Rotating magnetic field

[Ali Lama]

If I understand how induction motor works is that in a single phase induction motor there are no rotating magnetic field.  Of course the rotor rotates just fine if not as good or efficiently as the three phase motor.  My main point here is that the rotor does not need to follow a single rotating field for it to rotate, it simply follows the fields on the coils of the stator which is basically hardly more than a pair.

So my question is really this.   Lets indulge in a thought experiment and assume that a rotating magnetic field in a 3 phase induction motor is just a misnomer.  All we have is the three pair of coils and their localized alternating field..  How would the rotor behave?

Would the oscilloscope show something other than is  observed and would the location of rotor at any juncture be pointing somewhere else or freeze up or whatever other anomaly that may result..….  I am all ears..   Thank you very much  in advance.

[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.

Edited March 27, 2020 by Félix V.

[Félix V.]

1 hour ago, Félix V. said:

 I just notice mistakes : 

- "I assume that they  are localized"

- I also forgot the vector z in the expression of C (in my scheme)

 

Edited March 27, 2020 by Félix V.