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Forum Index : Windmills : visual effect of capacitors
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Gill Senior Member  Joined: 11/11/2006 Location: AustraliaPosts: 669 |
For those wanting more on the runaway phenomena here are some figures of it I collected whilst doing my seriesed back to back cap tests. As Herb stated, runaway starts around 300rpm though I have been able to stave that off to 700rpm by slow measured increases of applied power. It was further noted no runaway occurred when the 470uF caps were tested though rpm rose to 519rpm. 15KB .zip file contains 110KB .xls file. 2009-01-04_124501_TEST_Data.zip was working fine... til the smoke got out. Cheers Gill _Cairns, FNQ |
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| KiwiJohn Guru  Joined: 01/12/2005 Location: New ZealandPosts: 691 |
Adding capacitance across the phases provides a path for reactive currents to flow during that part of the cycle when the voltage is below battery voltage. Reactive currents cause IR and Fe losses. |
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andrewf Newbie  Joined: 24/12/2007 Location: New ZealandPosts: 15 |
yes, they do, your exactly right.. About 10% loss of efficency.. but an almost 200% increase in usable power. EVERYTHING SHOULD BE AS SIMPLE AS IT CAN, NOT SIMPLER - EINSTEIN |
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| woodchips Newbie Joined: 05/01/2009 Location: United KingdomPosts: 27 |
Hello, just found this forum, and otherpower, and what can I say but wow, amazing. Landed on this thread and had to read it all through, took 135 pages of printout though. Whilst I have read it all through I haven't really become familiar with it to know that a similar post has been made elsewhere, have to start somewhere. I have been looking at wind generators for some time, but only theoretically, not popular things in the middle of a housing estate. We also have a small wood where all power is currently from photovoltaic cells, but not enough in winter. I have just bought a Rutland 913 and was going to run some bench tests on it before putting it up a mast, and out of reach. I have the following comments: Bench testing is exactly comparable to real wind testing, and much more convenient. OK the mill speed might not vary unless you program the variable frequency drive, but it is irrelevant as to how the mill is rotated. The ability to set an exact speed, and then fiddle with any other parameters allows for efficient, not necessarily quick, characterisation. The back to back connection of polarised capacitors, typically as shown on page 22, is a real no no. The capacitors must have parallel diodes to stop them becoming reverse biased. A quick look through the Philips data book gives a maximum reverse bias on wound electrolytics of 1V, or one diode drop. As shown there is no point in connecting the caps in series, you might as well just use one cap and get double the capacitance. No diodes will result in the caps exploding one day. Many of the oscilloscope traces show spikes on the leading and trailing edges of the waveforms. Much of these are probably caused by the use of slow switching speed bridge rectifiers. Has anyone tried using the soft recovery diodes to see if the spikes dissappear? Spark quenchers are not the solution. Power factor is mostly irrelevant. The alternator will produce an output power factor as required by the load, if inductive then lagging, if capacitive then leading, the battery is resistive so unity. This raises an interesting question about the series caps, are they a load or not? Not certain, will have to think about that some more. Not convinced about the assumption that a 24V battery will clamp the voltage at 28V, or whatever. If a higher voltage is applied then the battery voltage will carry on rising, until it melts. There is no chemical voltage limiting process in a lead acid battery. If you charge a lead acid battery at more than 0.1C (tenth of Ah rating) then there are real problems with the speed at which the lead sulphate can be converted on each plate. Something never mentioned is iron loss. I assume the alternator poles are laminated to reduce this, but the loss increases as to the square of the frequency. To give some idea what this means, a vehicle alternator has an efficiency of less than 20% at high revs, and this is almost all down to iron loss. Hysteresis loss increases linearly with frequency, copper loss doesn't change. Enough for the moment, let's go and read some more posts. Bob |
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herbnz Senior Member Joined: 18/02/2007 Location: New ZealandPosts: 258 |
Hi Bob Welcome I see from your post You likely have similar views to me. I would suspect a Training in Electrical Engineering correct ? Herb |
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| KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691 |
