 Notice. New forum software under development. It's going to miss a few functions and look a bit ugly for a while, but I'm working on it full time now as the old forum was too unstable. Couple days, all good. If you notice any issues, please contact me. |
Forum Index : Windmills : What’s the limit on # of Poles?
| Author | Message | ||||
Glipski Newbie  Joined: 26/08/2008 Location: United StatesPosts: 10 |
What's the limit on the number of poles that a mill can have? I noticed that the F&P rewired to 7 poles seems to be designed more for low speed mills. Is there some reason that a mill couldn't be designed with say, 14 poles or 24 poles? Sorry for being a bother, just wondering...Thanks |
||||
GWatPE Senior Member  Joined: 01/09/2006 Location: AustraliaPosts: 2127 |
The rewire you mention is a 7phase conversion, not a change of the number of poles. The flux transition rate will determine the type of laminations on an iron cored machine, and rectifier type and speed. A 7phase conversion does not affect the number of poles. The F&P machine with a large pole count is suited to low rpm operation, as a large number of flux transitions occur per revolution. The emf is proportional to the number of turns in the same magnetic field and the magnet strength and the flux transition rate. I hope this helps. Gordon. become more energy aware |
||||
Gill Senior Member Joined: 11/11/2006 Location: AustraliaPosts: 669 |
G'day Glipski, No limits if one is making a generator from scratch. There are of course limitations when rewiring an existing unit. When making new, with many poles, a consideration is how large will the diameter be and will this interfere with other vital operations eg. wind flow over props. The larger diameter will further aid slower rpm generation. This is why I'd promote large diameter generators using finer wire and cheap as chips ceramic magnets on VAWTS and WaterWheels. It seems that human nature prefers to stick in the comfort zone of known, if not less suitable, design.  was working fine... til the smoke got out. Cheers Gill _Cairns, FNQ |
||||
| Glipski Newbie Joined: 26/08/2008 Location: United StatesPosts: 10 |
Thanks guys. My question was actually meant to focus on building from scratch. I only mentioned the F&P because of the large number of poles it has and the reference to low-speed generation due to the number of coils. That got me wondering if it would be possible to fabricate an axial flux with as many poles as the smaller F&P. I didn't consider the effect of blocking the airflow due to the larger diameter, though. How about making it larger for more poles and also making the root section of the blades longer to allow for the increase in diameter? It seems that the root section of the blades is just dead area to cover the generator anyway, right? I could just make the blades longer overall, allowing for longer root section...Agree or disagree? Will more poles cause the need for any more start-up torque on the mill? If there's no difference in the amount of power needed to actually turn the generator with more poles, is there a reason, other than airflow problems over the generator, for not utilizing more poles in the axial flux type generator? |
||||
| GWatPE Senior Member Joined: 01/09/2006 Location: AustraliaPosts: 2127 |
Hi glipski, My first prototype axial flux had 96 magnets, arranged as dual rotor, with 24 pole pairs, and 4phases. My mill is a scaled down version with 44 thicker magnets and 4phases. More magnets mean more flux transitions per revolution. Mechanical tolerances become problematic at larger sizes. matching of magnet strengths is difficult. Lots of small coils are difficult to assemble in an ironless design. the smaller the magnets, then the narrower the air gap needs to be. More problems here. I am very pleased with my axial flux mill performance. You need to be meticulous with all aspects in construction as the number of poles increases. Gordon. become more energy aware |
||||
| Glipski Newbie Joined: 26/08/2008 Location: United StatesPosts: 10 |
Does it help things to partition the generator into more phases? Using less poles per phase, but sending them into more bridge rectifiers to get the dc power output I'd be looking for...I'm trying to get my hands around the problem of low average windspeed. If the windspeed is 3-4 m/s on average, I wonder if I were to build a mill with larger blade area to capture the wind and pair it with a generator designed to output at lower speeds, if I'd have usable power output. I'm not trying to reinvent the wheel, just install a better tire, so to speak
