**What is Cogging in Induction Motor?**

Cogging of induction motors is
defined as the tendency of an **induction motor** to become magnetically locked,
which will occur when the number of slots on the stator and rotor are equal or
integral multiple of each other.

If the number of slots on the
stator (S_{1}) is equal or integral multiple of the number of slots on
the rotor (S_{2}) then the reluctance offered by the air gap is
quite pronounced, resulting in the strong alignment forces between stator and
rotor teeth. These alignment forces may create an alignment torque. This torque
dominates the accelerating torque or starting torque of the motor resulting in
the failure of starting of the induction motor. This phenomenon is called the
magnetic locking or cogging of an induction motor.

Cogging particularly occurs in
**squirrel cage rotor induction motor**. Because in these motors, external
resistance cannot be added to get high starting torque therefore, we have to
choose a reduced voltage starting method to start the induction motor. Due to
reduced voltage starting the starting torque of squirrel cage induction motors
is low and the alignment forces between stator and the rotor teeth dominate the
starting torque of the induction motor.

In slip ring rotor induction
motors, there is no need of reduced voltage starting method to get high
starting torque, instead of this we can get high starting torque by adding
external resistances to the rotor circuit by means of slip rings. Therefore, cogging
will not occur in slip ring induction motors.

**How to reduce Cogging in Induction Motor?**

To reduce cogging of induction
motors, the rotor slots are essentially skewed (also discussed in squirrel cage
rotor) and the number of rotor slots will be made different from the
number of stator slots, there is no common factor between them such as equal or
integral multiple.

Skewing means, shifting the rotor slots at some angle from the rotor shaft axis as shown in the given figure. Skewing of rotor slots also improves the flux distribution, reduces the harmonic content and produces better torque which makes the motor run smoothly with less noise called magnetic humming. (That's why slip ring induction motor’s rotors are also skewed).

**What is Crawling in Induction Motor?**

Crawling
of an induction motor refers to the running of the motor stably at low speed
i.e. N_{s}/7. This happens due to the influence of 7^{th}
harmonic frequency MMF distribution. Crawling is undesirable for an induction
motor because at such low speed the slip of the induction motor is
significantly high causing the motor to draw high current and produce large
noise.

As we know, the flux
distribution in the air gap of the induction motor is sinusoidal (ɸ = ɸ_{m}
sin(ωt)). However, due to the certain combination of un-skewed stator slots and
rotor slots, this flux distribution in the air gap is non sinusoidal or
distorted. This distorted flux distribution contains components
of fundamental plus higher order harmonic frequencies. These higher order
harmonics fluxes in the air gap are undesirable and may impact the motor
performance.

Generally, even-order
harmonics are not so dangerous because their average value is zero. But odd
order harmonics such as 3^{rd}, 5^{th}, 7^{th}, 9^{th},11^{th}
etc. can significantly affect the performance of induction motors (specifically
5th and 7^{th} harmonics).

As we know,
in **three phase induction motor** the fundamental components of flux distribution
is

ɸ_{a }= ɸ_{m} sin(ωt),
ɸ_{b} = ɸ_{m}
sin (ωt - 120), ɸ_{c} = ɸ_{m} sin (ωt - 240)

5th
harmonic component of these flux waves are

ɸ_{a} = ɸ_{m} sin5ωt,
ɸ_{b} = ɸ_{m}
sin 5(ωt - 120), ɸ_{c} = ɸ_{m} sin 5(ωt - 240)

ɸ_{a} = ɸ_{m} sin5ωt,
ɸ_{b} = ɸ_{m}
sin (5ωt - 600), ɸ_{c} = ɸ_{m} sin (5ωt - 1200)

ɸ_{a} = ɸ_{m} sin5ωt,
ɸ_{b} = ɸ_{m}
sin (5ωt - 240), ɸ_{c} = ɸ_{m} sin (5ωt - 120)

** **

If we
carefully observe the above flux distribution equations of the fifth harmonic
frequency component, we find that these flux distributions have three phase
nature, but their phase sequence is opposite to the original phase sequence. As
a result, this flux rotates backward with the synchronous speed of N_{s}/5.
Hence, this harmonic has no effect on the induction motor during the motoring
zone, but it has a significant effect on the breaking zone of the induction
motor.

As the
synchronous speed of this harmonics flux distribution is N_{s5} = N_{s}/5.
Therefore, the slip is s_{5} = 6/5.

7th
harmonic component of these flux waves are

ɸ_{a} = ɸ_{m}
sin7ωt, ɸ_{b} = ɸ_{m}
sin 7(ωt - 120), ɸ_{c}
= ɸ_{m} sin 7(ωt - 240)

ɸ_{a} = ɸ_{m}
sin7ωt, ɸ_{b} = ɸ_{m}
sin (7ωt - 840), ɸ_{c}
= ɸ_{m} sin (7ωt - 1680)

ɸ_{a} = ɸ_{m}
sin7ωt, ɸ_{b} = ɸ_{m} sin (7ωt -120),
ɸ_{c} = ɸ_{m}
sin (7ωt - 240)

** **

After carefully observing the
above flux distribution equations of 7^{th} harmonic, we find that this
flux distribution has three phase nature with the phase sequence being the same
as the original phase sequence. As a result, this flux rotates in the forward
direction with the synchronous speed of N_{s}/7. Hence, this harmonic
impacts the performance of an induction motor during the motoring zone.

For this harmonic flux, the
synchronous speed is N_{s7} = N_{s}/7 and the slip is s_{5}
= 6/7.

The above discussed harmonics
i.e. 5^{th} and 7^{th}, generate their own asynchronous torque
of the same general torque-slip curve shape as that of the fundamental.

The given figure shows the
**torque slip characteristic of a three-phase induction motor** with the effect of
5^{th} and 7^{th} harmonics frequency.

In the given figure, we see
that due to the presence of 7^{th} harmonic content a dip in the
resultant of the torque slip curve of the three-phase induction motor is
introduced in the motor motoring zone.

Therefore, if the induction
motor starts with reduced voltage starting (accelerated slowly to required load
torque, indicated by red dotted curve) then this effect will dominate at a
point A and make the motor run at stably at speed that is N_{s}/7.

At such a low speed, the slip
of the induction motor is very high causing the motor to draw high current and
make a large noise, which is not desired.

Similar to cogging, crawling
also occurs in squirrel cage induction motors because in this type of induction
motor we have to apply reduced voltage starting (starting torque is low).

Whereas in slip ring induction
motor starting torque is high, so there is no crawling effect.

** **

**How to reduce Crawling in Induction Motor?**

To minimize crawling of an induction motor, semi closed or closed slots are preferred over open slots, and the rotor slots are essentially skewed. This practice ensured a uniformly distributed air gap across the entire rotor length. This will ensure the uniform flux distribution. Additionally, short pitched windings are preferred to further enhance uniform flux distribution.

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