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.

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)