1. Which one of the following loads is suitable for load commutation in a Single-phase Half-bridge inverter?
A. R-L load
B. R-L-C overdamped
C. R-L-C underdamped
D. L-load
Answer: C
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.
2. R-L-C overdamped loads are generally lagging power factor loads.
A. True
B. False
Answer: A
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 an input voltage of 40 V and output voltage of 20 V. Calculate the value of the duty cycle.
A. 0.5
B. 0.2
C. 0.3
D. 0.4
Answer: A
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.
4. Calculate the De-rating factor if the string efficiency is 20 %.
A. .8
B. .4
C. .5
D. .6
Answer: A
The 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 %
Answer: B
The string efficiency is calculated for the 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 %
Answer: A
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
Answer: B
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
Answer: A
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
Answer: B
The range of turn-off time is 3-101 μsec. The time is very low as SCR takes a 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
Answer: A
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.
