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

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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
View Answer

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

Answer: a
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
View Answer

Answer: b
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
View Answer

Answer: c
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
View Answer

Answer: d
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
View Answer

Answer: a
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.
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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
View Answer

Answer: d
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
View Answer

Answer: d
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
View Answer

Answer: a
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
View Answer

Answer: a
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
View Answer

Answer: a
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
View Answer

Answer: a
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.
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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
View Answer

Answer: b
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
View Answer

Answer: a
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.

To practice all areas of Electric Drives, here is complete set of 1000+ Multiple Choice Questions and Answers.

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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He is Linux Kernel Developer & SAN Architect and is passionate about competency developments in these areas. He lives in Bangalore and delivers focused training sessions to IT professionals in Linux Kernel, Linux Debugging, Linux Device Drivers, Linux Networking, Linux Storage, Advanced C Programming, SAN Storage Technologies, SCSI Internals & Storage Protocols such as iSCSI & Fiber Channel. Stay connected with him @ LinkedIn