Jan Steinman via EV wrote:
I have a friend who owns a motor service/rewinding shop. He says it
would be cost-prohibative to do so, if you had to pay someone to do
it.
I'm sure that would be the case. If shop time is $100/hour and it took 4
hours, the rewind would cost $400. Nobody will pay that when they can
get a rebuilt one from Mexico for $150.
But, it's not all that hard. You have to cut off the old windings, which
are glued in place. Take *good* notes on exactly how it is wired!
If you're going to increase its voltage from 12v to 120v, then each coil
needs 10x the number of turns of wire 1/10th the cross sectional area.
Thick wire is hard to wind; but small wire is much easier.
The basic technique is to make a wooden jig that looks like the stator
iron but "laid out flat" like this:
_|-|_|-|_|-|_|-|_|-|_
Each of the wood "posts" is exactly the size of the iron teeth of the
rotor. You wind the requisite number of turns around each wood post. As
I recall, you'll have N turns around a post, then skip 2 posts, then N
turns around the 4th post, then skip 2 posts, etc.
Once wound, you tie each coil's turns together with string. Then slide
them off the wooden jig.
Cut pieces of insulating paper to slide into the teeth in the rotor.
They don't do this on car alternators, as they're only running at 12v.
But you'll need this extra insulation for a higher-voltage alternator.
It also keeps the steel from nicking the insulation during assembly.
Now wiggle the coils into the slots between the teeth of the stator.
Often, the entire bundle of turns won't fit in as a single group -- you
have to wiggle it in, a few turns at a time.
When they are all in, wedge them in with a wood or insulating paper shim
(so they don't escape while you are crowding the next coil's wire into
its slot).
All three phase windings are done the same way. You'll find that more
than one set of coils goes in each slot. If this is the first motor
you've ever rewound, it will help to deliberately use one size smaller
wire, just to make things easier. This will slightly reduce the maximum
current you can get.
Once you have all the windings in, you can glue them in place with high
temperature varnish or epoxy.
You'll want to replace the diodes. The stock diodes *might* survive at
100v or more; but they were chosen to be cheap, and only tested to 12v.
Use diodes rated for at least double your new maximum output voltage.
When you increase the voltage, you reduce the current proportionately.
I.e. a 12v 100amp alternator becomes a 120v 10amp alternator. Rather
than use the special oddball automotive diode packages, just bring out
the 3 phase wires, and use an external bridge rectifier. It's easy to
get a 600v 120amp 3-phase bridge rectifier.
I think all modern alternators are three-phase "Y" configuration,
which is a bummer. A delta-configuration might be simple to get
triple the voltage out of.
This is done on purpose. One reason is that wye gives you 1.7 times more
voltage for the same number of turns (so fewer turns are needed).
Another reason is that the sloppy construction of car alternators means
the voltages on the coils are not all that well balanced. In a delta,
the voltages *must* add up to exactly zero or you get circulating
currents around the ring (extra heat, low efficiency).
Use the field to control the voltage and current it generates, as
usual. With the field off, the alternator just freewheels. With the
field on, it generates.
Simplest case, run the field on your brake light switch, possibly
with the help of a relay.
That can work. But remember, the field current needs to change depending
on the RPM. The easiest way to control it is to keep the stock 12v
voltage regulator, but use it to sense some fraction of the new
alternator voltage. For example, if you have a 120v pack, then use a
10:1 resistor divide that tells the regulator the pack is at "12v" when
it is really at 120v. Power the field and regulator with a small DC/DC
converter (like a 90-264 vac/vdc "universal input" laptop supply with a
12v 1 amp output.
I saw one scheme where someone added an electric clutch, as used by
air conditioners. But is that really necessary, given Lee's assertion
that the field-less alternator has very little drag?
Try it; you'll find that an alternator "free wheels" very easily with no
field current. The belt will waste a lot more power than freewheeling
the alternator.
If you're going to have an electric clutch, it would be better to have
it on the motor, and not the alternator. That way, when the clutch is
off, you aren't running the belt, with all its losses.
I have a Nevile-Leece 2kW alternator that came off a fire truck, and
am planning to use that for regen braking.
Truck and bus alternators are decidedly more efficient (70-80%) than run
of the mill auto alternators (which are around 60% efficient).
--
If you're not stubborn, you'll give up on experiments too soon.
And if you're not flexible, you'll pound your head against the wall
and you won't see a different solution to a problem you're trying
to solve. -- Jeff Bezos
--
Lee Hart's EV projects are at http://www.sunrise-ev.com/LeesEVs.htm
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