On Wed, 21 Nov 2012 00:37:03 +0000 (UTC), Cydrome Leader
wrote:

Jon Elson wrote:
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 off-grid hydroelectric guys do this often. You have to put
large capacitor banks across the 3-phase leads to provide the self-exciting
current to build up the rotor field. You have to run the motor above
synchronous speed to get 60 Hz out, and the frequency regulation will be
pretty poor, and load-dependent. You will likely have to start the
alternator with no load except for the capacitor bank, and then connect
the load. Starting a large motor may cause a collapse of the excitation
and it won't restart until the load is removed. Voltage regulation
may not be real good, either, but can be adjusted by adding/removing
caps for the cap bank.

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.
A typical AC induction motor only develops torque at less than
synchronous speed. This speed is called the "Slip Speed" . The slip
speed is lower the more the motor is loaded. The motor will draw more
current the more it is loaded until it is overloaded when it stops
developing torque and stops. It will then be drawing maximum current.
The same motor used as a generator needs to be spun faster than
synchronous speed in order for there to be slip. The more the
generator is loaded the faster it must spin until it is overloaded and
it stops generating.
Eric