Let's consider just one pole pair.
I know from reading my book that a pm machine will have the angular speed of its rotor equal to the speed of the rotating magnet field that the three phase winding creates.
If this was not the case then the (average?) torque would be zero.
I did some research, and it seems that this behavior is due to the fact that two magnetic fields try to align, with a torque that's proportional to the product between their magnitudes and the sin of the angle between them.
Given that I have NO idea what that law is (its name, where does it come from...to be honest it seems an elementary explanation about how two magnets attract each other), I wonder if this is the case. does the motor torque come from this phenomenon? What would be zero (if anything, later on the book it states that if the speed are different I just wouldn't get a constant torque), the ''average'' (on what? a period?) torque, or the instantaneous? I'd say it's the average over a period of the sin that appears in that formula, but it's very tricky to reconstruct it to the time domain.
I also read that a pm machine will either run at the excitation field speed or not run at all. No idea why is that, tho. and if this was the case, how would I start them? as soon as I give them a rotating field, since they are not moving they have different velocities... then the torque would be zero (from the book)? It doesn't make sense.
And what if, by any chance, when I start the machine the two fields are aligned? Would I get the minimum possible torque (around 0) or something else would happen?