I am having trouble finding diagrams to show induction motors with poles not equal to 3x. Everything I find is 2 poles per phase. Can anyone explain and help me visualize, for example, a 4 pole 3 phase induction motor?
- Lest there be any confusion: poles always appear in pairs. The windings generate alternating N (north) and S (south) poles. As a result AC motors are 2-pole, 4-pole, 6-pole, etc. (It may help to remember that if you were to break a bar magnet in half to isolate each of the poles that new opposite poles would appear each side of the break, giving you two bar magnets each with a N-S pole pair.)
- On a three-phase motor the pole pattern has to be repeated for each phase. Therefore a 2=pole, 3-phase motor will have six poles.
Figure 1. A 2-pole, 3-phase motor.
Figure 2. A 4-pole, 3-phase motor.
Figure 3. A 6-pole, 3-phase motor.
Images from Basil Networks permanent magnet brushless motors page. This is worth a read as it shows the series motor connections.
Figure 4. 4-pole, 3-phase AC induction motor illustration. Source Wikipedia: induction motor.
Figure 4 is a little clearer regarding the flux path inside the rotor.
Because a magnet field has two poles N and S, or one pole pair. The poles number can therefore be just a number 2*N. And it is not poles per phase, but rather poles (or pole pairs). For example you have a two pole motor, or a motor with one pole pair, which is the same. The synchronous speed of a induction motor is N=f/(60*N_of_pole_pairs).
Four pole induction motor animation:
Edit: You should search for rotating magnetic field. The current in three phase winding generates magnetic field of constant magnitude that rotates if the currents are AC. The same could be done with two phase windings with 90 degrees appart, but we would need 4 wires, meanwhile the three phase system uses only 3 wires. Therefore the number of phases isn't related to the number of pole pairs.