What type of tolerances would come into play here? What would be the dimensions that I'd need to keep close for the larger diameter axial-flux? I'm thinking that I might be able to actually design the thing using AutoCad or Solidworks and then have a shop actually laser cut/water-jet/etc. the pieces for me prior to assembly for the really close tolerance stuff. I'm really leaning towards the axial flux design, just with a larger diameter utilizing more poles broken up into more phases like the F&P. That's why I have so many silly questions for you guys. One more thing: how are you testing the strength of the magnets? |
||||
oztules Guru  Joined: 26/07/2007 Location: AustraliaPosts: 1686 |
On an axial flux type of design, it is usually a function of disk size and magnet size that will determine the optimum pole number. For 4.5m/s wind speed go Here: and work out what size prop you need to get the power you want , at the windspeeds you have available, and design an alt to suit. The pole no. will fall into place when you know how big a set of blades you will use, and the diameter of the disks you will use to satisfy the power you anticipate. For me.. we select a disk of say 14" and use 2" round magnets, we space them around the perifery and try and get a little less than a magnet spacing between them. In my case it turned out that 12 magnets fitted in nicely.... so determined the no of poles. To be 3ph, it meant distributing 9 coils around the stator. Voltage is then determined by the rpm you expect for the prop size and cut-in speed you desire. For me it was 48v@ around the 110 rpm mark. You then make a test coil to deduce how many turns you need to get this voltage (48v) at the desired cutin speed (110rpm). My test coil gave me about 7.3vac (100 turns of wire)for 110 rpm. There will be 3 coils in series per phase, so each phase had 7.3 x 3 = 22v ac. I intended to connect in star, so 1.7 x 22 = 37v ac. per star phase. When we rectify we get 1.4 times the ac voltage (it's a peak thing) so 37x1.4=52v dc. Now subtract the diode loss (about 2v) and we get about 50v cut in at 110 rpm. Knowing I needed 100 turns, I then set about getting the thickest wire I can wind into 100 turns that will fit in 1/9th of the stator (because I need 9 of them). It turned out that I could get away with 1.8mm wire and still be only 14mm thick.... so that is what I did. This alternator did around 1.5kw at around 20 mph winds. So that is how you develop the alternator. Don't worry about the pole number, it will resolve itself when you decide on your disk size. (bigger the better for more power) If you want more volts, use more turns of thinner wire,.... but remember, your wattage rating will suffer as your wire size goes down. This is why low speed alts are bigger.... to get larger wire of more turns into the path of the magnets. Does this help? .......oztules Village idiot...or... just another hack out of his depth |
||||
| Glipski Newbie Joined: 26/08/2008 Location: United StatesPosts: 10 |
That helps alot, Oz. What if you were to design the disk size around the wire size? A sort of reverse engineering based on the size of the coils instead of sizing the coils to fit the disk...That would allow you to use the BIG wire, right? Thanks for your answer, I thought my question was dead, but you revived it. |
||||
| oztules Guru Joined: 26/07/2007 Location: AustraliaPosts: 1686 |
It is certainly possible to put the cart before the horse, but you could be changing a few horses before you are through. Until you get the geometry right for the rotors and magnets, you don't know what turns you will require.... bit of a problem really. Sort out the power requirements, the blade size necessary in your expected winds, and then put that against the funds required to achieve it... and then the compromises start from here.... and whether your allowed to put the monster up where you are. Mine is 4meters diam. I came home today to a virtually still clear day, and it was still putting out 150-450watts in the gentle gusts. Swept area of the blades is everything. ............oztules Village idiot...or... just another hack out of his depth |
||||
fillm Guru Joined: 10/02/2007 Location: AustraliaPosts: 730 |
Hi Oztules , I  your repily on this , well written and easy to understand , as some one who is not electrically wired I think a section on exactly what happens ( the basics ) when a magnetic field does its stuff and the different ways to make it happen in the info and projects section would be a valuable addition to "Home Grown Power" and to people like me who just wack more stators on to get more power , being able to desgn a generator or modify an existing gen one needs to understand the basics first and then the tech stuff might not seem so mind boggling .. Just a Thought ? .....Phill...PhillM ...Oz Wind Engineering..Wind Turbine Kits 500W - 5000W ~ F&P Dual Kits ~ GOE222Blades- Voltage Control Parts ------- Tower kits |
||||
 Print this page |
 |
The Back Shed's forum code is written, and hosted, in Australia. | © JAQ Software 2025 |
