 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 : Magnets and coils position on DIY PMA
| Author | Message | ||||
Diogo Newbie  Joined: 27/06/2009 Location: BrazilPosts: 3 |
Hello, I´m planning on building my first homemade windmill, and I have the following burning question on the PMA construction (by PMA I mean the basic PMA for DIY projects, where you have the disc shaped stator with the coils): My question is whether a given pole (magnetic field) needs to cut across one side or both sides (left and right) of a given coil at a time for optimal generation. I have done quite a bit of research on this, and my understanding so far is that one pole needs to cut across one side of the coil, while an opposite pole cuts across the other side of the same coil for optimal generation. My assumptions are based on the right hand rule (generated current direction), so that the current on he left and right side of the same coil would add up, and not cancel each other. However I'm having difficulties to find actual plans and descriptions that corroborate this theory, maybe someone here can help. Thank you very much. |
||||
Tinker Guru  Joined: 07/11/2007 Location: AustraliaPosts: 1904 |
Its hard to understand word pictures at times. Consider this simple explanation I was given years ago while studying electric power generation: Imagine a magnet with its N&S pole facing each other so that the magnetic field is in between them. You then need to move a coil through (cut) this field at right angles to generate power. When you do that there will be an instant when all the magnetic lines pass through the center of the coil. Just apply this to your set up. Right hand rules can be confusing, I would not bother with it. Just connect a meter at the coil terminals and see which way the current flows  - both ways, since its AC.Klaus |
||||
| windlight Guru Joined: 03/03/2007 Location: AustraliaPosts: 331 |
Buy a copy of Hugh Piggot's book. http://www.scoraigwind.com/ allan "I like this place and willingly could waste my time in it" - (Act II, Scene IV). |
||||
| Diogo Newbie Joined: 27/06/2009 Location: BrazilPosts: 3 |
Tinker, thank you very much for your reply. It´s very practical and hands on. However, I guess I´m trying to get deeper in the theory right now, and not in a trial and error construction. I understand the current generation by a coil cut at a right angle with the magnetic field, but what´s boggling my mind is what happens on each side of the same coil (left and right) when they cut the same magnetic field at the same time. In theory, since they are under the same magnectic flux and have the same "cutting" direction, they would generate current to the same direction (up or down). And if both sides generates current in the same direction (lets say "up"), they will tend to cancel each other. Thanks for the help, and LMK if you have other ideas. |
||||
Greenbelt Guru Joined: 11/01/2009 Location: United StatesPosts: 566 |
Electrons naturally have a negative charge, when they get bunched up they repel each other. Cathode ray tubes use magnets to move electrons. The positive pole, I believe the S ?? attracts electrons while the N repels them. the right hand rule shows you which way they will move, depending on which direction you choose to wind your coil), this only applies to DC current. one side of the coil is attracting electrons because of the S magnet pole? which moves them out of the way of those being pushed by the N pole. This is not all of the picture, When a coil of wire has a current flowing (induced by the lines of force in the magnetic field) this coil produces an electro magnetic field who's polarity is opposite to the field magnet that produced it. (pushes away) The instant the coil passes the center line of the Field magnets,( point of highest voltage), the current flow in the coil begins to slow down and its voltage drops to =0 , Now the coil is moving away from the magnets, coil voltage begins to build again but its polarity is such that it is attracted to the field magnets and resists moving away.I have described a 1/2 cycle alternating current This is why a good source of power is required to generate electricity. I like Tinkers answer Its graphic enough to picture exactly what is required. Time has proven that I am blind to the Obvious, some of the above may be True? |
||||
oztules Guru Joined: 26/07/2007 Location: AustraliaPosts: 1686 |
"I understand the current generation by a coil cut at a right angle with the magnetic field, but what´s boggling my mind is what happens on each side of the same coil (left and right) when they cut the same magnetic field at the same time. In theory, since they are under the same magnectic flux and have the same "cutting" direction, they would generate current to the same direction (up or down). And if both sides generates current in the same direction (lets say "up"), they will tend to cancel each other." Ok ... yes. If you can cut the two sides of the coil with an opposing field value (N and S) then the emf will be twice... they add. If you cut with N and N on both sides at the same time.. they cancel out. This is the basis for the good performance of the dual axial design. This link may give you a better visual of the 12 magnet/9 coils operation. zip files here.. open and run them consecutively with any viewer With thanks to Willib from fieldlines for that work. ............oztules Village idiot...or... just another hack out of his depth |
