Miss-print in Hugh Piggott book?

[machinist]

I've decided to build a 24v hawt. It takes a 4 magnet to 3 coil ratio. In Mr. Piggott's book there is a chart that shows wire size, turns, and number of coils. It shows that 16 coils will be needed for a 12' turbine. 16? Is this a miss-print or am I missing something?
Thanks
machinist

[Flux]

I don't know which book you have but if what you say is true it may be a misprint.

The 12ft machine uses 16 poles (32 magnets) and will have 12 coils.

The metric book that I have gives the number of turns per coil and wire size for the various voltages and the number of magnets for each size machine, nowhere does it say there are 16 coils.

If you are not confusing numbers of magnets with coils then it could be wrong.

Flux

[scoraigwind]

I confess there were several misprints in the early 'inches' editions of my Recipe book.  I believe I have fixed them all now. 

That particular one persisted for over a year.  Nobody ever pointed it out to me, but I fixed it accidentally in the August 2010 edition when I put both magnet and coil numbers in the table on page 36.

Profuse apologies to all readers.  And I really appreciate the ones who pointed out earlier errors.

[machinist]

A Wind Turbine Recipe Book  - The Axial Flux Alternator Windmill Plans, Aug 2009, English units edition
page 36
Chart at the bottom
"Number of Coils", one row under Turbine Diameter
Just wanted to clear it up before I start winding this evening.
Thanks
machinist

[machinist]

You must have posted while I was writing.
Thanks for all your hard work Mr.Piggott.
machinist

[scoraigwind]

once again sorry for those with older editions that show the wrong numbers of coils!

[scoraigwind]

The diagrams at the back clearly show the coils and the magnets for each size of machine but I am prone to being a bit random.  One time in Nova Scotia I presided over a magnet rotor with only six magnets, and it was covered in epoxy resin before somebody asked me a second time how many magnets it should be.  I got it right the second time and we had to tip out the casting and clean it up and do it all over again.  My brain cells are occasionally to busy to be useful.  Maybe next time i wll try seven magnets.

[machinist]

While I have wind turbine royalty on line, I would like to ask a question.
Would it hurt anything if my steel rotor plates extended past the outer edge of the magnets? I have a cnc mill and was thinking of miling shallow pockets in the rotor plates to capture the magnets.
Thanks
machinist

[prasadbodas2000]

You mean grooves on the steel disks to house the magnets? In this case some thickness of the magnet will be sunken inside the steel surface of the disk. IMHO probably a small depth of a milimeter or so may be ok, but more the depth it will short-circuit the magnetic path and will result in reduced output.

[Flux]

You can extend the disc as far as you want beyond the magnet. The groove is a different matter. Any part of the magnet within the groove is subject to excess leakage flux and if you make the groove as deep as the magnet you will loose out very badly. A small groove is permissible as the leakage will only be local to the groove wall and also the small section of iron in contact with the magnet will soon saturate.

I would suggest either a lip on the edge of the disc or a groove significantly wider than the magnet so it effectively acts as a lip. If you do this then you can probably have a lip up to about 1/3 of the magnet thickness before you see any big loss in flux. For mechanical constraint probably 2mm is sufficient and this will cause no trouble.

The alternative is a pair of roll pins for the magnet to fly out against and the area of these is too small to cause any loss even if they are carbon steel, you can in fact get stainless ones and have no loss at all.

Flux

[machinist]

I was thinking of a .06" pocket in the steel disk. This would provide a mechanical stop for the magnets and I could paint the rotors instead of encasing them in resin. The roll pins are a great idea. I should have thought of that. I guess I'm just trying to find something on this project to put on my mill.
I made some coils last night. I'm not happy with my winder and came up with some improvements to make a really tight coil. I'll post photos later.
Thanks
machinist