Predicting alternator output.

[olvvlo]

Predicting the propeller output seems fairly straight forward , thanks to software such as Ed's blade design program.

And is fairly easy to match to an alternators characteristics once known!

What I am wondering is, has anybody collected data together which allows one to predict the size of magnets,

 wire gauge etc. and the expected output in watts, as well as the input in watts required to drive the alternator.

What I'm trying to say, is, if I know the wattage out from the propeller at a given RPM.  And the load is to be 12v batteries then, is there a way to work out the rest without the numerous trial and error tests , winding coils experimenting with different sized magnets different gauge wire , etc.  This must have been done hundreds of times, and so I'm looking for somewhere where this info has been collected?

[windstuffnow]

  Predicting alternator output is a difficult set of circumstances because there are so many variations to achieving the same output goal.  The easiest way is to build something someone else has built ( tried and true ) and work from there.  I did make up a general prediction spreadsheet based on very basic input that works fairly well but it doesn't include the details ( size of magnet, wire diameter etc.)  That spreadsheet is more for prediction how an existing alternator would perform.   You can download if from... http://www.otherpower.com/images/scimages/15/Alternator_windturbine_outuput_prediction.xls

  I started to put together a spreadsheet that would help predict the outcome of a given set of circumstances like you suggested but it will be awhile before that is finished.

  Unfortunately, if your trying something new then its still a bit of a "trial and error" event with the coils.

.

[Propwash]

Get a copy of Hugh Piggotts book from the Otherpower site. All the data and specs are listed for four different sizes of mills as well as plans to build them.

                          Kevin

[johnlm]

I've put together a spreadsheet (not real pretty but it crunches the numbers) that will give the output V (ac and DC), output resistance contigent upon wire guage and length per turn number of coils and a gustimate of output impedance.  It will calculate amperage into a battery at specified voltage, calculate IR loss in the windings, total power needed from the prop for the given output etc etc.  The key parameter (v/turn/HZ) that needs to be input is not calculated from magnet caracteristics and gap but is come by experimentally by testing a test coil with a given magnet at any rotational speed. The v/turn/HZ is the ac voltage produced per turn of wire on the coil and the HZ variable takes into account RPM and number of poles.  I have the spreadsheet set up to calculate both standard 3 phase and single phase outputs.  Over time from experimenting with all the various magnets I have used I have recorded the v/turn/HZ outputs at various gap spacings and have done the same with various small box fan motor and car alternator conversions I have done so I have a feel for about what to expect with the given magnets.  I also made a spreadsheet to calculate the lines of flux put out by any given magnet (from formulas found on a magnet supplier's web site), so I can compare the calculated lines of flux of a known magnet I have already tested to the lines of flux calculated for a new un-tested magnet to be able to approximately predict the v/turn/HZ variable to input into the first spreadsheet.  One day I may try to combine the two spreadsheets to see if I can predict output just by inputing the magnet type and size, number of poles, spacing, number of turns, size of coil, and wire size.  Unfortunately the spreadsheets are not in a nice enough format that would incline me to share them freely.  Using these spreadsheets I can look at a given motor, estimate the wire size and number of turns and know pretty much what output I can get out of it with a given magnet size and type I select.  Or know how many turns of what size wire I can rewind it with to get the needed output voltage and output impedance to match a given prop design.  I really need to run a bunch more experiments and get more data on dual rotor designs to quantify v/turn/HZ and graph the results or develop equations to define output v/turn/hz for various spacings for various magnets.  There are some additional variables not accounted for in these spreadsheets such as coil shape and size relative to magnet size.  Most all my experiments have been done with coil shapes and sizes that pretty much fit the magnet shape and size.

Johnlm

Johnlm

[olvvlo]

The spreadsheet program sounds great , for people experimenting on the fringe areas

of design and I look forward to seeing it when you're ready.

I do have a copy of Hugh Piggotts book and an extremely good book it is.  But still can't find information I need , as I intend to build a

12 bladed prop, 1.2m diameter.

The reason for 12 blade is to keep the noise as low as possible as i am experimenting with wind turbines in more domestic setting.

I have built many two and three bladed props in the past, but they generally make a lot of noise and go dangerously fast when I have neighbours

quite close.

And so I'm going to 12 blades, with a tip speed ratio of no more than two. Building the prop and alternator should be straightforward enough.

But where and getting stuck and need advice is :- Size of magnets, size and wire, number of coils , to suit such a prop !

So if anybody can give me information on the design of alternator that perhaps would suit this prop I would be be very grateful ?

I do have a bit of experience with making alternators and have a workshop with lathes etc.

olvvlo.

[Flux]

If you do want to go down the tsr 2 route this should be near:-

8 pole dual rotor magnets 2 x 1 x 1/2", discs 9".

Coils 100 turns , 1/2" thick, probably get #16 wire in.

I really don't think you need to go down to tsr2 to keep it quiet.

I have one of Hugh's 4 ft machines running and as long as it is kept on load it is virtually silent. That has a tsr of 5 nominal and falls with wind speed. 3 blade.

I have a 5 ft machine using 5 blades tsr 4.5 in the same area and that is totally silent standing below it, it is at about 30 ft high.

Both of these are far from silent if you let them run away rather than hold their speed down.

I am not convinced your 12 blade design will be silent if you don't hold the speed down to tsr2, that is why I have suggested a winding that will hold it down, although the same magnets and rotor would normally be good enough for a 6 ft machine at tsr6.

Flux

[Flux]

Should have added 6 coils , 2 series per phase, star connected.

Flux

[olvvlo]

Thanks Flux I probably will increase the tip speed ratio maybe 3 or 4.

The alternator details make total sense , and I guess you reckon 100 turns per coil opposed to say 80 and dual rotor would

 be enough to compensate for the low tip speed.

I guess I will build into the design , easy adjustment of the air gap to give a little more scope.

And perhaps eight blades instead of 12.

olvvlo.