1. Calculate the value of output r.m.s voltage for a Single-phase Half-bridge inverter if the DC supply is 80 V.
A. 40 V
B. 20 V
C. 10 V
D. 60 V
Answer: A
The output voltage of a single-phase half-bridge inverter is a square wave with an amplitude equal to vD.C÷2.
The r.m.s value of output voltage is
vD.C÷2=80÷2=40 V.
2. High d(i)÷d(t) causes _______________
A. Local hotspots
B. Breakdown
C. High cost
D. Low cost
Answer: A
High d(i)÷d(t) causes local hotspots. Local hotspots are small areas with high temperatures due to overcrowdedness of charge generation. This causes high heat generation.
3. Calculate the value of output r.m.s voltage for a Single-phase Full-bridge inverter if the DC supply is 25 V.
A. 26 V
B. 25 V
C. 27 V
D. 28 V
Answer: B
The output voltage of a single-phase full-bridge inverter is a square wave with an amplitude equal to vD.C.
The r.m.s value of output voltage is vD.C=25 V.
4. High d(v)÷d(t) causes _______________
A. Local hotspots
B. False triggering
C. High cost
D. High-speed operation
Answer: B
High d(v)÷d(t) causes false triggering. The high value of d(v)÷d(t) causes the breakdown of junction J2. The thyristor will trigger automatically.
5. A step-down chopper has an input voltage of 12 V and output voltage of 2 V. Calculate the value of the duty cycle.
A. 0.26
B. 0.16
C. 0.33
D. 0.41
Answer: B
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 2÷12=.16.
6. Calculate the De-rating factor if the string efficiency is 90 %.
A. .4
B. .1
C. .7
D. .8
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-.9=.1.
7. Calculate the string efficiency if the de-rating factor is .22.
A. 78 %
B. 42 %
C. 20 %
D. 90 %
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-.22=78 %.
8. 25 V-rated n SCRs are connected in series. The operation voltage of the string is 300 V and the string efficiency is 40 %. Calculate the value of n.
A. 27
B. 30
C. 34
D. 36
Answer: B
The string efficiency can be calculated using the formula operation
voltage÷(Number of SCRs×Rated voltage).
The value of n is 300÷(25×.4)=30.
9. The most reliable method for thyristor triggering is thermal triggering.
A. True
B. False
Answer: B
Gate triggering is more reliable than any other triggering method. The risk of false triggering of SCR reduces in the case of gate triggering.
10. SCR is unipolar.
A. True
B. False
Answer: B
The SCR is a uni-directional, bipolar power electronics device. It is bipolar in nature because of its reverse and forward mode blocking capability.
