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Talk:Review:Homebrew Wind Power

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Poor Engineering, and High Risk

On Jul 16, 2009, "John L. Bass" <jbass {at} dmsd.com>

The Dan's generator design is just poor engineering, and high risk.

The DanB "heavy duty" 48V stator from his web site says "This stator is wound with #15 gage wire. It has 9 coils and each coil has 105 turns in it. [...] The coils are wired in 3 phase star configuration [...] In our tests we believe this stator will be about 50% efficient at 1000 Watts. Sustained output above about 1000 Watts may overheat this stator".

There are easy ways to get generator and rectification efficiency up above 97%, so that almost all of the harvested wind power available from the prop ends up in the batteries. Burning more than half of it in the generator and rectification is simply poor engineering. I can provide detailed plans for building a cheaper and more robust MPPT generator that is about 97% efficient at design load.

The DanB coil form, requires about 6" per turn average, so 105 turns is about 53 feet with pigtails. Each phase with 3 coils, is then about 160 feet of #15 wire, with a nominal resistance of 0.003184 Ohms/ft, or 0.50944 Ohms/phase at room temperature.

The nature of the diode rectifier switching, combined with the precise battery voltage cut-in while charging batteries, blocks the multiple interphase current paths that would normally exist in 3-Phase AC generation and AC load systems. As such, the two phases with the highest voltage sum, will be the only two conducting current the majority of the time. Near the phase switching point, there will be two phases in parallel briefly. Because of this, the majority of the cycle will have only two phases conducting in series.

The simple equations are V = I * R, and P = V * I = I^2 * R

A fairly drained battery bank, will have a cut-in charge voltage +/- about a volt from the nominal 48V, and rise to around 56V when the bank nears fully charged. 1000W at 47V, implies sustained charge currents will start around I = P/V = 1000/47 = 21 amps for drained battery banks. The stator copper losses are then P = I^2 * R = (21^2)*(2*0.50944) = 450W copper heating losses, plus another P = V * I = 1.4V*21A = 30W losses in the rectifier.

With the copper buried in the vinyl ester resin stator casting, there is no way to cool the coils. The resin is a fairly good thermal insulator, even with ATH filler added. A 500W heater, buried in a resin insulator, is going to heat up way above 80C very quickly. Above 80C where the N42 and N50 magnets, just a tiny fraction of an inch on both sides of the stator, are permanently damaged and lose their field strength. By 80C, the resistance in the copper has increased about 23% or so, and the stator is now consuming (21^2)(2*0.68)= 600W in copper heating losses. The stator at this point is well into thermal runaway, and will destroy itself at rated power. The stator heating, also risks pushing the magnets well above their thermal limits as well. This is anything but responsible engineering ... when many solutions exist to keep the product at safe operating limits by design.

Many resins, heated to several hundred degrees F, will burst into flames that will be fanned in this case by high winds and rotor windage. Before this design can be declared "safe", the thermal runaway MUST be resolved by proper engineering, so that internal and surface temperatures in the generator do not exceed 170F at any time. There is high risk of structure and wild land fires using this product in high winds, when furling fails. Since the design violates various NEC and UL guidelines and standards, there is a high chance that even if a home owner convinced a code inspector to sign it off, that the insurance would deny damage claims since it is not a listed product. The broad litigation that will fall out from a loss of life, or major fire, will include anyone and everyone even remotely involved in the production, sale or promotion of this product.

I do not think giving this product such high marks is "responsible" advocacy for the wind power industry. This product has a high risk for loss of life, and major forest or wild land fires, if this design continues to be built and installed. The back lash from such a major loss, would be heavy regulation that is likely to kill the homebrew market by setting expensive regulatory and permitting standards that would damage the market for safe listed wind power products as well.

I tried to politely raise these issues last year in DanB's forums, and found myself lynched by the DanB disciples, with TomW leading. I was kicked off the forums with DanB's blessings. DanB has been warned better designs exist, and willfully has refused to listen to efficient designs that do not fail like this.

I do not need a wind generator in my back yard, but I will provide plans and guidance to anyone that would like to build a better, cheaper, efficient MPPT wind generator. A design that is much safer by sound engineering design principles, totally lacking in the DanB design. It's interesting that designs that are 97% or better efficiency, also may need less copper and/or magnet material. So, selling poor 50% efficient designs, has the side effect of increasing magnet sales, which is where the profits are.

I will be releasing a broader analysis of the problems in the DanB generator design in a few weeks, that may catch some DanB advocates and partners in the cross fire because of their glowing reviews. We need "responsible" advocacy in this market, that includes looking well past what is accepted practice. Everybody has been doing it this way for a long time ... and it's just flat wrong.

John Bass
Senior Engineer

See also

GENERAL

TYPES

SPECIFIC

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