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Author Topic: Wind mill generator pulley ratio help  (Read 1927 times)
demodriver
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« on: November 07, 2011, 10:51:41 AM »

Ok I'm building a wind mill to run a alternator that will charge my battery bank while boondocking. 

Does anyone know how I can figure what my alternators rpms will be compared to the windmill.  I currently have a 11" pulley on my prop and a 2" pulley on my alternator. Can someone tell the correct formula to figure the ratio?

Thanks
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AndyG
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« Reply #1 on: November 07, 2011, 11:17:02 AM »

Should be 11:2 = 5.5:1  If the blades are turning 100 rpm then the alternator should be running 550rpm. 
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rv_safetyman
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« Reply #2 on: November 07, 2011, 11:46:00 AM »

My gut feeling is that you will need quite a bit more than 5.5:1 ratio.  You will have to look up the performance curve of the alternator to see what the output is at various RPM.  Recall that an alternator on a gas engine is overdriven by at least 2 times and usually closer to 3 times.  So, at an engine idle, say 600 RPM, the alternator is going at least 1200 RPM.

Depending on your propeller, you will have to be careful of "tip speed".  I know nothing about propellers, and more specifically what you are using.  That said, I suspect they are rated for maximum RPM and you will want to make sure that you do not exceed that value in a windstorm.

Jim
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Jim Shepherd
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« Reply #3 on: November 07, 2011, 11:49:38 AM »

Pulley And Belt Information Calculator

http://www.csgnetwork.com/pulleybeltcalc.html
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Jeremy
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« Reply #4 on: November 07, 2011, 11:50:50 AM »

Be aware that automotive alternators make very poor generators for wind turbines (and of no use at all in wind mills!). Much better is to use 'normal' permanent-magnet DC motors, and radially-wound motors are even better.

There's lots of very good advice on building wind turbines on the 'net - suggest you do some reading before before starting construction.

Jeremy

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« Reply #5 on: November 07, 2011, 12:01:10 PM »

You will be better off and have a better system buying yourself a 3.5 hp knockoff type engine from HF to power the alternator some alternators have to spin at 1/2 speed before it energizes the charging.

We had a 400 watt windmill on the boat completely worthless and very noisy   
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demodriver
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« Reply #6 on: November 07, 2011, 12:50:06 PM »

Be aware that automotive alternators make very poor generators for wind turbines (and of no use at all in wind mills!). Much better is to use 'normal' permanent-magnet DC motors, and radially-wound motors are even better.

There's lots of very good advice on building wind turbines on the 'net - suggest you do some reading before before starting construction.

Jeremy


Ive did alot of research and most do think that a alternator wont work but I have saw it work first hand. I just need to make sure that I get enough rpms out of it.


Clifford if all else fells I have a 3.5 horse briggs that I can use to turn the alternator if need be.  I would rather try to use "free" power tho.
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demodriver
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« Reply #7 on: November 07, 2011, 12:52:17 PM »

My gut feeling is that you will need quite a bit more than 5.5:1 ratio.  You will have to look up the performance curve of the alternator to see what the output is at various RPM.  Recall that an alternator on a gas engine is overdriven by at least 2 times and usually closer to 3 times.  So, at an engine idle, say 600 RPM, the alternator is going at least 1200 RPM.

Depending on your propeller, you will have to be careful of "tip speed".  I know nothing about propellers, and more specifically what you are using.  That said, I suspect they are rated for maximum RPM and you will want to make sure that you do not exceed that value in a windstorm.

Jim
  I am making it all from scrath, if it fails I will only be out my time. Where the wind mill will be mounted theres nothing for it to hurt if it self destructs.
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TomC
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« Reply #8 on: November 08, 2011, 07:44:27 AM »

I had a friend make a windmill for his hunting cabin.  Not sure what he used for the fan, but did say he had to use a jack shaft to up the rpms.  If you duplicate another 11" down to a 2"  then you'll have a total of 11:1 ratio.  Then at 100rpm, your alternator will be turning 1100rpm-which will get some juice out of it.  With most alternators capable of standing up to 6-7000rpm, I wouldn't worry about over speeding the alternator-the fan would probably come apart before that speed was reached.  Good Luck, TomC
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Tom & Donna Christman. '77 AMGeneral 10240B; 8V-71TATAIC V730.
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« Reply #9 on: November 08, 2011, 08:54:56 AM »

  Better to waste time reading and calculating than wasting time trying things that wont work well, which you then have to start over.

  Belt drive is less efficient than almost anything. Best is small 90 degree angle drive with an overdrive ratio. You have to determine average annual "mean" wind speed you expect, and calculate how large prop you'll need to drive to provide the energy HP required by the generator and angle drive at that selected wind speed, select proper gear ratio, and have some means of over-speed shutdown. Personally I think anytime a big blow is coming you'll want to pull the plug on it and stop the prop.

  The old Jacobs farm mills were 36 volt DC, and used some common rear axle differential, model T maybe? The reason to vertical mount the gen below an angle drive is you dont have to worry about wire wrap. 
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Jeremy
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« Reply #10 on: November 08, 2011, 09:17:42 AM »

[Pedant mode on]

The Jacobs farm devices were wind pumps. Devices with electrical generators attached are wind turbines. Neither are wind mills, despite that term being used exclusively throughout this thread.

[Pedant mode off]


Jeremy


PS. I've just had to Google the term 'jack shaft', as I'd never heard it before. So I have learnt something myself as well as annoying everyone else.


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« Reply #11 on: November 08, 2011, 09:21:23 AM »

Tom C - slight error in calculation.  It is 11 divided by 2 (roughly) and that is about 5.5 overdrive.  

My issue is not over-speeding the alternator, rather not getting enough speed.  I looked at the performance curve for my Leece-Neville alternator (auxiliary for the house batteries) and they do not produce their rated current until they are rotating at about 3000 RPM.  At 2000 RPM they are producing about 1/2 their rating and very little at 1000 RPM.  That is why car alternators have at least a 2:1 speed-up ratio - so that the alternator is producing reasonable current at idle.  The curves I have rate the alternator at 8000 RPM max.

If you stick with your current ratio, you would need to have the prop going over 500 RPM (3000/5.5) and that seems fast to me.

Jim
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Jim Shepherd
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« Reply #12 on: November 08, 2011, 11:15:26 AM »

I would start with the propeller, the size requirement to generate the required power (power need is probably pretty low) and speed ability per expected wind speed.  I would figure that out, the drive requirement falls easily out of that.  Mind you I have no clue how to start figuring out power ability and speed ability.

Brian
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« Reply #13 on: November 08, 2011, 02:47:37 PM »

[Pedant mode on]

The Jacobs farm devices were wind pumps. Devices with electrical generators attached are wind turbines. Neither are wind mills, despite that term being used exclusively throughout this thread.

[Pedant mode off]


Jeremy

  Pumps pump water, generators produce electricity. Jacobs may have made pumps (i believe those are a different manufacture), but I was specifically referring to Jacobs 36 volt DC current wind powered generators. Wind turbine is a modern term, Jacobs didnt call them wind turbines in the 1930's that I'm aware of.
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TomC
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« Reply #14 on: November 08, 2011, 03:36:04 PM »

Jim-maybe you mis understood.  I said to install a jack shaft that will give you two 11" down to 2".  So 5.5 ratio times two is 11 to one overdrive.  Good Luck, TomC
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