induction motormotormotor controllerpower electronicsthree phase
For a three phase squirrel cage induction motor (IM,) we can have 2 or 4 or 6 etc. pole pairs on its stator and when these stator windings are energized by 3 phase mains supply then we get a rotating flux inside the motor.
Can we say the same for the rotor of a squirrel cage IM also?
Please correct me if I am wrong that the rotor of an IM always has 2 poles regardless of the poles on its stator.
The rated speed is usually about 90-95% of synchronous speed. You can guess the synchronous speed and number of poles by looking for a number of poles that gives a rated speed slightly less than synchronous speed.
In this case, I would guess you are dealing with a 60Hz 12-pole motor, which would have a synchronous speed of 600 rpm.
I've done a quick lookup table below for reference.
Poles 50hz 60hz
2 3000 3600
4 1500 1800
6 1000 1200
8 750 900
10 600 720
12 500 600
16 375 450
20 300 360
24 250 300
30 200 240
40 150 180
The problem is that the rotor and stator must have spaces called slots, in the iron for the conductors. The conductors and air in the slots have a lower reluctance than the reluctance of the iron. There are more slots than there are poles in the stator. The windings are distributed among the slots to form distributed poles. Their is a different numbers of slots used for the stator and rotor. Those two features prevent the rotor from becoming locked or "cogged" in the minimum reluctance position that would otherwise exist.
Even with proper selection of the number of stator and rotor slots, there can be reluctance variations at different rotor angles that cause torque fluctuations and inflections in the torque vs. speed curve. Skewing the rotor slots mitigates that effect.
Here is a picture of the rotor and stator of a 4-pole, permanent-split capacitor, single-phase induction motor.
Best Answer
Induction motors have an even number of poles or any number of pole pairs formed on the stator by the winding pattern. The stator field induces currents in the rotor that form a matching number of poles on the rotor.