The alternator requires two rotors to be built. The first step
is to build a wooden mold. Each rotor will be made from 12" diameter
steel disks each with 12 magnets on it.
Once we've built the rotors we'll cast polyester resin around them to
keep the magnets in place and prevent corrosion. In order to make a
neat casting we need to build a mold out of plywood. Like many other
things in these plans, there are alternative ways of doing things. Here
we'll detail exactly what we've done. Some of the pictures show two
molds in use, but you only need one. See the picture above for
dimensions of all the parts
- 3/4" thick plywood, 2 squares 14" x 14" - 1/2" thick
plywood disk, 6.5" diameter - 1/4" plywood, 1 disk 2.75" diameter -
about 20 1" long wood screws
The mold is 14" square. The hole in the center is 12.5" in
diameter and 3/4" deep. In the center is an island 2.75" diameter and
1/4" thick. The lid is the scrap which we cut from the center of the
mold (a 12.5" diameter disk). The center of every part has a 1/4" hole
drilled through the center that we use to align everything during
assembly. All the disks are easy to cut out with a bandsaw, or a jig
saw. With some patience a coping saw would do fine as well. All the
disks and the inside of the top that we cut out should be sanded
smooth, and preferably slightly tapered so that the top of hte hole is
slightly larger diameter than the bottom. Once all the parts are cut
out, screw the 3 square pieces together so that the one with the 12.5"
diameter hole is on top. The screw the small 2.75" dia disk down in the
center - using the 1/4" drill bit as a pin to center it perfectly.
Caulk all the seams inside the mold so that resin can't run into
cracks. This, the sanded surfaces, and the tapered hole in the mold
will make the molded pieces easy to remove.
The magnet template
The next tool you need to make is a template for placing the
magnets. The one pictured was made at a local machine shop with a CNC
water jet cutter out of 1/8" thick aluminum and the cost was quite
reasonable (about $25 USD). But you can just as well build it from thin
plywood or plastic. It's a 12" diameter disk, with 4 1/2" holes on a 4"
diameter (just like our magnet rotors) and 12 equally spaced cutouts
the size of our magnets (1" x 2" x 1/2" in this case). We do sell
CAD-cut aluminum templates that are perfectly aligned for this -- check
our online store.
Building the magnet rotors
- 12" diameter mild steel disk, 1/4" thick, qty 2 - 1" x 2" x
1/2" N35 grade NdFeB magnets, qty 24 - cyanocrylate glue with
accelerator - fiberglass cloth or mat, 2 square feet - 1/2 gallon
Start with two steel disks, 12" diameter. Each disk should
have 4 1/2" holes on a 4" diameter circle(a touch larger to provide
some clearance is nice) and a 2.75" hole in the center. You can build a
rotating table and do a pretty neat job cutting these out with an
oxy-acetaline cutting torch, but we usually have ours cut out by a
fabrication shop. A CNC laser cutter, plasma cutter, or water jet
cutter will do a very nice job. If you have it done, you might have
them cut all the holes for you - it saves a lot of time and assures
that things are accurate. Otherwise, if you decide to machine your own
rotors, the first step is to cut a 2.75" hole in the center of both
rotors. Use a high quality bi-metal hole saw and a drill press for
this. The drill press needs to be run very slowly and you need to use
lots of oil. Easiest is to clamp both rotors together and cut both at
the same time. Save the scraps from the center, we can use one of those
later. Next, keeping both disks clamped together, drill out the 4 .5"
diameter holes equally spaced around a 4" diameter circle. The easiest
way to do this is to put the wheel hub on the rotors and clamp it
there, and drill right through the holes on the wheel hub. This saves a
lot of layout and assures accuracy. One of the disks needs 4 more holes
7/16" dia (also spaced around the same 4" diameter and located between
the .5" diameter holes we just finished) which we'll tap 1/2" - 13 for
jacking screws to aid in the assembly/disassembly of the alternator.
Again - we usually have this done at a fabrication shop - when they
cutout the disks for us it's easy for them to use the same CNC machine
to make all the holes.
Pictured above we're tapping the 4 smaller holes 1/2" - 13.
