How to make a Permanent Magnet Alternator. Get any alternator in good working condition, (Make and size is not important, just be sure it works) disassemble by removing the big nut on the pulley end, and sliding the nut, washer, pulley, spacer, cooling fan, second spacer off the Rotor (Remember it ROTates) shaft, they should slip right off, If you’re unfamiliar with alternators then line the parts up on the workbench, you’ll be using them later, an air impact wrench is the best way to remove the nut,(Addendum, Don’t hold the fan, you’ll cut your fingers, use gloves and hold the pulley loosely, the idea is to create a drag, then the impact wrench does its job a lot faster, don’t try to clamp the pulley in a vise, you’ll just destroy the pulley those wrenches are powerfull enough to rip it out of a vise) but is not absolutely necessary, there is a hex in the center of most alternators to take an Allen wrench, clamp an Allen in a vise, slip the hex into the hollow shaft, and loosen the nut with a large Box wrench, do not use an open end, or a crescent, the nut will distort and be ruined.(No hex? go to your local garage, the mechanic will usually be happy to take a few seconds to unscrew the nut with his air wrench, You might want to tip the guy) After removing the pulley etc., turn the alternator over and remove the three (Sometimes four) screws holding the two case halves (Front and rear) together, note that the alternator is three parts, the rear Housing (Where the wires go in) the center Stator, (remember it stays STATionary, made of Laminated steel) and the front housing, (Aluminum with air slots) sit the alternator on it’s rear with the shaft sticking up, and use two small flat screwdrivers to gently pry the FRONT end housing toward the shaft end away from the Stator, (There are small grooves or slots between the steel and aluminum just for the screwdrivers to fit in) it should come off either with the rotor, or without, but do not pry on the half toward the rear, leave these two parts (Stator and rear housing) together for right now.

Lift out the rotor, if it didn’t come off with the front housing, and ignore the scary small things that jump out, those are the slip ring brushes and springs, and will be discarded anyway. Prying will not be needed it should slip out easily. Set the bottom half (Wires and copper coils) aside for now, take the rotor (Center part that turns) and check, usually there is another spacer on the front, this locks the bearing so the rotor will not slide back and forth in the case when running, slip it off, and put it with the pulley etc., Now take the rotor assembly to a large hydraulic press, and press the shaft out of the windings and slip rings, Some will only press one way, so look for a shaft that is bigger on one end, if bigger, press away from the big part ) this will usually destroy the slip rings, that’s OK we don’t use them again. Now you should have just a plain shaft, usually knurled to hold the rotor tightly, set the center part aside for other scrap use, it has a copper coil in the center worth keeping for other uses, or just as

Measure the shaft where the rotor was, write this dimension down. Measure the inside of the wire coil in the back half, where the outside of the rotor would normally turn, write that on the same paper as the shaft size. Also measure how wide the Stator coil is,
usually around an inch wide. Go to Radio Shack, or where ever they sell “Button” magnets in your area, you are looking for square magnets like the ones that you use to stick notes to the ‘fridge, you want the ones about 1/8 thick, and about 1 inch square, (Correction, it will work with any shape magnets ) rectangles will work better if they are available, get the strongest magnets you can find, about a dozen will do, and a package of “5 Minute” Epoxy Glue, if you can get a small roll of fiberglass package strapping tape fine, if not we’ll use something else, go back to your workshop.

Addendum, I also recommend that you can get small “Button” (Round) magnets and use two of them in a row, (Smaller, more powerfull)or if a small alternator round magnets can be “Staggered” in the cut groove using a “North” on the left then a “South” on the right for a more compact alternator rotor.

Get a chunk of aluminum (Correction, steel not aluminum, it’s much cheaper and as pointed out to me the flux needs to travel through a Ferrous medium for greater power) that’s the rough size as the outside measurement, and a bit longer than the magnet’s long dimension 1/8 larger and 1/8 longer will do nicely, I usually cut a slice off a round aluminum (Steel)bar, about a 1/8 inch bigger than the inside will be, and about 1/8 longer than the inside of the coils are wide front to back, you will turn this down, so get it bigger than needed.