||||
| Greenbelt Guru Joined: 11/01/2009 Location: United StatesPosts: 566 |
oztules explains. Greenbelt... the earths field is meaningless in this operation. I'll buy that! Jupiter perhaps.  I will edit the post
and remove the faulty choice of words. Time has proven that I am blind to the Obvious, some of the above may be True? |
||||
| woodchips Newbie Joined: 05/01/2009 Location: United KingdomPosts: 27 |
Imagine you have a horeshoe shaped magnet, so one arm is N and the other S. Position this above a sheet of iron to complete the magnetic circuit from pole to pole. Now wind your coil so the mean diameter of the coil is exactly the same as the centre to centre distance between the horseshoe magnet poles. This is the optimum size of coil to generate the maximum voltage from that magnet. The voltage induced, generated, on each side of the coil adds. Yes, with a coil some turns will not be exactly under the centre of each pole, doesn't matter. The voltage induced in each turn will add to the voltages induced in every other turn. The adding is done vectorially, not algebraically. It is because of this vector addition of voltages that series connection works whereas parallel connection results in circulating currents. Never wind the stator with any turns in parallel. The simple rule is, each side of any one coil must be equally positioned under a N pole one side and a S pole the other. It is irrelevant how many N or S poles there are in between the two over the coil. Buy a cheap oscilloscope and look at the waveforms, it does help. Oh, a real oscilloscope, not one of these USB things, you want real analogue signals on the screen. Bob |
||||
| oztules Guru Joined: 26/07/2007 Location: AustraliaPosts: 1686 |
If you are going to brew your own coil arrangement, there are a few other things to take into account. Windmills are compromise machines, and the coils are no exception. As usual there are more things to take into account. As Bob implies, the optimal inner coil size for max emf production for minimum turns... should be the same as the magnet size........ however, you will do better by decreasing the inner coil diameter by about 20%, and using more turns....eeeekk The problem with mills, is to get power out at low rpms... and this is where the problems start. To get decent power out at low rpm, we need lots of turns and a very strong magnetic field. There is a limit to the winding space before coils begin to touch each other, and so we tend to have to decrease the wire diameter to get the turns in we need. The problem with this is that the resistance then goes up, and our losses go up as a square of the current... So we don't want excessive resistance or we heat the stator rather than charging the batteries. One way around this in the real world, is to get less volts per turn than ideal, and use more turns... (seems a little silly so far)... but we do it in the inner diameter. This way there are less inches per turn than adding to the outside, so the extra turns, while slightly less volts per turn (some cancellation), we more than make up for with less resistance per turn.... we can win a bit here. The practical limit to this caper is about 20% from my testing, and this seems to be borne out by others. (for 50mm magnets, I used 40mm inner diameter in the finish) The gap between disks is also a compromise. It has been found that if your magnets are say 1/2" thick, then the optimum air gap is about 3/4". You get much more flux in a smaller gap, but you get less winding space. I tried reducing the flux gap to 6mm from 19mm... only to find that even with the extra flux lines in the new gap width, you could not hope to get the current carrying capacity and voltage that was required... so another compromise.. and another rule of thumb. Using those two rules of thumb, you can set up your disks and wind the test coil.... and then try to get the maximum weight of copper in there for the minimum turns you need... (thickest wire you can manage to get in the winding window) get the resistance down as best you can... you can always add some later if your crushing the TSR too much (stalling)...but do it outside the stator where it wont heat up your resin and destroy your hard work. Have fun with it .............oztules Village idiot...or... just another hack out of his depth |
||||
GWatPE Senior Member  Joined: 01/09/2006 Location: AustraliaPosts: 2127 |
The approach to design a windmill alternator for lowest coil resistance is improved again if a capacitor voltage multiplier, external to the stator, rather than additional wire turns, is used to produce the extra volts for low end rpm battery charging. Hopefully there will be some experimental data published soon from a forum member with a recently finished dual rotor AxFx machine incorporating a capacitor voltage doubler in this way. Gordon. become more energy aware |
||||
| Diogo Newbie Joined: 27/06/2009 Location: BrazilPosts: 3 |
Thank you guys very much for the great answers. I think Oztules and Woodships nailed my needs, and I have learned quite a bit from this. I have a lot still to learn, so we´ll keep in touch! |
||||
 Print this page |
 |
The Back Shed's forum code is written, and hosted, in Australia. | © JAQ Software 2025 |
