klem kedidelhopper wrote:
My friend has a 40HP 3 phase motor which he wants to try to use as an
alternator. He seems to think that if he powers this monster from a
diesel engine he can generate enough useable AC power to run his small
factory and heat it as well. Even if you could get some AC out of the
motor I don't think this is possible. Any thoughts? Lenny

you can do it, but the performance sucks.

I've made generators using induction motors attached to engines, just to
see it work.

they act really weird, and work at some weird RPM nowhere near the value
of the faceplate produce around their rated input voltage at about 60Hz.

There's usually enough residual magnetization in the rotor or laminations
to self excite, but not always. Runnign 12 volts across the windings from
my engines starter battery for a second would always work when they
didn't.

The last test with a 3/4 HP 1725 RPM motor with about 200uF of caps across
the power cord that would usually run it seemed too be able to run about
400 watts of light bulbs, but only when run at 2200 to 2800 RPM. The
output frequency did stick to about 60Hz, somehow. The output voltage
regulation wasn't that bad, and was probably "regulated" by some sort of
saturation or leakage in the motor design itself.

I can't explain how the whole thing worked, but trying to drive an
inductive load would make the thing stop acting as a generator, and the
engine would then rev up as the load seemed to go away.

Also, if you're dealing with single phase motor used as generators this
way with 1:1 speed coupling to the engine, you get a very strange flicker
or beating to to output voltage. I could see light bulbs pulsate in
intensity, which was horribly obnoxious.

then I realized that it was because I was using a 4 cycle engine, so the
power stroke was brighter than the rest of them. a 15Hz flicker seemed
about right. It still doesn't really jive with what the tachometer said
for 2200 to 2800 RPM. I was using a wrap-around the ingnition cable unit
from Sendec.

Anyways, it was an interesting project.