11. Calculate the line voltage for 3-∅ 120° VSI if the value of DC supply is 96 V.
A. 68.25 V
B. 67.87 V
C. 66.25 V
D. 62.14 V
Answer: B
The line voltage for 3-∅ 120° VSI is
(VDC÷√2)=.707×96=67.87 V.
In 3-phase 120° VSI each phase conducts for 120° and only three switches from different phases conduct at a single time.
12. Calculate the phase voltage for 3-∅ 120° VSI if the value of DC supply is 78 V.
A. 30.45 V
B. 15.18 V
C. 31.84 V
D. 48.15 V
Answer: C
The line voltage for 3-∅ 120° VSI is (VDC÷√6)=31.84 V.
In 3-phase 120° VSI each phase conducts for 120° and only three switches from different phases conduct at a single time.
13. Calculate the r.m.s value of thyristor current in 3-Φ Full-wave converter for the load current=2 A and α=59°.
A. 1.15 A
B. 2.55 A
C. 2.07 A
D. 2.89 A
Answer: A
The r.m.s value of thyristor current in a 3-Φ Full-wave converter is Io√1÷3. It is the r.m.s value of the thyristor current.
I(r.m.s) = Io√1÷3=1.15 A.
14. Calculate the average value of thyristor current ia n 3-Φ Full-wave converter for the load current=729 A and α=3°.
A. 254 A
B. 285 A
C. 247 A
D. 243 A
Answer: D
The average value of thyristor current in a 3-Φ Full-wave converter is Io÷3. It is the average value of the thyristor current.
I(r.m.s) = Io÷3=243 A.
15. 1600 V rated 8 SCRs are connected in series. The operation voltage of the string is 4400. Calculate the string efficiency.
A. 34.37 %
B. 30.15 %
C. 38.48 %
D. 35.84 %
Answer: A
The string efficiency can be calculated using the formula
operation voltage ÷ (Number of SCRs×Rated voltage) = 4400÷(1600×8) = 34.37 %.
16. The use of the inductor in CSI is to make the source current constant.
A. True
B. False
Answer: A
The inductor in CSI is used to make the source current constant. The input current of CSI should be constant.
17. The output voltage of CSI depends upon the load.
A. True
B. False
Answer: A
The output voltage of CSI depends upon the load. The output current of CSI is fixed as the input source current is fixed.
18. Which one of the elements is used to make R load commutating?
A. Inductor
B. Capacitor
C. Resistor
D. Coil
Answer: B
The capacitor is used to make the R load commutating. It is used in parallel with the resistive load.
19. Calculate the minimum period for ASCI using the data: R=12 kΩ, C=45 μF.
A. 2.58 sec
B. 2.16 sec
C. 2.23 sec
D. 2.48 sec
Answer: B
The minimum time period for ASCI is a
4×R×C
= 4×12×45×.001 = 2.16 sec.
The total time is the sum of charging and discharging time.
20. Full form of ASCI is ___________
A. Auto sequential commutated inverter
B. Auto starter commutated inverter
C. Auto sequential current inverter
D. Auto sequential current inserter
Answer: A
ASCI stands for Auto sequential commutated inverter. No external forced commutation is required for ASCI. The capacitor itself works as a load-commutated inverter.
