Stator winding movements 2

[lyen12]

With regards to caged induction motor,
How does winding move within the slots when voltage is applied? Can this be explain by the left hand fleming?
Does the rotor bars play any role in "pushing" the stator coil to either side of the slot as to minimise its movement?

 

[7anoter4]

You may explain the force direction using Fleming's left hand rule if you know the magnetic field direction and the current direction. In a slot,since all the currents go in the same direction [sometime] the force direction will be in the opposite sense. The magnetic field generated by rotor current produce another force in connection with stator current flowing through stator slot wire, but this time in the attraction direction. The force producing rotor rotation will be equal to the force acting on the stator windings.
But, anyway, the winding has to be well fastened in the slot so that it does not move.

 

[Gyo]

Since the rotating magnetic fields are sinusoidal in time, does it mean that at opposite polarity, the force produced by the magnetic field generated by the rotor current will push the stator coil away? Is that the reason coils vibrate?

The magnetic field generated by rotor current produce another force in connection with stator current flowing through stator slot wire, but this time in the attraction direction.

Does it mean that the rotor bars are now like a magnet which attracts the stator slot wire?

 

[electricpete]

You cannot determine forces for conductors in slots based on currents ignoring presence of the iron.

If you have two parallel conductors in air carrying current the same direction, they are attracted toward each other.

But if you have conductors in a slot which are all from the same phase, the force is primarily down deeper into the slot. The reason is that the flux in the slot is primarily cross slot leakage flux. It interacts with the current that caused it to cause force into slot by your normal right hand rule. When current reverses, the flux also reverses and force direction remains downward into the slot. The frequency of the vibration is twice line frequency, plus the dc component ..for example F = F0*[0.5 + 0.5*cos(2*2*pi*LF*t)] in the deeper-into-slot direction.

That is steady state within the slot, all conductors same phase, the easiest case to predict. Faults become more complicated. There can be a dc component. There can be saturation and the conductors may act more like conductors in air in that case. There are other effects I don't know much about but I think I recall hearing force can be upward out of the slot during a fault (not sure if that is limited to slots with multiple phases). The endwindings are also very difficult to predict.


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[electricpete]

The endwindings are also very difficult to predict.

As a first step they are conductors on air. And after a fault it is sometimes seen the knuckles of coils within a group pull together and groups push apart from each other.

But the flux distribution in the endwindings includes many conductors in 3-d space and adjacent iron with fringing flux... lots to consider.

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[7anoter4]

I have to apologize, my exercise with Fleming's left hand always failed. There is a rule, more simple, I think, using entire hand but I cannot find where is it.

 

[electricpete]

Here's further discussion of why force is always down using the rules.

Imagine we're looking at a slot with gap at the top, iron on bottom and sides..
...current flowing in the slot out of the page toward you.

The leakage flux from that current wants to encircle the current flowing ccw (right-hand thumb in direction of current, right-hand flux in direction of fingers).

The flux that encircles at the bottom stays in iron. The flux that encircles as the top has to cross an airgap some way. Some of that leakage flux will flow right-to-left accross the slot.

Right hand rule for force on conductor:
Thumb in direction of currrent (out of page)
Pointer in direction of flux (to the right)
Middle Finger points in direction of force (down, toward bottom of slot).

Reverse the current, reverse the flux, force still points down (toward bottom of slot).
This is the predominant direction of force on conductor in slot (only one phase per slot) under load.

You may ask why we didn't consider any magnetizing flux. That flux flows primarily accross the airgap, in the teeth, and in the backiron. There is nothing to drive it accross the slot. The flux in the slot under load arises primarily from "cross slot leakage flux" (google that term and I think you'll find more).







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[7anoter4]

Thanks,electricpete!

 

[lyen12]

Thank you, electricpete.

1. My fingers are aching as I tried to follow closely with the right hand Fleming. You said "Pointer in direction of flux (to the right)", but I read that the flux encircles but which direction we should be looking at?

2. Where are slot wedges normally located (or should be located) - at the bottom or top of the slot? If the slot wedges are removed, the turns at the slot will move upwards towards the airgap, is that right?

 

[electricpete]

Where are slot wedges normally located (or should be located) - at the bottom or top of the slot? ....

At the top in my discussion above.

...If the slot wedges are removed, the turns at the slot will move upwards towards the airgap, is that right?

Not under normal load and assuming there are two conductors of the same phase per slot.
But under fault I believe it may occur.

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(2B)+(2B)' ?

 

[electricpete]

Imagine we're looking at a slot with gap at the top, iron on bottom and sides..
...current flowing in the slot out of the page toward you.

The leakage flux from that current wants to encircle the current flowing ccw (right-hand thumb in direction of current, right-hand flux in direction of fingers).

The flux that encircles at the bottom stays in iron. The flux that encircles as the top has to cross an airgap some way. Some of that leakage flux will flow right-to-left accross the slot.

Right hand rule for force on conductor:
Thumb in direction of currrent (out of page)
Pointer in direction of flux (to the right)
Middle Finger points in direction of force (down, toward bottom of slot).

1. My fingers are aching as I tried to follow closely with the right hand Fleming. You said "Pointer in direction of flux (to the right)", but I read that the flux encircles but which direction we should be looking at?

Attached is a figure illustrating what I described in the quote above.

The top figure shows notation for current out of page.
The next figure down shows flux associated with current out of page in air.
The next figure down shows flux from current in an iron slot with airgp above.
The bottom figure shows current in slot, flux in slot, and resulting direction of force using the right hand rule (including a right hand in the picture).



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[electricpete]

Sorry, here is the file

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