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

[Eddy Current]

Do induction and a wound AC motors use the same type of stator with the poles 120 degrees apart?

 

[Haji]

Magnetic field is induced by changing current in the stator.

The magnetic field FROM the stator induces changing current in the rotor.

The induced current in the rotor creates a magnetic field.

The rotor and stator magnetic fields are of the SAME polarity, therefore 'repulsing' each other and THAT creates the rotary motion.

Stop misleading: If the stator and rotor magnetic fields repulse each other, the rotor should move in the opposite direction to that of the stator magnetic field. But in reality the rotor rotates in the same direction as that of the stator magnetic field but at a lower speed.

 

[ggunn]

Stop misleading: If the stator and rotor magnetic fields repulse each other, the rotor should move in the opposite direction to that of the stator magnetic field. But in reality the rotor rotates in the same direction as that of the stator magnetic field but at a lower speed.

That cannot be true. If the rotor spun more slowly than the field, the field would "lap" it and sometimes oppose the rotation of the rotor.

 

[Haji]

That cannot be true. If the rotor spun more slowly than the field, the field would "lap" it and sometimes oppose the rotation of the rotor.

If you were an Electrical Engineer, you would have very well known about the SLIP of an induction motor.

 

[ggunn]

If you were an Electrical Engineer, you would have very well known about the SLIP of an induction motor.

I am an electrical engineer.

 

[Haji]

Well. Then what is the slip of an induction motor?

 

[ggunn]

Well. Then what is the slip of an induction motor?

What is the speed of an unladen sparrow?

 

[GoldDigger]

Well. Then what is the slip of an induction motor?

OK.
1. The induced field opposes the CHANGE in the applied field, and is therefore of opposite polarity to the stator field. We are dealing with, Lentz's Law, not ferromagnetism. The force is attractive.
2.
If you look from the point of view of the rotor, there is a slowly rotating magnetic field relative to the rotor which is exerting a force and also causing inductive heating. You can look at this as slip from the stator's point of view or drag from the rotor's point of view.
3.
The original discussion did not necessarily involve an induction motor. The mention of synchronous specifically excludes an induction motor. It is possible to construct a synchronous motor using a shaped ferromagnetic rotor instead of a permanent magnet. The field is then still of the opposite

 

[GoldDigger]

Missed the editing timeout. (Dang PDA input methods!) See revised form below:

OK.
1. The induced field opposes the CHANGE in the applied field, and is therefore of opposite polarity (in space) to the stator field. We are dealing with, Lentz's Law, not ferromagnetism. The force is repulsive, since we have N adjacent to N and S adjacent to S. (In one case we are looking at the field outside the source magnet, in the other case the field within the induced magnet.) This is a brain twister....
2.
If you look from the point of view of the rotor, there is a slowly rotating magnetic field relative to the rotor which is exerting a force and also causing inductive heating. You can look at this as slip from the stator's point of view or drag from the rotor's point of view.
3.
The original discussion did not necessarily involve an induction motor. The mention of synchronous specifically excludes an induction motor. It is possible to construct a synchronous motor using a shaped ferromagnetic rotor instead of a permanent magnet. The field is then of the SAME polarity, but there is attraction because we have the rotor field between the pole pieces of the stator field, putting N adjacent to S and S adjacent to N.
4. Drawings help, but I think I will quit while I am only this far behind and reason it out completely before posting again.

 

[Besoeker]

Do induction and a wound AC motors use the same type of stator with the poles 120 degrees apart?

If you mean wound rotor as opposed to a cage rotor, then both are induction motors and yes, the stator winding is the same. You can short the slip rings and you have much the same as a cage rotor but there are performance differences.
Starting torque is very much affected by rotor circuit resistance. With a wound rotor machine that can be, and often is, controlled by external resistance banks. With the cage version, that isn't an option so the rotor has to be designed with sufficient resistance to produce the torque.

 

[Besoeker]

That cannot be true. If the rotor spun more slowly than the field, the field would "lap" it and sometimes oppose the rotation of the rotor.

if the rotor ran at the same speed as the field, no torque would be produced.. The rotor conductors HAVE to moving at a different speed to the field for current to be induced. No current, no torque.

 

[GoldDigger]

if the rotor ran at the same speed as the field, no torque would be produced.. The rotor conductors HAVE to moving at a different speed to the field for current to be induced. No current, no torque.

