Drilling Magnets it can be done
Hi guys , its been a while since I have posted , just been to busy with an endless list of projects and jobs in progress.
The latest project is a dual rotor axial machine and I am almost ready to wind the coils but I thought I would run the figures past the experts.
As usual I never do things the easy way, I like to use/reuse stuff I have at hand if possible. My previous builds where 8,10 and a 12 footers but mostly followed the plans from Hugh, this one was inspired by Hughes 14ft machine but with more magnets.
So here's the spec
Dual rotors 18 inch by 1/2inch
16 poles per rotor each made up from two magnets T shaped the mags are 2x1x½ n42 with 32 per rotor. Each pair of magnets are like poles forced to sit in repulsion.
The magnets are glued with slow set epoxy and a single 4mm stainless csk screw to secure each one in position (tapped into the rotors). I don't think I will pot the magnets , I have had problems with rust in the past , I think the polyester holds any moisture that gets in and it just eats into the mags. I have applied several coats of paint over an anti rust primer. Time will tell I suppose!
The bearing and flange are from the front axle of an old series 3 Land rover with new bearings and seals ..
I got a very good deal on the magnets but they were plain and without holes. I had a few spare so I decided to experiment with drilling them. I know some say you cant drill them while others will say you shouldn't drill them.
So good or bad here is how I did it. I first tried normal hss bits which was useless so I then tried a masonry bit, it did cut but got hot very quick even with coolant. So then I ordered a very cheap set of glass/ceramic bits from eBay
These are cheap and nasty and I didn't hold out much hope but against the odds they went through them like a hot knife through butter. I cooled the test piece with water, the bit and the magnet was stone cold though I did go very slowly. The hole(4mm) was cut clean with no chipping or flaking and didn't break out the bottom .
So as the test was such a success I decided to go for it with the set of 64. I set up a jig with in a tray on the vertical drill but instead of water for the coolant I used white spirit , I didn't want to introduce water into the magnet. The jig insured each one was drilled dead centre and the tray held enough coolant to cover the magnets, I also kept a flow of coolant on the bit as I wanted to be sure there was no heat to demagnetize the magnets. The countersinking was done using the 10mm bit from the same set. The 4mm bit has a very flimsy shaft and I managed to drill 62 before it bent slightly and I had to buy another set for the remaining 2, such is life lol. I don't have a meter to test the mags but they don't seem to have suffered from the process.
The cuttings form a magnetic mud and is a bit of a pain to remove, I then cleaned them with surgical spirit and coated the inside of the holes with epoxy.
So there you go that's how I did it, whether it is worth the trouble or if it has harmed them is another matter?
I assembled the rotors and wound a test coil from 1.6mm wire, 80 turns filled the coil size nicely though I could squeeze in a few more turns if needed, I fitted this coil in a plywood form and inserted it between the rotors. My stator will have 12 coils 1/2inch thick, air gap mag face to mag face is 5/8 allowing 1/16 clearance at each side, I can adjust this later if needed by skimming a little of the aluminium spacers(the nuts in the pic are just temporary) I then spun the rotors and measured the ac volts from the coil. Speed was measured with a digital tacho so should be close. With the rotors spinning at 50 rpm I was getting 5vac (4.93)
This is where I need some help, as math is not my strong point, so from the Dans excellent book it gives the formula as
Vac * 1.4=peak vac *no. coils per phase * 1.73 for 3 phase -1.4 rectifier drop
So that would be as follows (I think)
5*1.4 = 7 *4 = 28 *1.73 = 48.44 - 1.4 = 47.04 with 80 turns @ 50rpm with 1.6mm wire
This machine is for battery charging at 48v, I was hoping for 16ft dia blades, I'm not sure as to a good cut in speed for 16ft I am guessing at about 100rpm if that's the case would 40 turns of 2 in hand 1.6mm seem reasonable for the coils? I am in a high wind area and not really interested in a few watts on a rare calm day but I would like to avoid stall, I can add resistance but changing the rotor gap would be tricky once installed due to the aluminium spacers instead of nuts.
I have a stock of 1.6mm wire but if I need a different size its not a problem Im not even sure 16ft would be a good match, the only thing I cant change at this point is the magnet rotors, the offset etc has been left for now and will be determined by the recommended blade size.
I think I have included all the details if any further info is needed just let me know!
As always many thanks in advance
Russell hilltopgrange
Originally posted on fieldlines May 23, 2009,
