# Electric Drives Questions and Answers – Induction Motors – Current Source Speed Control

This set of Electric Drives Multiple Choice Questions & Answers (MCQs) focuses on “Induction Motors – Current Source Speed Control”.

1. Calculate the value of the short circuit ratio if the per unit value of synchronous reactance is 1.5 Ω.
a) 0.66
b) 0.33
c) 0.17
d) 0.12

Explanation: The value of the short circuit ratio determines the stability of the synchronous machine. It is inversely proportional to the per unit value of reactance. SCR=1÷Xs(p.u)=1÷1.5=.66.

2. A higher value of armature reaction means the lower value of the short circuit ratio.
a) True
b) False

Explanation: SCR is inversely proportional to the per unit value of synchronous reactance. Armature reaction depends upon the value of the synchronous reactance. A higher value of armature reaction means the lower value of the short circuit ratio.

3. The unit of SCR is Ω.
a) True
b) False

Explanation: SCR is the ratio of the field currents required to produce open circuit voltage and short circuit current. It is a ratio. It has no unit.

4. Calculate the power factor if the power angle is 12°.
a) .87
b) .47
c) .97
d) 0

Explanation: Power factor is the ratio of the real power to the apparent power. It measures the useful power contained in the total power cosΦ=cos(12°)=.97.

5. Calculate the reactive power in an 84 MΩ resistor.
a) 1.47 VAR
b) 45.6 VAR
c) 0.59 VAR
d) 0 VAR

Explanation: The resistor is a linear element. It only absorbs real power and dissipates it in the form of heat. The voltage and current are in the same phase in case of the resistor so the angle between V & I is 0°. Q=VIsin0° = 0 VAR.

6. Calculate the value of the resistance of a wire of length = 12 m, area = 2 m2 and Ρ = 6 Ω-m.
a) 36 Ω
b) 6 Ω
c) 26 Ω
d) 10 Ω

Explanation: Resistance is the opposition offered by the body to the flow of the current. It is expressed in terms of Ω. R = ΡL/A = 36 Ω.

7. Calculate the reactive power in a 29.6 Ω resistor.
a) 45.1 VAR
b) 41.1 VAR
c) 46 VAR
d) 0 VAR

Explanation: The resistor is a linear element. It only absorbs real power and dissipates it in the form of heat. The voltage and current are in the same phase in case of the resistor so the angle between V & I is 0°. Q=VIsin0° = 0 VAR.

8. The slope of the I-V curve is 160°. Calculate the value of resistance.
a) .36 Ω
b) .41 Ω
c) .89 Ω
d) 2.74 Ω

Explanation: The slope of the I-V curve is the reciprocal of the resistance. The slope given is 160° so R=1÷tan(160°)=2.74 Ω. The slope of the V-I curve is resistance.

9. Calculate the value of the power if voltage=4.2 V and current=10 A.
a) 42 W
b) 12 W
c) 16 W
d) 18 W

Explanation: Power is the product of the voltage and current. It is generally expressed in term of W. P=vi=4.2×10=42 W. 1 horsepower=746 W.

10. 1 calorie is equal to ________
a) 4.18 J
b) 5.23 J
c) 6.23 J
d) 8 J

Explanation: The calorie is the unit of energy. It is equal to 4.18 Joule. It is the amount of heat energy required to raise the temperature of one 1 g of water by 1° Celsius.

11. 1 Horse-power is equal to _______
a) 745.7 W
b) 741 W
c) 747.7 W
d) 740 W

Explanation: 1 Horse-power is equal to 745.7 W. It is equal to the rate at which work is done. Horse-power is the amount of work done by horse in carrying the weight of 90 kg for 50 meters in 60 seconds.

12. Calculate the value of the time period if the frequency of the signal is 58 sec.
a) .017 sec
b) .014 sec
c) .045 sec
d) .077 sec

Explanation: The time period is defined as the time after the signal repeats itself. It is expressed in second. T=1÷F=1÷58=.017 sec.

13. Calculate the value of the short circuit ratio if the per unit value of synchronous reactance is 4.2 Ω.
a) 0.26
b) 0.23
c) 0.67
d) 0.72

Explanation: The value of the short circuit ratio determines the stability of the synchronous machine. It is inversely proportional to the per unit value of reactance. SCR=1÷Xs(p.u)=1÷4.2=.23.

14. Calculate the value of the short circuit ratio if the per unit value of synchronous reactance is 1 Ω. Assume armature resistance is 0 ohm.
a) 1
b) 2
c) 3
d) 4

Explanation: The value of the short circuit ratio determines the stability of the synchronous machine. It is inversely proportional to the per unit value of reactance. SCR=1÷Xs(p.u)=1÷1=1.

Sanfoundry Global Education & Learning Series – Electric Drives.

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