Posted by Ulysses on May 31, 2009, 2:46 pm
I was hoping that I could achieve a low enough voltage to use it for a
backup charger for my 48 volt battery bank. When I measured over 600 volts
the engine was running at a very low speed and I was expecting it to be a
lot less than 200 volts so I thought my capacitor would be OK just for
experimental purposes. Oops. Another option I have is the use the
alternator the way it was intended by connecting it to the original
inverter. This works fine and I can adjust the engine speed to accomodate
the anticipated load. I can then use it as a generator to power up to 2000
watts (actually I think it will go to about 2400 with the 4 HP engine but I
need to recheck that) and I can use it to charge my batteries at about 18
amps at 50+ VDC by going through my OutBack inverter. The coils are wired
in a Y configuration and I could rewire it to Delta and get roughly half the
voltage but if the Y output is over 600 volts then that won't do me any good
plus I'd lose the option of powering the inverter. Probably.
Posted by Neon John on May 31, 2009, 6:19 pm
On Sun, 31 May 2009 07:46:49 -0700, "Ulysses"
Why don't you simply rewind the stator. It's a very easy task
physically, and only a problem of proportions mathematically.
The easy way is to develop an alternator constant. Then for any RPM,
you simply multiply the constant by the RPM to get the new voltage.
To do that, unwind one pole. Place 10 turns of any size wire on the
pole, install the stator, spin the motor up to 1000 RPM (to make the
math simple) and take a voltage reading.
The constant in turns per volt-rpm is voltage divided by (10*1000).
From that constant you can calculate how many turns you'd need to give
you the desired voltage at any RPM. Since each electrical phase has
multiple poles on that style stator, divide the constant by the number
of poles to get the phase constant. Calculate the total number of
turns and then divide by the number of poles to get the turns per
The amperage obviously goes up for a given power so the wire size must
be increased. The desired outcome is to fill the stator with the same
amount of copper. You can use either a fill table (available on the
net) or you can figure it as a ratio of cross-sectional area of the
Take the existing wire diameter, figure the cross-sectional area with
the pi*r^2 formula. Multiply that by the number of turns of wire
removed. Then pick a new wire size that gives you the same volume of
copper (area * turns) for the new, fewer number of turns winding. The
result won't be exact because the heavier wire will have more air gaps
from imperfect stacking but it'll be darn close.
In practice, I do a little math to get close and then trial-and-error
on one pole to get the exact solution. Then wind the rest of the
poles the same way.
I have done just this process on this little generator
This is a miniature (300 watt) version of the EU. The PM alternator
is the same. I rewound it charge and extend the range of the 48 volt
battery in my hotrod scooter. It doesn't have a governor so I have to
manually set the throttle.
With your setup using an engine with a governor, you'd set the
governor to produce the desired max absorption voltage. Assuming the
engine is overpowering for the alternator size, the engine will then
load down at wide open throttle during the bulk stage. Once the
voltage has risen to the absorption/float transition, the governor
takes over and regulates the speed (voltage) to that value until you
turn the unit off.
The advantage of this approach is that there are no electronics
involved other than the diodes. Nothing electronic to fail or get
zapped by a transient.
Posted by Ulysses on June 1, 2009, 4:41 pm
Thanks NJ. I tried looking up how to do this and kept coming across "post
was removed due to request by so and so." It seems "so and so" is SELLING
the information and somehow managed to get web site hosts to go along with
it. So much for the free exchange of ideas and information.
Even I can understand that ;-) Looking at the coils it appears that it
would be one of those things that requires a lot of concentration, lots of
coffee, and no interruptions. Plus the right frame of mind. But I think I
can do it with your instructions.
I also want to get rid of the pulleys and belt on my engine-driven
alternator that is my main battery charger. I have it geared about 2:1
right now and that seems about right, or close enough. I want to either
rewind the stator coils or get a new stator and use a direct-drive coupler.
From what you said (I'll read it some more to make sure) it sounds like I
need twice as many turns of one half the mass of wire in order to achieve
twice the voltage at a given RPM. How am I doing so far? I have had
nothing but problems with my belt drive ever since I moved up to a bigger
engine (6.5 HP Chinese OHV). It destroys belts in a short time. Some belts
actually turn inside-out. I have very carefully aligned the pulleys and
adjusted the belt tension and tried various belts and pulleys. I'm thinking
maybe the crankshaft on the engine is not perfectly straight but I can't see
any wobble while it's turning and the alternator seems to be OK too.
