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Forum Index : Windmills : Inspiration needed
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piercy007 Newbie  Joined: 27/06/2008 Location: United KingdomPosts: 28 |
Well I finally got a mill up at the weekend (Mk3!) and we had some decent winds. 
This is a simple DC motor (4800 rpm at 180v)with set of 90ish cm PVC blades. No idea what sort of RPM the mill was getting but it seemed quite fast (over 300rpm I would say). Problem is the power coming out from it is no where what had expected or hoped for. 
Before it went up I did some hand cranking - at a low turn (say 60rpm) it was generating 7/8 V and with a good spin it was getting 15/16. However now it is up it rarely gets near 6 and is more like 3 in a normal wind. I have no idea were the loss is and am hoping for some forum inspiration?? My only other thought was building a voltage doubler circuit (or buying one of ebay as my eletronic skills are not that good) though I have found a couple of what looks like easy diagrams. I am measuring the voltage directly (no capacitors/diodes in the way) via a cable attached to the 2 outputs from the motor. I have also put a picure below of the Mill. Ideas anybody? 
My middle name is luck mind you my first is bad! |
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Gill Senior Member  Joined: 11/11/2006 Location: AustraliaPosts: 669 |
G'day Piercy, I can see the effort you have put in and it is disheartening when performance is so dismal. My first impressions were, What long skinny blades,and Gee that's low to the ground considering the surrounding environment. Yet you estimate 300rpm and with open circuit you are only getting 3 to 6 Volts whereas when spinning by hand you got 7 to 8 Volts???? Seems to me you've got some wiring wrong. Is it a brushed motor? I wouldn't go building a voltage doubler until I found the fault. Don't try to optimise the prop for example until you are getting an output consistent with your hand cranking bench tests. With what you have described it points to being something that has changed between hand cranking and flying. This means it can be found and fixed. Let's see what other members say. A different approach, or simple suggestion might be just what is needed. Gill EDIT: P.S. The prop shaft seems offset to the gen body. Is it geared??? was working fine... til the smoke got out. Cheers Gill _Cairns, FNQ |
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Gizmo Admin Group  Joined: 05/06/2004 Location: AustraliaPosts: 5117 |
Hi Piercy. Yeah I agree with Gill, I think you need to do some fault finding to see whats changed since your hand cranking test. I would get it back on the bench and try the hand crank test again. Judging windmill RPM by eye is difficult, but here's a little tip that might help. Wrap some black tape around ONE blade only. It divides the number of blades you need to count by 3, trust me it makes it a lot easier to comparatively measure RPM. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| piercy007 Newbie Joined: 27/06/2008 Location: United KingdomPosts: 28 |
Gill Thanks for the fast response. The offset is due to the bearing the shaft is on. Its a bolt on job but the bearing also has rotation (a bit like an elbow joint) so it keeps moving in its socket. Mind you it did not seem to matter as it was still getting a decent spin! Yes its a brushed motor - I was wondering about the actual cable - I am sure I read somewhere you can lose some Voltage across specific lengths and size? My middle name is luck mind you my first is bad! |
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| Gill Senior Member Joined: 11/11/2006 Location: AustraliaPosts: 669 |
Sure, Voltage drop for long runs or fine wire is a problem. However, your voltage drop is too huge to be that. Rather than guessing, how about an ohm meter testing for a high resistance somewhere in the circuit? was working fine... til the smoke got out. Cheers Gill _Cairns, FNQ |
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Dodgeman Newbie Joined: 19/07/2008 Location: United StatesPosts: 33 |
Piercy, I am having the same delima as you are, With bench tests, I turned my (3/4 hp-90 volt-1725 rpm dc, brush) motor by hand , first just the shaft with my fingers, and again with a 4 inch pulley attached for a "good grip" to spin the motor, I too read 15+vdc open volts, and lit an 1157 automotive tail light bulb easily. After getting mine atop the pole (about 20 feet up, but next to a tree) and flying, I am having exactly the same problem that you are describing. I haven't had much wind to speak of since it went up last weekend, but I am only in the 5-6 volt range with the load of the stop light bulb, one time I saw 13.2 volts but it was for a very short but powerful gust of wind. (which also showed me that I have a bit of blade balancing to do before the windy season really hit again) I have to believe that the RPM's which I was -hand spining- on the bench must have been much more than I am getting from the wind, I am wondering if my "long skinny" blades are not too long and too skinny (mine are 4 ft 4 inches long, cut from 6 inch PVC) to achieve the needed 300 plus RPM's that I guesstamated I would need for constant 12 plus volts of output. I haven't tried shorter, fatter blades yet, but I am leaning that way if I cannot find a way to produce at least a decent charging voltage with the PVC blades I made (which look alot like yours) I have just received my digital tach but have not had wind enough to try it on the mill. I will eventually be posting those RPM readings as soon as I have the wind to do some tests. |
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| KiwiJohn Guru  Joined: 01/12/2005 Location: New ZealandPosts: 691 |
One comment, the length of the cable will make little or do difference for an open circuit voltage test. |
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oztules Guru Joined: 26/07/2007 Location: AustraliaPosts: 1686 |