The next part requires a lathe, chuck the (Steel) aluminum disc, and drill or bore a hole in the center that’s about 0.010 (Steel requires a looser hole, about 0.005 smaller not the 0.010 for aluminum) SMALLER than the outside of the knurled part of your shaft, If not knurled only 0.003 smaller((Steel, only 0.002 smaller) take the disc out and press the shaft into the hole, it should seize nicely (That’s what the grooves are for) if not, you got the hole wrong, and will need to resize (If too small) or get another aluminum (Steel) disc (If too loose), measure the distance from the case rear to the end of the windings, and press one way or the other until the disc will be where the rotor used to be, (Center of the Stator windings) Don’t forget to add the shaft length that will go into the rear bearing, return to the lathe.

Now chuck on the rear shaft where the bearing rides, (Wrap a small piece of brass shim stock around the shaft for protection on this bearing surface) true the shaft, and use a live center on the nut end, when the shaft runs true turn the outside (Correction, the outside of your rough rotor disc)to about 0.010 smaller than the inside of the Stator inside diameter,(Correction, measure the old rotor that you pressed off, and turn your rough disc to the same size outside diameter, much easier) this is not critical, try to keep it near the steel laminations, but a gap here only decreases the output slightly, so feel free to use a larger space, up to about 0.060 will not hurt.

Next cut a groove to fit your magnets (Bet you wondered why the trip to the store came first) if rectangular, make the groove big enough to lay the magnets in it sideways (With the long way front to back) and hand fit a magnet into the slot, making sure that the slot is deep enough to have the magnet entirely inside the slot at both ends, remember that the slot is curved, but the magnet is flat (On my first one I had to scrap the finished rotor because the ends stuck up and hit the stator)

Addendum, if too small to take twelve magnets go to the staggered magnet idea with the round magnets, I did not emphasise just how important it is to be sure you have the same number of magnets as the alternator has field poles, they usually have twelve, six “North” and six “South”, look at the old rotor you pressed off, and count the triangular pieces, make the same number of “Poles” on your disc

Now you’re ready to put the magnets in the groove, take each magnetand as you slip it in the groove you just cut, reverse the magnet, you want a north-south-north-south, etc., it does not matter which is “First” just be sure to alternate (Alternator, remember), they will try to jump out, tape them in with the fiberglass tape, it’s very unlikely that the magnets will come out exactly an even number, or that the magnets will have no gap (I got lucky on two assemblies, the rest were “Gapped”) (Error, you MUST have the same number of magnets as the original rotor had, if you’re using two magnets side by side for added strength it counts as one magnet, just be sure both are either “North” or “South” and not one “Flipped” the wrong way) so adjust the magnets until they are roughly even (Space between the magnets is roughly the same) and you have an even number of magnets (Mine usually hold from eight to twelve, that’s why we got a dozen magnets, save the rest for other projects) (Correction, if you can’t get the same number of magnets as poles on the original, go to smaller magnets, “Staggered” magnets, or two “Button” magnets turned the same way)

Once you have the magnets spaced, use another magnet to check polarity, it should “Flip” going from any magnet to the next adjacent magnet, when you’re sure, epoxy the magnets down,

I found it easier to wrap a strand of tape around the rotor, then “Peeling” the magnets off in a strip, putting epoxy in the groove, then wrapping the magnets back in place, tying another strip of fiberglass tape around them tightly and squeezing the magnets into the epoxy goo with my fingers. Addendum, if you can’t get fiberglass package tape, wire will do, but other tapes are not strong enough to hold the magnets down against the pressure they exert trying to “Flip”. Whatever method you use, be sure that no magnet ends stick up, those things are very hard, and can’t be cut off without using a diamond wheel.

