Converting common induction motors to low rpm alternators
Induction motors are commonplace, you'll find them
on tools, furnaces, blowers...etc. It is possible to convert them
into effective brushless low rpm alternators by installing permanent
magnets in the armature. We have performed experiments installing
surplus computer hard drive magnets into the armatures. Undoubtedly,
a "surplus" magnet is probably not optimized for the application, but the
results have still been promising and we think it might be a cheap, quick,
and practical approach to building a low rpm alternator. Unless extremely
creative, one would probably need to have, or find somebody who does
have, a metal lathe...that makes it a half hour project.
Para Español, traducción de Julio Andrade.
NOTE 06/25/2003 -- We've stopped experimenting with these conversions. They work, but the windings of the motors are just not made for producing lots of power.
The resistance is way too high, which makes them VERY inefficient once they reach higher speeds. They also cog, which causes slow startup. We've found that it's the same amount of work to build an efficient alternator from scratch as it is to convert an induction motor, and the home-made alternators perform MUCH better. You can see some of our latest, most powerful, and most efficient from-scratch PM alternators on our HERE.
Pictured above are the magnets we've used, though
undoubtedly any small magnets could be arranged to work reasonably well.
The magnets we used are rectangular, and arched such that 8 of them fit
to form a ring approx. 3 3/4" diameter(pictured above), which seems to
be a fairly good match for induction motors from 1/2 hp to 2 hp.
The magnets come out of a computer hard drive, ours are surplus, and are
available from our products page. The come magnetized with either
the North or South pole on the concave surface. These are NdFeB(Neodymium
Iron Boron) magnets, of extremely high grade - much stronger than normal
ceramic or AlNiCo magnets.
UPDATE 6/25/2003 -- These magnets were surplus, and they have SOLD OUT. We cannot get any more. We do have some rectangular block magnets that should do the trick, and still fit inside the armature....you'll just have to turn out the slot differently.
The armature will need to have a slot cut, to accept
the magnets. We think they should press in fairly tightly, and then
glued applied(epoxy is probably best). Odds are the curvature of
the magnets won't match up perfectly with the diameter of the armature,
so the slot needs to be deep enough so that the highest point of the magnets
is flush with the surface of the amateur. In the armature above,
there are 6 magnets used. We used feeler guages to even up the gap
between the magnets. Undoubtedly one could drill out the armature
to accept disc magnets, but disc magnets are not ideal, and some performance
would be compromised. Of course, the number of magnets used depends
upon the number of poles in the motor. A 3600 rpm motor would have
2 poles, 1800 rpm 4 poles, and 1200 6 poles. Voltage is dependent
upon the speed at which the magnetic field changes, so...the more
poles, the better candidate the motor would be for a low rpm alternator.
The lower the rated speed of the motor, the better it will work at low
rpm. In our tests, we always used the same number of magnets as poles,
except in a 2 horsepower motor, which had 4 poles. In that motor
we installed 8 magnets, but in sets of two such that there were 2 North
and two South poles on the armature.
The first motor we tested was a 1/2 hp furnace blower
motor, rated at 7 amps and 1050 rpm. It had 6 poles and we installed
6 magnets in the armature, equally spaced. It cogs(when the magnetic
field locks in with the slots in the motor stator) enough such that it
is difficult to turn the shaft of the motor. It hits charging voltage(12
volts) at approx. 80 rpm! At 400 rpm, it will charge 12 volt
batteries at over 10 amps. We tested this with a wind generator propellor
- click here to see more on that! Basically, it worked fairly well,
but he wind speed had to hit about 10 mph before the propellor would start
turning. Once it started turning...it kept spinning and generated
well. We also hooked this one up to a bike- it would easily put 10
amps into the battery with pedal power. I suspect this one would
probably peak at 15-20 amps, but it becomes inefficient after about 10.
This could be changed if one could make a regulator which would take the
coils in the motor(alternator) out of series and put them in parallel at
a certain rpm. Another drawback of this motor...it was a fairly cheap furnace
blower motor, with bronze bushings, it might last longer with higher quality
The second motor was a 2 hp single phase 1800 rpm,
rated at 15 amps. In this motor we installed 8 magnets. Oddly,
these computer hard drive magnets just happen to be a perfect fit, there
are no gaps, and no overlap - the diameter of the ring is exactly that
of the armature. The magnets on this motor are in sets of 2, so...we
put 2 magnets with North up, then 2 magnets with South up...etc, so that
there are 4 magnetic poles on the armature. This alternator doesn't
cog nearly as bad as the first one, and would certainly work well on most
windmills. It doesn't reach charging voltage until about 150 rpm,
but...at 400 rpm it charges my batteries at over 15 amps, and would probably
be efficient up to 20-30 amps.
Both of these test alternators become very difficult
to turn by hand if the wires are shorted...even a couple of rpm by hand
will produce a very noticeable spark at the leads. This might be
an excellent alternative, considering the difficulty and labor required
in making an alternator from scratch. Considering just how slow these
alternators are charging, they may have the most potentual of any low rpm
alternator we have yet to run across. We'd like to know what other
folks have done in this area, so please send us an email if you have any
©2003 by OTHERPOWER