+1.
If you think about a permanent magnet AC motor, or one where the rotor windings are driven by an external voltage rather than relying on induced current, then there does not need to be a rotational velocity slip, just an angular offset. That would then be a synchronous motor.
But that is not how the typical AC motor is made.

The angular offset in the case of an induction motor is produced by the constant motion of the induced magnetic field in the rotor relative to the structure of the rotor itself. And that in turn depends on the LR time constant of the rotor. A non-conductive rotor would have complete slip and still no torque. A superconducting rotor would lock the induced field rigidly to the rotor and would, IMO, result in a zero-slip, high torque situation. Have to think about that one some more though.

 

[ggunn]

if the rotor ran at the same speed as the field, no torque would be produced.. The rotor conductors HAVE to moving at a different speed to the field for current to be induced. No current, no torque.

OK, I stand corrected. What about my question about the unladen sparrow, though?

 

[GoldDigger]

OK, I stand corrected. What about my question about the unladen sparrow, though?

"On a wagon, bound for market...." (Dona, Dona)(or Donna Donna, or Dana Dana)

Based on that source, the answer for a swallow is "swiftly." Does that generalize to sparrow also?

 

[ggunn]

"On a wagon, bound for market...." (Dona, Dona)(or Donna Donna, or Dana Dana)

Based on that source, the answer for a swallow is "swiftly." Does that generalize to sparrow also?

I screwed it up. It should have been "What is the airspeed velocity of an unladen swallow?"

 

[Besoeker]

OK, I stand corrected. What about my question about the unladen sparrow, though?

I'm going to have to wing it..........
As I wrote in the ornithologist column of a local newspaper, the passer vulgaris is known for its short, sharp, jerky movements......in flight.....

 

[JDBrown]

I screwed it up. It should have been "What is the airspeed velocity of an unladen swallow?"

African or European swallow?

 

[Haji]

1. The induced field opposes the CHANGE in the applied field, and is therefore of opposite polarity (in space) to the stator field. We are dealing with, Lentz's Law, not ferromagnetism. The force is repulsive, since we have N adjacent to N and S adjacent to S. (In one case we are looking at the field outside the source magnet, in the other case the field within the induced magnet.) This is a brain twister....

Lenz law talks about the reduction in the relative motion between stator and rotor and not about the attraction or repulsion between stator and rotor magnetic fields which is misleading.

2.
If you look from the point of view of the rotor, there is a slowly rotating magnetic field relative to the rotor which is exerting a force and also causing inductive heating. You can look at this as slip from the stator's point of view or drag from the rotor's point of view.

If you take the position of an observer on the rotor or stator magnetic field, either field appears stationery to the other.

 

[GoldDigger]

Lenz law talks about the reduction in the relative motion between stator and rotor and not about the attraction or repulsion between stator and rotor magnetic fields which is misleading.

Actually, the Lenz's Law I remember from Physics is that any change in a magnetic field in a system will try to create an opposing influence which acts to reduce that change.
The specific mechanism in this case is that the changing magnetic field from the stator induces a current in the rotor which acts to lessen the change in the overall magnetic field.
In the case of an inductor, it tells us that increasing the current through an inductor creates an electric field that opposes that increase in current. (i.e back EMF.)

If you take the position of an observer on the rotor or stator magnetic field, either field appears stationery to the other.

I prefer not take a position on a magnetic field. I was referring to taking a position on either the rotor itself or the stator itself.
From the point of view of the rotor, the physical lump of metal, the induced field in the rotor is moving slowly with respect to the rotor itself and is exactly keeping sync with the rotating field produced by the stator. That is how the two fields can stay aligned even though the rotor is moving at less than synchronous speed. I guess your wording could be interpreted as trying (and IMHO failing) to express the latter statement.

 

[Haji]

Actually, the Lenz's Law I remember from Physics is that any change in a magnetic field in a system will try to create an opposing influence which acts to reduce that change. The specific mechanism in this case is that the changing magnetic field from the stator induces a current in the rotor which acts to lessen the change in the overall magnetic field. In the case of an inductor, it tells us that increasing the current through an inductor creates an electric field that opposes that increase in current. (i.e back EMF.) .

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.

I prefer not take a position on a magnetic field. I was referring to taking a position on either the rotor itself or the stator itself. From the point of view of the rotor, the physical lump of metal, the induced field in the rotor is moving slowly with respect to the rotor itself and is exactly keeping sync with the rotating field produced by the stator. That is how the two fields can stay aligned even though the rotor is moving at less than synchronous speed. I guess your wording could be interpreted as trying (and IMHO failing) to express the latter statement.

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