How does a stator's magnetic field rotate in an AC motor?

[GoldDigger]

Your clarification does not explain why the induction motor runs or why an inductor stores energy in its magnetic field i.e it is vague.

Lenz's Law itself is vague, but very powerful as a result!
I was not trying to explain why the motor rotates or why torque depends on slip for an induction motor. Just to correct some misstatements which had been made by others.

Then there is no sense in stating stator magnetic field opposes the rotor magnetic field or vice versa in an induction.

If you look at the total resultant magnetic field at an area in space (for convenience we assume an area not occupied by a ferromagnetic material), then you can use the following terminology:

1. A single sum magnetic field vector can be decomposed for convenience into several components which are generated by two or more different sources. The linearity of electric and magnetic fields allows us to do this decomposition and superpostion.
2. If the field contributed by source B acts to cause the overall magnetic field to be lower than it would be if only source A is present, I say that the field of source B opposes the field from source A. Makes perfect sense and is a quite useful concept. Another way of looking at it is that the field vector from B is in the opposite direction to the vector from source A. Or at least that the component which is parallel to field vector A is opposite.
It is a convenient way to determine the direction of the induced field without spending several minutes applying the Right Hand Rule and getting confused about the direction of conventional current versus electron flow.

 

[Haji]

Lenz's Law itself is vague,

No. Just a misunderstanding of it.

If you look at the total resultant magnetic field at an area in space (for convenience we assume an area not occupied by a ferromagnetic material), then you can use the following terminology:

1. A single sum magnetic field vector can be decomposed for convenience into several components which are generated by two or more different sources. The linearity of electric and magnetic fields allows us to do this decomposition and superpostion.
2. If the field contributed by source B acts to cause the overall magnetic field to be lower than it would be if only source A is present, I say that the field of source B opposes the field from source A. Makes perfect sense and is a quite useful concept. Another way of looking at it is that the field vector from B is in the opposite direction to the vector from source A. Or at least that the component which is parallel to field vector A is opposite.
It is a convenient way to determine the direction of the induced field without spending several minutes applying the Right Hand Rule and getting confused about the direction of conventional current versus electron flow.

One sided explanation. For the Lenz law also permits enhancement of inducing magnetic field by the induced magnetic field when the inducing magnetic field tends to decrease in value.

 

[GoldDigger]

8nmk

No. Just a misunderstanding of it.

One sided explanation. For the Lenz law also permits enhancement of inducing magnetic field by the induced magnetic field when the inducing magnetic field tends to decrease in value.

Quite true, which is why my original description referred to opposing a CHANGE in the. original field.

 

[Haji]

8nmk
which is why my original description referred to opposing a CHANGE in the. original field.

My point is that the stator and rotor magnetic fields not only oppose but aid each other along the periphery of the rotor of an induction motor in such way as to cause the rotor to move in the same direction as that of the stator field of the induction motor.

 

[Besoeker]

An attempt at a simple, if not rigorous, qualitative explanation.....

The stator windings are arranged in order physically around the stator.
The magnet field produced by phase A is in a particular location. When it decays, the prevailing field moves to B which is further round the stator. Thus the field moves round to B location and so on.
Rotation. The rotation is at synchronous speed. The synchronous speed depends on how many pole pairs, sets of A,B, and C windings. For simplicity, consider two poles.
The rotational speed of the magnetic field is 3600 rpm (60Hz * 60 seconds.)

Remembering the simple equation for a conductor moving in a magnetic field:
e=Blv, e being the induced voltage, B the flux density or field strength, l the length if the conductor, and v, its velocity through the field.

It doesn't matter if the conductor is moving relative to the field or the field relative to the conductor.
For an induction motor the relative movement is the field with respect to the rotor.
Electrically for calculation purposes, the rotor us usually represented by two components, R2 and X2. Resistance and reactance.

At standstill (ie start conditions) the induced rotor voltage is highest, the greatest relative v and at the highest frequency.
High currents result but the high frequency means that the X2 component dominates resulting a non torque producing component of current.
As the motor speeds up, the relative value of v reduces, the frequency reduces, and the X2 component reduces and the proportion of useful current increases.
Then beyond a certain point, the rotor speed is such the the relative velocity reduces to a point where the induced voltage is low enough for current and torque to start dropping. This typically up yo a fey percent of synchronous speed on a medium to large motor.

At synchronous speed there is no relative speed difference and no induced voltage. For e=Blv, if v=0, then e=0. Above synchronous speed the relative speed is negative and the induction machine acts as a generator.

Actual motor data: