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different methods for 'decogging'
By jacquesm, Section Wind Posted on Tue Sep 7th, 2004 at 07:15:18 AM MST
how many differnent methods are there for decogging a pma ?

If you have a permanent magnet alternator with some kind of decogging measure then please describe it for me.
Aircore is pretty obvious (no decogging neccessary) but there must be plenty of other interesting ways to do it.

different methods for 'decogging' | 10 comments (10 topical, 0 editorial)

Re: different methods for 'decogging' (3.00 / 0) (#1)
by Flux on Tue Sep 7th, 2004 at 07:46:32 AM MST
(User Info)

Cogging occurs with iron cores when there teeth or other discontinuities that line up with the magnetic field. The way to avoid it is to average the force out over the complete circumference.
With radial designs the standard method is to skew the slots or the magnet faces by approximately one slot width. Another method is to alter the spacing between magnets to less than the theoretical value so that you end up with one wide space, this works ok but it may need balancing.

With axials you can make the slots non radial, or choose a magnet shape to have the same effect with radial slots.
Another way is to stack single phase stators at 120 electrical degrees, this would work with the Fisher & Paykel and similar machines.

Unless it cogs very badly it is not generally the cogging that causes starting problems with iron cored machines. A much more serious problem is the iron loss during start up if the core material is not of the highest grade.

It is often possible to find a machine that will turn slowly and still not get out of stall at a reasonable wind speed.
Flux

Re: different methods for 'decogging' (3.00 / 0) (#3)
by jacquesm (j@ww.com- I run a whitelist, add 'stjoes' to msg) on Tue Sep 7th, 2004 at 11:05:45 AM MST
(User Info) http://www.greenbits.com/

I like the gap spacing one ! I think I may have even seen that (now that you've described what it does) in the the guts of an Air 'X', it has straight laminations and a bunch of tiny (really tiny, no kidding) neos in there. around them is a tiny steel strap (the strap lives in the airgap) and the strap is *bolted* on in a gap between two magnets.

At the time I thought the gap was to accomodate the bolt. Now I see it might serve dual purpose. The RPM of that thing is huge (2000+ if I recall correctly), it has no furling other than some electrical trick and thin injection molded blades.

I figured they needed the straps at that RPM to keep the magnets from flying of the rotor and that's where the bolt came from. I didn't understand why they did not space out the magnets equally and use a shrink strap instead. Your explanation seems to solve that question.

Thank you !

btw the neos in that machine are really neat. They are shaped just so that the airgap is constant all the way around the circle.

I used the skewed slot trick on my alternator, but there must be tons of other ways of doing the same thing.

The skew works ok, but it causes several problems:

The slot liner has a tendency to crawl out of the slants during the winding and handling because the one side is higher than the other

It makes winding that much harder, the edges no longer sit above each other but every lamination is skewed a bit which causes a little 'ledge' to form

The lifespan of the lamination is probably shorter, vibration will have much more opportunities to eat through the slot liner than it would otherwise because of all the little spaces and edges.

I think ofsetting the magnets might be a better way of doing this. Maybe there are even better ways !
www.greenbits.com
[ Parent ]

Re: different methods for 'decogging' (3.00 / 0) (#2)
by DanB (danb@*no spam*otherpower.com) on Tue Sep 7th, 2004 at 07:57:37 AM MST
(User Info) http://www.otherpower.com/

Ive always thought it would be interesting... (and fun) if one had a nice PMA that cogged badly - to simply build a "decogging wheel" that you could stick on the shaft. It'd just be a disk of magnets perhaps, that interacts with some stationary magnets (or bits of steel) - it would be really easy to adjust, and I think you could get by in most cases with fairly small inexpensive magnets. Basicly - jsut a magnet rotor that interacts with other magnets and does nothing other than "Cog" - and you would offset the cogging points between the alternator, and the wheel.

Re: different methods for 'decogging' (3.00 / 0) (#4)
by jacquesm (j@ww.com- I run a whitelist, add 'stjoes' to msg) on Tue Sep 7th, 2004 at 11:39:24 PM MST
(User Info) http://www.greenbits.com/

hehe, that's clever ! Two lefts making a right. I can see how that would work, throw in another stator and get free power to boot :)

Neat !
www.greenbits.com
[ Parent ]

Re: different methods for 'decogging' (3.00 / 0) (#5)
by Flux on Wed Sep 8th, 2004 at 01:43:57 AM MST
(User Info)

Would it be a cog wheel or an anti-cog wheel.

It's a neat idea if you ended up with a machine that was otherwise useless.

I also thought you might get some power from it, but if you intended that, it would be 2 stacked machines again and you would probably do it differently.

To use it just as a de-cogger you would have to watch the eddy current loss. Ferrite on ferrite magnets would be one way.
Flux

[ Parent ]

Re: different methods for 'decogging' (3.00 / 0) (#6)
by DanB (danb@*no spam*otherpower.com) on Wed Sep 8th, 2004 at 07:23:00 AM MST
(User Info) http://www.otherpower.com/

I think - looking at how lots of motor conversion I've done cog (and I had big magnets layed in there straight so they really did cog!)... you could make a "cog" (or "anti cog") wheel - that would do the job with very small magnets - the magnets would not have to be so large that we'd even consider making power with them - I was thinking like... 3/8" diameter, or perhaps 1/2" diameter disks (something small and cheap).

[ Parent ]

Re: different methods for 'decogging' (3.00 / 0) (#7)
by jacquesm (j@ww.com- I run a whitelist, add 'stjoes' to msg) on Wed Sep 8th, 2004 at 07:38:51 AM MST
(User Info) http://www.greenbits.com/

you could the do the 'spacing' trick referenced above, I'm quite sure that would work for a motor conversion (the Air-x is basically a re-packaged car alternator with neos in it).
www.greenbits.com
[ Parent ]

Re: different methods for 'decogging' (3.00 / 0) (#8)
by TomW on Wed Sep 8th, 2004 at 11:46:57 AM MST
(User Info)

Folks;

One thing I did on my Garbogen which would make power in winds it would not start in:

I just sent a slight "bump" of power up the line which seemed to cancel the magnetic attraction just enough to release the cogging.

It was purely manual and a test but it worked flawlessly. i simply put a momentary switch across the blocking diode and if it seemed it should be turning but wasn't I just tapped the switch. Not a theory or an idea, a method that I can prove anywhere. Automating it would not be too difficult either.

Cheers.

TomW

"Education consists mainly of what we have unlearned."--Mark Twain

Re: different methods for 'decogging' (3.00 / 0) (#9)
by jacquesm (j@ww.com- I run a whitelist, add 'stjoes' to msg) on Wed Sep 8th, 2004 at 12:09:34 PM MST
(User Info) http://www.greenbits.com/

that's clever ! how was that machine vibration wise ?
www.greenbits.com
[ Parent ]

Re: different methods for 'decogging' (3.00 / 0) (#10)
by TomW on Wed Sep 8th, 2004 at 12:34:43 PM MST
(User Info)

acquesm;

WEll it has a very noticeable brrr'ing sound at lower speeds which was very noticeable in my shop since it was attached to the wall poles of a metal building. Not annoying just obvious. The sound and vibration disappeared at any reasonable rpm however.

"Education consists mainly of what we have unlearned."--Mark Twain
[ Parent ]

different methods for 'decogging' | 10 comments (10 topical, 0 editorial)

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