Predicting output watts

[reinoud]

Dear folks,

as you know, i'm working on a prototype windturbine with a dual rotor axial flux alternator using smallish magnets. The alternator is now getting in its final stages only the stator needs to be wound still but i have a few coils to experiment with. It is to have 15 coils in a 5-phase configuration; star propably.

The latest coil i tested has 300 turns of 0.35 mm wire and produces around 2.4v AC at 105 rpm. I hear a slight humming when i put a load on it and a small vibration can be felt.

Measuring over a 220 ohm resistor at unknown rpm (150-200?) I read 4v DC over the resistor and that would mean an amperage of just 0.0183 trough it due to Ohm's law. (Only have one meter). That would indicate a 73 mWatt consumed... not really much to even consider use full and i doubt if i could extrapolate it.

What is a preferred method of predicting/measuring feasable output wattage? or is its common to just build an alternator full and measure over some resistor at various rpm?

Thanks in advance,

Reinoud

P.S. wouldn't this make a good entry for the FAQ?

[SamoaPower]

How did you come up with 220 ohms for a test load? I estimate that a load of 5-10 ohms would come closer to matching the coil resistance. You may get 0.5 watt or so.

At that low RPM and small magnets, don't expect too much.

[dinges]

Reinoud,

Have you considered using stepper motors? This may be easier & cheaper, esp. for the lower powers you want.

That resistor of 220 ohm is a bit high. I'd give it a try with a 12V/.1A lamp, or 6V/.1A

You are using just one coil; I'm assuming you will be building an ordinary 9coils/12 magnets 3phase machine? If so, you get 3 times the AC voltage; in star, multiply by 1.7; As DC, you get 1.4 times as much as AC voltage.

Try calculating the internal resistance of your single coil and/or complete generator:

Measure unloaded voltage and loaded voltage (say, 8V unloaded, and 5V at .1A load)

R= (change in voltage)current

R= (8-5).1 = 30ohm.

The bigger & better your genny, the lower this internal resistance.

Anyhow, your 220 ohm resistor is way to high; it will seriously limit the current you can get out of the genny (in your case, 18mA). Try again with 5-10ohm, like someone else said. Or use a lightbulb, they also give a visual indication.

No worries, it'll take a while before you get the hang of designing and correctly sizing gennies, but it's definitely good fun (in my book).

Looking forward to new measurement results.

Peter (NL).

[johnlm]

You have not given much information to go on to help with any answers.  If you had  measured the coil resistance and posted that info I might be able to start calculating some kind of expected output.  However; to me 0.35mm diameter wire implies 27 guage which is really small lightweight wire to be making any alternator out of especially if you are intending to use the output at 12 or even 24 V.  With that small of diameter wire I suspect you will need to operate each phase in parallel similar to the Jerry Rigging technique to realize any useful output current.  Just a wild guess off the top of my head, but from the numbers you have given I suspect you will never get more than 10 to 20 W out of your design.  Also from the skimpy amount of info I suspect you will be looking at about a 2 to 3 ft diameter prop to try to keep the rotational speed up to hit some kind of reasonable cutin wind speed for even 12 V output.  It sounds as if you are using pretty insignificant magnets or else your gap that the coil resides in is pretty wide or something.  I havent gone back to try to find a previous post if you gave that info before.  Is this just a practice alternator to get the knowledge of building a genny?  At least you are making electricity so you must be having some amount of fun.

Johnlm

[dinges]

Oops, noticed just now; it's a 5phase machine. In that case, don't multiply by 1.7 but by 1.9;

Johnlm is right; a some extra info wouldn't hurt much. A picture of the coil & magnet would be also very nice. The more info you provide, the more & applicable feedback you will get. It also helps in getting answers as people see you are serious about it all. I've seen your 'propellor' (how does one call such a thing anyway?), looks like a lawn-ornament to me but hopefully you will get some useful power out of it.

[reinoud]

Thanks a lot for your feedback; I've made some additional measurements with my alternator loaded with a 6V light bulb.

At a speed of 160 RPM the unloaded DC voltage was 5.3V, the loaded DC voltage was 1.25V at an amperage of 170mA. This would indicate an R_int of 4.05V / 170mA = 23.8 Ohm :-S This yealds only around 30% is used and the rest is going up in heat. So much for performance.

The coil resistance I measured is 7.1 Ohm and yes the distance between the magnets is around 12-13 mm so pretty big. I'm trying to reduce it a bit but i dont expect much. The complete set will consist of 15 coils in star (or delta?) configuration. I'm thinking of maybe go for 500 turns of 0.10mm for with 0.35 mm it could get too big to fit.

I think that one of the main problems with this measurement is the fact that with such low voltages the rectifier is dropping the voltage too much. Should i compensate for this?

With regards,

Reinoud