11. Calculate the r.m.s value of the output current having an amplitude of 2 A for CSI.
A. 3 A
B. 4 A
C. 2 A
D. 1 A
Answer: B
The r.m.s value of the output current having an amplitude of 2 A for CSI is 2A. The waveform for output current is a square wave. The r.m.s value of the square wave is its amplitude.
12. Calculate the r.m.s value of the fundamental component of the output current having an amplitude of 3 A for CSI.
A. 2.80 A
B. 2.70 A
C. 2.54 A
D. 3.48 A
Answer: B
The r.m.s value of the fundamental component of the output current is
2√2I÷π=8.48÷3.14=2.70 A. The r.m.s value of the output current is I.
13. Calculate the string efficiency if the de-rating factor is .76.
A. 24 %
B. 25 %
C. 26 %
D. 27 %
Answer: A
The string efficiency is calculated for the series and parallel connection of SCRs. The value of string efficiency is 1-(De-rating factor)=1-.76=24 %.
14. 20 V rated 8 SCRs are connected in series. The operation voltage of the string is 50. Calculate the De-rating factor.
A. .67
B. .68
C. .69
D. .70
Answer: B
The string efficiency can be calculated using the formula
operation voltage÷(Number of SCRs×Rated voltage)=
50÷(20×8)=.31.
The De-rating factor value is 1-.31=.68.
15. Forced commutated CSI require _________
A. Diode
B. Resistor
C. Inductor
D. Capacitor
Answer: D
Forced commutated CSI requires a capacitor. The capacitor acts as a commutating element. It is connected in parallel with the load.
16. In a Half-wave controlled rectifier calculate the average value of the voltage if the supply is 13sin(25t) and the firing angle is 13°.
A. 4.08 V
B. 4.15 V
C. 3.46 V
D. 5.48 V
Answer: A
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.
17. Calculate the extinction angle as a purely inductive load if the firing angle is 13°.
A. 328°
B. 347°
C. 349°
D. 315°
Answer: B
The extinction angle in the purely inductive load is
2π-∝=360°-13°=347°.
The extinction angle is the angle at which the current in the circuit becomes zero. The average value of the voltage in a purely inductive load is zero.
18. Calculate the conduction angle as a purely inductive load if the firing angle is 165°.
A. 78°
B. 55°
C. 30°
D. 19°
Answer: C
The conduction angle in the purely inductive load is
β-α=2(π-∝)
=2(180°-165°)=30°.
The conduction angle is the angle at which the current exists in the circuit. The average value of the voltage in a purely inductive load is zero.
19. R-L-C underdamped loads are generally lagging power factor loads.
A. True
B. False
Answer: B
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.
20. In a 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
Answer: A
In a 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.
