USING AN AC motor as an alternator.

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

 

He needs a magnetic field. I don't think such a motor has permanent -- or
electro- -- magnets.

 

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.

 

The secret to a successful engineering project is the ability to generate
many ideas and QUICKLY eliminate the bad ones. Most ideas are so
incredibly bad that you don't need a detailed analysis to reject them.
Gross approximations will do just fine.

So, let's head down that path. Stop when you get an answer you can't
live with.

Assume that a 40hp motor can produce 40hp worth of electricity.
That's about 30kW.
Will 30kW do the job? Is it in the right format?
Is 3-phase power what you want? Remember that you may need much
more than 30kW peak to start a large machine. If the diesel dies
or you have a brown-out
under the peak load, things get ugly.

Pick a round number. Say that it takes 90kW worth of diesel fuel to
produce 30kW of electricity. I don't know what that works out to in
dollars per kilowatt-hour, but does that number compare favorably
with your other electrical energy alternatives? The good news is
that, if you can capture it and it's at a temperature
you can use, you have 60kW of heat for the factory,
summer and winter.

By this point, it should be obvious that the generator costs way more
than power from the grid. I think my estimates were very generous.

So, if you can stand the cost and the power is in the format you can use,
you get to determine exactly how the motor is wound and start trying
to separate out the field and power it.

I'm betting you don't have to do all that work to reject the idea.

 

It seems alot more efficient when the grid is down.

J

 

What *kind* of motor? If it's a typical squirrel-cage induction motor,
your friend is SOL.

 

this exactly matches what I came across, with a single phase induction
motor. do you know why such a great speed overrun is needed to hit 60Hz
and why the motors seems to try to output only it's rated Hz and input
voltage?

My generator project was going to have a marine alternator to make 24
volts, but somehow I ordred the wrong type (12 volts) and shelved the
project after my bank of batteried from a charger mishap.

 

"do you know why such a great speed overrun is needed to hit 60Hz
and why the motors seems to try to output only it's rated Hz and input
voltage? "

The field actually rotates in the rotor. It does the same thing when it is a motor, when you load it and it pulls more current, you are fighting the magnetic field's desire to not rotate in the rotor. At 60 Hz it should be 1,800 RPM but it isn't. It's 1,750. That is actually nominal but it is accepted as the normal loss for motors that run machines rather then say, clocks.

This is the same shit only opposite. It's also a hell of alot less eficientwhen not made for the purpose. In a way it's like running an internal combustion engine backwards, just not as bad.

J

 

Rather THAN say

 

Not at all. But exciting it and regulating the output is a bit tricky.
Common as wind power systems.

?-)