# Electric Drives Questions and Answers – Solid-State Devices – Power Electronic Devices Ratings

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This set of Electric Drives Multiple Choice Questions & Answers (MCQs) focuses on “Solid-State Devices – Power Electronic Devices Ratings”.

1. Which one of the following load is suitable for load commutation in Single phase Half-bridge inverter?
b) R-L-C overdamped
c) R-L-C underdamped

Explanation: R-L-C underdamped are leading power factor loads. They do not require any forced commutation technique. Anti-Parallel diodes help in the commutation process.

a) True
b) False

Explanation: R-L-C overdamped loads are generally lagging power factor loads. They require forced commutation. Anti-Parallel diodes do not help in the commutation process.

3. A step-down chopper has input voltage 40 V and output voltage 20 V. Calculate the value of the duty cycle.
a) 0.5
b) 0.2
c) 0.3
d) 0.4

Explanation: The output voltage of the step-down chopper is Vo = Vin×(D). The value of the duty cycle is less than one which makes the Vo < Vin. The step-down chopper is used to step down the voltage. The value of the duty cycle is 20÷40 = .5.
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4. Calculate the De-rating factor if the string efficiency is 20 %.
a) .8
b) .4
c) .5
d) .6

Explanation: De-rating factor is used to measure the reliability of a string. The value of the De-rating factor is 1-(string efficiency) = 1-.2 = .8.

5. Calculate the string efficiency if the de-rating factor is .48.
a) 48 %
b) 52 %
c) 50 %
d) 60 %

Explanation: The string efficiency is calculated for series and parallel connection of SCRs. The value of string efficiency is 1 – (De-rating factor) = 1-.48 = 52 %.

6. 400 V rated 5 SCRs are connected in series. The operation voltage of the string is 500. Calculate the string efficiency.
a) 25 %
b) 40 %
c) 38 %
d) 35 %

Explanation: The string efficiency can be calculated using the formula operation voltage÷(Number of SCRs×Rated voltage) = 500÷(400×5) = 25 %.

7. 100 V rated 3 SCRs are connected in series. The operation voltage of the string is 200. Calculate the De-rating factor.
a) .35
b) .33
c) .38
d) .18

Explanation: The string efficiency can be calculated using the formula operation voltage÷(Number of SCRs×Rated voltage) = 200÷(100×3) = .66. The De-rating factor value is 1-.66 = .33.

8. Gate triggering is more reliable.
a) True
b) False

Explanation: Gate triggering is more reliable than any other triggering method. The risk of false triggering of SCR reduces in the case of gate triggering.

9. The range of turn-off time is _______
a) .51-3 μsec
b) 3-101 μsec
c) 9-400 μsec
d) 7-787 μsec

Explanation: The range of turn-off time is 3-101 μsec. The time is very low as SCR takes low time to turn-off. Commutation time should be greater than circuit turn-off time to avoid false triggering.

10. The range of On-State voltage drop is _______
a) 1-1.6 V
b) 0-1 V
c) 1.6-2 V
d) 2.4-3 V

Explanation: When SCR is in forwarding blocking mode it does not conduct. When the gate pulse is applied, it starts conducting. The On-state voltage drop is very low around 1-1.6 V.

11. Thyristors are used in series to complete ________
a) High voltage demand
b) High current demand
c) Low voltage demand
d) Low current demand

Explanation: Thyristors are used in series to complete high voltage demand. In series voltage across the components can be the same or different but the value of current remains the same.

12. Thyristors are used in parallel to complete ________
a) High voltage demand
b) High current demand
c) Low current demand
d) Low voltage demand

Explanation: Thyristors are used in parallel to complete high current demand. In parallel current in the components can be the same or different but the value of voltage remains the same.

13. Calculate peak-peak voltage if Vmax=200 V and Vmin=190 V.
a) 20 V
b) 10 V
c) 60 V
d) 80 V

Explanation: Peak-Peak voltage is equal to the difference between the maximum and minimum voltage. It is mathematically represented as Vp-p=Vmax-Vmin=200-190=10 V.

14. Which harmonic is not present in 3-Φ fully controlled rectifier?
a) 27rd harmonic
b) 15th harmonic
c) 17th harmonic
d) 12th harmonic

Explanation: Triplen harmonics are absent in the case of 3-phase fully controlled rectifier. Because of the absence of triplen harmonics THD of the rectifier 31%.

15. Third lowest order harmonic present in 3-Φ fully controlled rectifier is __________
a) 11th
b) 17th
c) 13th
d) 22nd

Explanation: Only 6k±1 order harmonics are present in 3-Φ fully controlled rectifier. The lowest order harmonic is 11th harmonic for k=2. Third order harmonics are absent in 3-Φ fully controlled rectifier.

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