coil and mag testing

[madkane]

i have to admit i am doing my gen build the wrong way ie gen first blade second to try and match but the other day testing the single rotor gen during coil testing i had a nice large coil about 40 mm in dia  on 1.25 mm cable putting out 6 to 9 amps  ant 1.25 volts  at a very close air gap  but just for a laugh i pulled the mags back to about 150 mm air gap and found .01 volt at just under the same amps  delivered from coil

if this helps i have a 266 mm disc 12 mags 40mm in size 5 mm thick

it was just an interesting observation volt way down but a little drop in amps

and when building gen trying to get 12.5 volts  my 2 mm cable give near 12amps at 350  and at 7 to 800rpm  35 amps with my lack of knowledge just seems like a very big blade needed to drive it

[Flux]

I think you are measuring short circuit current. Sorry it is totally meaningless, I just can't persuade people not to do it.

You can arrive at voltage from test coils but the short circuit current is not worth measuring. You can predict output from voltage and the measured resistance of the winding.

Your little thing with a single rotor and tiny magnets is not going to produce much and it will not take a very big prop to handle it. I should imagine this would suit about a 4ft prop but I have no idea of the resistance of your coils.

Flux

[madkane]

thanks for the input i will buy an acurate ohm meter as the one i have is just pants i take on board that shorting coils mean nothing in testing

so it would be a simple v/r = I and then *volts give power delivered or generated as heat

[Flux]

This is not so simple. Unless you get a milliohmeter with 4 lead Kelvin connection it will not be very accurate at reading the low resistances of the coils you need to be using. Normal multimeters are not much good below 5 ohms. You can measure by the volt and amp method with normal meters.

Assuming you choose coils for the cut in speed you want then for 12 magnets and the common arrangement you will have 9 coils ( 3 per phase).

 Two volts ac per coil will get you cut in at your chosen speed at 12v in 3 phase star with rectifier.

If you know the winding resistance  R between a pair of output leads ( 6 coils in series) then you can predict the current into the battery at various speeds.

At 3 times cut in speed you will have 36v dc open circuit. You have 24v above battery volts so in theory the current will be 24/R amps. In reality it will be much nearer 24/R*1.3 Amps.

The power lost in the alternator will be three times the loss per phase. Each phase will have a resistance of R/2 ohms. So power lost will be 3*I^2(R/2) watts where I is the rms winding current( using dc battery current will be near enough).

Flux

[DanB]

Like Flux has said, the ohm meter is useless for this.  He suggested running a known DC voltage into the stator and measuring current - that works well.  Another easy way to get close is to weigh the stator before you cast it and use wire gage charts that tell you the weight of a given length of wire and the resistance of a given weight of wire and work it out that way.