21. 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 %.
22. 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.
23. 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.
24. 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.
25. 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.
26. Thyristors are used in series to complete ________
A. High voltage demand
B. High current demand
C. Low voltage demand
D. Low current demand
Answer: C
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.
27. Thyristors are used in parallel to complete ________
A. High voltage demand
B. High current demand
C. Low current demand
D. Low voltage demand
Answer: C
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.
28. Calculate peak-peak voltage if Vmax=200 V and Vmin=190 V.
A. 20 V
B. 10 V
C. 60 V
D. 80 V
Answer: B
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.
29. Which harmonic is not present in 3-Φ fully controlled rectifier?
A. 27th harmonic
B. 15th harmonic
C. 17th harmonic
D. 12th harmonic
Answer: A
Triple harmonics are absent in the case of a 3-phase fully controlled rectifier. Because of the absence of triple harmonics, the THD of the rectifier is 31%.
30. Third lowest order harmonic present in 3-Φ fully controlled rectifier is __________
A. 11th
B. 17th
C. 13th
D. 22nd
Answer: A
Only 6k±1 order harmonics are present in 3-Φ fully controlled rectifiers. The lowest order harmonic is the 11th harmonic for k=2. Third-order harmonics are absent in 3-Φ fully controlled rectifier.
