What is cogging and crawling in case of 3 phase induction machines?

Answer 1

CoggingInduction 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.

CrawlingAnother characteristic of induction motors, is crawling. There are harmonic fluxes developed in the gap due to the magnetics 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.

Answer 2

The cogging effect is the attraction between the magnets and the stator poles when at rest; this attraction and cogging is easily felt when trying to turn an electric motor by hand. The rotor resists the turning, the magnets attempting to stay bonded to the current poles, until just over halfway through the air gap, where the attraction to the next pole is stronger than that of the previous pole, resulting in a "clunking" or jumping effect at low Speed (such as when trying to turn by hand). This is the cogging effect, resulting in a large torque required to begin rotation.

Answer 3

crawling- when motor not able to accelerate at its full speed but only runs at 1/7 of its rated speed due to presence of harmonics.

can be removed by fractional slot winding

Answer 4

rotor is... skewed to reduce cogging