Cogging and Crawling of Induction Motor

In Electrical Engineering Induction Machine is an important part of industrial purpose but we don't know the reason for some of the basic problems while starting a motor such as Cogging and Crawling of an Induction Motor. 

Cogging:-

           In an electrical circuit current always takes the minimum resistance path like that in a magnetic circuit flux takes the minimum reluctance path. We know reluctance = ℓ/μA, so if the length of the air gap is minimum then the reluctance of that path also becomes minimum.

               There are teeth & slots present in both stator & rotor so the length of the air gap between them is not uniform. Now if the teeth of the stator & rotor are perfectly aligned then the air gap length between stator & rotor teeth is minimum and between the slots of stator & rotor is maximum. So the reluctance between stator & rotor teeth is minimum, for this flux has got the path of minimum reluctance when the teeth are aligned. Now if the rotor tries to move right or left the reluctance of the path increases but the flux will oppose this because it has got the path of minimum reluctance which is the most stable position and in any other case the reluctance increases which is the unstable position. The strong alignment force is the reason to keep them aligned, so the motor does not move or it fails to start. This is called cogging.

                 It can only happen if, the number of stator slots=number of rotor slots
or, the number of stator slots is equal to the integral multiple of the rotor slots.

                Cogging can be prevented by avoiding such designs to reduce the parasitic torque. Skewing of the stator or rotor slots is the best method to avoid cogging, usually rotor slots are skewed. By skewing flux will never get the minimum reluctance path so the machine will never settle.

Crawling:-

                The time harmonics of impressed voltage wave have some effect on the Induction Motor performance but their effects will not be considered here, because their influence on the operating characteristics is insignificant.

                If the space distribution of flux wave along the air gap periphery is sinusoidal then the torque-speed curve of 3-phase Induction Motor is also smooth & if the flux wave is non-sinusoidal then the torque-speed curve is not smooth (saddle region comes into the characteristics) at low speed. These harmonics due to non-sinusoidal flux are called as belt harmonics or space harmonics.
Reason for the production of belt harmonics-

• variation in the air gap reluctance due to stator & rotor slots.
• distribution of the stator winding. 

The order of belt harmonics is = 6n±1

              The synchronous speed of belt harmonics = Ns/the order of the harmonics, the reason for this is, if the order of harmonics is higher then the frequency of current reversal is also higher and  as   the current reverses poles are created because between two oppositely current-carrying coil always   there will be a pole. The synchronous speed is inversely proportional to the number of poles, so the synchronous speed of belt harmonics getting divided by its order.

for n=1,
⇒6x1+1=7, which is a positive sequence harmonic.

The synchronous speed of the 7th order belt harmonic=Ns/7.

⇒6x1-1=5, which is a negative sequence harmonic.

The synchronous speed of the 5th order belt harmonic=Ns/5.

            The net flux produced by the triplen harmonics(3n) is zero as they behave as zero sequence so they do not produce the rotating magnetic field (rmf). So the torque is produced by the fundamental & belt harmonics, not by triplen harmonics.

          Now for the presence of harmonics in the flux waveform affect the torque as well as the performance of the motor because the positive sequence harmonics(6n+1) rotates in the same direction of the fundamental and negative sequence harmonics(6n-1) in the backward direction of the fundamental. So the net torque is also affected.

          The total torque consists of fundamental torque & parasitic torque, for this reason, the net torque consists of a saddle region near the neighborhood of 7th harmonic synchronous speed.

There are two stable region of operation - 

1. negative slope region in the saddle region.
2. negative slope region where the slip of the fundamental synchronous speed is low.

           Now due to the harmonics starting torque of the motor become low for this the machine has slow acceleration. When we start on load, the machine slowly approaches the negative sloped saddle region and it can be stuck in the saddle region(due to slow acceleration of the machine) as it is also a stable operating region and the speed of the motor is close to the synchronous speed of 7th belt harmonic, which is much slower than the speed of the fundamental or the rated speed of the motor. This is called crawling because the motor crawls or runs at a very slow speed.

• Disadvantage -

             When the motor crawls the slip is high so at the low slip we know the current becomes high. The power loss (I²R) is high which generates more heat in the machine. If this happens for a longer duration causes damage to the machine.

• Solution -

          Crawling can be avoided if we eliminate the 5th & 7th harmonics. This can be done if we design the machine properly such that flux becomes sinusoidal by which starting torque can also be improved. Some of the methods to reduce the starting torque are - a. Proper choice of coil span, b. integral slot windings, c. skewing of stator or rotor slots, d. choice of stator and rotor slots.

          Crawling is not observed in the slip ring Induction Machine as it has higher starting torque which accelerates the motor to overcome the saddle region.