Welcome to the Back Shed Bob. Bench testing is not quite the same as real world testing, you might have noticed where I commented on that, I said we can simulate a battery bank or even use real batteries in the bench test but the bench drive (i.e. lathe) does not simulate the actions of the wind turbine. For example, if the blades are overloaded at low RPM we can actually get more power by reducing the load, momentarily, and allowing the mill to spin up to a more effecient RPM, how is that simulated on the lathe? I agree with your comments regarding back to back connection of capacitors. Soft correction diodes would be nice but most of us will first turn to whatever is on hand, becides, as far as I know R-C snubbers ("spark quenchers") have been used for about a century to arrest the sudden rise of voltage resulting from the sudden interruption of current flow in an inductive circuit. |
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| woodchips Newbie Joined: 05/01/2009 Location: United KingdomPosts: 27 |
Hello all. Herb Yes, spot on. Used to have a nice business designing and manufacturing electronics measuring systems, but the EU doesn't want that any more. One foolish regulation too many and I can't make a living, so find other things to do. Play with the digger in the wood, alternative energy, lots to keep me interested. KiwiJohn A test bench can simulate anything. By measuring the speed and power into the alternator then from calculations on the blade output charateristics the stalling can be identified. Of course it isn't that simple, the characteristics of the blade are probably unknown. Because the electrical characteristics are so much easier to measure then probably working backwards from alternator to blade is better. I have bought a commercial alternator tester, but it is huge, the size of two wardrobes and nowhere to put it at the moment. So it is stuck on the drive, under a tarpaulin, on a housing estate, ummm. This has a 7.5kW variable frequency drive motor, so should be interesting to get it running. Think I have caused confusion with the diodes. Every cap must have a reverse parallel connected diode for their own protection. Using soft recovery diodes as the main rectifiers is a separate option. The spikes are not due to the inductance, they are due to the slow switching speed of the diodes, on or off. Briefly when a diode is forward biased then it will start to conduct, but there is a delay between the application of the forward biased voltage and the diode current starting to flow. For rectifiers intended for simple 50/60Hz operation then this can be microseconds. This is much more easily seen on a decent, Tektronix, analogue oscilloscope, digital scopes tell awful lies at times. New to these forums, seem to find my brain is four sentances ahead of my fingers, result = gobblegook. Bob |
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GWatPE Senior Member Joined: 01/09/2006 Location: AustraliaPosts: 2127 |
Hi bob, some new blood. If you can find a diode with a lower forward bias than the caps, I would agree with you re back to back caps. The crossover voltage for the caps reversing polarity is < <0.1V. There are no diodes in Electrolytic NP caps either. Gordon. PS Edit: The capacitor type issue is likely to keep coming up. I have performed some simple tests on the DC electrolytic caps I have available for testing in the back to back arrangement. I have found many different types of behaviour. first of all, high voltage types, 450VDC @ 470uF, of the type Bryan started with. These behaved like a non polarised cap. They charged up similarly when voltage was applied in both directions. When reverse biased, the voltage dropped slightly faster. I measured with a high impedance multimeter. The drop was , approx 0.05V/sec forward biased, 0.2V/sec reverse biased. There was <0.01A current draw at 30VDC either bias. I tested 2700uF @ 160VDC with similar results. I tested 470uF @ 180WV, of one type I am using. These had current of 0.05ADC when a reverse potential of 30V was applied, and a current of <0.01ADC when 30V normal bias was applied. These caps get slightly warm at the high power levels. I tested power supply type caps, 10000uF @ 100VDC and these had almost a short cct when reverse biased, and an open cct forward biased. I tested many more types, with results falling between these extremes. There are obviously large differences in construction with electrolytic marked DC caps. some appear to be like NP types, others well anything seems possible. Bryan and I may have been lucky with the caps we are using. It would seem that for testing, at least, that if there is any significant current when the cap is reverse biased, then it is probably unsuitable. This will make it difficult if sourcing on EBay, etc. The final cct should use NP type caps, of suitable AC ratings, both voltage and frequency. become more energy aware |
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| Don B Senior Member Joined: 27/09/2008 Location: AustraliaPosts: 190 |
