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From: "Jones, Martin" <email@example.com>
Date: Tue, 31 Aug 1999 16:56:19 -0500
The best I've found for use as low-RPM generators are the
permanent-magnet "servo" motors such as those sold by Surplus
Sales in Lincoln, NE. For about $30 they will sell you a permanent-
magnet motor that will put out about 5-10 amps at battery-charging
voltage, at an RPM consistent with direct drive-props. Also, most
of these motors have large-diameter (~3/4") shafts and a four-hole
face-mount. Typical size is about 4" dia by 7" long.
If you want the details of how my homebuilt works, using the servo
motor, email me privately as it is a long file. Also, I have some
detailed characterization data for the motor's output vs RPM and
load, from which I derived a generator voltage constant and the
output impedance. It was fairly linear.
Marty again.. (13 Oct 1998)
For someone into this as a hobby, I think using a PM motor rather than
building your own generator/alternator is the best way to get your feet
If you are going to build a generator/alternator from scratch, there
many references. Hugh Piggot has published several pamphlets, which he sells
(not cheap, considering how small they are), on home-building low-RPM
generators using brake drums and auto axles.
If you're interested in rewinding an auto alternator for low-RPM operation,
one good reference is "Electronic Design of Alternate-Energy Projects", by
R. Andrew Motes. He has some other interesting items in there as well, though
much of the material is dated as far as electronic technology is concerned.
I built my wind generator using a permanent-magnet DC motor as the generator.
My particular model was a 48-volt motor, cost $29 (I think) about 3
years ago, and has a 5/8" shaft. The case is 4" by 7", has a 4-hole
face-mounting pattern, and is totally enclosed. I use it to produce
about 100 watts at 15 volts in a moderate wind.
I bought two of these motors, and coupled the shafts so that one could
serve as a variable-speed drive for the other while I took output data
vs load and RPM. I varied the speed using a variable-output-
voltage power supply. I can get 12 volts (open-circuit) at around 300 RPM.
As electrical load is increased, the shaft speed must also increase
to maintain output voltage.
One advantage of using this type of motor as a generator is that it
produces very "high-quality" DC - the large number of poles on the
commutator produces low ripple that is very easily filtered. If someone
wanted to power communications equipment, this would probably
be a good choice. In my application, I'm limiting current to 5-10
amps, which is in the range of the motor's operating current under
some load conditions. I've pulled 5 amps into a dummy load
for several hours and noticed no heating of the generator. The "motor"
of the experimental motor-generator pair runs substantially warmer
than the "generator".
A trick I found helpful when testing the motors was to place both
of them in the "vee" of a piece of angle iron. This holds them (they
are almost perfectly cylindrical) such that the shafts are along
a common axis, and the shafts can actually be pushed nose-to-nose and
coupled temporarily using strong duct tape. A couple wraps of the
tape also holds the motors in the angle iron. Measure RPM using
a taped-on magnet and solenoid with a frequency counter. If you
can't get this stuff together, a crude estimate can be gotten with
a stopwatch and a long piece of string - time how long it takes to
wind up on the shaft.
The measured generator constant for open-circuit voltage is 26.5 rpm
volt, and output resistance is 1.24 ohms.
So if I wanted to produce 5 amps at 12 volts, I would need to generate
open-circuit voltage of:
12 + (5)(1.24) = 18.2 volts
and the rpm required is:
(18.2)(26.5) = 482.3 rpm
This is in a very good range for a direct-drive propeller. For
information on propeller speed ranges and output-power potential
versus diameter and blade design, you can read almost any text on
wind generators. I think the Motes book I referenced earlier also
has this information.
Anyway, I got these motors from Surplus Center in Lincoln, Nebraska.
This particular motor is no longer listed in Surplus Center's current
catalog, but I think they have several similar ones.
Weight is about 10 pounds (my guess).
One downside of using a PM motor as the generator is that there is
no way to internally regulate the output voltage. You have to
have an external regulator, or just use a battery bank large
enough that you don't worry about overcharge. If you don't have
anything else between the battery and the generator, you need a
diode to block reverse current. Remember, this thing is really a
motor, and in absense of enough wind to generate charging current,
the battery will power the motor and spin the rotor.
Another downside is that such a motor is generally not designed for
thrust loads on the shaft, so if it were a permanent installation I'd
try to rig a thrust bearing to prevent wearout of the motor bearings.
Also, the brushes will eventually wear and need to be replaced, but
since my application (and presumably yours) is temporary power,
it's not a problem.