AC coupling of windmills

[GWatPE]

Hi Flux,

I have been testing AC coupling of windmills to battery loads.  Ruddycrazy[alias Bryan1] was fortunate to AC couple his windmill and found a marked increase in output power in higher winds.  [approx doubling of output current]  These initial tests results were disbelieved.  I had replicated the outputs on my windmill in the same configuration.  These were F&P permanent magnet alternators, with a high pole count.  There has been discussions related to this gain being a result of lower leakage flux in this capacitor arrangement, resulting in more flux acting on the coils etc.  I believe there is more to it, nothing to do with free energy though.

The capacitors on a wind generator debate has a new direction to follow, that of wind energy power tracking.  I have adapted multiple capacitor/rectifier configurations to give increasing loading with mill rpm.  I have recorded benefits to any windmill, not just high pole count machines like a F&P.  The capacitors in the simplest configuration boost top end power.  Modifications of the basic cct give voltage doubling, and even quadrupling etc, that allows a lower cutin rpm.  I had relied on electronic boost maximisers to perform this function, but simple capacitors and diodes work better.

I have published schematics on TheBackShed forum.  I have a working cap setup on my mill, where the same windings simultanously charge a 24V battery and power a 48V grid connect inverter, with no electronic DC-DC converters.  The mill was designed as a 24V system.  The addition of capacitors initially increased output from 280W, to 605W, and now this same mill, without changing any mechanical components has produced 890W.  I am awaiting some more test data from a typical chinese windmill of 1000W rating that is being trialed with a capacitor doubling cct.  

My aim is for a windmill to produce current to the load if it is spinning, and for this current to be proportional to the wind energy.  I have restricted my testing to battery systems.

There is no doubt that series capacitors will play a bigger role in the difficult task of maximising windmill outputs and matching machines to a load.

I do not believe there is any limit to what alternator is used in a windmill, for a benefit to be seen.  The low cutin benefits are additive to the normal output.  

I have measured an increase in output of 300%.  I would not consider flux leakage alone was sufficient to explain this.  There are probably benefits with blade efficiency and transmission.  

The voltage doubling function of the basic design, would benefit any windmill.  I am sure any windmill owner would benefit from a boosting cct that did not rely on electronic DC-DC conversion or Microprocessor control to achieve an increasing loading system for a windmill, that was able to be set to very closely match the wind energy, with little more than capacitors and diodes and interconnecting wiring.

Do you have any problems with this information being posted here?  I am presently testing a quadruple system that I believe will make a MPPT for a windmill of the electronic switching design unneccessary.  

Basic gross mismatching of windmill to load may still require transformers etc.  I have published a lot of my findings on TheBackSed, but thought this forum may also be interested in these latest findings.

Gordon.

[Flux]

Gordon

I have no idea why you directed this at me. I have no say in what you do or do not post on this board.

I have followed some of this stuff in comments to Oztules and at least for the F & P machine it makes sense. I can see why it may be of benefit.

As to your wider claims about any machine then at present I have doubts, I can't see any basic reason for there to be wonderful improvements with axial machines that will make results comparable to voltage conversion mppt possible. If you carry out your tests and come up with credible figures then you will be entitled to that view.

I don't believe that capacitors boost top end power ( or any other power) at best you can only change the machine characteristics to better match the prop power curve. If you can do that and maintain alternator efficiency then you have achieved your aim.

Not having any access to F & P motors I am really in no position to comment but their characteristics normally are very poor for wind power. I can see perfectly good reasons why cancelling the inductive reactance will increase the output over normal operation and if this also reduces the slope of the characteristic in low winds then I can see good reasons why you may get a good match.

The same trick may work with some other iron cored machines but probably with much increased capacitor values. At present I can't see any possible way it be useful for axial machines unless the pole number is drastically increased as the capacitor values will be very large. I don't share your enthusiasm for huge banks of capacitors, the values needed for the f & P are bad enough.

I don't have sufficient large banks of capacitors to even try it on an axial machine at present but I can see that the falling reactance with frequency may be useful in reducing stall in the mid wind region. In the case of a machine of this type whose characteristics are dominated by winding resistance rather than leakage reactance I really don't see any great benefit.

Flux

[Sly]

Hi Gordon,

I tried to find your schematic on thebackshed but can't seem to find it. Please point me in the right direction. (moderators if this is not acceptable let me know and I will take a different approach to find it.)

