1 A
View Answer
Explanation: In Half-wave uncontrolled rectifier, the average value of the current is Vm÷πR=34÷3π=3.6 A. The diode will conduct only for the positive half cycle. The conduction period of the diode is π.
10. In Half-wave controlled rectifier calculate the average value of the current for 2.5 Ω resistive load if the supply is sin(5.2t) and firing angle is 26°.
a) 0.8 V
b) 0.15 V
c) 0.12 V
d) 0.21 V
View Answer
Explanation: In Half-wave controlled rectifier, the average value of the current is Vm(1+cos(∝))÷2πR=(1+cos(26°))÷6.28×2.5=.12 V. The thyristor will conduct from ∝ to π.
11. Calculate the circuit turn-off time for Half-wave controlled rectifier for a ω=5 rad/sec for resistive load.
a) .62 sec
b) .42 sec
c) .58 sec
d) .64 sec
View Answer
Explanation: The value of the circuit turn-off for Half-wave controlled rectifier for a ω=5 rad/sec for the resistive load is a π÷Ω=π÷5=.62 sec.
12. Calculate the string efficiency if the de-rating factor is .429.
a) 48.1 %
b) 57.1 %
c) 47.8 %
d) 46.5 %
View Answer
Explanation: The string efficiency is calculated for series and parallel connection of SCRs. The value of string efficiency is 1-(De-rating factor)=1-.429=57.1 %.
13. Calculate the output frequency for the six-pulse converter if the supply frequency is 10 Hz.
a) 40 Hz
b) 30 Hz
c) 60 Hz
d) 80 Hz
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Explanation: The output of a six-pulse converter consists of six pulses in one cycle. The output frequency of the six pulse converter is 6×supply frequency=6×10=60 Hz.
14. Calculate the pulse number if the supply frequency is 2π and the output frequency is π÷6.
a) 4
b) 12
c) 16
d) 8
View Answer
Explanation: The pulse number can be calculated using the ratio of input frequency to the output frequency. The value of pulse number (P) is 2π÷π÷6=12. It is a twelve-pulse converter.
15. Volt-sec balance method is based on the principle of the energy of conservation.
a) True
b) False
View Answer
Explanation: Volt-sec balance method states that the net area over the cycle will be zero. It is based on the principle of the energy of conservation. The amount of charging is equal to discharging.
Sanfoundry Global Education & Learning Series – Electric Drives.
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