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Forum Index : Windmills : visual effect of capacitors
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andrewf Newbie  Joined: 24/12/2007 Location: New ZealandPosts: 15 |
No offence taken or intended. I just did'nt want to hijack your thread, i had misunderstood its intention. I've been quietly tinkering away with this problem for the best part of 2 years now, and belive i have some insights on it. It is my belief that the improvement seen by installing caps, comes down to improving the power factor of the system. My apologies to those who this is too simplistic, but i did'nt want to loose anyone, and i dont want to do the math today. Power factor is the ratio of the real power flowing to the load, in relation to the apparent power. REAL power is the actual work being done in a particular time, and apparent power is what you get if you measure current and voltage and Multiply them. In systems that are just resistive ( say a light bulb ) real power and apparent power will pretty much be the same. ( ie a powerfactor of 1 ) This is because the voltage and the current are in sync with each other ( ie, when the voltage is zero, the current is zero, and when the voltage is at its peak, current is also at its peak ). In systems such as a generator, real power often is much less than the apparent power. The inductive components of the mill ( the coils ), cause the current to "lag" the voltage. Its not uncommon for the PF of a generator to be around .7-.8.. In the case of the F&P. If the PF was .7, the apparent power is 1.4 times the real power. Current flowing in the circuit would also be 1.4 times the curent if the the PF was 1. Now heres teh crunch point. Loses in teh circuit are doubled, as they are proportional to the square of the current. In the case of the FP mill, the loses are significnat, i expect could be as much as half the power avaialble! ( don't take that as gospel, i need much more data than i have ). Linear circuits ( such as an induction motor on the mains, or perhaps the most common example, flouresent lights) can easily have the pf 'corrected' by a passive network of capacitors ( or inductors ). This is done by placing capacitance to create enough reactance to counter the reactance created by the inductanace. The net effect is that the current lag or lead is dragged back into or as close as possible to the voltage, giving you a PF of 1. The F&P setup probably can't be considered linear unforutantly. For starters the frequency of the AC, is best described as 'wild'.. the frequency is proportional to the rpms of the mill. theres a lot of other factors as well that are non-linear. ( i've just ordered a book that describes a modeling system for generators, i might get a better idea one day. The other non linear component is the recifiers. A passive PFC network of inductors/caps is "tuned" to a specific frequency, where it will bring the PF as close to 1 as possible. At frequencys less/greater than that frequency, the PF will decay away from 1. The rate at which it decays may be shallow enought that you can get an acceptable range. ( everythign with windmills is a comprimise is'nt it? ). My best guess, is that bryans got an appropriate value of caps for his mill, running in a resonable range of wind-speeds. It would be really interesting to test this in a controllable environment such as a wind tunnel.. Though its probably feasible to test it in a lathe rig.. given the very large range of power under which the mill has to operate, using an Active PFC rather than a Passive PFC ( read caps ) *may* give further improvements. This coudl be as simple as switching in additional caps depending on the load/power at a given time.. However theres other techniques available, that probably can yeld higher efficencys, by the use of PWM switching schemes.. Its quite viable to get a constant voltage out of the end of a Active PFC. A constant voltage source is only partially useful.. From there you need to control the current flow into a battery ( or into the grid in my case ). Thats the job of a MPPT type arrangement.. You want to be extracting all teh power that is available, ( not more and causing a power deficit, slowing/stalling the mil ) or less than available ( causing overspeed, and imminent death of the mill and anything in the path of a flying blade )... In summary, I think its well worth exploring the use of an Active PFC scheme, alongside the already fantastic work thats been done with the passive PFC ( the caps ).. And this would also apply not just to F&P mills, but all generators in general. BTW, Google, "Power Factor Correction wind generators" theres lots of references to all this stuff. I may be able to send you copies of some IEEE papers, that i have ( you need a subscription ), most of them are quite indepth and require a resonable grasp of some engineering maths.. ". ( ie its pretty geeky, and not very practical ).. EVERYTHING SHOULD BE AS SIMPLE AS IT CAN, NOT SIMPLER - EINSTEIN |
