What should a testcoil read?

[tomloods]

 dear all,

I am in progress of building the Piggotts 3 meter turbine.
First some info

rotor diameter 3,4 meter
2 Stator disks 300mm x 10 mm thick
12 magnets per rotor NEO  N40  46 x 30 X 10 mm
9 Coils  1,7mm (about awg14), 57 turns 2 in hand, star connection.
estimated TSR of blades =7
3 blades
Coil thickness 17mm
Coil leg with 30 mm

I would like a cut in speed of about 165 rpm. If i put a testcoil on the lathe and turn 165 RPM on the stators, how much should one coil read?
24 volts +1,4 (rectifier) / 6 coils = 4,2 volts open circuit? correct?

Some more doubts:

1)Coil leg with is 30 mm like should be as per Piggotts recipe book. seems a lot to me. when one magnet is just on top of one coilleg another magnets starts to be above the first leg... somewhere i read voltages wil cancel each other out... (see pic)
2)Coil thickness is 17 mm + 1 mm for fiberglass cloth + 2 x 1,5 mm airgap =21 mm. compared to magnet thicknes of 2 times 10mm.  slichtly above max. i read a lot of theories that space between magnets schould be 1,5 or sometimes 2 times the magnet thickness... what is correct?
3) coil legs schould be parallell (shoulder to shoulder). although I made coil litlle tapered (30 mm top width and 25 mm bottom width) the coils wil not lay parallel... could that be a problem?

Thx in advance for replies/advice. will keep u posted about progress but waiting for my stainless steel studs for the bearings.
Tom

[Flux]

For 24v dc, rms line volts = 24/1.4 or 17.14v

Phase volts rms = line volts/1.73 or 9.99v

This phase volts is divided amongst 3 coils so we have 3.3 volts per coil.

I have ignored the diode drop but the waveforms are not likely to be ideal and will cause more error than ignoring the diode drop.

I would have thought the cut in at 165 rpm was a bit on the high side unless you are in a good wind area for 3.4m.

The rest of your concerns are personal issues for which there are no right answers. Hugh has done a lot of work on these designs and I see no real point in making changes unless you are fully aware of the consequences. Everything is a compromise, there is no correct coil shape.

Regarding the stator thickness it affects the flux density in the gap, within sensible limits it doesn't make much difference except for the number of turns. By doing a test coil you are in effect settling your winding for the flux density of the magnet set up during the test.

I would be tempted to keep the stator a bit thinner but make that decision before you do your test coil.

The trade off is for a few less turns with a narrower gap but with less winding space and possibly having to use thinner wire, as long as you don't stray too far from something sensible it becomes a trade off that is beyond my maths to predict, again I see no reason to do other than Hugh suggests and for that matter I have no real reason to doubt his number of turns either unless you want to change the cut in speed. I see no problem with your coil shape with 30 wide at the outside and 25 at the inside, it's a compromise I would probably make myself. Just choose a wire size so that the coils virtually touch with your chosen number of turns from the test coil.

Flux

[tomloods]

Thx for ur reply flux,

The generator will be used to charge a battery bank of 880 Ah (together with 2,7 kw solar). Especially during daylight i Expect the batteries to be at about 25 volts. So i guess that will raise the cut in RPM's. I will try with the testcoil that is made now at 55 windings and see what is does. Most likely i will have to try few more turns to lower cut in speed. Any idea how much more turns i have to make to lower cut in from the 165 to about 150 rpm?
The line from windgen to battery bank is more or less 40 meters and i have wiresize of 6mm². guess that big wire will not add any resistance to the line... Is it correct that i could lower the cut in speed by adding resistance in the line? (lost power but...)
From the equations you gave for the testcoil i can determine my cut in speed? See what RPMs i need to have 3,3 volts on testcoil to have 24 volts output?
Once i made the test I'll post results...
Grts
Tom

[Flux]

The number of turns is directly proportional to speed. If you have the number for 165 rpm and want to reduce speed to 150 you use 165/150 x the original turns.

You will have to reduce wire size to get many more turns in.

40m of 6mm2 wire is going to be significant even at 24v so I doubt that you will have stall issues.

No you can't lower cut in speed by adding line resistance, that is determined by the number of turns, but if you mean that you can use a lower cut in speed and use the line resistance to avoid stall then that is correct. So much depends on your wind speeds and when you capture most power. If you have consistantly high winds you will do better with a higher cut in speed to get more top end power. If it is a low wind area you will be better off with regular lower output for more of the available time.

Flux

[tomloods]

thx Flux