Félix V.

Hello Ali, I'm french, so I apologize in advance for my language mistakes. However, it is an interesting subject, so I will try to answer your question. In this type of motor (synchronous motor), as you said there is two parts : the rotor which is like a magnet (represented by a magnetic dipole moment, oriented from south to north) and a stator which have 3 coils (in the simplest case) envenly distributed in a circle. In your though experiment, if you accept that the 3 coils are supplied by a three-phase electric current (alternating currents), they create 3 magnetic fields. Each of them has the same phase as the current which is supplied its coil. I will assume that there are localized as you want. Let's number them : n°1 (phase=0, magnetic field=B1), n°2 (phase=-2*pi/3, B2), n°3 (phase=-4*pi/3, B3). Laplace actions create engine torque which tends to align the rotor's magnetic moment with the magnetic field (for each magnetic field). Let's suppose that when time=0 the magnetic moment is between B1 and B2, so B1>B2 (because B(t)=Bm*cos(w*t+phase) with w=2*pi*frequency), so the engine torque of B1 (C1) is weaken than that of B2. So the rotor go toward B1. But a few moment latter : B2>B1 (for exemple when t=pi/(w*2), B1=0). At this moment C2>C1, so the magnetic moment go toward B2. A few moment latter : B1>B2 ---> C1>C2 ... To conclude, in your though experiment, I can say that the rotor will do oscillations between 2 coils, or maybe will pass between an other interval of magnetic field (B1-B2 and after B2-B3) and does a few oscillations... It's true to say that I didn't take in account B3 when I studied the rotor's mouvement between B1 and B2, but we now that C3=M*B3*sin(theta), M=vecteur_norm(magnetic moment), and theta is the angle between the magnetic moment and B3. So sin(theta) is small and C3 can be neglect when the rotor is between B1 and B2. I made a scheme of the situation. Finally, I made a simple study, it can be wrong, more study is needed to have the exact answer ! But I think that is the general idea. I hope that I helped you and I will accept all observations with pleasure ! Félix V.