It's important to use lots of oil (or tapping fluid) when running the
tap in. Try to keep the tap as straight as possible. Once it starts to
cut threads, turn it just till things start getting tight, then back up
a bit and 'break the chip'. Continue this untill the tap goes all the
way in and spins freely. Never force the tap in if things get too tight
- always back it up, break the chip and then go foward again.
Use a countersink to chamfer the edges of the 1/2" diameter
holes. This makes things assemble more easily and helps protect the
threads on the studs that hold the alternator together. Once this is
done, all the 'metal work' is finished for our rotors. Both rotors are
oily (finger prints and oil from drilling and tapping) so we need to
clean the carefully with some kind of solvent. We usually use laquer
thinner. After that - try to keep grease off them, handle them with
clean hands. You're about to start putting magnets on them, so this is
a good time to clean the work area. Metal chips from the drill press
and grinder should be cleaned up, or you should move the work to a new
The steel disks don't always come out perfectly flat. I
expect some sheets of steel get bent in handling by forklifts and such
and when the disks get cutout sometimes we find them slightly warped.
Check for this with a straight edge. Flatening them can be done but
it's tricky. We usually locate the dimension in which it's warped and
we put our magnets on the most convex surface. (the surface facing up
towards the straight edge in the picture is the surface we'd put the
Put the magnet template down on one of the steel disks and
line the holes up.
Place the other steel disk down on top of that, with the
holes line up and pin the sandwich together with two 1/2" drill bits
(or wooden dowels or bolts or whatever).
Looking from the side you can see the 'gaps' in the template
where the magnets will fit. Pick one gap and use a permanant marker to
mark both sides of the gap. This is where we'll place the first magnet
on each disk.
(drilling into the top magnet rotor)
(drilling into the bottom magnet rotor)
Use a small drill bit (3/16" is a nice size) and drill a
divit (a dent - not a hole, don't drill through) into both disks
between the marks we made. These will be on the outsides of the magnet
rotors and will serve as 'indexing marks' so that when we assemble the
machine we'll know how to line up the two rotors. Alignment of the
rotors is critical in the operation of this alternator, they must
always go together the same way with alternating magnetic poles facing
one another. Once we've done all this we can take the top rotor back
off the stack and put it aside in a safe place (away from the bottom
rotor because were about to play with magnets).
For this alternator we require qty 24 Grade 35 NdFeB magnets
1" x 2" x 1/2" thick. These are available from many vendors, they
usually come either epoxy coated or Nickel plated, either way is fine.
These are very powerful magnets and need to be treated with extreme
focus and caution! Two coming together on your finger could hurt very
badly and leave blisters easily. Once we assemble these on the rotors
we have some very powerful/dangerous magnetic assemblies. Two finished
magnet rotors coming together on your fingers could easily break them!
Build one magnet rotor at a time. When it's finished- put it in a safe
place. When building these be sure that all ferrous (anything
containing iron which includes steel tools, wrenches, knives scissors
etc) are away from the work area. Only handle one magnet at a time and
always grip them firmly. If a magent flys onto a piece of steel or into
another magnet at high velocity, it may break and send shards flying!
Handle one at a time, handle them with a firm grip. Store them in a
safe place away from kids and folks who don't realize what they might
be getting into. Keep them away from electronics/video tapes and other
forms of magnetic storage medium. These magnets are perfectly safe when
handled properly, but most folks are not familiar with the dangers and
there can be surprises.
The magnets are so strong they can be tricky to seperate off
the stack. The best way is to place the stack on a wooden workbench and
hold the stack firmly. Then grasp one magnet firmly with the other hand
and slide it off. (you'll not be able to just pull them apart, you have
to 'shear' them apart)
Now we can place the first magnet on the bottom magnet rotor.
The template is pinned to it and made of wood or Aluminum so it won't
move. But the magnet is strongly attracted to the steel disk so we need
to hold that down with one hand. While firmly gripping the magnet in
the other hand, bring it towards the edge of the rotor and 'slide' it
into the slot. (don't just try to put it down on - it will pull out of
your hand and hit the rotor hard - possibly breaking the magnet!)
