Discussion
Crawling of induction motor occurs due to :
Answer: D
The keyed answer is both harmonic induction torque and harmonic synchronous torque.
Crawling is the tendency of a squirrel cage induction motor to settle at a low speed, around one seventh of synchronous speed, instead of accelerating to full speed. It comes from space harmonics in the air-gap flux. The 5th and 7th harmonic fields act like small induction motors of their own, and the 7th harmonic torque produces a dip in the torque-speed curve near one seventh of synchronous speed, in which the motor can stick. Interaction between stator and rotor harmonics also produces harmonic synchronous torques at particular speeds, which can lock the motor there.
The standard account treats the two parasitic torques together as the cause of the motor's failure to run up, so the options naming only one of them are incomplete. 'Vibration torques' is not a recognised cause.
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