Hi Bob, I would like to add my welcome to this forum and thread, and a few comments. First regarding bench testing, fixed speed testing only lets you investigate what happens at some particular shaft speed. A proper rig would need to simulate the dynamics of a wind generator by having a control that would give a power limit that followed the cube of the shaft speed. I would need to think even more about a curve for the torque input, which would be a function of the shaft speed, and the simulated wind speed. The available torque would need to be continually varying, to simulate the effect of fluctuating wind speed. Ideally, there would also need to be a factor for the inertia of the rotating parts, and the increased aerodynamic losses at higher blade speeds. A rig that could do this would let you investigate stall, runaway, and optimising power output from any particular alternator, load, and circuit combination. Regarding the back to back connection of polarised electros, I believe that you need to start from the point that it has been demonstrated to work in this application, and has been working in a few installations for some time. The voltage rating of the caps used is also conservatively high, which presumably helps. It may be that the "diode effect" of the capacitor with reversed polarity, or perhaps the fact that it is then fully discharged, might be part of why this arrangement works. It would certainly be interesting, however, to see whether this duty results in any long term capacitance value or ESR degredation (or cap failures). Regarding the voltage "spikes", I suspect that this is just part of the way that this circuit works. If you look at the current traces that Gordon took in his post of 31-12, these are what seems to be the normal discontinuous sine wave forms of a load supplied through a bridge rectifier. What the voltage waveform does to achieve this current flow is immaterial, so long as the components are not overstressed. I also don't think that the current trace shows any energy "leakage", as suspected by some, as there would have to be associated current flow. It is also interesting to note that the end of the current peak for that phase coincides with the end of the "spike". Presumably, this also coincides with the beginning of the "spike" on the next phase?? The power factor, or, if you prefer, the angle between the phase voltage and current, is important in beginning to understand how this circuit arrangement works. If the series capacitors offset the voltage drop across the alternator reactance, then their effect is to lower the circuit impedance, and more current and power will flow. If the power factor is raised to 1, then the impedance will be a minimum, maximum power will flow and, incidentally, the condition for series resonance will have been established. In this circuit arrangement, I think that the "Q" is low enough that the benefit extends over a reasonable spread of shaft speeds, and hence frequencies. I also think that this is where the theory for the use of shunt capacitors fall down, as this tends to create a parallel resonant situation, and increases the circuit impedance. Lastly, on the matter of batteries, in a simple alternator, bridge rectifier and battery circuit, the battery does clamp the alternator output voltage for all practical outputs. My visualization of a battery equivalent circuit is a voltage source and a resistance in series. Ohms law would suggest that the battery resistance seen by the charging circuit is the battery voltage divided by the battery current. As the charge current increases, the apparent resistance of the battery decreases. While I don't disagree that extreme charging rates could cause a battery to ultimately rise above its charging voltage range, in the short term at least the voltage will hold at some point. A common effect of this is that the wind generator shaft is consequently held at a speed below its maximum power capability for the wind speed at that instant. This is, of course, dependent on many factors associated with the number and diameter of the blades, and the alternator characteristics. As you will have noted Bob, this forum has a place for a considerable spectrum of opinions, and you will no doubt have spotted several holes in my comments. I would welcome a response from you (or any others), as this is what a forum is all about. I believe that a real benefit has been demonstrated (particularly in higher voltage systems) from the use of series capacitors, and I think that understanding the mechanisms by which this benefit is obtained will help to permit this to be optimised, leading perhaps to squeezing just a little more of the free stuff out of mother nature. Regards Don B |
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SparWeb Senior Member Joined: 17/04/2008 Location: CanadaPosts: 196 |
Hi Don, I've been trying to sort this stuff out, too, for a while, so thanks for that great explanation of the point of view that the capacitors are correcting power factor. Is there any chance that you also understand Gordon's contrary point of view well enough that you could explain that as clearly? 
