Alternator - Magnet fit to crankshaft

I finished the crankshaft adapter this evening and put the two anti-rotation pins in it.  The holes are tapped for 10-32 grub screws, and they are set in place with loctite.  I changed the design slightly while parting the adapter from the lathe.  It was easier to leave a 1/2" diameter raised ridge around the center of the adapter on the outside to allow perfect centering of the magnet cup, than creating a special washer for centering.  The bolt is 3/8" and the hole in the magnet cup is 1/2", so a method of getting it centered was required.  The run-out was acceptable, and I'm starting to think about creating the flange to hold the stator coil.  I still haven't decided on exactly what to make it from.  Aluminum would be easiest to machine, but I don't have a piece wide enough without having one cast to approximate shape.  I may make it in two parts and just bolt it together, but this becomes more challenging to keep it centered.  The original back piece of the Kubota alternator is too thick, but I might be able to machine it down.  The problem is that it still needs a wider plate to fasten to the edges of the housing.  The critical part will be to keep the stator perfectly centered, as not to touch the magnet.

I test fit the stator to check the magnetic center.  If I think the output is too high (since its designed for 12v output), I might be able to make the field strength adjustable by allowing the stator to pull out toward the front of the bike and weaken the magnet's grip.  I suppose I could use spacers or washers to accomplish this to get it just right when its all done.  It did light a 6 volt bulb readily only turning the alternator by hand. Hmmm.

I still need to make the inside frame for the housing so it can be fastened to the front of the engine, and give something to bolt the cover to.  That will be the next task.  One part of the frame inside the housing may need to be removable to install the magnet cup once the frame is in place.

Here's my buddy Mark testing out his plasma cutter and making some parts for the internal frame of the alternator.

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Alternator - The Build

From the last post, I've got pictures of the Kubota alternator all dismantled.  Next comes the modification pieces.

I've made an adapter ring plate to which flat bars will be welded to hold the case of the dynamo to the engine block.  This has the challenge of interference with the side of the case.  The cap screws that I will use will have to be ground down somewhat, and the side of the dynamo case ground down to accommodate the screws.  This should hold the case firmly to the engine block.

The next big item is machining the crankshaft adapter.  This will be a copy of the back of the armature, which will allow the magnet of the alternator to bolt directly to the crankshaft.  Here's a picture of it just started out of a piece of roundstock.  I'll need to get a parting tool for the lathe before I finish the part.

Here are the plans for the finished part.

Alternator - Proof of Concept

I'm fed up with the original Sunbeam S7 Lucas dynamo, and the MC45L dynamo that I bought. Neither one produces enough electricity in a foolproof fashion.
I don't want to have to worry about if there's enough battery to get me home anymore, and I want to run the lights all the time. I also don't want an after-market alternator conversion that does not look like the original.

The plan is to create an alternator conversion that produces 6 volts, is fully encased in the original housing, and works reliably.

I've seen others who have used a Kubota tractor permanent magnet alternator to do the job. Its only 3-1/2" in diameter and shorter than the MC45L dynamo. That means it should fit inside the casing. The trick is to mount it up.



It comes as a flange mount unit for belt drive, with its own bearings.

[edit]  I actually did use the bearings after all to keep the stator assembly centered in the magnet.  The magnetic forces are VERY strong radially, and will pull the stator into the magnet if there is the slightest mis-alignment or looseness.  A spacer piece was machined to hold two of the bearings into the stator and then the bearings were placed on the center bolt with their own machined spacers.

The permanent magnet rotor can attach directly to the S7 engine crankshaft, and the stator can mount to the engine block, or dynamo casing. All that is critical is that it runs centered and true. Should not be a problem for a guy with a lathe in his workshop.

Here's the sample fit up.  The center bolt presses out of the rotor cup easily.  This will allow me to make a bolt (left-handed Whitworth of course) to fit in there and mount to the crankshaft.  The question is whether 'tis better to bolt the stator to the engine block, or to bolt the stator to the outside of the dynamo, and put the rotor cup "upside down" against the crank.  I'll have to make an adapter to provide the seal to the front of the engine regardless.  The stator to the front makes it easier for the wires to come out to the terminal block on the dynamo casing.

The final part is the regulator / rectifier from a scooter.  I've bench-tested the rectifier / regulator up to 6 amps load.  It works fine with 14 vac input, and does not overheat.

Here's a VERY ROUGH sketch of the assembly.  I keep changing the design, so my dimensioned sketch is pretty bad.  I will require a minor modification to the dynamo case.  The cap screws will interfere with the side of case, so the case will need to be ground away in two small spots not visible from the outside.  The cap screws can be modified slightly to reduce the amount of grinding to the case.