My site conditions are like so many i.e. low winds most of the time followed by good winds for a few days then the cycle repeats itself. Hence I have been looking at various approaches for boosting. After discussing with Flux I think I might have something to experiment with but would like to see how you go about it.

tks

sly

[imsmooth]

Gordon,

What values (mfd) were you using for the F&P?  You connected the caps in parallel with each phase?

[Tritium]

I think this is the page with the schematic referenced:

http://www.thebackshed.com/Windmill/FORUM1/forum_posts.asp?TID=1138&PN=1&TPN=15

It took me two days of reading to find it.

Thurmond

[tanner0441]

Hi

I have a Chinese Turbine, Albeit AC it seems to be a very low frequency, in fact I can't remember ever seeing 50/60Hz.  The sums tell me I need very big capacitors.

I don't dispute it could work on higher frequency alternators, Do the capacitors run hot at all? It is the thought of a polarised capacitor constantly being reversed. Also you mention gains of 300%.  X size wire will handle X amount of power, as in fuses, how do the coils handle the extra work.  There are people on here able to explain the physics better qualified than I am, but I remain sceptical, I have read lots of posts on this site (going back years) regarding the use of capacitors, I cant understand why it has never been done before. As flux states I can see various advantages under some conditions, but not generaly across the range of alternators.

Sounds wonderfull but with the gains you quote, also a bit like a free lunch.

Brian

If it was this good why don't car manufactures use it and reduce the size and copper content in their alternators?

[imsmooth]

What I would like to know is for a fixed quantity of work energy for turning the generator how do the capacitors affect the output.  One needs to distinguish between reclaiming wasted reactive wattage versus requiring more input at a given rpm, giving more output.  With a test bench and the ability to measure input amperage and voltage one can determine how the capacitors truly affect the output.

[Sly]

Thanks Thurmond much appreciated. sly

[oztules]

The bench test will do as you say, but on a mill, you get extra tsr because in this case (Gordons) it well mimicks mppt tracking, so the prop can get about doing it's own thing... like it should (but rarely does with stall limiting.... unless you have too much resistance in the stator.... then it's - away we go!)

You don't get to see this advantage with a bench test.

...........oztules

[hmccoy99]

I would like to see some real tests

wind speed

dia of turbine

rpm

rms power out

load voltage

load current true power not reactive power

with caps

and a test without caps with the same wind conditions or rotor rpm.

same load

Henry

[GWatPE]

Hi imsmooth,

The cap values for voltage doubling as in the schematic on the link are approx 100uF for my 100S F&P in delta.  I have additional components in similar arrangement directly from the output phases to the normal 3phase battery rectifiers.  These are 470uF.  The capacitor sizing will be determined by the amount of power that you select the cct to transfer.  This can be calculated quite easily if the frequency and capacitor size are known.  The impedance of the arrangement can be determined and this coupled with the load voltage, will allow the power to be calculated.

The combinations that work on a chinese 1000W windmill to reduce the cutin for a mill that spins but produces no current are 470uF.  These are connected as in the voltage doubling arrangement.  These values would work on an axial flux mill in the same way to reduce cutin voltage.  A mill that sees a wide range of winds, where a lot of the time just spins, without reaching cutin will benefit.  

The real benefits come if the mill is wound for a mid range wind cutin.  The low wire resistance, coupled with the low cutin and power matching ability offered by the capacitor combinations in the voltage doubling and quadrupling arrangement allow the mill to be matched more accurately to the wind power curve.

There may be many capacitors and rectifiers needed but connecting would be well within the abilites of windmill RE enthusiasts.  The requirement for the windmill loading to be increased in a way to follow the wind energy curve is well reproduced by the capacitor arrangement, without fancy microprocessor control of DC-DC converters.  All of the components are located close to the battery in my arrangement, with only 3phase AC coming from the mill.  The mill output reaches 40VAC or more at the higher power levels.  I have theories as to what is happening, and it seems as though the energy that is passed through the capacitor function is added to the normal current that is transferred by the rectifiers.  No doubt this will be explained by someone in the future, with more testing.  

I see no reason that capacitors need to handle the full power of a windmill.  I would suggest that the voltage boost function would be more than adequate for many windmill RE enthusiasts needs.  

On my windmill, before the addition of a capacitor/rectifier arrangement, power output was restricted to 280W.  This same windmill has now produced up to 890W.  I have made no changes to the blades, or system voltage, or furling, etc, but now my system benefits from increased power output with a lower cutin and aditional power at the top end.  

I cannot predict the exact benefit you may obtain from your windmill.  This is an arrangement that can be placed in parallel with existing rectifiers, so power output would not decrease.  My system has benefit at the low and top end of windmill output power.  