(PS Gordon, thank you for trying to explain so much before, but I must be thick or something... I just don't follow well enough, so I am asking for a fresh opinion.) Steven T. Fahey |
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| Don B Senior Member Joined: 27/09/2008 Location: AustraliaPosts: 190 |
Hi Stephen, I am not suggesting that the additional power necessarily comes from correcting power factor, although that may well be a significant part of it. There is also the likelihood that judiciously selected capacitors can improve the coupling between the generator and the load such that it can output more power at both lower and higher speeds, or to a range of more than one load as Gordon is doing. In order to understand how capacitors do this, it is necessary to know the relationship between the voltage and current in at least one phase (as Gordon has recently recorded). Ideally, simultaneous voltage and current recordings in all 3 phases would permit more ready analysis. Hopefully, it will be possible to get this additional detail at some time. Regards Don Don B |
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oztules Guru Joined: 26/07/2007 Location: AustraliaPosts: 1686 |
Non polarised AC capacitor is simply two electrolytic (matched) capacitors back to back. The positive terminals are poking out of the can, and the two negatives are built in conjunction with the electrolyte inside the can. This works perfectly well with two capacitors in separate cans. Simply join the two negatives together and enjoy. Diodes are a waste of time and money, and do nothing if in circuit, although, if a diode should fail, it will blow the opposing cap up. On the alternate cycles, the "plates" (for want of a better word) charge up to potential, and the alternate oxide strips act as blocking diodes to protect the opposing capacitors oxide layer. The small asymetric leakage keeps the solution charged negatively and it is this that ultimately protects the layer from reverse current eroding the oxide before the charge builds. This stops a redox reduction which would remove the insulating oxide layer,and ultimately shorting the thing out .... blowing it up. What if they are unmatched??? The smaller value of the two will dictate the capacitance, and the larger capacitance will form a slightly thicker oxide layer to compensate, until they are matched. ..........oztules Village idiot...or... just another hack out of his depth |
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fillm Guru Joined: 10/02/2007 Location: AustraliaPosts: 730 |
Hi Oztules Great explanation on Capacitors  , it would definatly make reading and understanding a lot of what is said on this thread if we could have a gosslary of terms , eg - the PFF , MMF , etc , that are argued over so often and that some of us can be a little more enlightened on what goes on when a magnetic field cuts through copper/iron and what the capacitors are trying to capture ( I realise the last bit of that might be a bit hard to aggree on ) .. I definatly would like to be able to understand the BIG PICTURE a little more , may be there is some links to other sites that you and the brains trust know of to help us novices get a better picture .. I then probably would not have to refer back to my "Star Trek" manual so often  PhillM ...Oz Wind Engineering..Wind Turbine Kits 500W - 5000W ~ F&P Dual Kits ~ GOE222Blades- Voltage Control Parts ------- Tower kits |
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| GWatPE Senior Member Joined: 01/09/2006 Location: AustraliaPosts: 2127 |
Hi oztules, there is an aspect of using polarized caps that is still unanswered. The DC rated caps I tested of high voltage rating appeared to behave like Non polarised types. This is probably why when Bryan measured two back to back DC caps, the capacitance was half the individual rating, like you would expect with non polarised types. Other caps appeared to be a short cct when reverse biased. I would expect that if these caps were placed back to back, then the effective capacitance of the combination would not change from the marked value on an individual capacitor. I expect that placing diodes across the caps has the effect of maintaining the capacitance at the marked value, with the additional power dissipation of a diode at the cct current to achieve it.. I suspect that some capacitors of a type that I measured that came from a power supply might benefit in this arrangement with diodes as well. The chemistry of a particular cap may dictate the cct components to make it work. I suspect that if there is a requirement for large capacitance, then diodes might help. Bear in mind that these diodes will be required to handle the full, possible resonant, currents that can occur. This will probably add confusion to cap sizing. The caps on my mill and Bryans have been in operation for months. I have spare caps. that I am able to check with those in operation, to see if any changes have occurred. I could do this if needed. Gordon. become more energy aware |
