Epoxy with steel

[Harold in CR]

 Made up my first Alternator conversion to Permanent magnet rotor. It is a 12V 20A out of a Toyota from more than 10 years ago, when I got it from a rebuild shop. This is what the shop identified it as.

 I realize that these Alternators are not real efficient. I used what I had laying around, mostly, before I try to do an Axial Flux Alternator.

 Took a 3" dia X 1½" length of Aluminum round stock. Center drill a hole for the rotor shaft, and then turned a ¾" wide X ¼" deep groove on the edge of the disc, to pot 12 magnets. Wrapped 2 layers of galvanized strapping around the groove and epoxied it to the aluminum. That was back iron for the flux, as I read another forum where this was discussed.

 Then, epoxied every other space with North face magnets facing out from the disc. When they were dried I repeated with South face magnets facing out from the disc. Then, I filled all around with a steel powdered Epoxy. Not realizing this was steel embedded in the Epoxy, I have filled over half the spaces.

 Before I had enough to finish the job, I decided to put the Alternator together and spin it up using the drill press on direct drive gearing (1725 RPM ) ?  Spun it up, but, not sure it got to max RPM's and the best voltage I could see was 10.93 as I hooked the output to my DVM, no battery was used.

 Does this sound right at a no load voltage IF the RPM's were close to 1725 ??

 Did I screw up using that steel embedded Epoxy to kind of kill the flux from magnet to magnet ??

 I bough more of the same Epoxy today, to try to keep this thing balanced at high RPM's ?

[joestue]

thicker back iron would help. how thick are the magnets.

[Harold in CR]

 Magnets are 8mm thick X 20mm long X 12mm wide  M42 grade.

[Flux]

I agree that the back iron is probably inadequate. I doubt that there is enough iron in the epoxy to make any significant difference.

I suspect you are significantly down on the flux from the wound field if your speed is to be believed but without a tacho belt slip can confuse things.

If you can drive it fast enough it should work well enough. I think you probably have the right number of poles or it would be a whole lot worse.

Flux

[Harold in CR]

 So, am I correct in assuming, that epoxy is not "shorting" out the magnetic field of the magnets ?? I might cut the epoxy out from between the magnets and lift out the magnets and place more back iron on the spool. How much would be adequate ? I still have about 3mm gap from magnets to stator teeth. I was also wondering if getting the magnets to just clear the teeth would help any ? I didn't think the clearance I have now was a factor ?

 Thanks for the input guys. It's tough being old and learning these things. 

[Flux]

The epoxy may be loosing you a bit of flux but I suspect it will not be a big factor. If you dig the magnets out and re-do it then a plain epoxy may be worth using.

Back iron thickness for 8mm magnets ought to be about 5mm ( absolute minimum 4mm ). 6mm will be better if you can find the space.

Reducing the air gap to a minimum helps output but it will be a trade off against cog. A gap of 1mm at the closest point should be a good compromise, perhaps 1.5 to 2mm average.

If you can manage these things you ought to be up to the point of saturating the core teeth and that is about as good as you can get.

Flux

[Harold in CR]

 Thanks Flux. I will ponder this today, to see what I can possible modify. IF I can get the magnets out without breakage, I can turn off a little more from the bottom of the channel that the back iron, magnets and epoxy occupy.

 Might be a challenge, but, I believe it is worth the risk. Does that back iron need to be joined at the seam, where both ends will meet ?

[Flux]

No the back iron doesn't need to be joined. Probably it will be easiest to make a cylinder and split it so you can put both halves on without having to do a lot of bending. Just put the joins under the middle of a magnet to get the best flux path.

Flux

[Harold in CR]

 Excellent idea. Today, I stripped everything off the Aluminum hub. Cleaned up the magnets and need to figure out how much deeper I need to cut the channel to set the thicker back iron and epoxy and magnets.

 Thank you very much, Flux.

 Harold

[Harold in CR]

 Here is an update. I stripped the aluminum spool and all I could find was a galvanized water pipe for back iron. SO, I cut 2 rings off and ground them symmetrical to just over ¾" wide. Split one, then cut a piece off the other, to make them long enough to fit around the spool. Hammered them on a piece of 2" solid bar so the curves would be correct. Finally got them where I wanted them, and cut them to fit, cutting both ends on an angle, so they "overlapped" so a magnet would cover them as advised.

 The pipe was right at 4MM. It's the best I could do. I have a drill press, a 4½" angle grinder and hand tools, so, making this was time consuming. FINALLY removed enough aluminum from the ¾" wide groove, to allow the split iron ring, magnets and epoxy to all fit within the stator teeth.

 Last night, epoxied all the magnets on, and, this morning, cleaned up all the excess epoxy and assembled the alternator. Chucked it up in the drill press, did the 1725 belt gearing, clamped the housing in the cross feed vise, and hooked up alligator clip wires to the output and case. mashed the start button, and the DVM says --- 20.87V.    I left the windings connected to the diodes, for the 12V output.  Nice to see this thing finally working.

 Thanks to Joestue and Flux for pointing out where I went wrong.

 Now to see if it will motor.    IF it will ever stop raining, I will dig holes for the frame work for the Lenz2 turbine and see what I can get for output. Wind this past wee has been in excess of 17 MPH average, with gusts to over 35 MPH.