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Using magnets we already have for a larger rotor
By cslarson, Section Wind Posted on Mon Nov 5th, 2007 at 06:17:52 AM MST
can i put two rings of smaller magnets on a larger rotor?

Hi all,
We have a number (500?) of 46x30x10mm, grade N40 (supposedly) neodymium magnets that we are using for 500W, Hugh Piggot-style wind turbines. I am building a 4m, 3-bladed prototype and am looking for some advice on the design of the alternator. Thanks very much to Steven Fahey for putting together "Basic Principles of the Homemade Axial Flux Alternator". The blades were designed using PROPID, and according to that program the blades should produce the following power curve.

The magnet rotor diameter will be around 500mm. Hopefully I can design it so that it is producing high voltage (200+), 3-phase ac. I am wondering if I can put an inner ring of magnets to increase the magnetic flux and thus match the power curve of the blades more appropriately (i am aware that some may feel i am doing things backwards). This way, using 12 stations might produce a rotor like this:

or with 16 magnet stations:

Am I correct in thinking that using more magnets, and thus more coils (like the 16 station version) would be better because there would be less resistance in each coil? Or can I go with 12, reduce the cost a little, and just put more windings on each coil? Should there be a gap between the outer and inner row of magnets for this to work? Or should i just order larger magnets?
Thank you.
Carl

Using magnets we already have for a larger rotor | 10 comments (10 topical, 0 editorial)

Re: Using magnets we already have for a larger rot (3.00 / 0) (#1)
by Flux on Mon Nov 5th, 2007 at 12:46:59 AM MST
(User Info)

I won't say that you are doing things backwards but you will get little help until you tell us how you intend to load this thing.

Your predicted prop characteristic will be at mppt, if you load it as such then you may be able to get a good match.

With the disc size and number of magnets you are looking at you will have a powerful alternator and if you try direct charging to a high voltage battery or using a lower voltage with a transformer then you will just stall.

We also need prop speed or tsr to be of any help.
Flux

Re: Using magnets we already have for a larger rot (3.00 / 0) (#2)
by hiker (hiker(at)adnmail.com) on Mon Nov 5th, 2007 at 02:15:47 AM MST
(User Info)

i doubled up on the mags like that a while back..
with the same amount of windings--the voltage was about the same but the wattage went up..in other words the long leg coils put out more watts then the short leg coils-
with windings the same on each coil-
i had my mags end to end-no gap.....
WILD IN ALASKA
[ Parent ]

Re: Using magnets we already have for a larger rot (3.00 / 0) (#4)
by cslarson on Mon Nov 5th, 2007 at 07:44:02 AM MST
(User Info)

brilliant. thanks. by the way, was up in Alaska last summer for the first time - Sitka. we had some great weather and one of the best vacations i've ever had. stunning place. can't wait to go back!
Carl

[ Parent ]

Re: Using magnets we already have for a larger rot (3.00 / 0) (#3)
by cslarson on Mon Nov 5th, 2007 at 05:56:34 AM MST
(User Info)

Hi Flux, thanks for the response. Those values are calculated at a tip speed ratio of 6.5. Here is a chart for wind speed vs rpm:

I certainly can tell you about how I hope to load this up. I'm hoping to connect it through a controller to a 3-phase water pump. As described in the following publication, the controller will switch the pump on and off based on a cut-in/cut-out low frequency and cut-in/cut-out high frequency, and only on as long as the voltage/frequency ratio stays in a predetermined range:
http://www.cprl.ars.usda.gov/REMM%20Pubs/1996%20A%20Smart%20Controller%20for%20Wind%20Electric%20Wat er%20Pumping%20Systems.pdf

It looks like an off-the-shelf 240v ac pump will operate at different voltages and frequencies, and even efficiently, as long as V/f stays at a certain preset ratio. Let me know if anyone has already designed a controller like this that might share the schematics!

With a high-voltage ac alternator it should be straightforward to use a normal water heating element as a dump load. Secondarily, it should charge batteries, though for the moment I am more interested in seeing how viable a solution we can make for pumping water.

You said it looks like a powerful alternator. Which configuration? One of the things I am having trouble with is sizing the alternator. I care more about producing power at the lower wind speeds. If a smaller alternator would do this (maxing out at 3kw or so), then that is fine as it would be cheaper and I can furl earlier to protect it (if that even makes sense).

Thanks,
Carl

[ Parent ]

Re: Using magnets we already have for a larger rot (3.00 / 0) (#5)
by Flux on Mon Nov 5th, 2007 at 07:54:22 AM MST
(User Info)

A pumping load should work ok over a modest speed range. The volts/cycle should remain constant and the power absorbed by the pump should follow speed cubed I think.

I haven't had time to look in depth at your article. If you are interested in lower winds then you should not need a monster alternator.

When properly matched 16 pole with single magnets should manage that with reasonable efficiency up to near 2kW perhaps even more. The more magnets and copper you through at it the higher the efficiency and the lower the stator heating, so the higher wind speed you can go to before furling.

