2. is the arrangement I have developed. This uses combinations of capacitor voltage multipliers, in a 3 phase parallel arrangement. The simplest form is a capacitor voltage doubler in parallel with std rectifiers to the battery load, as in this circuit.
[ This circuit would suit a alternator that has a high cut in speed, compared to the circuit above that suits alternators with a low cut in speed. For example a Smartdrive reconfigured as a 2 pole SP would cut in at a much higher RPM than a standard stator. This circuits uses a voltage doubler to harness the low RPM power that would normally be lost. The circuit below uses the common 35Amp bridge rectifier instead of individual diodes ]
The capacitor voltage multipliers are placed in parallel with the main rectifiers, and there can be doublers and triplers and quadruplers etc. The capacitor sizing for each type is different, even though the voltage rating would be the same.
A quadrupler would have smaller caps than a tripler, and these would be smaller than a doubler. The power handled by each arrangement is different.
The caps effectively convert the abrupt loading charastic of a windmill connected to a battery, into one that the alternator is loaded in a manner that is proportional to the wind energy that is passing. The rotor speeds up and down proportional to the windspeed and the changing characteristics of the capacitors with windmill AC frequency and the combination of ccts, gives a varying loading with a close to cubic power relationship to doubling of windspeed. The choice of capacitor size determines how close to optimum the loading is.
The loading should be slightly underloaded so the blades TSR continues to stay just slightly above the optimum. This ensures a smooth response of the loading to the changing windspeed.
The calculation of capacitor sizing continues to be the most difficult task. I size the various arrangements this way.
The main rectifiers are sized to pass full rated output current of the windmill plus some.
A voltage doubler would be sized to handle approx 15-20% of this maximum power.
A voltage tripler would be sized to handle approx 5-10% of this maximum power.
A voltage quadrupler would be sized to handle approx 1-2% of the maximum power.
In the arrangement I have on my Dual rotor AxFx windmill. I have a voltage quadrupler, a voltage doubler, and series caps. I do not have a direct rectifier on the windmill to the battery. My system is 48V nom battery, and the capacitors on the quadrupler are 200VDC back to back 220uF, with 10A 200V bridge rctifiers. The caps on the doubler are 350VDC back to back 560uF, with 35A bridge rectifiers. There are 2 sets of series caps. One set is 350VDC back to back 820uF with 35A rectifiers, and there is a second parallel set of 180VDC back to back 1500uF with 35A bridge rectifiers. As it turns out, all the caps need only be 200V rated. My windmill is rated 1500W.
The reason I have series caps, is that my windmill is stalled when connected directly to the 48V battery.
In my arangement, the quadrupler passes approx 1A at full power, the doubler passes approx 5A, and the series caps pass 25A.
My rule of thumb is for a windmill of the same power rating, that for a halving of the system voltage, the capacitance is doubled. The capacitance is proportional to the power rating of the windmill.
The power handling of the capacitors determins the physical size required. The voltage usually determins the physical size of the same capacitance value. Lower system voltage does not dictate lower voltage caps. I would not use caps rated at lower than 180VDC in a back to back arrangement.
12V systems suitability is often asked. I have no data, but it should still work, but with a higher proportion of diode loss.
The best system is one where the alternator is wound for a normal battery cutin rpm of approx half the intended maximum rotor rpm. This allows normal rectifiers and a cap voltage doubler, and quadrupler. This reduces the effective cutin to approx 1/4. In this example, say the alternator was expected to have a 500rpm max rpm. The normal cutin would be approx 250-300rpm, so the alternator would be wound for this. The voltage quadrupler would bring cutin down to 60-70rpm. The output power would increase up to 120-140rpm, where the voltage doubler would start to contribute. As the rpm increased up to 250-300rpm, the power output would still be increasing, until the normal rectifiers started to pass current as well. All the time, each of the cap cct is adding its additional increasing power contribution.
The capacitance sizing determins the best cubic power fit. Normally, the blades will operate across a wide windspeed range with similar efficiency within a tight optimum TSR.
The range of windspeeds that the windmill can be expected to produce battery charging current is increased by a factor of at least 3. The alternator loading can be relatively easily tuned to the rotor externally.
The electrical solution replaces an electronic windmill MPPT system with only capacitors and diodes.
Gordon ( GWatPE )
To find out more about the cap mod, see the following thread in our forum....
Sourcing the capacitors.
It's highly recommended new capacitors are used for Gordons cap mod. Used capacitors from PC power supplies are usually not equal in value and will fail very quickly. The caps are available from the following suppliers, keen an eye out for specials/sales, and its also worth looking on e-bay.
http://www.farnell.com/ http://www.rscomponents.com http://www.rockby.com.au