what is cogging and crawling
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Answer Posted / nikhil
Cogging
Induction motors have a series of slots in the stator and in
the rotor. These slots should not be equal in number because
if they are, there is a good chance that the motor will not
start at all due to a characteristic known as cogging. The
slots will align like a stepper motor.
For this reason, there are an unequal number of slots in the
rotor and in the stator, but there can still be situations
where the slot frequencies coincide with harmonic
frequencies and this can cause torque modulations. The slots
are skewed to keep an overlap on all slots to reduce this
problem.
Crawling
Another characteristic of induction motors, is crawling.
There are harmonic fluxes developed in the gap due to the
magnetic of the motor. These harmonics create additional
torque fields. A common problem is with the seventh harmonic
where the seventh harmonic creates a forward rotating torque
field at one seventh of the synchronous speed. There will be
a maximum torque just below 1/7 Ns and if this is high
enough, the net torque can be higher than the torque due to
the line frequency where at 1/7 Ns, the slip is high. This
can cause the motor to crawl at just below 1/7 synchronous
speed.
There is another crawl speed at 1/13 Ns.
| Is This Answer Correct ? | 12 Yes | 2 No |
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