Dodgeman, I think the tacho will tell the tale. I am unable to offer help in regards to output with plastic blades, so we will have to wait for the plastic Guru's to turn up. But with the latent iron losses and brush friction, it may take a fair sort of wind to get it out of stall.... if it is an aerodynamic lift type (I don't know about those plastic ones, they seem to have higher than 1 tsr, but I am not sure how they arrive at the calculations to do it). 007, The inspiration speech starts here: Stall does not mean not turning, just means that it is not turning at the best TSR for that particular blade. So it can turn quite happily, yet be making nowhere near the power it should for that diam at that windspeed. This is all about matching the load. You can only hive so much power out of the spinning blade, before you have such an effect on it that you impair it's ability to stay at the best TSR. As it slows away from best, it gets bad quickly, until it just spins at a fairly ordinary rate, turning, but not working very well at all. Sometimes, the motor you use may be so heavy to turn, that it needs a cyclone to get it going, or it needs more blades to give it more torque... which you have to balance with rpm, as a general rule.. the more blades, less speed. So if speed is an issue, then this is problematic. Rarely is a backyard design an optimal match of blades to load requirements. Usually the first will be a bit of a disappointment, as it has a habit of performing not up to expectations..... or just plain not work at all. But don't be discouraged. This is the time to learn the relationship between you and the wind.... and it the most complex thing I have come up against yet, so don't think your the first in the world to not get it right the first time. The motor you now have, so it is a question of matching it to something... now the fun begins. If I knew a bit more about plastic blades I could be more useful, so I will make some sweeping statements so that someone who does know will sort us both out. If we aren't making the rpm we want, and we have sufficient wind and the right diameter that should have done the job, we have stall. If these are high TSR blades (for arguments sake), we might add another blade from 3 to 4.... what will this do..should slow it down! Well if they were say TSR 8, and you stall them, they may be operating at maybe 1 or 2. If we add another blade, our bladeset TSR, may come down to 6, but it may have the torque to actually get there. In this instance, we have dropped the design TSR down to get torque, and that has increased our chance of getting out of stall and just may deliver a tsr of 5 or 6..... much better than 1 or 2, by doing the opposite of what seems right.... did I say complex, I meant downright difficult. Or we could make the chord a bit wider, and catch a bit more wind, and get a little more torque (little bit like an extra blade, just increasing the square inches differently to adding another blade.) It could be that we are not stalled at all, but have chosen a blade diameter that is too big for the rpm we want. In this case, we do the opposite to what seems sensible, and shorten the blade. This will keep the tip speed the same, but the rpm will have to increase to run around the smaller circumference to match the number of inches per second the tip speed dictates, and so rpm increases. In this example, we had ample wind to drive it, but the wrong "gear" so to speak. There are numerous other combinations as well. Maybe the match between blade size and motor drag is alright, but the motor is a bad choice for 12v.... in this case you need to charge a 6v battery, or use a booster. As you can start to appreciate, this is not the end.... just the beginning of a journey of one of the most complex systems you will come into intimate contact with..... Another example is my 13 footer. In the relatively mild winds the other day, it easily did 1000w into a 48volt battery (well it was for a short time anyway). Out of curiosity, I connected to 36v instead.... well.. The blades slowed markedly, and the watts dropped down to about 400, I then jumped it to 2 batteries (24v), and the power dropped to 3-5A@30v... 100 odd watts, and finally to 12v... 1A@14or so volts.... a whole 12 watts. So by introducing various stages of mis match, we got increasing stall. Same alternator, same blades, same winds... only the load changed in voltage (impedance actually) and we went from a ripper performer to a absolute dud... in the space of a minute. So by changing the load you change everything dramatically .. from over 1000w to struggling to support 12w. There is a lesson to be learned here. It is a system from start to finish. Any mismatch will change things to better or worse, depending on the start point. I'd like to take some kind of credit for it's first up performance.... but it surprised me more then you. I thought it would require some more tuning than doing nothing... I think in truth there was an element of dumb luck there as well.. Keep smiling, it just keeps getting better, and the more you learn the more you will appreciate just how amazing the whole relationship is between the prop and load. Inspiration speech ends now...... get back to work you lacky's 
And of course, sometimes you just need more damn wind
..........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 |
With a DC brush motor, a test could be. Short the blocking diode and see how fast the blades spin as a motor. If the blades spin a bit faster, then blade stall may be a problem. If the mill spins quite a bit faster then there may be another problem as well. The motor output may be a poor match to the battery as well. Gordon. become more energy aware |
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| piercy007 Newbie Joined: 27/06/2008 Location: United KingdomPosts: 28 |
Ozules Wow - what inspiration! No worries though already planning the next mill (think I will have a go with a stepper motor) as well as an ongoing homebrew twin rotor axial generator. Also just putting together an anemeter and playing with some solar panels (as it aint to windy some days!) My middle name is luck mind you my first is bad! |
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