Electrical Machines Questions and Answers – Losses and Efficiency

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This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Losses and Efficiency”.

1. Insertion of external resistance in the squirrel cage induction motor _________
a) increases torque
b) decreases torque
c) increases torque for some seconds and then decreases
d) does not affect
View Answer

Answer: a
Explanation: Adding the external resistance reduces the starting current and increases the current.

2. Insertion of external resistance in the squirrel cage induction motor _________ the current ________ the starting current.
a) reduces, increases
b) increases, reduces
c) increases, increases
d) reduces, reduces
View Answer

Answer: a
Explanation: Adding the external resistance reduces the starting current and increases the current.

3. Rotor ohmic loss is ________ slip.
a) proportional to
b) inverse of
c) inversely proportional to
d) hyperbolic to
View Answer

Answer: a
Explanation: Rotor ohmic loss is direct proportional to slip.
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4. A 3-phasle, 6-pole, 50 Hz induction motor has a full load speed of 960 rpm with its slip rings short circuited. The motor drives the constant load. If the speed is reduced to 800 rpm, then ohmic losses will _________
a) reduce
b) increase
c) reduce first then becomes constant
d) remains same
View Answer

Answer: a
Explanation: The ohmic losses will increase as the slip will increase due to reduction of slip.

5. A 3-phase, 6-pole, 50 Hz induction motor has a full load speed of 960 rpm with its slip rings short circuited. The motor drives the constant load. If the speed is reduced to 800 rpm, then ohmic losses will be?
a) 5 times that at full load
b) 0.2 times as that at full load
c) 15 times as that at full load
d) same as that at full load
View Answer

Answer: a
Explanation: Slip at full load = (1000-960)/1000 = 0.04
slip at 800 rpm = 0.2
ohmic loss at 800 rpm = (slip at 800/slip at 960 rpm)*ohmic loss at full load = 5.
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6. A 3-phase, 6-pole, 50 Hz induction motor has a full load speed of 960 rpm with its slip rings short-circuited. The motor drives the constant load. If the speed is reduced to 400 rpm, then ohmic losses will be?
a) 15
b) 1/15
c) 5
d) 1/5
View Answer

Answer: a
Explanation: Slip at full load = (1000-960)/1000 = 0.04
slip at 400 rpm = 0.6
ohmic loss at 800 rpm = (slip at 400/slip at 960 rpm)*ohmic loss at full load = 15.

7. The starting torque of the squirrel-cage induction motor is _________
a) slightly more than full load torque
b) slightly less than full load torque
c) equal to full load torque
d) zero
View Answer

Answer: a
Explanation: To overcome the inertia the starting torque of the induction motor is slightly more than full load torque to start.
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8. For achieving high starting torque and high operating efficiency an induction motor should have _______ rotor circuit resistance and _______ circuit resistance under operating condition.
a) high, low
b) low, high
c) low, low
d) high, high
View Answer

Answer: a
Explanation: At starting the rotor circuit resistance must be small and while operating under normal conditions the resistance should be low to reduce losses.

9. In a 3-phase slip ring induction motor high starting torque is achieved by _________
a) increasing supply voltage
b) increasing supply frequency
c) connecting a capacitor across the motor terminals
d) connecting a star connected resistance across slip ring terminals of motor
View Answer

Answer: d
Explanation: To increase starting torque, we need to increase the rotor resistance.
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10. In a three-phase induction motor, the starting torque is maximum when?
a) r2 = x2
b) r2 = 1/x2
c) r2 = (x2)2
d) r2 = 1 pu
View Answer

Answer: a
Explanation: The maximum slip is obtained at the condition of start is when r2 = x2.

11. The losses that occur in induction motor are _________
a) stator copper loss
b) rotor iron loss
c) rotor copper loss
d) all of the mentioned
View Answer

Answer: a
Explanation: All of the losses mentioned occur in an induction motor.

12. Iron loss in the rotor is negligible. This is due to _________
a) very low frequency of emf induced in the rotor
b) very low emf induced in the rotor
c) very low flux density in rotor parts
d) laminated core
View Answer

Answer: a
Explanation: Due to the low frequency of the emf induced in the rotor, the iron losses are reduced.

13. In an induction motor, what is the ratio of the rotor copper loss to rotor input?
a) s
b) 1/s
c) 1-s
d) s/(1-s)
View Answer

Answer: a
Explanation: P(cu)/P(in) = slip.

14. The rotor power input of 3-phase induction motor is 15 kW at a slip of 5%. The mechanical power developed at the shaft is?
a) 14.25 kW
b) 7.5 kW
c) 4 kW
d) 700 W
View Answer

Answer: a
Explanation: Pm = (1-s)Pin = 0.95*15 = 14.25 kW.

15. What is the synchronous watt?
a) the torque under which synchronous speed would develop a power of 1 W or power input to rotor in watts
b) shaft output in watts at sub synchronous speed
c) input power to stator at synchronous speed
d) unit rating of synchronous machine
View Answer

Answer: a
Explanation: It is the torque under which synchronous speed would develop a power of 1 W or power input to rotor in watts.

16. If the rotor circuit resistance is increased in an induction motor, maximum torque will occur at _________
a) lower speed
b) higher speed
c) same speed
d) none of the mentioned
View Answer

Answer: a
Explanation: Increasing the resistance will reduce the speed at which maximum torque will occur.

Sanfoundry Global Education & Learning Series – Electrical Machines.

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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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