Electrical Machines Questions and Answers – Losses and Efficiency

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
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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
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3. Rotor ohmic loss is ________ slip.
a) proportional to
b) inverse of
c) inversely proportional to
d) hyperbolic to
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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)
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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
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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
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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
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Sanfoundry Global Education & Learning Series – Electrical Machines.

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