when an induction motor refuses to start even if full
voltage is applied to it, this is called as cogging. this
happens when the rotor slots and stator slots are same in
number or they are integer multiples of each other. due to
this the opposite poles of stator and rotor ome in front of
each other and get loked.
on the other hand when an IM runs at a very low apeed
(precisely speaking 1/7th of synchronous speed)even if full
rated voltage is applied to it,then it is called at
crawling. this happens due to harmonic induction torques.
in which torques due to 7th harmonic overpower the driving
torque(fundamental component torque).
When rotor bars are made to run parallel with stator , the
torque rises & falls correspondingly causing more
pulsations. This is termed as cogging in other qords
magnatic locking. This is reduced by making the rotor bars
run at an angle to the stator ie crawling in order to make
the torque uniform.
Crawling on the other hand signifies running of motor at
almost one seventh of the rated spped due to interference
of seventh harmonics.
When the voltage is low, the rotor of Squirrel-Cage motor
refuses to start at all. This happens when the number of
stator teeth S1 is equal to the number of rotor teeth S2
and is due to the magnetic locking between the stator and
rotor teeth.This is why this phenomenon also known as teeth-
locking or cogging.
The Squirrel- Cage type motor sometimes exhibit a tendency
to run stably at speeds as low as one-seventh of their
synchoronous speed Ns. This phenomenon is known as Crawling.
when the full voltage applied to the stator winding ,the
rotor of 3-phase cage induction motor fails tostart.This
happenes whe the no. of stator and rotor slots are equal
or an integral ratio.so due to strong alingment of forces
are produced between stator and rotor at the instant of
starting. these forces create an alingment torque greater
than the accelarating torque.The phenomenon of magnetic
locking between stator and rotor teeth is called cogging or
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 RPM (such as when trying
to turn by hand). This is the cogging effect, resulting in a
large torque required to begin rotation.
The motor runs at one seventh of the synchronous speed if
the full rated supply voltage is applied is known as
crawling. This is happens because of 7th harmonic induction
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
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
There is another crawl speed at 1/13 Ns.
the no of stator&rotor slots are equal in quantity,then the
flux producing each are same so the rotor&stator are runs
with same speed after little some time motor stops rotating
that means motor is stops automatically.they are locking
each other then it will be refused to run. this is
the rotor rotates below the sy.speed,i.e (one seventh speed
of sy.speed),this effect cause by the harmonics).this cause
by 3rd harmonic,this is crawling
if the no. of stator poles are been equal to the no. of
rotor poles in an induction motor then the machine refuses
to start. then that reason is called as cogging.this is due
to the failure of machine elements(i.e,bearings) of an