Anand.machine
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The document discusses cogging and crawling phenomena in induction motors. Cogging occurs when the number of rotor and stator slots are equal, causing the motor to not start. Crawling is when the motor runs stably at speeds near one-seventh of the synchronous speed due to harmonic fluxes. To avoid these issues, the number of stator and rotor slots should not be equal and their difference should not be ±3P, ±1, ±2, or (±P ±1), (±P ±2). Skewing the slots also helps reduce cogging.
Anand.machine
- 1. Induction Machine Presentation • Submitted By- Anand kumar Roll no-10MUBEEE008
- 2. Cogging and Crawling • Proper numbers of rotor slots are to be selected in relation to number of stator slots otherwise undesirable effects will be found at the starting of the motor. • Cogging and Crawling are the two phenomena which are observed due to wrong combination of number of rotor and stator slots.
- 3. 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.
- 4. Cogging • 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.
- 5. Crawling • Another 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. It has been observed that in induction motors especially the squirrel cage type motors, there is a tendency of the motor to run stably as speeds which are nearly one-seventh of the synchronous speed and this phenomenon is known as crawling.
- 6. To avoid cogging and crawling • (i) To avoid cogging and crawling: (a)Ss ≠ Sr (b) Ss - Sr ≠ ±3P • (ii) To avoid synchronous hooks and cusps in torque speed characteristics ≠ ±P, ±2P, ±5P. • (iii) To noisy operation Ss - Sr ≠ ±1, ±2, (±P ±1), (±P ±2)