# Electric Drives Questions and Answers – Solid-State Switching Circuits – Single Phase, Half-Wave, AC/DC Conversion for Inductive Loads With Freewheeling Diode

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This set of Electric Drives MCQs focuses on “State Switching Circuits – Single Phase, Half-Wave, AC/DC Conversion for Inductive Loads With Freewheeling Diode”.

1. Calculate the voltage across the freewheeling diode when the output voltage is 24 V.
a) -15 V
b) -24 V
c) 28 V
d) 39 V

Explanation: The freewheeling diode is used to provide a freewheeling path. It is connected in the anti-parallel direction of the load. The voltage across the diode is -Vo=-24 V.

2. Calculate the value of the conduction angle for SCR for R-L load with a freewheeling diode if the value of β and α are 65° and 12°.
a) 168°
b) 170°
c) 130°
d) 180°

Explanation: The conduction angle for SCR for R-L load with a freewheeling diode is π-α=180°-12°=168°. R-L load is a current stiff type of load. The current in the SCR only flows from α to π.

3. Calculate the value of the conduction angle for diode for R-L load with a freewheeling diode if the value of α is 45°. (Continous conduction mode)
a) 220°
b) 225°
c) 230°
d) 280°

Explanation: The conduction angle for diode for R-L load with a freewheeling diode is π+α=180°+45°=225°. R-L load is a current stiff type of load.
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4. Calculate the value of the conduction angle for diode for R-L load with a freewheeling diode if the value of β is 226°. (Discontinuous conduction mode)
a) 40°
b) 45°
c) 50°
d) 46°

Explanation: The conduction angle for diode for R-L load with a freewheeling diode is β-π=226°-180°=45°. R-L load is a current stiff type of load.

5. Calculate the value of the conduction angle for R-L load if the value of β and α are 56° and 18°.
a) 48°
b) 38°
c) 57°
d) 15°

Explanation: The conduction angle for R-L load is β-α=56°-18°=38°. R-L load is a current stiff type of load. The current in the circuit only flows from α to β.

6. Calculate the value of the fundamental displacement factor for 1-Φ Full wave bridge rectifier if the firing α=17°.
a) .98
b) .19
c) .56
d) .95

Explanation: Fundamental displacement factor is the cosine of angle difference between the fundamental voltage and fundamental current. D.F=cos(∝)=cos(17°)=0.95.

7. Calculate the value of the fundamental displacement factor for 1-Φ Full wave semi-converter if the firing angle value is 95o.
a) .60
b) .68
c) .62
d) .67

Explanation: Fundamental displacement factor is the cosine of angle difference between the fundamental voltage and fundamental current. The fundamental displacement factor for 1-Φ Full wave semi-converter is cos(∝÷2)=cos(47.5o)=.67.

8. Which one of the following load is suitable for lagging power factor load in Single phase Half-bridge inverter?
b) R-L-C overdamped
c) R-L-C underdamped

Explanation: 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.

9. Which one of the following load is suitable for leading power factor load in Single phase Half-bridge inverter?
b) R-L-C overdamped
c) R-L-C underdamped

Explanation: 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.

10. A step-down chopper has input voltage .1 V and output voltage .01 V. Calculate the value of the duty cycle.
a) 0.1
b) 0.2
c) 0.3
d) 0.5

Explanation: 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 .01÷.1=.1.

11. Calculate the De-rating factor if the string efficiency is 98%.
a) .04
b) .02
c) .05
d) .03

Explanation: De-rating factor is used to measure the reliability of a string. The value of the De-rating factor is 1-(string efficiency)=1-.98=.02.

12. Calculate the string efficiency if the de-rating factor is .50.
a) 28 %
b) 42 %
c) 80 %
d) 50 %

Explanation: The string efficiency is calculated for series and parallel connection of SCRs. The value of string efficiency is 1-(De-rating factor)=1-.50=50 %.

13. 70 V rated 6 SCRs are connected in series. The operation voltage of the string is 130. Calculate the De-rating factor.
a) .70
b) .73
c) .78
d) .74

Explanation: The string efficiency can be calculated using the formula operation voltage÷(Number of SCRs×Rated voltage)=130÷(70×6)=.30. The De-rating factor value is 1-.30=.70.

14. d(v)÷d(t) is more reliable for SCR triggering.
a) True
b) False

Explanation: Gate triggering is more reliable than any other triggering method. The risk of false triggering of SCR increases in the case of the d(v)÷d(t) triggering.

15. Calculate the output voltage of the Buck converter if the supply voltage is 13 V and duty cycle value is .16.
a) 2.08 V
b) 2.24 V
c) 2.58 V
d) 2.54 V

Explanation: The output voltage of the buck converter is Vo = Vin×(D)=13×.16=2.08 V. The value of the duty cycle is less than one which makes the Vo < Vin. The buck converter is used to step down the voltage.

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