Author Topic: Tiny 3 phase radial alternator needs decogging? (Read 6784 times)

gotwind2 · « **on:** November 04, 2007, 05:00:11 PM »

Hi Guys, I need a bit of clever help please...

I have got a tiny 3 phase radial axis steel core alternator rated at 6 watts (although I have measured 16 watts from it).

Lovely bit of engineering 6 pole Neodymium magnets with skewed stator laminations - it is a bit similar to the Fisher & Paykel smart drive motors seen in Australia.

It needs to spin at 2000 rpm to hit 12 volts but suffers from cogging.

1 3/4" in diameter - the pen is pictured for scale purposes.

Does anyone have an idea how the cogging can be reduced?, I have read Dinges's PDF on decogging and found this interesting article also.

http://www.thebackshed.com/windmill/articles/coglessFP.asp

Maybe I could skew the magnets somehow?

Thanks

Ben

Flux · « **Reply #1 on:** November 04, 2007, 10:35:37 AM »

You don't look to have many options. Wit 12 magnets and 9 poles with skewed tops I wouldn't expect much cog anyway.

All that I can suggest is that you file away the leading and trailing edges of those pole shoes very carefully, probably you will not need to attack the main part of the shoe, just those triangular bits. If you look at the decogging process for the F & P and see what they do I think it will help. In your case with skewed poles I doubt that you will see much loss of performance. The amount you need to remove will be critical and you will need to treat each pole the same amount.

You have no room to skew magnets or use differential spacing.

Flux

gotwind2 · « **Reply #2 on:** November 04, 2007, 02:03:53 PM »

Thanks Flux for your prompt response.

I will get the Dremel drill working on rounding those corners on the pole shoes - Carefully not to damage the windings.

Ben.

gizmo · « **Reply #3 on:** November 04, 2007, 08:23:46 PM »

I would suggest you add more skew to the stator by removing material like the drawing below.

Other than that, all you could do is remove the magnets and try different combinations/phases. You could try a 5 or 7 phase conversion, they do work well for the F&P.

Glenn

tecker · « **Reply #4 on:** November 04, 2007, 10:58:59 PM »

Looks like a motor with the magnets that close together . The thing has some larger wire looks almost like 20 .Anyhow to do any good at gaining 12 volts you'll need to series it . Best bet is to seperate each coil and rectify .

Jon Miller · « **Reply #5 on:** November 05, 2007, 06:13:51 AM »

Hi Ben,

Dont Know where you got that little generator from but I have got hold of one from a photo copier that is also three phase.

The one i have dosnt have neos just ceramic magnets. It has roller bearings, It also looks a little bit bigger then your one. Its rated at 24volts 1.8amps.

What have you got planned for your one?

The clogging on mine is not that bad.

Sorry i cant help, interesting seeing you one as well.

Heres some pics of it

ghurd · « **Reply #6 on:** November 05, 2007, 06:33:42 AM »

Might try feeding that to a 2:1 or 3:1 3-ph transformer to get the cut in lower, then use slower higher torque blades.

Less % diode losses, less drag from the blades, etc.

That's my plan for something like that, but no neos in it. 3~4A Isc, but only 3.5VDCopen at 600RPM.

Cool motor!

G-

Jon Miller · « **Reply #7 on:** November 05, 2007, 06:43:17 AM »

Hi

Got some better pics, got hold of a better phone camera

gotwind2 · « **Reply #8 on:** November 05, 2007, 08:03:34 AM »

Thanks for the advice guys.

Jon. your motor doesn't have any stator skewing, so it should cog more - I am guessing it is the lower strength ceramic magnets that are making it 'feel' like there is less cogging. Mine can generate 16 watts which is pretty impressive for such a tiny alternator.

I will try filing a bit of each stator shoe with a dremel drill soon.

Ben.

Flux · « **Reply #9 on:** November 05, 2007, 11:55:38 AM »

Jon's motor is single phase, I assume Ben's must be a sort of 3 phase with that magnet to coil ratio.

I would have expected the single phase one with no skew to cog worse, but it does not have discrete magnets and the poles on that magnetised ceramic ring will not be well defined. The lower flux of the ceramic will cause far less cog.

All you can hope to do is make the attraction between the discrete magnets and those laminated poles average out over one rotation.

Changing the skew as Glenn described or making the edges of the polar projections less defined as I suggested should both work, but it will be experiment, no theory is going to get you to the exact point, I expect the makers attempt has removed 90% of the cog.

Ghurd has a point about trying to increase the rotor starting torque with lower speed, the starting losses do become a big issue with tiny machines if you need to start in low winds.

Flux

gotwind2 · « **Reply #10 on:** November 05, 2007, 01:14:47 PM »

Thanks again Flux.

Would an air core design at this scale be possible - I'm not after more than 2 watts.

b.t.w are you U.K based Flux?

Ben.

Flux · « **Reply #11 on:** November 05, 2007, 01:48:00 PM »

Yes eliminating the iron will prevent all cog and drag at start up.

You will need more magnet for the same output but for 2 W the magnet cost will be small.

True radial air gap machines are a bit difficult to produce without machining facilities, you may manage with a slotless design or you may be forced to build an axial but you may need to use a bigger diameter to accommodate some form of hub.

Yes to the last bit.

Flux

gotwind2 · « **Reply #12 on:** November 05, 2007, 02:03:16 PM »

Agreed.

Flux:

You clearly have a great knowledge on this subject and most other stuff.

Can you contact me via email please - no pressure at all, I may have a mini project for you

ben[at]gotwind.org

Thanks

Ben.

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