Crawling is a phenomenon in which Induction Motor operates at very low speed. Crawling is caused by the presence of space harmonics in the air gap flux.
For understanding Crawling, there are some points which are very important to know. Those points are as follows:
- Speed of Rotating magnetic flux = 2f/P, where f is frequency of Supply and P is the number of Poles
- Harmonic magnetic flux travel at a speed of 1/k time that of fundamental magnetic flux.
- Harmonic magnetic flux rotates in the direction of rotation of fundamental magnetic flux if k = (6m+1), where m = 1,2,3……
- Therefore, 7th,13th, 19th etc. harmonic magnetic flux will rotate in the direction of rotation of fundamental magnetic flux.
- Harmonic magnetic flux rotates in the opposite direction of rotation of fundamental magnetic flux if k = (6m-1), where m = 1,2,3……
Therefore, 5th,11th17th etc. harmonic magnetic flux will rotate in the opposite direction of rotation of fundamental magnetic flux. Keeping all the above mentioned facts in mind, we will discuss Crawling.
As stated earlier, Crawling is caused by the presence of space harmonics in the air gap flux. Following are the reason for the presence of space harmonics in the air gap flux:
- Air gap harmonics due to uneven distribution of Stator winding.
- Due to variation in air gap reluctance because of Stator and Rotor Slots. Space Harmonics due to this is also called Slot or Tooth Harmonics.
The analysis of Stator mmf wave by Fourier Series shows that it consists of fundamental mmf wave and other odd harmonic mmf wave. As the sum of Triplen (multiple of 3) harmonic mmf wave due to all three phases sum up to zero therefore resultant air gap flux is free from 3rd and Triplen harmonics.
The most predominant space harmonics are 5th and 7th, therefore in our discussion only these two harmonics will be considered.
Thus, 5th harmonic mmf wave will rotate at a speed of Ns/5 rps in opposite to the fundamental mmf wave and 7th harmonic mmf wave will rotate at Ns/7 rps in the direction of rotation of fundamental mmf wave.
These two harmonic mmf will produce their own torque in Induction Motor. The torque produced by 5thharmonic mmf will be breaking in nature as the wave rotates in opposite direction of rotation of fundamental mmf wave while torque produced by 7thharmonic will aid the torque produced by fundamental mmf wave. A polt of Torque Slip characteristic is drawn for each as shown below.
From the above torque slip characteristics, it is clear that if Load Torque is assumed TL then Motor will have stable operation at a slip of 6/7 and therefore Motor will operate at a speed of Ns/7 [As 6/7 =(Ns -Nr )/Ns , so Nr = Ns/7]
This stable operation at a speed of Ns/7 is called Crawling as this speed is much lower than the rated speed of Induction Motor.
Crawling in Induction Motor leads to higher Stator Current, noise and vibration.