The magnets need to be spaced around the disk with
alternating poles facing up. All magnets have two poles, a North and a
South. Opposite poles (North and South) attract one another, like poles
repel. It doesn't matter how we put the first magnet down so long as
things alternate from there. The safe way to place the rest of the
magnets is as follows: Hold the magnet rotor down firmly to the work
bench with one hand which should be placed over the magnet that's next
to the one you're about to place. Then, holding the next magnet firmly,
bring it over your hand which is holding down the rotor. If the bottom
of the magnet in your hand is repelling the one on the rotor, then
slide it into the slot carefully in it's current position. (Because we
know that if the bottom of the one in your hand is repelling the top of
the one on the rotor then we have like poles facing each other, so the
one in your hand has the opposite pole facing up as the one on the
Once all the magnets are placed on the first rotor you can
remove the pins and pry the template off. Do this carefully so the
magnets don't slide around.
Run a bead of thin viscosity cyanocrylate glue (Super Glue)
down both sides of each magnet. Large bottles (2oz usually) are
available at most hobby stores. It's also handy to have 'accelerator'
which will force the glue to harden immediately. The accelerator
usually comes in a small spray bottle. We don't rely on this glue to
hold the magnets down forever, it's a temporary means to keep things in
place till we finish the casting. If cyanocrylate glue is not available
then other glues should also work fine. Epoxy would probably be fine it
just takes longer to dry.
I expect you could skip this part all together, but I believe
it offers some insurance that our magnets will never fly out. Also - if
the resin cracks this will keep things together for us. Take a roll of
fiberglass drywall tape (this stuff is sticky on one side) and cut the
roll with a razor knife so that you can peel off a strip of the tape
about 1/2" wide.
Wrap the tape around the magnets several times. Be sure that
none of the tape sticks up above the top of the magnets.
Now that the first magnet rotor is finished, drive a nail
somewhere in a wall in a high/safe place and hang it there. It's a
somewhat dangerous thing and should be kept in a safe place.
Now to begin the second magnet rotor. Put the template on it
so that the 4 holes line up and one of the slots lines up with the
marks we made earlier. This assures us that the magnets we place will
be facing each other when the rotors are assembled. The top of the
first magnet on this rotor must be the opposite pole as the top of the
first magnet we placed on the first rotor. In other words, the two
surfaces facing one another must attract one another. Once you get the
first magnet down, follow the same proceedure as you did with the first
Check your work!
You can easily double check your work now. Find a small
magnet and hold it in your hand (dont turn it over - hold it in the
same position always for the testing). Each magnet rotor has one magnet
(the 1st one we placed) between the marks we made. The test magnet
should attract this magnet on one magnet rotor, and repel it on the
other. Then we can go around each magnet rotor and the test magnet
should attract one magnet, repel the next one, attract the next one
etc. If you made a mistake, you need to knock loose the offending
magnets, put the template back on and get them right. Once all the
magnets are placed and the tape is wrapped around them it's a good idea
to clean the magnets and the rotors one more time with laquer thinner
to make sure there's no grease. This will help the resin stick to the
Cut out two rings from fiberglass mat, or fabric. They should
be 12" in diameter, with a 6.5" diameter hole in the center.
Grease the mold everywhere (Except on the bottom - that's not
necessary). A good mold release is car wax, or Johnsons wood wax. We've
also used shortening from the kitchen and axel grease (axel grease is
kind of gross and messy but it works). Grease it really well especially
the first time you use the mold. The first coat tends to soak into the
wood but after several applications it gets better. The point here is
to make the mold greasy so the resin won't stick to it. Once all the
parts are greased well then run a bead of caulk around the outside of
the 12.5" hole in the mold. Also run a thin bead around the outside of
the 1/2" thick 6.5" diameter disk. If it's not still there, stick the
1/4" drill bit in the center hole, we'll need this for alignment.
Drop one of the magnet rotors into the mold carefully. It fit
nicely on the smaller 2.75" disk in the center of the mold so that the
magnet rotor is a good fit and about pefectly centered.