I would have been tempted to try 20 poles of single magnets rather than double up on the magnets. You make better use of the magnets by increasing the pole number with decent spacing rather than using very long thin magnets that are too widely spaced at the outside and crowded at the centre.

You will need to choose your voltage to match the volts/cycle of the motor ( aim for 60 Hz at 240v if it is N.America).

It is unlikely that you will manage to start the motor in low winds, I just have no idea what the starting requirements will be at low volts -low frequency. You may need to let the blades have a run for it and bring the motor on line to use the inertia of the blades to start it. Once started it may run down to speeds where the pump can no longer manage the lift. You will have to do a fair bit of experimenting, which you may be able to do with a conventional alternator driven at the same volts/cycle from a variable speed source rather than build the pmg first.

Once you have sorted the pump characteristics then you have a far better idea what the alternator needs to do.

You will have to limit the motor to full load determined by the pump, beyond that you will need to bring in a dump load or furl. Choice of pump characteristic and motor size may be quite critical. I think with careful choices you can produce a perfectly satisfactory scheme but expect a lot of unforeseen things, this is probably new ground.

Flux

[ Parent ]

Re: Doubling magnets for a larger rotor. (3.00 / 0) (#6)
by finnsawyer on Mon Nov 5th, 2007 at 09:24:09 AM MST
(User Info)

When the rotor advances the magnets from being over one coil to the next the change in flux will be the same for the two cases. With the 16 magnet layout the time it takes at a given RPM will be 3/4 as long. This would translate into 1/3 more average voltage per coil if the coils had the same number of turns in the two cases. Also, there will be 1/3 more coils. This would result in 16/9 times the voltage per phase if the number of turns didn't change. Of course the coils won't have the same number of turns as the spacing gets smaller. Or will they? It's up to you. Reducing the wire size increases the resistance per unit length of wire, but reducing the coil size has a strong effect on the total amount of copper used per coil and hence the resistance per coil since both are proportional to the area of the coil minus the area of the central hole. If you know the wire size and area of the coils for the 12 magnet 9 coil case you should be able to predict the effect of different coil sizes and wire sizes in the 16 magnet 12 coil case.

In looking at your numbers it appears your diagrams are not to scale, that you could go to 24 magnets with 18 coils (1.8" x 1.2" magnets doubled on a 20" diameter rotor). Your aspect ratio is rather severe (3 to 1), but with the 20 inch rotor you should do ok. Of course, you are showing the magnets right to the edge of the rotor, which would have the coils extending past the stator rim. You need to sit down and do some analysis of your own.
GeoM

Re: Doubling magnets for a larger rotor. (3.00 / 0) (#7)
by Flux on Mon Nov 5th, 2007 at 10:08:21 AM MST
(User Info)

I never checked whether the things were to scale. You could go up to 24 magnets on a 500mm disc if you want to. I would certainly do that rather than stacking 2 magnets in line.

24 poles would give more alternator capacity than you need without doubling up on magnets.

I can't help much more without knowing the pump and motor characteristics.

Flux

[ Parent ]

Re: Doubling magnets for a larger rotor. (3.00 / 0) (#8)
by finnsawyer on Tue Nov 6th, 2007 at 08:16:31 AM MST
(User Info)

With a single row of magnets he frees up more space, enough so that it looks like he could go to 28 magnets with 2.4 inch wide coils (0.6 inch wide coil legs) or very nearly so. Doubling the magnets doesn't look like that good an idea.
GeoM
[ Parent ]

Re: Doubling magnets for a larger rotor. (3.00 / 0) (#9)
by cslarson on Tue Nov 6th, 2007 at 11:07:34 AM MST
(User Info)

you all are absolutely right. 24 (or as you say maybe even 28) would work as a single ring. i was going to post that last night, but our internet got cut off (as it does at 11pm every night). i think i was hoping to get away with a fewer number of coils. anyway, today i found out i'd have to order a new batch of magnets for this alternator and can specify any dimension i want. i'd like to decrease the magnet rotor diameter and use fewer, larger magnets. maybe 16 of 60x40x15mm on one of the magnet rotors that would now be 400mm rather than 500mm.

the induction motor conversion idea just won't leave my head, either. there seem to be pluses and minuses to both ideas. i think the axial flux (when finished) is elegantly simple, weather-proof, and efficient. but the idea of buying a motor, modifying it slightly, and away you go... rather than winding all those coils. then there's the cost. looks like the magnets alone for an alternator like this are around $250, not to mention the copper. so, today i picked up a 3-phase, 1400rpm, 3kw induction motor. it was about $130. i think i may end up making the axial flux alternator, and playing around with the induction motor, too.

thanks very much for the help.

[ Parent ]

Re: Doubling magnets for a larger rotor. (3.00 / 0) (#10)
by cslarson on Tue Nov 6th, 2007 at 09:40:50 PM MST
(User Info)

ha! so it looks like we paid way too much for that motor. the guys are taking it back and will pick up one from Pakistan, which are supposed to be better quality...

[ Parent ]

Using magnets we already have for a larger rotor | 10 comments (10 topical, 0 editorial)

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