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1) Slip of a induction motor will be zero, a. At start b. When machine is rotating at its maximum speed c. When it is rotating at half of the maximum speed d. Cannot be zero
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2) A 4 pole, 3 phase, 50 Hz star connected induction motor has a full load slip of 5%. Then full load speed of the motor is a. 1500 b. 1400 c. 1405 d. 1425
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3) A 50 Hz, 3 phase slip ring induction motor, has 6 poles on stator and 4 poles on rotor. Then the machine will run at a. 1000 rpm b. 1500 rpm c. 1400 rpm d. Machine will not run at all
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4) A 3 phase, 50 Hz squirrel cage induction motor, has 6 poles on stator and 4 poles on rotor. Then the machine will run at a. 1000 rpm b. 1500 rpm c. 1750 rpm d. Machine will not run
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5) The rotor frequency of a three phase induction motor is a. Slip times the supply frequency b. 1 / slip times the supply frequency c. Equal to the supply frequency d. None of these
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6) The rotor resistance per phase on standstill and rotor reactance per phase on standstill of induction motor is a. Slip times the rotor resistance at standstill and slip times rotor reactance at standstill b. Same as rotor resistance at standstill and slip times rotor reactance at standstill c. Same as rotor resistance at standstill and same as rotor reactance at standstill d. Slip times the rotor resistance at standstill and same as rotor reactance at standstill
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7) Nature of the rotor power factor in running condition is always a. Leading b. Lagging c. Both (a) and (b) d. None of these
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8) If a voltmeter when connected to the rotor of an induction motor gives 150 oscillations per minute and stator frequency is 50 Hz. Then the slip of induction motor will be a. 3% b. 4.5% c. 5% d. 5.5%
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9) For a three phase induction motor, the two rotor frequencies are given by a. sf, (1 - s) f b. sf, (2 - s) f c. (1 - s) f, (2 - s) f d. Sf , sf
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10) The torque produced in the induction motor depends on the following factors 1. The part of rotating magnetic field which reacts with rotor and is responsible for producing emf. 2. The magnitude of rotor current in running condition. 3. The power factor of the rotor circuit in running condition. Which of the above statement is true? a. Only 1 b. 1 & 2 c. 2 & 3 d. 1, 2 and 3
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11) The torque produced in a three phase induction motor is a. Directly proportional to the square of the induced emf in rotor b. Directly proportional to the induced emf in rotor c. Inversely proportional to the square of the induced emf in rotor d. Inversely proportional to the induced emf in rotor
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12) For maximum torque, the only parameter which controls the torque is slip. The maximum torque is a. directly proportional to the rotor resistance b. inversely proportional to the rotor resistance c. directly proportional to the slip times the rotor resistance d. independent of the rotor resistance
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13) When the torque produced by the induction motor is at its maximum, the slip is given by a. X2 / R2 b. (R2 + X2) / X2 c. R2 / X2 d. R2 / (R2 + X2)
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14) A 400v, 4 pole, 3 phase, 50 Hz star connected induction motor has a rotor resistance and reactance per phase equal to 0.02 ohm and 0.2 ohm respectively. Then the slip at which maximum torque occurs and the corresponding maximum torque is (assuming stator to rotor turns ratio as 6) a. 10%, 159.08 N - m b. 20%, 318.16 N - m c. 11%, 159.08 N - m d. 10%, 318.16 N - m
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15) When an induction motor is loaded from no load to full load, its speed and slip a. Increases, decreases b. Decreases, increases c. Both increases d. Both decreases
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16) An induction motor when loaded from no load to full load, its slip also varies from s = 1 to s = 0. Then the torques in the low slip region and in high slip region is a. Both are directly proportional to the slip b. Both are inversely proportional to the slip c. Directly proportional to the slip, inversely proportional to the slip d. Independent of slip, proportional to the slip
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17) The curve obtained by plotting torque against slip from s = 1 to s = 0 is called torque slip characteristics. The nature of the graph in the low slip region and in the high slip region is a. Rising exponential, decaying exponential b. Both will be straight line c. Straight line, rectangular parabola d. Straight line, decaying exponential
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18) If an induction motor is operating at a point in the high slip region, then the motor will be a. Stable b. Unstable c. Either stable or unstable depending on the torque d. None of these
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19) The value of slip for motoring, generating and braking region respectively are a. S > 1, 0 = s = 1, s < 0 b. S < 0, 0 = s = 1, s > 1 c. 0 = s = 1, s > 1, s < 1 d. 0 = s = 1, s < 0, s > 1
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20) When the slip of an induction machine is negative, it is running as a a. Motor b. Generator c. Synchronous machine d. None of these
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21) A 16 pole, 50 Hz, star connected three phase induction motor has rotor resistance of 0.012 ohm per phase and rotor reactance of 0.220 ohm per phase at standstill. Its full load torque is at speed of 220 rpm. Ratio of its starting torque to maximum torque is a. 0.1208 b. 0.1087 c. 0.1102 d. 0.1012
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22) If the rotor resistance of an induction motor is doubled, keeping the other parameters constant, then the maximum torque of the induction motor will become a. Double b. Halved c. One fourth d. Remains same
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23) “By adding external resistance to rotor till it becomes equal to rotor reactance, the maximum torque can be achieved at start”. This statement holds for a. Slip ring induction motor b. Squirrel cage induction motor c. Both a and b d. Neither a nor b
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24) A 4 pole, 50 Hz, wound rotor inductor is supplied at rated voltage and frequency with slip ring open circuited. If the rotor is driven by an external means at 1500 rpm opposite to the direction of stator field, the frequency of voltage across slip rings will be a. 50 Hz b. 80 Hz c. 100 Hz d. 0 Hz
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25) The approximate value of torque angle at which maximum pull out torque will occur under practical circumstances is a. 90 degree b. 75 degree c. 60 degree d. 30 degree
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26) For slip s of an induction motor, the ratio of rotor copper loss to the gross mechanical output is a. 1 / ( 1 - s) b. 1 / s c. ( 1 - s ) / s d. s / ( 1 - s )
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27) While delivering an useful power of 20 kW to the full load, a 3 phase, 50 Hz, 6 pole induction motor draws a line current of 50 A. it is connected to supply of 420 V and runs at a speed of 950 rpm. If mechanical losses are 1000 W, then its rotor copper losses and shaft torque are a. 1105.26 W, 201.03 N-m b. 1125.26 W, 201.03 N-m c. 1105.26 W, 208.09 N-m d. 1125.26 W, 208.09 N-m
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28) A 4 pole, 25 kW, three phase, 420 V, 50 Hz induction motor operates at an efficiency of 90% with a power factor of 0.8 lagging. The current drawn by the motor from the mains is a. 42.89 A b. 45.56 A c. 47.73 A d. None of these
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29) In a three phase induction motor, the mechanical load should be such that the equivalent load resistance referred to stator is equal to the a. Total leakage reactance of the motor referred to stator b. Total leakage reactance of the motor referred to rotor c. Total leakage impedance of the motor referred to stator d. Total leakage impedance of the motor referred to rotor
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30) When an induction motor is working under loaded condition, its induced voltage will a. Leads the magnetic flux by 90 degree b. Lags the magnetic flux by 90 degree c. Lags the magnetic flux by 30 degree d. In phase with the magnetic flux
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