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| andrewf Newbie Joined: 24/12/2007 Location: New ZealandPosts: 15 |
And this is why you charge batteries using a current source, rather than a voltage source.. I woud'nt ever contemplate just putting a rectifier inline with the mill and attaching a battery.. Its not magic either. Control the current flowing into the battery matching the power thats avaiable from teh mill. EVERYTHING SHOULD BE AS SIMPLE AS IT CAN, NOT SIMPLER - EINSTEIN |
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herbnz Senior Member  Joined: 18/02/2007 Location: New ZealandPosts: 258 |
Andrew Most seem to follow that a lagging pf is the reason the caps work. I myself do not. Generateors supply loads, loads set the demands on the generator. Motors create lagging pf that need correcting sothe lines and ultimatly the generator supply less reactive power(BTW this power takes no power from the input to the generator other than the losses due to resistance ) In our case there is in th erory no inductance in the load the FP does not need to produce a reactive power. The only inductance that can be in the output is in the stator windings themselves and this inductance is not the standard inductance of the winding, this can not have any effect as the mmf produced by current flowing in the winding cannot produce flux as it is opposed by a greater rotor mmf. However if some flux does get produced by the stator current due to a lose magnetic coupling yes then we have inductance in our output. I know this does exist but the degree is only speculation from various ppl. I tend to the view that the caps create a leading pf this calls on the generator to produce a leading pf this forces the rotor to advance in its timming and by a very complex set of events kick more excitation into the rotor mmf (note that in power stations we manually increase excitation to force a unit to take mor reactive power also done with rotary condensers at local substations to simulate capacatance. Its this increase in the rotor flux that produces a higher output that we can gain from. I am out on a limb here all others think as you do pf correction. The use of auto switching in of caps is still fesiable and will work but using pf to control the switching IMHO would not work. but there is plenty of scope to use pre set load control Herb Ps you will see in Gordons scope pics no lagging current only leading when he adds caps |
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| KiwiJohn Guru  Joined: 01/12/2005 Location: New ZealandPosts: 691 |
Herb, I would appreciate you comments on my topic regarding power factor. Thanks. |
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| andrewf Newbie Joined: 24/12/2007 Location: New ZealandPosts: 15 |
you know what i've had a eureka moement tonight. Sometimes you have to have a good hard look at your thinking and look for your faulty assumptions... . I've gone back and read and reread everything that everyone had to say. herb, You are quite correct about there being minimal inductance in the F/P stators. And Gordons traces did'nt support that theory either... So, out with PF. its just wrong. Heres my new theory, which shoud'nt be too hard to prove or disprove, I think bryan will be itching to try this out with gordon. The stator windings have reistance, Inducatance and Capcitance, enough that they form a tuned LCR circuit, that oscillate every time the diodes turn on and off as the direction of the current changes. The recifiing diodes cause a step load becuase the effectively are not being turned on and off at zero voltage/zero current. This 'step' excites teh natural ossilations in the LCR circuit. These oscillations will be at too high a frequencey for the core to handle. So these oscillaitons become dissipated by the core instead. So what we see ( and you can see this in gordons first scope traces ), is a voltage spike, followed by an exponential decay ( that was the big clue btw! ). This dissipation of energy results in pretty significant core losses, and as a result a lower power output. Perhaps as much as 50%. So what happens when you add the caps? The caps are simply detuning the LCR circuit to a point where its natural resonance is signifncantly lower than a point where it can get kicked off by the diodes turning on and off.. The result is that these high frequency osccilations are not occuring and that power is'nt being wasted. There is a down side, with caps installed, the current through that stator will increase, incresing losses int eh copper.. However that loss should be much smaller than the gain made. There will be an 'optimal' value cap given for different speeds, but it shoudl be possible to get a good comprimise that will sit across the usable speed range of the mill. I started