Stator Teeth

[vawtman]

Instead of using them to lock the coil in place couldnt they be cutoff and sharpened and the coil be held by the housing to reduce cog?Is it worth a test or is just me trying to get back at Sparky because i gotta get a tooth pulled tomorrow?

Ikes!

 Thanks

[Countryboy]

The stator teeth attract the magnetic flux, since they are closest to the mags.  

You are correct that it would eliminate cogging if we removed any metal the magnets were attracted to.  Unfortunately, that would also greatly diminish output from the coils, as there would be much less flux moving thru the coil.

[vawtman]

Im just thinkin the outer lip not the whole tooth.Ouch!Thanks and output loss estimate.Any ideas

[windstuffnow]

  You can remove the cogging almost completely by using a number of magents that don't match the teeth properly.   My last post on pole to coil count.  I've come up with a way to rewind just about anything with strange pole/coil counts that will eliminate or very closely eliminate the cogging from laminants.  I've wound up several combinations and the results are quite impressive so far.  I've only had 2 complete failures out of all the combinations I've tried, actually not complete failures because they did actually produce some power, other combinations worked far better.

  How many slots are in the stator?   I'll see how many combinations I can come up with that will work good.  

.

[Flux]

Apart from the impossibility of removing the tooth bridges you will make the cog many times worse.

The normal semi closed slots are a fair compromise between ease of winding and reduction of slot ripple and tooth loss . The slot ripple is closely associated with cog.

The main advantage of open slots is the ease of winding and they sometimes have to be used with large machines wound with bar coils. The alternative of threading bar conductors through tunnel slots is very costly.

Flux

[vawtman]

Thanks Ed

 Would be very interested in your ideas.The stator has 36slots and was a 3hp baldor 4pl.The stator is skewed and my current mags will work for 12pl and am following Zubblys method currently.

 I currently have a 5hp 4pl with very light cogging but enough to really effect startup.When i put an unconverted motor on it starts nice.Its my 8x8 vawt.

 An axial flux geared up would probably work nice but would like to decog a conversion has much has possible.

[vawtman]

Ok now we got another disease.What is and causes slot ripple.Oh and also hysterisis if i may.Sure am learning alot about these.Thanks

[Flux]

I think i would do better to write a book on machine design, but never mind, i will give it here free of charge.

Magnetic flux passes through iron about 1000 times more easily than air, so that is why we use iron circuits in electrical machines. Even the dual rotor has an iron circuit ( the discs) that increases the flux, but the coils are in a long air gap and it needs a lot of magnet to produce a strong field in a large air gap.

If you make a tiny air gap you have no room for wire of useful thickness. This problem was solved by Pacinotti about 150 years ago by making slots in the iron core and winding the coils in the slots. You now have to look at things rather differently as the coil legs are in a region where there is virtually no flux. The flux passes through the iron teeth, but on its journey from N to S pole it links the coil via the iron core. This flux linkage reverses when the magnets move to the next pole pitch (magnet S to N ).

As the flux is passing from the magnet via the air gap to the teeth you have it split up into discrete lumps and something strange has to happen when a tooth comes from under the magnet. The air doesn't readily support the flux so the flux in the tooth drops rapidly as it leaves the magnet and transfers to the nearest one left under the magnet. If it is an open slot then this transfer is very snappy and there is a sudden disturbance in the flux path and a rapid change in flux results in a fast ripply peak in the coil voltage ( slot ripple). If the tips of the slots are bridged by a thin strip of iron then part of the flux transfer is via this bridge and the fall in flux in the leaving tooth is fairly gradual. Semi closed slots are a compromise that lets you wind it easily and have a compromise slot ripple.

It is this snapping flux and associated change in magnetic pull that causes cogging so the things are related.

Iron is very beneficial to the conduction of flux but it does cause other losses ( iron loss). Energy is needed to take the magnetic domains( the little bits that line up and carry the flux) round the magnetising loop. this is hysteresis loss and depends on the type of iron alloy. The other loss is caused by circulating currents in the iron and can occur in any conductor, this is eddy loss and can be almost completely prevented by breaking up the current paths so the voltage is too small to circulate significant currents. This is why we laminate cores.

I hope this is of some interest.

Flux

[Flux]

I hope this ends up in the right place, if not you should work out what it is about.

Are you sure your start up trouble is really caused by the cog and not by iron loss.

If it turns at all and doesn't gather speed then it is not cog.

A standard motor with no magnets has no iron loss. Even if you can eliminate cog completely you will be left with iron loss, that just acts as a drag and has a similar effect to thick grease in the bearings. I think a lot of the worry about cog here is caused by a failure to realise that this drag from iron loss will inhibit starting.

When I played with motor conversions years ago using magnet steels, I found that cog was never the issue unless it was drastically bad. On the one that I de-cogged completely by careful adjustment of core slot skew the starting was no better.

This was a HAWT and using a gear drive but the issue is the same.

Flux

[vawtman]

Thanks again Flux

If you ever decide to write a book ill be first inline.

Has for the reply being of interest I think ill frame it and put it right next to my monitor.

Everything is much clearer now.

I guess thats why most Vawts use what is called a Magnetic Levitation Alternator which i guess is a fancy name for the axial flux.

Ill continue on with the 3hp and promise not to butcher anything.

    Have good day