Cogging of Induction motor
The induction motor consists stator and rotor. In the stator, the slots are available on the inner portion and in the rotor, slots are available in the outer portion. The copper windings are placed in the stator and rotor.
When a three-phase ac supply is given to the stator winding, the rotating magnetic field is produced around the stator winding. These rotating magnetic filed cut by the rotor winding. There are emf induced in the rotor winding as per faradays laws of electromagnetic induction. The voltage will be induced in the rotor and the current flows through the rotor winding.
Due to the circulating current in rotor winding, a magnetic field is produced around the rotor winding.
When the rotor slots and teeth and stator slots and teeth are at equal levels, the flux produced by the stator and rotor winding interlock with each other. As a result rotor of the induction motor can not rotate and the induction motor produces noise. This phenomenon is known as cogging of induction motor.
When the magnetic flux of the stator and rotor of the induction motor interlock with each other is known as cogging of the induction motor. In cogging, there are magnetic locking takes place between the rotor teeth and stator teeth. The cogging is also called teeth locking. If we supply three-phase full voltage to the stator of the induction motor, but the rotor of the induction motor can not rotate. The effect of the cogging or teeth locking takes place in the induction motor when the number of rotor slots is equal to the number of rotor slots.
The number of rotor slots and the number of stator slots are aligned to each other and a strong magnetic locking force takes place between them. In such a case, rotor teeth tend to remain fixed in front of stator teeth which is called magnetic locking between the rotor teeth and stator teeth of the induction motor. So induction motor can not start and remain at its standstill condition. This effect is more marked at low voltages.
When the rotor teeth and stator teeth of the induction motor face to each other, the reluctance of the magnetic path between the rotor teeth and stator teeth are minimum. During this condition(reluctance between the stator teeth and rotor teeth is minimum) magnetic locking takes place between the rotor teeth and stator teeth of the induction motor.
Cogging is reduced by
1. The number of rotor slots and the number of stator slots should not be equal.
2. The effect of cogging is reduced by skewing the induction motor.
3. To reduce the cogging effect, a squirrel cage rotor is used. The construction of the squirrel cage rotor is simple and the bar of the squirrel cage rotor is skewed.
Crawling of Induction motor
In the squirrel cage type rotor of induction motor, there is variation in air gap reluctance that takes place between the stator and rotor and due to this some odd harmonic like 3rd, 5th and 7th are present in the air gap flux.
The rotor of the squirrel cage induction motor runs at a speed low as 1/7th of their synchronous speed is known as the crawling of the induction motor. Its means the 7th harmonic effect is present in the three-phase induction motor and due to this crawling effect produced in the induction motor.
Crawling of induction motor occurred due to the presence of odd harmonic between the air gap of stator and rotor.
The odd harmonics are the 3rd, 5th and 7th harmonics. In a balanced three-phase system, 3rd harmonics are absent. So no 3rd harmonics current is circulated and no harmonic torque is produced. So effects due to 3rd harmonics are neglected.
The 5th harmonics have a negative sequence current and produce a negative magnetic field. The torque produced by the 5th harmonic is opposing the torque produced by the fundamental harmonics torque. Hence this Torque is the opposite of rotor rotation. This Torque is called breaking torque or reverse braking torque of induction motor.
The torque produced by the 5th harmonic is small. So it can be neglected.
Now torque produced by the seventh harmonics is in direction of the rotating magnetic field or torque produced by the fundamental harmonics at a speed of Ns/7.
Now total torque is a sum of torque produced by the 7th harmonics current and torque produced by the fundamental harmonics currents.
Due to 7th harmonics, torque main resultant torque becomes less than load torque. So the motor runs at 1/7th of full speed which is less than its normal speed. An induction motor crawling less because an induction motor has higher starting torque.
Reason for crawling in induction motor
1. The crawling occurred in the induction motor due to the presence of harmonics between the air gap of the stator and the rotor of the induction motor.
2. Air gap harmonics occurred in the induction motor due to the uneven sharing of the stator winding coil span.
3. The air gap reluctance difference occurred in the induction motor due to the difference in slots of stator and rotor parts.
Methods for reducing crawling in induction motor
1. The crawling in an induction motor is reduced by designing a stator winding with proper coil distribution and coil span. The harmonics are produced between the stator and rotor air gap due to the reluctance variation. This is reduced by this method.
2. To reduce the crawling effect, length of span less than 180 degrees between two adjacent poles.
What are harmonics?
The multiple integers of frequency are known as harmonics.