looking up the math for this.. its horriffic.. It can't be considered a linear system really.. An experiemental method may be better than a calculated one! I'll do some work on this tommorrow and see what i can come up with though. My white board is covered in stuff tonight and my head hurts now... I think that the optimal place to put the caps would be between the phases. This would work well for an unmodifed system, such as a 100S, wired in delta. I figure you'll only need a small amout of capacitance to acheive the desired result in this config, 1-10uF shoudl suffice. I figure in the re-wired systems, that the natural capacistance between poles will be much higher than in the unmodified system. Anyway, theres some testing to do, whcih will require a pretty decent high bandwidth scope, so i can see if i can spot the high frequency 'blips'.. This of course is just a theory as yet. I've not had the time to test it out, but based on some of the stuff that i've already seen i think this is a path well worth pursuing.. Any comments? EVERYTHING SHOULD BE AS SIMPLE AS IT CAN, NOT SIMPLER - EINSTEIN |
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| herbnz Senior Member Joined: 18/02/2007 Location: New ZealandPosts: 258 |
Hi Andrew Yea its a path worth following in fact you better re read The Dr Chako's report if I remember this is his theroy. All you say is factual but the degree is like leakage reactance unknown . I have never liked parallel caps as the units become unreliable due to on run down after the driving force is removed the natural inductance (can now come into play as mmf from rotor is not the driving mmf ) and caps can and do create resonance in a local closed cct Byrans series means the cct is still got the battery load that damps out high surges. |
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| KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691 |
That sure turned a light bulb on for me Andrew, then I thought, 'hey wait a moment'. Do the diodes really stop and start the current in that way? Sure, with a resistance load they will but when charging a battery bank what current is there until voltage reaches battery voltage? |
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| herbnz Senior Member Joined: 18/02/2007 Location: New ZealandPosts: 258 |
Hi Again Just another fact that I will throw up. The inductance of the stator coils can not cause any effect during the period it is under influence of rotor mmf but after this passes it is able to perform all its tricks with any caps present either stray or introduced. Also Andrew on talking last night on irc you questioned Gordons measurement current scopes only measure voltage to measure current common practice to insert small insignifigenr R to sense current Gordons cct Ok by me Herb |
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| andrewf Newbie Joined: 24/12/2007 Location: New ZealandPosts: 15 |
Yes, John, they do. the the case of an unmodifyed 100S, 6 times a second. EVERYTHING SHOULD BE AS SIMPLE AS IT CAN, NOT SIMPLER - EINSTEIN |
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oztules Guru Joined: 26/07/2007 Location: AustraliaPosts: 1686 |
I think your theory falls down here. The power is not being "wasted", it is being ignored completely... not as losses, just not used at all. The caps don't seem to be solving an efficiency problem, they are solving a not using it problem. So... I still think that these alts are just the very thing for lots and lots of leakage. They have long poles, spaced well apart, and weak magnets closely packed. This is a recipe for high inductive leakage reactance. These graphs of Gordons/Phils on the alt shows to me that they fold over at about 300 (or less) RPM. From this point hereabouts, inductive reactance probably equals Copper R, the inductive reactance becomes the dominant resistance as the frequency rises further... and we exhibit all the hallmarks of a reactance limited machine... or basically a constant current machine. As we add capacitors, we do create an RLC circuit (not the one Andrew indicates but a much bigger one). As we start to approach resonance, we will reduce the effects of the XI, until at resonance, we should only be dealing with R copper. (forget armature reactance for the moment) As we speed up further from this point, then the resonance will taper off and the resistance/reactance will build up once again. I don't think I've seen this happen yet, as no-one has reached this higher fold over point as I recall. From a lot of trawling through lots of greek letters, it would appear that the armature reactance will decrease the flux into the stator fingers with a trailing PF load, and will magnetise with a leading PF load. This would mean that EMF will rise with rpm (with a capacitive load), and even better if we decrease XI at the same time...we will see the output volts go up  instead of stagnating.
