Both crawling and cogging effect is only observed in squirrel cage induction motor as in wound rotor inductor motor, this effect is not observed as toque is very large.
Check the meaning of crawling in dictionary .what you find ..crawling means “move forward on the hands and knees or by dragging the body close to the ground”
In a smiliar way, when an induction motor does not run at the rated speed but runs at a speed little lower than (1/7th) of the synchronous speed (Ns=120f/P) than machine is said to be crawling ..means you can try a joke on motor and say motor is running on its knees.
So why this happened? Why motor failed to attain the rated speed..what causes machine to crawl.
When stator of an induction motor is connected to ac supply. It produces magnetic field ..this magnetic field contains fundamental value of flux ( 50 Hz) and odd harmonics (3rd, 5th, 7th and so on )
In a balanced three phase system, 3rd harmonics are absent..and higher order harmonics effect is very less on working of indiction motor .
Now we have left with two harmonics i.e. 5th and 7th order harmonic
So stator magnetic flux is the sum of following fluxes .
- Fundamental flux and rotating at synchronous speed Ns.
- 5th harmonic field ..rotating in opposite direction with speed of Ns/5. It phase angle will be 120×5=600 = 2×360-120= -120 degree so the it produces minimal breaking torque in the rotor that can be ignored.
- 7th harmonic field ..rotating in forward direction with speed of Ns/7. Its phase angle will be 120×7=840 =2×360+120= +120 degree..it produces torque that will add to the synchronous torque created by the main field and its only this harmonic, due to which induction motor crawls (motor runs at one seventh of the synchronous speed, Ns/7).
So It is only the 7th harmonic that cause all this shit. 7th harmonic causes machine to crawl. How it does this.
So net motor torque will be toque due to fundamental flux plus 7th harmonic torque. 7th harmonic toque reaches it maximum value just before one seventh of synchronous speed, Ns/7. If the motor load is of constant torque load..motor will not attain speed more than one seventh of synchronous speed as shown in the figure below. This condition is called crawling.
Cogging or Magnetic locking of induction motor :
It is observed sometime with induction motor that if teeth of stator and rotor are equal and face each other ..than motor fails to start due to magnetically locking this phenomenon is known as cogging.