EM Field

[artv]

Hi All,...here is a pic out of Jerry's photo files,...Jerry I hope you don't mind",
I think this is the perfect example of three phase...which coil is at peak output? Is it the lower left?
Also how far out from the coil does the EM fieild emanate??? I assume it is strongest , at peak outout.
thanks.....artv

[richard]

  No.   Top one.   richard canon city of Co,

[Janne]

Lower left would be the correct answer if the coils had iron cores, that linked all the coil turns together. In this case the flux mostly radiates vertically from the magnets, and as such the voltage peaks when the opposite coil legs have different magnet poles under them.

If one would try to calculate the EMF of an axial flux air core generator, it would work out by using the E= vbl equation. v = speed, l = wire lenght inside the magnetic flux and b = flux density.

[Flux]

Not possible to see the positions exactly from the angle of the photo but it looks as though the bottom left has the coil directly over the magnet.

Assuming this is the case, the flux will be fully linking that coil and the rate of change will be zero, it will have no emf induced in it. As the three phases of a 3 phase system sum to zero it follows that the other two coils will have an equal voltage, one positive and one negative.

I don't see where EM comes into this, but if you are thinking of magnetic field then flux will leave N poles and return to S poles. With no return disc, most of the flux will loop close to the magnets, the flux from the centre will take longer paths and at least in theory  the centre lines will go straight outwards. Most will miss the coil unless the coil is very close to the magnet and the coil is thin.

Flux

[artv]

Thanks for the replies,..This is what I am working on right now
the stator has 3 layers of coils ,12in each layer, 12 magnets on the armature ,I assume all coils are wound the same direction??
So there is always 2 legs of 2 layers seeing max flux, the 3rd layer will always be the hole of that layer over the magnet.
When the coils are producing, they have their own EM field, does it radiate out far enough to affect the coil thats at zero output??
So for Jannes' equation you would need to calculate the length of wire in 2 layers for "l",  "v" speed is easy enough , but how do you figure out the flux density "b"??.....
Also the Em field of the coils is it, 90 degrees opposite of the mag rotor (armature), or 180 degrees??
thanks for your time......artv

[bj]

  Art, not completely sure, as I didn't try rewinding that particular alt, but in the ones I did, one stator coil was always wound backwards to the
other two.  I tried winding and hooking all three phases the same, and it was dismal.  Never put much thought into why, just wound all in the same pattern
as factory windings on future tries, and it worked.
   Hope I'm not just confusing the issue.

[Flux]

This is really too complex to explain here.

You have 3 windings in that alternator, spaced 120 degrees electrically apart. If you consider a single phase then if it has 12 coils and is 12 pole then alternate coils must be reversed for the thing to add.

Just to add to your confusion it is highly likely that the star point is made on 3 coils 60 deg ( elect) apart as it is often easier. In that case the entire middle phase will be reversed to give the 120 deg displacement. I think this is where bj is coming from.

That machine has an iron core and the flux will have a short path across the tiny air gap so you needn't worry, the flux will go where it needs to, it won't radiate out and affect anything else. You can treat the 3 individual phases separately, it is only when you come to the rectifier that one phase has an influence on another and it is far too complicated to deal with rectifier conduction patterns here.

Just regard it as 3 alternators sharing one rotating field and treat each winding as a conventional single phase one and it will sort itself out. For any normal load other than a rectifier the 3 phases behave individually in the alternator and the load. The only advantage of 3 phase is that you can share things in the connecting leads and use 3 instead of 6.

Starting from the strange things that happen here is not a good way to learn basic machine theory, thse wind machines feeding rectifiers and batteries are a bit odd in many ways.

Flux

[artv]

bj, Flux,...I didn't rewind this alt ,it has its' stock windings.It had 3 pairs of wires connected to its bridge ,I cut the bridge away,and un-twisted the 3 pairs to end up with 6 wires. I then did an ohm test on the leads to find the 3 phases, each pair of leads gave a reading of .8 ohms.There wouldn't be any more wires than this would there??
 Each phase measured, 1 at a time, reads 3.5VAC (max rpm of my cordless drill).
By using 12 magnets on the armature, that means one layer of windings is always at zero output, while the other two are at max output.When 2 of the phases are at max output, the 12 coils of each phase, will be producing EM fields.
Do these fields not affect the 12 coils that are at zero output???..........just wondering.....artv

[Flux]

From your description it seems as though you had a delta winding and have separated the 3 phases correctly if you get 3.5v on each phase.

I can't follow the rest, it is true that if you look at any instant in time you can have a point when one phase volts is zero and the others are high (but not full volts), that is the nature or 3 phase, but it is an instantaneous thing and looking at it longer term all three phases will be active.

The instantaneous voltages in the phase windings have absolutely no effect on your rotating magnetic field ( certainly true on no load). The field from your 12 pole magnet just rotates and the field induced in the core just rotates with it, the windings just happen to get cut by the flux, they don't alter it.

You must not try to add all 3 phases in series as they are displaced in time. If you try connecting them in series then you will get zero if the phasing is correct as it is a feature of three phase that the instantaneous sum of the phases is zero.

If you are trying to add them to get more volts then you must do as Jerry does and rectify all three and add the dc values in series with very large capacitors across each rectifier.

Flux