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| oztules Guru Joined: 26/07/2007 Location: AustraliaPosts: 1686 |
Gordon, and Phill, I was about to write a long laborious thing on electrolytic s, when I stumbled onto this entry in wikipedia. It should answer all your questions.... and throw the diodes out Gordon, they wont do as you suspect I'm afraid. There is still a lot of incidental stuff not in this article, but it is a good start. (like the magnetic fields created during the electron build up etc, which can induce hysteresis in the Al and asymmetric leakages associated... but I'm weak on that also)) I am still trying to fine some stuff on fieldlines (difficult at the best of times) regarding this also. ULR did some excellent stuff on this, which I took full notice of. wiki entry .........oztules Edit... and I think Herb did an excellent thing on MMF.. and I can't find that either. In understanding how the caps actually work, it is clear any extra current gained, comes from the stator getting a better MMF and so EMF. Herb has explained this (and I have given my take on it as well) I still think that the synchronous impedance decreases as well through a decrease in leakage reactance as well as armature reactance but I haven't sold Herb on this ... yet. Village idiot...or... just another hack out of his depth |
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| herbnz Senior Member Joined: 18/02/2007 Location: New ZealandPosts: 258 |
'Synchronous impedance' I Know now you been reading text books this will really confuse the natives. No if leakage flux is as large as some ppl think we will have inductance and therefore impedance (inductive reactance )that will cause pf but no one shown this yet all speculation by Flux. armature reaction is the term books give to what I see this term actually combines a lot of complex going ons in the rotor flux and phase shifts etc. found on fieldlines post No 59397. Ruddycrazy posted BTW I have lots problems with back to back caps too , when one cap is charging +ve surely the Mr electron must flow out of negative of that one must enter the -ve of the next cap . My trials on the bench have shown Ac on both caps. I dont realy want to get into this I personally will not use them I have my own bank Ac caps Scored from old teaching days. Herb Ps 6 Jan Note further on p26 Oztules has shown that manufacturers approve back to back Herb |
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| woodchips Newbie Joined: 05/01/2009 Location: United KingdomPosts: 27 |
Hello all, thanks for the welcome and feedback. Wife suggested that getting into arguments with long serving thread contributors wasn't the best way of winning friends and influencing people, better not read on then..... I don't see the point of keeping quiet, we all know different things, if I can help then why not try. So, to continue with the application of polarised caps in AC circuits. As I said, Philips give a maximum reverse voltage of 1V, so any diode can be used. Back to basics. The internal structure of an aluminium electrolytic is that the positive foil is anodised, aluminium oxide, with this becoming the insulator. With a reverse voltage applied this anodising is starting to be reduced, in fact being transferred to the other foil. An AC electrolytic has both foils anodised so this damage, the drift of the anodising between the foils cancels out. It is possible to completely reverse an electrolytic by reverse biassing it with a current limiting resistor. Damage to the insulating layer is most easily detected, and measured, by measuring the forward leakage current at varying voltages, with it exploding when it gets near the working voltage for serious cases. The bit in Wikipedia about the foil strips acting as diodes is rubbish. A diode requires all sorts of clean room processing of silicon wafers with precise amounts of impurities. A bit of aluminium foil it isn't. Yes, I have heard about selenium and copper oxide rectifiers, but they are both relatively low voltage, you need a stack of 20-30 for 240V. If you have supporting evidence that this is true then I would be very interested to read it. The AC capacitors work because of the anodising film on both foils. But this makes for a terrible capacitor, figures I have come up with, (both books and a Chinese manufacturer) give a tangent loss angle of 0.15. This means that they are rated for about 3 minutes in every hour before exploding. The fact that back to back caps have worked, mostly without exploding, is a tribute to their quality of manufacture, not to the quality of design. Every capacitor must have a diode connected across their terminals, cathode