Posted by Neon John on June 1, 2009, 8:37 pm
On Mon, 1 Jun 2009 09:41:53 -0700, "Ulysses"
Same mass, twice the turns, about half the cross-sectional area of the
wire. Rule of thumb used in the electric motor shop is to go down 3
wire gauges to double the area. That doesn't hold at the extremes but
it's good for the wire size typically encountered in motor winding.
My approach is to find the right number of turns and then use the
largest wire that will physically fit. I don't much mind if I have to
strip off the trial winding to go to slightly smaller wire. I'm
working in a motor shop with every gauge wire known to man right there
on one rack :-)
What kind of alternator is it? Conventional rotor-through-the-stator
type alternators will be a challenge to rewind. Almost always the
stator is dipped and baked after winding which renders the stator a
solid mass. At the motor shop, we cut the end turns off with an air
chisel and then burn the windings out in a burn-out oven.
After the old windings are out you'll need to form-wind the
replacement coils to match what was removed. That's a challenge to do
without a winding machine and a taper form. Finally you'll need
formed slot papers to insulate the slots and a bake and dip afterward.
I suggest finding a motor shop that will do this for you at a
reasonable rate. If the shop is a member of EASA (most are), he can
call them and one of their engineers will calculate exactly what
changes need to be made to change the voltage. We do that all the
time at the shop where I moonlight. There is no charge to the shop.
Of course, if you're using an automotive alternator, ignore all the
above. They're easy to do. Unless you're using a PM alternator,
it'll be a lot easier to change the field excitation than to rewind
That said, I think that you'll be happier with the belt drive than
with a Lovejoy or similar direct coupling. I always have been. A
small engine with light flywheels (essentially all of 'em) delivers a
large shock loading to the coupler on each power stroke and that tends
to hammer out the rubber from Lovejoys. It is worse if the generator
has much inertia of its own.
Why not post a picture or two of your belt drive? Maybe we can figure
out what it wrong with your setup. I've had great success with this
That's a 27 hp diesel engine running a 10 kW head. IT has several
hundred hours, all at nearly full load, with no indication of wear on
the pulleys or belts. You should get the same kind of results if your
pulleys and belt are sized right and your mount is rigid.
Posted by Ulysses on June 2, 2009, 1:36 pm
After I wrote that I realized I should have said "area" and not "mass." I'm
having a little difficulty in my mind trying to figure out how to measure
the area of a circle that is in the thousandth's of an inch. In any case I
understand that the total mass of the wire needs to be about the same as the
I'm using a basic Delco 10SI 63 amp alternator. There seems to be a limit
on how much field excitation the whole assembly can withstand. The nichrome
wire I'm using to control the field current will burn out if I reduce the
resistance too much but this can of course be corrected with bigger wire.
The stress on the belt seems to be the main problem--when I increase the
field current the load on the engine/belt/pulleys increases and the belt
starts flopping around. It doesn't seem to matter how much tension I have
on the belt--too much and the belt starts wearing quickly and becomes shiny
on the sides and starts to lose rubber. My mount is three laminated layers
of 5/8" exterior waferwood which is attached to a tubular steel frame with
1/4" bolts. I *think* it is rigid but perhaps it's not. I have tried
various different ways to adjust the belt tension. I was using 200# chain
(guessing on the chain strength but that's probably close) and it broke. I
have broken a 1/4" turnbuckle. Obviously something is amiss. Right now I'm
using 1/4" chain and it seems to be strong enough. The same belts that can
last for years driven by a 250 HP car engine last only a week on my setup.
I was wondering about just what you are describing but it seemed like it
might hold up better than what I have been doing. What about something more
solid? Is it simply too difficult to get everything aligned perfectly? I
was thinking that an alternator with a a rotor that was made to attach
directly to a 3/4" keyed crankshaft would be nice. Or one that attaches to
a tapered generator-type shaft.
It's torn apart right now but I'll see if I can reassemble it enough for a
portrait. It might be a couple of days as all of my cars decided to need a
lot of work all at once.