Torque Ratios in Three Phase Induction Motor - MCQs with Answers

Torque Ratios in Three Phase Induction Motor - MCQs with Answers

Q1. If s_fis the full load slip and s_m is the slip for maximum torque, then the ratio of full load torque to the maximum torque is given by

a. 2s_fs_m / ( s_m² + s_f² )
b. 2s_f / ( s_m² + s_f² )
c. 2s_m / ( s_m² + s_f² )
d. s_fs_m / ( s_m² + s_f² )

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Q2. If s_st is the starting slip and s_m is the slip for maximum torque, then the ratio of starting torque to the maximum torque is

a. s_m / ( s_m² + 1 )
b. 2s_m / ( s_m² + 2 )
c. 2s_m / ( s_m² + 1 )
d. 1 / ( 1 + s_m² )

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Q3. 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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Q4. 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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Q5. “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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Q6. 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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Q7. 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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