to positive, anode to negative. This allows one cap to charge through the other cap's diode on one half cycle, and swapped on the other half cycle. This is required to stop the caps exploding. To explain why this is important consider the oft used 470uF, charged to 100V gives a charge, Q=CV, of 0.047 coulombs. This is 0.047 Amp second. Power is VI so 4.7W in one second, but the explosion will only take 1ms so this becomes 4700 Joules of energy (1W = 1 Joule / 1 second). Your safety boots are designed to cope with 200 Joules, so it is more than 20 times as much. Do you really want that right in front of your face? Oztules comment that non-polarised caps are simply two electrolytics connected back to back is wrong see above. To repeat, the anodising on the film is polarity conscious, reversing the polarity will result in failure. AC electrolytics are only intermittently rated. Diodes are not a waste of time and money, they are what, in the end, protects your eyes from an explosion. To take this further, the proposed back to back capacitor has 200V across it, which means each series capacitor has 100V. In the middle is a piece of aluminium film, connected to nothing. So what is charged negatively? If these series caps are identical to standard polarised types, then this supposed diode will be easily detectable by testing any electrolytic. Unfortunately nothing can be found, because they don't exist. Go and test one, increase the reverse voltage on a cap and measure the current that flows, it will be an almost straight line. Try it with a diode and it won't be a straight line. So the last question is, why do they appear to work? Mostly quality manufacturing with a significant amount of luck. Don't forget that aluminium is extremely reactive with oxygen, it forms an oxide immediately, hence the problems of soldering. This means that whilst the positive plate has lots of oxide, to act as an insulator, the negative plate will also have oxide simply because you can't get rid of it. It is this negative plate oxide that is keeping it all working. Some makes and types simply have more or less of the oxide, and hence either will withstand the reverse voltage or won't, bang! Enough, have to fix the central heating boiler. Bob |
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| herbnz Senior Member Joined: 18/02/2007 Location: New ZealandPosts: 258 |
full support and agreement here Bob. I said initially we have seem to have similar backgrounds, I even have a digger to but no central heating. Your explaination of AC caps the best Ive seen I sort knew about the oxide layers on both plates but not well enough to debate on here. Dont let anticipated comments put you off I just ride with the blows here.In fact usually get stoney silence. Herb Ps It appears the opposition to back to back caps is approved by manufactures as proven by Oztules in following posting p26 A definite win for Oztules Herb 6th Jan |
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| SparWeb Senior Member Joined: 17/04/2008 Location: CanadaPosts: 196 |
I have found All About Circuits to be an excellent learning tool. All About Circuits www.allaboutcircuits.com There are many pages about AC circuits, broken down in logical steps that build up to a "big picture" understanding that has helped me immeasurably. It even has worked examples for those who miss the heady days of university and college! And!... those who have found AAC in the past and read through, you should go back because many things have been changed and added over the years. (Sorry if it sounds like work for them or something, but when I find something good, I like to tell others about it.) Steven T. Fahey |
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| fillm Guru Joined: 10/02/2007 Location: AustraliaPosts: 730 |
Bob, Welcome to this site , you have given an in depth discription of the workings of the capacitor , knowing the basics of how something works sometimes starts to clear the fogg .. I hope I get this right in what you are saying is that it is safer and better to make our non polarised caps from two polarised back to back , so then with my tests I used 6 x 400vac 50uF caps. To duplicate the test if I was to use 450vdc 100uF x 12 and back to back they would they be simular and be rated at 50uF 225vac . The max ac unloaded voltage I read was 130vac @ 571 Rpm in star , does the saftey margin on the cap V need to be 3 times the expected max.. If this is so it definatly brings the cost of using caps down as higher uF Ac rated are expensive , which brings me to another question , are all dc caps the same or is there a certain type that are more suitable ?... PhillM ...Oz Wind Engineering..Wind Turbine Kits 500W - 5000W ~ F&P Dual Kits ~ GOE222Blades- Voltage Control Parts ------- Tower kits |
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