Put the 6.5" diameter disk down. The drill bit will serve to
center it on the disk. The side that we've run caulk around should face
down and we need to press it down onto the magnet rotor. The caulk will
assure that no resin can run under it.
We use polyester resin to cast the rotors. We get this stuff
from almost any autoparts store. It's best to buy it by the gallon (it
takes about exactly one gallon to build this whole machine). It comes
with hardener in small plastic tubes.
It's nasty stuff. It smells bad, the fumes are toxic. Best to
work outside or in a very well ventelated area. Use safety glasses (the
hardener is especially dangerous if you get it in your eyes), rubber
gloves, and a respirator.
It takes almost exactly 1 quart of resin to make 1 magnet
rotor. (maybe a touch less) Usually a gallon of the resin comes with
two tubes of hardener, each containing .77oz (22ml). When casting this
stuff the resin tends to warm up and get hard much faster than it would
in normal applications - especially if its warm outside and if the
resin is warm to start with. We usually use about half the hardener
that the instructions call for. This lets it harden more slowly - I
believe it helps it to be stronger, shrink less and make things less
likely to crack. There have been times when we've used half the tube
for 1 quart (what the instructions call for) on warm days and the resin
has become hard in 15 min or less! (it was hard before we could even
pour it!) If it goes off too fast, there is also the risk of it
catching fire. So be careful...
If you like, there are powders available to color the resin,
or you can just use a little bit of acrylic enamel to give the resin a
color. If you use enamel, I would use about 1 part paint to 50 parts
Pour resin into the mold and over the tops of all the
magnets. The mold needs to be level and it should be completely filled
Place the fiberglass ring over the top and work it in with a
stick so it becomes saturated with resin. Work the air bubbles out as
best you can.
Pour a bit more resin over the top and work that in. At this
point it doesnt hurt to beat on the mold or vibrate it (with a
vibrating sander or something) to work air bubbles out. Air bubbles
won't really hurt it, but they don't look nice. We always get a few.
The lid also has a 1/4" hole in the center. Place it down
over the drill bit and on top of the magnet rotor. You can clamp it
down with magnets, or bits of steel (wrenches etc..) because they'll
all stick to the magnets. C clamps are fine too but more work than
necessary. Keep an eye on the resin that spills out of the mold. When
it starts setting up clean the outside of the mold. Don't take the lid
off though untill you feel the resin is good and hard. In practice,
depending on the tempurature and the amount of hardener we used I find
this takes anywhere from 1/2 hour (which is scary fast - I expect
cracks and shrinkage when it goes that fast) to 24 hours. It seems the
slower the better with regard to shrinkage and cracking.
Once the resin is completely setup we can remove the lid from
the mold. Usually (if we made the mold well and greased it well) the
rotor will just fall out of the mold when we turn it over. If it
doesn't, tap it on the back with a hammer and it should. Sometimes
things get tricky and we have to pry it out, or even take the mold
apart but this shouldn't happen if we did everything correctly. The
wooden disk on the inside of the rotor should knock out easily with a
hammer through the hole on the back side of the magnet rotor. The edges
of the rotor will be rough. We can cut the excess resin off with side
cutters - or remove it with a sander. A belt sander works real well,
but be sure to wear a dust mask. Clean up all the burrs so that nothing
sticks up beyond the surface of the magnets.
There we have a finished magnet rotor! Once one is finished -
then repeat the process with the second one in the same mold. We
actually use two molds (molds are easy to make) so that we can get all
this done in one shot but if you're not in a rush one at a time works
It's only for cosmetic reasons and not important at all...
but if you have a lathe available that's large enough it's fun to clean
up all the edges that way. If you do, be careful and remember how
feircely these rotors are attracted to steel. When cleaning up edges or
deburring on the lathe, *DO NOT* use a steel file. Use sandpaper. A
small flat sanding block is a handy tool on the lathe. A file, or any
other steel tools that are not 'part' of the lathe should be kept away
because of the magnets. I've seen magnets yank steel tools out of
peoples hands on the lathe, it can be very dangerous. Again, this step
is not needed - one does not require a lathe to build this wind
turbine, it just makes things look nice.
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