As I have said before, I suspect that the benefits are a combination of alternator and rotor matching improvements to the wind power, so wind energy harvest efficiency is improved.  

Gordon.

[dnix71]

I haven't taken electrical engineering courses since college a long time ago, but I do remember that impedance matching is key to efficient power transfer. Any builder of audio speakers or power amps knows how tricky it can be.

http://en.wikipedia.org/wiki/Impedance_mismatch

is a good place to start.

http://en.wikipedia.org/wiki/Maximum_power_theorem

discusses the choice between maximum power transfer versus maximum efficiecy in a system (like a motor driven by a battery).

[GWatPE]

Hi Henry,

I wish it was this simple to get test data.  I do not have the facilities to simultanously record all these parameters.  I would suggest that in the first instance that you add the doubling function configuration  This only operates when the windmill is normally producing no power.  For a 1kW mill this may amount to up to 50W instead of 0W.  This all adds up over the course of a day, if the windmill is not in an ideal location.

Gordon.

[imsmooth]

Ok, Gordon, I put my test bench together to do a preliminary study.  I have a rectifier box that I made that will read DV volts and amps.  I mounted a F&P 80s with a standard ceramic magnetic rotor.  I used a variac with an AC volt readout and I used an AC ammeter to record input volts and amps.

First, I ran the system without caps into a 50 ohm load.  At 50v x 7.4A (370W) powering the motor the generator put out 205v x 1.7A (348W) for a 94% conversion.  Please take these numbers in a relative sense.  I really don't think I was getting this efficiency, and I am comparing AC watts to DC watts.  With approximately 200uf AC capacitors in series with each phase I got 410W input and 396W output (96.6% conversion).

I then ran the motor faster.  Without the caps 703W in and 667W out (94.9%).  With the caps 741W in and 720W out (97%)

So far I can see an improvement when I compare power put in to power put out.  However, the gain is small.  I did not try low RPM, yet.

As far as the theory as to why this works:

I use hard-start caps for my air conditioner to lower the amperage requirement for my backup generator.  The caps are in parallel with the motor winding.  Caps in parallel increase the impedance as one approaches the resonant value, which lowers the net current.  This is good when trying to reduce power.  For the generator I have the caps in series with the inductor.  A series arrangement reduces impedance as one approaches resonance and this will increase current, which is good when generating power.

I will try difference cap values and low, medium and high rpm settings and get back to you when I tabulate the results.

[GWatPE]

Hi Thurmond,

I am new to the syntax re what can be posted on this forum, with links etc.  The link above is the schematic for the voltage doubling function, to lower the mill cutin rpm.  This is only a start to what is possible.  In this instance the rectifiers are still normally connected.

Gordon.

[gizmo]

Hi Jonathan

Do you have a 100 series F&P lying around? I know Gordon achieved the best results with a 100 series, the 80 series only gave little gain, as your results have shown, and a 60 series isnt worth converting to series caps. The 100 series has the lowest inductance be far.

Dont be too surprised with F&P efficiencies over 90%. 80% is considered the norm for a throw together machine, higher with a little tuning to the blades, series caps, etc.

Glenn

[hmccoy99]

Gordon,

The results seem to be machine dependent, I would like to try it on a AIR 403 but I have yet to understand the variables to deterimine the capasitance value.

is it inductance?

pole count?

frequency?

that determine the capasitance value

Henry

[Flux]

I don't think you are going to do any good with bench tests unless you plot a complete load curve for the alternator over the windmill speed range and you do the tests into a battery of the intended voltage.

This is not entirely a question of electrical efficiency. It is far more related to the aerodynamic efficiency of the prop. With the F & P there is also an issue on high load with leakage reractance and a conventional rectifier will not let you load it effectively when the reactance becomes significantly greater than the resistance. Series capacitors neutralise this reactance at resonance and the machine behaves as though it is purely resistive. This ought to correspond to your prop peak power point at furling.

You said your efficiency figures may be off, in fact I suspect they are way off, but comparing electrical efficiency into some sort of resistive load is not going to be useful.

For those wanting to try the doubler circuit on an axial machine it is no good trying the same machine at the same voltage. If you use a 12v machine and run it at 24v then you will probably get a decent result as long as you can find big enough capacitors. Starting from scratch it would be better to use heaps of smaller magnets and get the frequency way up.

If you use a machine at the same volts you will gain nothing, most axials are wound for low wind cut in already and for this reason they are inefficient at full load ( deliberately). If you take a 12v machine and run it at 24v the I^R losses are reduced to 1/4 and the full load efficiently is raised very significantly and will mean that the load curve falls fairly well on the prop curve in high winds. There will be a big increase in output mainly from the removal of stall of the prop but partly due to the significant gain in electrical efficiency. This is the basis of the argument I proposed long ago in that matching the load diary entry.