For those who haven't followed this long road, the armature reactance is: When the magnets(rotor) induce an EMF into the stator coils, and we draw a current, then this current will set up it's own opposing MMF. This MMF tries to oppose the MMF from the rotor. With strong magnets, and a large airgap, this "back MMF" will not have much effect on the main magnetising field (rotor)... so no drama......but of course, the F&P has weak magnets and small air gap, skinny well spaced fingers....and so this back MMF has a larger effect on the main field than we might think. There's plenty of places for errant field lines to hide out. Without caps, we will have a trailing PF , and so this back MMF will detract from the main field. With caps providing a leading PF load, it will add as a magnetising field to help get more flux into the skinny stator fingers and so increase available EMF. Now, this effect will be the same for shunt caps as for series caps. (Herb found it with shunt caps I think) In Gordons tests, the shunt caps still current limited, although they gave a good account of themselves initially. So, I contend that I must agree with Herb, that the MMF increase induced driving a leading PF load, allowed more MMF to be passed into the stator core and so improve EMF.....(what you saw with your strobe) However it still folded over early and Gordon had the mill runaway with the shunt caps. With the series caps it would appear that the resonant effects of the series caps decreased the XI so that more current could flow with this increased MMF. So, without the series caps but with the shunt ones, we got the MMF increase, but could not use it higher up the rev range... where the power is, because XI had taken hold. Series provides the same MMF relief, but with the advantage that the fold over is much higher up the rpm band, and we get rpm dependent relief from the inductive reactance (XI) as freq increases for part of the journey. Of interest to those driving resistive loads is the decreasing XC as F goes up,allowing the blades to clear stall before driving the R load (water heating etc). In summary, series caps do a few things, allow better matching of the load, by allowing the blades to clear stall, increase MMF (as do shunt caps), and decrease XI at a freq dependant rate. Hopefully we furl just past resonance, and every one is happy Well.... it's time for another oztules bashing session I suspect. ............oztules 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 |
Hi oztules Now your on the right track by my reckoning. We have three threads going now I have just post on FP one similar line i think Herb |
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| andrewf Newbie Joined: 24/12/2007 Location: New ZealandPosts: 15 |
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| oztules Guru Joined: 26/07/2007 Location: AustraliaPosts: 1686 |
Yes Andrewf, The mill runs away, ie the stator is not accepting anymore input power, regardless of how much faster you turn it. (except for the increase in bearing and iron losses going up accordingly) You can go from say 400 to 800 rpm with only a minimal increase in drag from the alternator. The stator barely heats up anymore from after the fold over point onwards. This provides self protection from burnout in high winds (you can't burn a F&P out), but provides no protection to the blades and associated turning pieces. The input power associated with this doubling of rpm increase is 8 times as much power available as at 400 rpm (wind is cubic function). As you could imagine, this would result in a puff of smoke or two if it was to utilise this extra energy in the form of losses in the stator. It just doesn't. ........oztules Village idiot...or... just another hack out of his depth |
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GWatPE Senior Member Joined: 01/09/2006 Location: AustraliaPosts: 2127 |
Hi oztules, This seems to be the conclusion I have as well. You have probably saved me buying a lathe. Gordon. become more energy aware |
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fillm Guru Joined: 10/02/2007 Location: AustraliaPosts: 730 |
So it's like on "Star Trek" when Capt Kirk asks Scotty for "more power" and Scotty has to say "Ive goot neu more to giive Cap" and the Enterprise and its crew are then at the mercy of the emerging danger unless good old "Scootie" can hit the right buttons  ..
Sorry guys , for my interpration of what is being said here but I just dont understand what is being said ,and I'm not trying to make a joke out of what is being said, Im just a dumbie with all this mmf emf pfc pwm stuff , all I know and have seen in my first set of tests is that a lot of extra power is there for not much more effort, and most of us already have overpowered F&Ps flying and as long as we can come up with a cost effective way of adding a doubler and caps the power is there for the taking , to put it in a nut shell. So I will leave all the tech stuff and explanitions on how and why and just do what I can do to to help in getting " More Power ".... Cheers... Phill PhillM ...Oz Wind Engineering..Wind Turbine Kits 500W - 5000W ~ F&P Dual Kits ~ GOE222Blades- Voltage Control Parts ------- Tower kits |
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| herbnz Senior Member Joined: 18/02/2007 Location: New ZealandPosts: 258 |
Well.... it's time for another oztules bashing session I suspect. ............oztules Least you get response usually I get stoney silence. looking again at your thesis some right some not quite there yet. The current limiting can be due to impedance mainly caused by leakage flux creating reactance. or mainly i think due to the killing of the rotor flux by the opposing stator mmf. Its happening very early in these graphs cause they are grossly over wound for the voltage. this shows up in less in delta (less turns) and 48v . As yet for all the talk of leakage flux no one including me has proved or disproved. Gordon your traces on single phase are nearly there if yo could replace your voltage channel to sense a few sensing turns on the a stator pole of the phase you are using this will show the true induced emf . at present we are seeing only the load voltage . and lead lag is only the load. Herb |
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| andrewf Newbie Joined: 24/12/2007 Location: New ZealandPosts: 15 |
EVERYTHING SHOULD BE AS SIMPLE AS IT CAN, NOT SIMPLER - EINSTEIN |
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| GWatPE Senior Member Joined: 01/09/2006 Location: AustraliaPosts: 2127 |
Hi herb, the single phase was 0.8A from a series80S. This winding would be over rated as well I imagine. Gordon. become more energy aware |
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| KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691 |
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| andrewf Newbie Joined: 24/12/2007 Location: New ZealandPosts: 15 |
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