PMG design advice please

[stevesteve]

Hi Everyone,

Please can you have a look at the bits I have and advise. I am in the position of not having any serious metalworking machine tools so I cannot make any of the nice custom metalwork that some of you produce. My supplies will tend to be 'found' ie whatever I can obtain. This will be an urban turbine so I can't build a 12" disk machine (as per Hugh's book) as the blades will have to be smaller and would stall too easily. I bought a couple of steel rings on ebay (so dual rotor) and some 25x5mm (1"x1/4") disc neos.

OK option #1 12 magnets 9 coils 3 phase.





Note: this leaves a significant gap between the individual magnets.

Option #2 16 magnets 12 coils 3 phase.

Is the small gap between the magnets a disadvantage? If so will any lost power as the coil reaches this position:





(ie both legs of the coil momentarily over similar poles) be compansated for by having the extra magnets?

Cheers Steve

[Flux]

The 12 magnet spacing will use your magnets much more efficiently, but in general the more magnet you throw at it the more you get out so you will almost certainly get more from the 16 magnet layout but nothing like as much as you ought to get for a 50% increase in magnet volume.

You need to keep the holes in the coils about the size of the magnets for efficient use of magnets, but with round ones you can reduce the width of the hole a bit since the bulk of the flux is in the centre part.

Even making the coils a bit narrow you will end up with thin coils with the 16 magnet layout which restricts the size of wire you can use. Also the closer magnet spacing will require a smaller gap between rotors to keep leakage flux low, so again you will restrict winding area.

You give no idea of your proposed prop size or speed. If you can put up with a fast noisy prop you will get a fair bit more power than with a nice slow prop.

If you are in an urban environment, don't expect too much, it may seem windy but there is likely to be little power in it.

Flux

[stevesteve]

Thanks Flux,

Fast silent prop would be good ;-) Perhaps 4' to 5' diameter. I will cheat slightly by seting up the rotors (as they are the parts I can change the least) then I will try winding a single multi-tapped coil and test it at various speeds to try to get a handle on how to set up the wire size/turns.

I guess that I am most concerned that I can get a high enough voltage to charge a 12V system and the current will be whatever I can get. I will probably use a star/delta switch to get as low a cut-in as I can but still utilise days when there is more wind.

Anyone else have an opinion before I get the epoxy out?

Cheers, Steve

[Flux]

I think I would stick with 12 poles. Those magnets should be ok for a 5 ft prop with tsr 6 or 7.

You can always get the volts by increasing the turns, that is never an issue. Don't go crazy with the turns, it's not realistic to cut in much below 6 mph and to try will spoil the whole result.

As a starting point I would use coils about 6 or 7mm thick. Try your experiments at about 100 turns per coil. When you have settled the turns, choose the thickest wire you can get in with the coils just touching.

With star delta you may get up to about 12A in a good wind.

best of luck

Flux

[Flux]

Steve, make that starting point 60 turns for starters, 100 will be to slow for the prop tsr I suggested.

Flux

[JW]

Stevesteve,

 Rotor(magnet) diameter is considered a good solid 'centripose' for a design allowing one, too experiment with 2 other major factors of 'urban turbine design'.

 But, there is no such thing as a 4foot(prop overall diameter) blading set, that will be quiet, at a useful rpm range band... to be in the useful rpm powerband range, it will make a chopping noise like a heliocopter. But this is highly dependent of what, the watt capacity desired from the machine is.

 Flux makes alot of sense with what he's sayin in regards to the 'magnet rotor diameter.

 The 2 other variables are related too separate aspect's, of a complete urban turbine. While, the whole time, the same machined parts can be used with different coil wire diameter's and bigger or smaller overall diameter props. Of coarse if you've have followed some of the interesting results and findings of actual experimentors who share there results with us. one could come to the conclusion thinner wire in the windings is better for high rpm "pmg' speed. since thicker wire wound coils, in fast rpm situations[regardless of magnet rotor diameter] is bad because it enhances voltage drop in the coils, since they get hotter faster. Sometimes 'multi-strands' of thinner wire are wound to give more current capacity while minimizing voltage drop due to hystersis,[ hystersis is dealt with relation too these problems, thru laminations in construction of the iron core's in conventional 'pmg's or dc motors].

 So the metal backing (magnet rotors) is at proper axis to reduce hystersis heating,(moving with the magnets) the problem arises at 90 degrees rotaton position location such as how the coils are inserted into this. (sp?)   fariday's principal?

 A larger diameter prop, will be quieter, since it can spin slow, produce torque without fast rpm, "and no gearing, direct drive is used instead". But this depends on what Flux was saying, about magnet spacing with the "given" mag rotor diameter, with relation to how to best use the copper related to all this.

To everyone,

 Is tsr related to torque and rpm for windspeed? just would like to be sure, on this, as to 'my own' curiousity. anyone?

Stevesteve does your pmg "pmg versa pma" use mechanical commutation to produce  dc voltage? Also what are you thinking in the way of bearings? for the magnet rotors.

JW

[JW]

Just would like to elaborate on this star/delta, S/D switch, etc buisness.

- any 3 phase stator with 9 coils wired in 'parallel' is called 'delta circuit'. It is not possible to have a 'center' tap with this wiring scnario. However, it is possible to have a 'center tap' with a wye/star circuit with, again 9 coils... Technically speaking star is designated as three AC output leads with the 'center tap' not being used. if the center tap is used(for example as a neutral) the 9 coil circuit[in the stator] is condsidered 'WYE' not 'STAR'. so a star wired stator circuit has 3 ac output leads, just as delta does. but a wye wired stator has 3 ac out put leads and one 'neutral' lead, a total of 4 wires. And a AC 'ground wire is something different as well. Generally the diode bridge ensures the AC output leads become DC output of only 2 wires, positive and negative at whatever volts and amp rating.

 So 'in theory' delta makes more amps(because its internally wired parallel) at low speed rpm of the mill, and acomodates the lower voltages such as 12dc volts. Star on the otherhand is considered series circuit 3 phase, and the diode bridge doesnt care either way. I think the idea behind the S/D switch is to make the most of the windspeed at any given time.

Just having some fun thinking about this...

jw

[stevesteve]

Flux,

Thanks for the advice, now going to rn with 2 x 12 magnet system. I will try from 60 turns upwards.

JW,

Thanks for your thoughts. I will bear them in mind.