Homebrewed Electricity > Wind

alternators: does power potential need to be designed to match the intended load

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brandnewb:
I have 16 x 3.2V 280Ah Lifepo4 cells that I intend to configure in series.

The charge rating of the cells is 0.5C and the discharge rating of the cells is 1C.

I also have a wind charge controller that starts relaying power at 54 volts and hits the brakes (shorts the 3 phases) when the voltage reaches 60 volts.

Currently I am using a 0.4mm diam wire in the alternator knowing full well that this wire is too thin. I just wanted to showboat amazing open circuit voltages at 60 rpm a bit on another forum. I am aiming to replace the wires with 1mm diam wires and calculate how many winds i'll need if the target voltage should be a bit under 60 at average wind speeds we have here on this location.

But in general I am rather happy with what you guys have told me here already. It could mean that the brakes aren't constantly being hit on rather windy days, only that the battery array will get charged faster yes?

Adriaan Kragten:
Correct matching in between a certain generator, a certain wind turbine and a battery with a certain nominal voltage is the biggest problem in wind turbine design. You only can find the optimum number of turns per coil of the generator if you measure the generator on an accurate test rig for a large range of loaded voltages and if you know the optimum cubic line of the rotor of your wind turbine. The optimum cubic line is given by formula 8.1 of my public report KD 35. You have to use that voltage for which the Pmech-n curve of the generator is lying close to the optimum cubic line of the rotor. If this voltage isn't the same as the loaded battery voltage, you have to change the number of turns per coil. Another point is that the generator must have a certain maximum torque level which matches with the torque level of the rotor at high wind speeds. So if the generator is too small, it can be matched well with the rotor at low wind speeds but not at high wind speeds. I have tried to explain this procedure in an earlier post called "Design of a PM-generator for a wind turbine" of november 2021.

If you design a generator with a random number of turns per coil, it will have a certain open DC voltage after rectification at a certain number of turns per coil. This can easily been measured but the open voltage can't be used to determine the matching. The generator efficiency depends very much on the ratio in between the loaded voltage and the open voltage (see measurements given in my public report KD 78). You only get a high efficiency if this ratio isn't very small. If you use the generator to charge a battery, the loaded voltage is almost constant. This means that the ratio in between the loaded voltage and the open voltage is only high at rather low rotational speeds. So only at rather low rotational speeds, the generator will have a high efficiency. The efficiency will always be low at high rotational speeds even if the matching is optimal. For high efficiencies at high rotational speeds you need increase of the loaded voltage but that isn't possible for a battery load. If the wind turbine is connected to the grid by an inverter, this inverter accepts a large variation of the input voltage and so the voltage can be high at high rotational speeds resulting in a much higher effciency at high rotational speeds. 

brandnewb:
I am happy to admit that I am not in this game to seek for maximum efficiency.

All I care about is to make a turbine that produces usable power and that is easy for everyone to copy paste.

So Adriaan, I am still not advanced enough to fully grasp your in depth comment, to which I am always grateful for of course!

Would you mind boiling it down a bit? I am looking for asnwer more along the lines of
*) no, the alternator needs to be perfectly fine tuned to your battery array. (opening up a whole world of hurt)
*) you can get away with some mismatch as the amps will do some of the finetuning while in operation.

Adriaan Kragten:
The point is that if you don't know the optimum cubic line of your wind turbine and if you don't know the characteristics of your generator, combining both for a certain battery voltage is purley a gamble. The best thing you then can do is measuring at which wind speed the generator supplies the open battery voltage (so when the rotor is running unloaded). If this happens at a wind speed of about 3.5 m/s, it might work if the generator isn't very small with respect to the geometry of the wind turbine rotor.

It surprises me how much time, work and money people spend in building wind turbines and generators and how little time one wants to spend in aquiring the knowledge to make the proper choices. Without the knowledge given in at least my public report KD 35, it is all try and error. If you have designed something which is far away from the optimum, it always means that you have a high investment per kWh of generated energy. A poor overal efficiency makes that much more material is used and some wrong choices even make that the whole thing isn't working at all. The world has limited recources and people should not use these resources for something which ends at the rubbish dump.

brandnewb:
Dear Adriaan, I have the utmost of respect for you and all you have done for the field.
If only I could ever be as knowledgeable as you are.

But please understand that I am on a mission, and that mission does not involve complex aspects.

It should be simple. Big turbine, Big alternator output some usefull power.

I am more than willing to debat the fine points once I have my post apocalyptic turbines spinning and producing power.

And in answer to your question why there are still people that invest blindly into a path that seems a dead end.
Well I speak for my self and in my scenario it is rather simple. Money and time is what I have in abundance. Knowledge and skills to learn more I am rather limited at. SO here is me going all in with a simple goal;

Big turbine, Big alternator => output some usefull power.

I my self find it extremely hard if not impossible to wade through the more advanced research papers and for me trying any longer does more harm than good. As in it will grind me to a halt while if I just plow through stupidly then at least I am able to get some results.

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