The issue with using a 12v machine at 24v is that the low wind performance is messed up. Star/delta type schemes help a bit but are rarely in the right mode to suit the wind and usually make the alternator too powerful in low winds and induce stall.

I proposed a boost converter with current sensing to deal with this low wind end and it certainly works . Bergey used it on the XL1 in a different form. Everyone else seems too afraid of electronics to try it but I believe one or two have had some sort of try.

The capacitor voltage doubler has often been suggested for machines that don't reach charging volts in low winds and I have been doubtful as they were proposed in single phase form and the capacitor values to do anything reasonable for low frequency alternators are very large.

Gordon seems to have modified it to 3 phase which is immensely better. If you can live with the capacitor values and can get the values right to track the prop curve then it may be a practical alternative to a very simple small and reliable boost converter. If you can make it work you will be able to manage a very considerable increase in output in high winds and you solve that old stator heating problem.

The F & P by nature of its long iron poles is a strange case and in its original form it is pretty bad, I have little doubts that the series capacitors will drastically improve its characteristics on full load. If you can combine this with a scheme to reduce stall in the lower winds then you gain even more.

I think you need a different approach for the axial compared to the F & P and any bench testing needs to be very carefully done to mean anything.

I hope someone follows this up with a large pole count axial.

Flux

[imsmooth]

Yes I do.  I will have to get to testing this later.

[imsmooth]

I think I need to take a step back and calculate the inductive reactance of the coils.  Many may think this is hard because simply placing an inductance meter won't duplicate the conditions when the generator is spinning, which is correct.  However, there is a formula using a shunt resistor that will enable one to accurately determine the resistance and inductive reactance of an active coil; and, using this information, one can determine what capacitor is best for any desired frequency of operation.  If anyone wants the derivation I can fax it to them or respond by email.

What one does is connect a resistance, Rshunt, of known value in series with the inductor.  The inductor impedance is comprised of two values: Xr and Xl.  Xr is the real resistance, and Xl is the inductive reactance. Make sure the shunt resistor  can handle sufficient wattage. Now connect a variable ac voltage source to the RL circuit.  Connect an ammeter so the current can be measured.  Connect a volt meter across Rshunt-L which we will call V2; connect a volt meter across the inductor (two leads of the three phase generator), which we will call V1.

Slowly, increase the variac.  I increased mine so I had 1 amp flowing through the circuit, since this makes the math easier, but you can use any values as long as they are big enough to reduce measurement errors.  I got I = 1A, Rshunt = 98.5ohms, V1 = 5v, V2 = 104.5v.

The formula: Xr = ((V2/I)^2 - (V1/I)^2) / 2*Rshunt

Knowing Xr one can calculate the inductive reactance:

Xl = sqrt((V1/I)^2 - Xr^2)

Xl = 104.3ohms

Knowing Xl, one can calculate the inductance, Xl = 2*pi*F*L

[GWatPE]

Hi henry,

Does the Air403 have a 3phaseAC output?.. I think these have DC output.

Gordon.

[Flux]

Normally what matters for alternators is LEAKAGE reactance. What you are measuring is normal inductance. The figure for a good alternator will be way higher than the leakage reactance. For things like the F&P there may be much less difference as it is inherently a dreadful alternator ( deliberately as it was designed as a synchronous motor with electronic commutation).

Probably the only way to get at the leakage inductance of a permanent magnet alternator is to measure its dc resistance and then to do a load run into a short circuit. At first the current will rise with speed and then it will level off to effectively constant current. The knee in the curve is where XL equals R. From frequency ( speed and poles) you can determine L.                

Flux

[hmccoy99]

Hi Gordon,

Yes it is 3 phase, wye..

[gizmo]

You really dont like the F&P do you.

Its cheap, maybe $20 2nd hand, or can be bought new over the counter for less than $200.

Its rugged and reliable. Never heard of a F&P buring out or flying apart.

Its easy to retrofit to a turbine or adapt to any system voltage.

Its efficient, pony tests show over 80%, though yes there are some iron losses.

Old problems like cogging have been solved, and the efforts of Gordon and others have increased efficiency and output to better match the turbine, both low end and high end.

There are hundreds, if not more, of them used in windmills and hydro applications all around the world, totaly reliably and successfully.

So how do you define a "dreadfull alternator"? I would call it a very successfull alternator!

Glenn