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Forum Index : Windmills : Power Calculation
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Tim_the_bloke Senior Member  Joined: 15/11/2009 Location: AustraliaPosts: 105 |
I have been thinking on how to calculate the power my windmil generates. Now I know power in watts is amps x voltage. I can measure the current in amps with a meter between my rectifier / contoller and the batteries being charged. But with voltage should I measure? Is it the voltage difference between what comes out of the recifier and the voltage of the batteries? I could place a meter across a diode in the circuit on the way to the batteries. Is my theory correct? |
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| davef Guru  Joined: 14/05/2006 Location: New ZealandPosts: 499 |
If you want to know how much power is going into the battery just measure the voltage across the battery and the current flowing into it. If you want to measure the power the windmill is capable of delivering I would suggest the following: - every minute and for a short period of time switch in a resistive dump load, disconnect the battery, measure the current, re-connect the battery and finally disconnect the dump load. To do this will require some rather complicated switching devices. You want to ensure that the windmill is always loaded. Others may have further useful comments. |
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| Tim_the_bloke Senior Member Joined: 15/11/2009 Location: AustraliaPosts: 105 |
I want to know how much power is going into the batteries. So, it does not matter how high the voltage gets that the windmill generates? |
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| VK4AYQ Guru Joined: 02/12/2009 Location: AustraliaPosts: 2539 |
Hi Tim A generator voltage is load and speed dependent, a generator connected to a 12 volt battery generating 20 amps as its amp limit will be around 300 watts charging output. The same generator spinning faster will charge a 24 volt battery at its amp limit of 20 amps and will produce charging output around 600 watts. The speed changes the voltage and the wiring limitation controls the amps to some degree. As Davef suggests the restive load will demonstrate the voltage possibilities of a generator, but can overheat and burn out if high output is produced. All the best Bob Foolin Around |
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mac46 Guru Joined: 07/02/2008 Location: United StatesPosts: 412 |
Tim_the_bloke, Simply hook up a amperage meter and observe the reading, you already know you'r approx. voltage...simple math will convert this to watts if thats what you'r asking for, or let you'r batteries rest for a perriod and do a specific gravity test on the battery cells, this will give you the "state of charge" of the batteries. Knowing what you'r "load" was over a given time, and how long the batteries charged with the resulting cell test results will tell you approx. how many watts were returned to the batteries. The differance of what you do have, vrs what you should have, will tell you how "healthy" you'r system and batteries are, a big discrepancy or differance, means theres a problem someplace. I hope this helps... .....Mac46 I'm just a farmer |
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| Tim_the_bloke Senior Member Joined: 15/11/2009 Location: AustraliaPosts: 105 |
So any voltage over the batteries is wasted? Umm, I suspect there is a problem. I am chargeing 12V batteries. Testing with a meter shows never more than 2amps going into the batteries from the windmill, even though it spins fast. I wish I had more measurements (like is it getting hot up the top of that tower?) or knew more about what it is meant to produce. My windmill is mongrel melding of several different Chinese models. Hopefully I will get time to arrange and post up some images for you all to comment on (under a new topic). |
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| mac46 Guru Joined: 07/02/2008 Location: United StatesPosts: 412 |
Tim_the_bloke, (so any Voltage over the batteries is wasted) No this is not true Tim...the batteries will "clamp" or keep the generator voltage down to thier levels...or at least close to their levels. Probably already checked this but make sure all connections are clean and tight, What is the size and age of the batteries, how are they hooked together, I'd especially look the ground connections over. Specific gravity tests on each battery will help narrow down if its the batt. bank, or one battery in paticular, or the system, or the generator. Mac46 I'm just a farmer |
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| Tim_the_bloke Senior Member Joined: 15/11/2009 Location: AustraliaPosts: 105 |
What ground connections? 3 phases into the rectifier/charger, +ve and -ve to the batteries. Should I connect the -ve to ground too? BTW: I did started another topic with images of my windmill. Responses make me think perhaps I need bigger blades to produce more power. |
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| mac46 Guru Joined: 07/02/2008 Location: United StatesPosts: 412 |
Tim, The connections I was refering to was the battery terminal connections. .....Mac46 I'm just a farmer |
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| Tim_the_bloke Senior Member Joined: 15/11/2009 Location: AustraliaPosts: 105 |
Ah, thanks for the clarification. Those connections are good. I just think the windmill doesn't produce much power. It has been good fun getting it up and working, and looks impressive, but disappointing after all the effort. I avoid telling my family how little electricity if produces compared to the solar panels I also installed. Perhaps the next stage should be a new F&P windmill to replace the Chinese one. |
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| mac46 Guru Joined: 07/02/2008 Location: United StatesPosts: 412 |
Tim, As VK4AYQ has stated is you'r other post..."The blades need to run very fast to make useful power". You may want to check on a faster blade set, the other possibility is trying to gear the generator up somehow, but I don't see how this would be done without a complete rework of the mill caseing. .....just my thoughts. .....Mac46 I'm just a farmer |
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| Tim_the_bloke Senior Member Joined: 15/11/2009 Location: AustraliaPosts: 105 |
By faster blades do you mean similar blades but out to a larger diameter? |
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Tinker Guru Joined: 07/11/2007 Location: AustraliaPosts: 1904 |
Generally, blades arranged with a larger diameter spin >slower< than those with a small diameter. Consider the pirouette of an ice dancer  , this basic principle applies to windmill blades too. So if you want the thing to turn faster fit shorter blades with a fast profile.Klaus |
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| Tim_the_bloke Senior Member Joined: 15/11/2009 Location: AustraliaPosts: 105 |
I would need some convincing of this. It would be true if we were running a motor of fixed power. However, I would have thought larger diameter in a windmill means more energy captured out of the wind, leading to faster spin. |
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Gizmo Admin Group  Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
Good argument Tim, but while there is more power in the larger diameter, there is another limiting factor, tip speed. The blades of a turbine are designed to run at a certain speed into the wind. There is a sweet spot, where they are making max power. Any slower and the blades loose power and start to stall, any faster and the blades loose lift, going into a dive, as it were. So when we go bigger diameter, the blades will still travel at that sweet spot, which is the same speed as it were for a smaller turbine. Bigger turbine, more distance to travel at same speed, lower rpm. Its not the RPM, its how fast the blade is traveling through the air. TSR is the term ofen used, or Tip Speed Ratio. 5 to 7 is just nice for a efficient turbine, meaning the tips are traveling at 5 to 7 times the wind speed. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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