While the goo cures, go back to the Alternator’s back half on your workbench, look inside and find the two springs that always jump out when the rotor is removed, put them in your junk box (What? you don’t have a junk box, shame on you) and see how the carbon brushes are fastened inside, if soldered, just cut the braided wires as near the end as possible, You want the wires to touch nothing, or there will be a short, so remove them, If fastened with small bolts, nuts, etc., just unscrew them and into the junk box they go, the brush housing can be removed or left, it makes no difference as long as it doesn’t hit anything when reassembled.

By now the Goo should be cured enough to remove the fiberglass tape without the magnets jumping out, wait if not hard enough, then strip all the tape, and clean away any epoxy that sticks out.

Ready to reassemble, slip the Rotor into the back half, (Stator and rear Housing, make sure it doesn’t hit the rotor against the Stator, if so clean epoxy off until it fits, Replace the front bearing washer (If it had one) and slide the front housing on, it probably will not rotate, don’t worry, just fasten the three (Or four) screws that pull the housings together then check, a lot of the times the bearings will “Cock” and not spin freely until bolted up. If it won’t spin, pull it back apart and find what’s hitting, then proceed.

Spin the rotor, Spins freely?(Error, the rotor will “Cog” (Bump as it turns) this is desired, and the stronger the “Cogging” the more power it will develop, don’t mistake “Cogging” for hitting something, the rotor will not spin freely) No noise of hitting something? Good proceed with the spacer, fan, second spacer, pulley, washer and nut, tighten the nut, (Addendum USE AN IMPACT WRENCH) check again, still no hitting, you’re done.

Addendum, you may be tempted to lave off the fan, don’t do it, cooling is needed.

Addendum, the alternator will develop power when turned either direction, but unless you turn it clockways (Looking at the pulley end) THE NUT WILL EVENTUALLY LOOSTEN AND UNSCREW destroying all your work. A very few alternators have a left hand thread, but you’re highly unlikely to run across one of those, they are used on Corvettes, and are very expensive, if you do find one with a “lefty” thread, sell it to a corvette repair shop and probably buy yourself a brand new windmill with the money.

Your Alternator now needs no power input to generate electricity, (Addendum, and is not now limited to 12VDC, it will produce any voltage Just spin it faster to produce 24-36-48 VDC etc, but it will have to be connected to a correct battery bank first, just spinning the alternator unconnected to a load will give some very high voltages, but no real power is being produced, Example, I hand-spun a Nippondenso I modified and got the voltmeter to read 45 Volts) but you also have no field control, and the output is solely based on RPM and power you put in the Alternator, you’ll have to experiment to find the RPM needed to put out the power you want, but that’s as simple as connecting the alternator to a battery (Case is Ground, Big terminal is output, (Addendum, USE A 60 AMP FUSE IN THE BATTERY WIRE, as close to the battery as practical, then a short circuit won’t blow the battery up in your face) no other wires, connections etc needed, ignore them) Put an ammeter and a voltmeter on the wire, and spin the alternator with a motor.

Enjoy your new toy.

A few added late thoughts, this whole idea came to me while modifying an old Sears Generator (2.5 KW 110 VAC)that I bought cheap with a blown gas engine, I disassembled it and made a short shaft to use it as a belt drive, and found a small magnet epoxied into a hole drilled in one of the generator’s field pieces (Rotor) I figured they were using the small PM to excite the field enough to start the generator producing power then just tapping the output for the field coils to continue at rated output. Sorta like Bootstrapping the field, Maybe this neat trick could also be used to excite an automotive alternator, as they will not generate any power unless first power is applied to the field windings (Rotor in an automotive alternator).

So somebody want to try this? instead of gutting an alternator and building a new PM rotor , maybe just drilling a small hole in one of the rotor’s “Fingers”, and epoxying a small button magnet will make the alternator self-exciting, at worst you can then gut the rotor and procede with a PM rotor (Be carefull of the field’s direction, or you’ll cancel the magnet when running, make sure that the “North” or “South is the same as the rotor’s “Finger”)

Another thought, after pressing the old rotor off, pull the two halves apart, then remove the wire coil and put a couple of those big speaker magnets where the coil was, press back on the shaft and remove the brushes as directed above.