Modifying the Rotor

and final assembly

Like the stator, the rotor has six pole sets distributed as "claws" around it's circumference. These claws have a strong electromagnet formed on a core inside, with the windings oriented axially with the rotor shaft. One set of claws is thus a "north" pole when this electromagnet is energized, and the other set is a "south" pole. We will now remove the field winding electromagnet from the rotor and substitute a permanent magnet in it's place that also has axially oriented poles. The permanent magnet used is much stronger than the electromagnet that is removed, but the magnetic path through the claws remains unchanged except in flux density.

The iron used for any alternator rotor is always chosen to be a highly permeable type, so the magnetic flux lines within the rotor are largely constrained to flow from one pole to the other through the claw structures at each end of the electromagnet, folding around the internal (electro)magnet and passing between the claws through a zig-zag shaped circumferential air gap. At the dimensions we are dealing with here there is no reverse flux flow through the rotor shaft itself, because the field strength overall is strong enough to repel any such effect. That is, the rotor shaft will become magnetically entrained and act as yet another part of the rotor (electro)magnet. The shape of the zig-zag gap between the claw pieces, being air and much less permeable than iron, directs the magnetic flux into a similar zig-zag shape. So as the rotor rotates there will be a position where this gap exactly bisects a stator segment and both the "north" and the "south" claws on either side of the gap will have an equal magnetic influence on it. As it continues to rotate, however, a greater and greater area of either the "north" or "south" claw will come adjacent to this particular stator segment, thus transferring the particular magnetic field polarity into it. This zig-zag "claw" design therefore prevents abrupt magnetic transitions from taking place in the stator segments as the rotor is turned. By preventing abrupt transitions this elegant design thus largely eliminates any tendency to have a location, or a series of locations, where the magnetic path is more permeable and thus more attractive than adjacent locations. It is these variations in permeability around the circumference that are the cause of magnetic "cogging."

Once we understand how the rotor is designed, it is an easy to see that the axially wound electromagnet that comprises the field winding can easily be replaced with a permanent disk-type magnet with the same axial polarity and a suitable hole for the rotor shaft. Well, perhaps "easily" is not quite the right word. A further benefit is the astounding fact that permanent magnets such as the various rare-earth types have much stronger magnetic fluxes than can be practically achieved with electromagnets. By "practical" I mean that superconducting approaches are beyond most of our means. And, as it turns out, not even necessary.

So let's proceed to the details. After one last warning:

The magnet used here is VERY DANGEROUS TO HANDLE. It has a pull strength against iron of over 150 pounds. If your finger is between it and that iron it will be like someone who weighs that much standing with all their weight on that finger, except that there will probably be some inertia added to the effect as the screw-up that leads to this situation will happen extremely fast. The magnetic flux from this magnet will also damage any magnetic media it comes remotely close to, most watches, and quite probably a lot of other things as well.

So, before doing anything with it take your watch off and put it somewhere safe, only work on a wooden or other non-magnetic table and generally keep all other metals away from this magnet as much as possible. If nothing else, I feel I can safely guarantee that this magnet will teach you respect. Oh, and


The first thing you must do is disassemble the rotor. For this you will need a shop press, and for that you will need to set up a jig something like the one shown here. I used the hold-down clamps from a milling machine, and drilled and tapped holes in the shop-press plates to accommodate the bolts. jig
You will be pressing one of the "claws" off of the shaft, which is splined, and you will want to replace it later so the first thing you need to do is make some marks somewhere so you can get the same alignment between them. Then set up the rotor as shown. Note that you will be pressing the end of the shaft that the pulley mounts on through the claw, NOT the other way around. press
A piece of cake, if you do it right! You can see the field winding on the left claw. It's in a greenish plastic piece. a piece of cake
We may inconvenience our electrons, but we don't waste them, so remove the field winding and discard it. As you can see in the photos, the field winding is on a very stout pole piece which is an integral part of the "claw." The magnet we are going to put in here is roughly an inch thick, so that means that you must shorten that pole piece by one half inch. You will need to get some help machining this, unless you have a lathe. Anyway, assuming you do have a lathe, place the claw with the shaft still in it and the brush slip-rings in a three-jaw chuck as shown and cut off the slip rings. Be careful not to tighten the chuck too much, as you may damage the splines on the shaft if you do. remove the slip rings
Now switch to a four-jaw chuck and mount the same claw with the inside of the claw facing out. Use an indicator as shown to make absolutely sure that the shaft is centered, and the face cut you will be making is perpendicular to the shaft. Any deviation from perpendicularity will result in an air gap when the magnet is installed, and this will cut down on performance considerably. claw setup
Cut the face back as shown, exactly one-half inch. cut the face back one half inch
Now do the same thing with the other claw. Set it up with a dial indicator as shown and make sure that the face you are cutting is perpendicular to the lathe's rotation axis. Centering is not as important this time as you will be cutting past an empty shaft hole. pully side setup
Cut back the face as shown, exactly one-half inch. pully side cut back one half inch

You now have a rotor that, if assembled, would have a one-inch gap between the claws, into which we will insert a magnet. At this time you may want to clean up the parts a bit and even spray-paint the claws, being careful to mask the faces you have just cut and the shaft. If you do paint it, use a very light coat so it doesn't drag against anything when inside the stator. Assembling this rotor will require a drill press, some table clamps, an eye bolt, some grease, a half-inch copper pipe sweat coupler, about 20 inches of very strong plastic strapping tape or something similar, and a strong hose clamp that can open up enough to go around the magnet.

Please do not think that you can skip the following steps and put this thing back together by hand. You are NOT strong enough. Not only that, but these magnets are brittle and if you let one slam into something it is very likely to break.

The magnet has been nickel plated and is quite weather resistant, but the alternator shaft is not. A good way to protect it is to slather it well with grease, which is the red stuff in the photo. Then mount it on a sturdy drill-press table as shown here. That is, with the rotor shaft directly under the chuck. Make sure that the chuck is very firmly seated if it uses a taper to attach to the drill-press. You don't want it pulling out at a bad moment. claw on drill press
Pull the strapping tape through the eye bolt and attach the ends to the magnet as shown, then put the eye bolt tightly in the chuck. Be very careful to not let the magnet get close to any iron when you are getting set up to lower it. ready to lower
Now use the drill-press handle to carefully lower the magnet onto the rotor shaft and the claw. safely down
Remove the hose clamp and strapping tape. Using a screwdriver or similar lever, force the magnet to a center position around the shaft. Cut the half-inch copper coupler to a one inch length and split it lengthwise. Then by whatever means is handy spread the coupler until it will fit over the rotor shaft and press it completely down between the rotor shaft and the magnet. insert copper bushing
Set up the claw and the magnet on the drill press as before. Set up the second claw with the hose clamp and strapping tape on the drill press the same way you did the magnet, align the two claws with the alignment marks you made earlier and lower the second claw over the shaft and down until the magnet is firmly clamped between the two claws. The splines may be stiff and require you to press the claws together, but once they are down they will definitely stay there. The photo shows the assembled rotor. assembled rotor

Isn't she a beauty? Proceed to "Assembly" next.

Identifying the Appropriate Alternator.


Modifying the Stator.

Modifying the Rotor.

Assembly and Voltage Regulation.

Resources and Services.

Purchase our Special Magnet.

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