Coil Size for 12-feet wind turbine

[bhatman]

Hey  Folks, Thanx a ton to all otherpower members for loads of information on simple homebrewn wind turbines. My 10-footer made out of Hugh Piggots axial plans is flying since last three months and its spinning away with lotsa amps and voltage. Now I am planning a 12-footer. I have N35 1'x 2'x 1/2" dual rotor 16 x 2 magnets. Planning a three pahse 12 coil stator in series/ star i.e. 4 coils in series per phase. Looking for a 24 Volt machine. What should be the wire size. Should I wind a test coil of 50 turns of single 17 AWG wire and see the results. I have plenty of #21 and #15 gauge wire with me in my cupboard. What should be the cut-in voltage. The wind speeds around my site is approx 7 m/s for almost 10 hrs a day. Alternatively should i go in for a 12 volt or 48 V machine. What is the main  factor for deciding to make a 12V or 24V or 48V machine. I am a little confused on this issue. Also I am into a some unknown arena about TSR. How do i decide what is the best TSR for my blades. I have read at many places that a TSR of 6 or 7 is quite acceptable. But how and why, I m just not able to comprehend.    

Bhatman

[Titantornado]

I hear ya bhatman.  I'm still uncertain my rotor blade diameter is properly sized to the alternator.  With all the extensive building of axial flux turbines here over the years, there's got to be a rough formula to determine a decent match.  Something like the amount of magnet material volume to rotor blade size.  I'm sure there are other variables, like coil mass that need to be taken into consideration.  Sure would be nice if a chart could be available to make a pre-determination of building one's own turbine, and cutting out a lot of the trial and error.

[Flux]

50 turns would be a good starting point for 24v but with your wind you may want to reduce the turns a bit. Perhaps with N35 magnets you had better stick with 50 turns at least for a test coil.

You will need far heavier wire for the final thing, 2 in hand #15 would seem a good starting point if that is what you have. I think you could get thicker in.

 "What should be the cut-in voltage"        24v DC   Did you really mean that or did you mean cut in speed?  With your wind I wouldn't aim for cut in below 140 rpm but others will tell you to go lower. With steady winds at 7m/s regularly I would raise it to 150 rpm. you know if your wind estimate is correct.

 "What is the main  factor for deciding to make a 12V or 24V or 48V machine. "

The voltage of your existing machine . If it is 12v I would think again, if it is 24 then that seems fine.

Tsr   Well that is a compromise. High tsr results in fast running and a smaller alternator for a given output. It means noise, blade erosion and critical blade profiles for good results. Low tsr is quiet, blade profile is not very critical, starting is never an issue and the low speed means virtually no erosion. It means a large and costly alternator.

A nominal tsr between 6 & 7 seems a good compromise. You will not very often work at the design point, at cut in you will be up to at least tsr 8. On full load you will probably be below tsr5. If you drop below 4 with a nominal tsr 6 blade you will be likely to stall in the winds that you have frequently ( one reason I suggested a higher cut in speed).

The high tsr blades are best left to the commercial manufacturers with facilities to evaluate clever profiles and produce them in volume. For home build you don't want the trouble, uncertainty and noise. Very low tsr such as the Windflower work well but the low speed rules out direct connection with a sensible cost alternator.

Flux

[jmk]

 I made mine with fifty wraps of #13 for 24 volts same machine.

[iamdewayne]

what would be a good starting point for wire size and turns if you were going for a 48 volt machine? ( same design )

[bhatman]

Thanx folks for all the valuable clarifications,

I actually meant "cut-in speed" and not "cut-in voltage"...blame it on a little mistyping. Actually my basic doubt about 12V, 24V and 48V power systems is to understand which one of these would be advantageous. I have been wondering that with a 48 V alternator, I have the option to charge three types of battery banks depending on the wind speed conditions i.e. 12V charging at lower wind speeds, 24V and/or 48V at higher speeds. Though I do understand that this will also require changing the type of battery banks as regularly as the wind speed variations. Also how is the efficiency affected due to heat losses at higher RPMs between 12V, 24V and 48 V machine.

Well to start with I have made a test coil of #14 gauge wire with 50 turns of single strand for a 24V alternator. The test coil results on a lathe at 140 RPM gave me 2.5 volts AC. Thus, in a three phase, 12 coils in series/star configuration, I should be getting 4 x 2.5 = 10 Vac. Thus the Voltage of the Phase = 10 x 1.73 = 17.3V.

Using the formulae Vac x 1.4 = Vdc, I get exact 24.22 Vdc, which in my opinion, is what I am looking forward to presently. This also means that I can start charging a 12 volt battery bank at 140/2 = 70 RPM (very low wind speed condition). Now is this flexibility an advantage or a disdvantage. I've just read the paragraph on battery bank voltage at http://www.otherpower.com/otherpower_wind_tips.html#cut and it has only confused me a little more.

I also checked the test coil resistance as 0.2 ohms. While calculating the output of the generator, Will the stator resistance wired in series/star total up to 0.2 x 4 = 0.8 ohms or is it 0.8 x 1.73 = 1.384 ohms.

Bhatman

[Lumberjack]

Dont forget to allow for voltage drop between the generator and the battery bank. For a 24 v bank you might want to plan for a 26 -28 volt system.

[dinges]

He'll need higher voltage than that;

Just to properly charge a '24' V battery, he'd need about 27.6V (at least) to 29V.

Add to this any losses due to resistance of the line.

If he wants to do the occasional equalization charge as well with his windturbine, the voltages would have to be even higher.