On 11/17/2012 2:45 PM, 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

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.