[AT] More On Electric Motors

[Bustedtractor at aol.com]

In a message dated 1/24/2006 2:55:16 PM Central Standard Time, 
sheppard at indiana.edu writes:
On the discussion recently about three-phase motors:  I have seen many
articles about making a rotary converter from an old three phase motor
that "idles" in the circuit.  There are several ways to get this motor
turning initially like a single phase motor that can be disconnected.
The idler motor must be at least equal to the largest motor intended for
use (I think).  My question:  Can power be extracted from the three
phase idler motor?  The static converters use some sort of capacitors to
make a three phase motor run so it seems possible that you could use
capacitors to get the idler motor going and use it to power something
while serving as a rotary converter, too.
This type of phase converter uses banks of capacitors for both starting and 
running. A 3 phase motor runs on 3 phases of power that are 120 degrees apart. 
The capacitors produce an artificial third phase since capacitors cause a 
phase shift in the power applied to them. Three phase motors start without the 
need for the centrifugal switches that are used with single phase motors. A motor 
that is larger than the motor to be operated runs in an idle mode and does 
not generate any significant torque, so it can not be shaft-coupled to the load! 
A second 3 phase motor is wired to the 3 leads of this first motor to drive 
the load.

To start the idler motor, 2 of the leads to this 3 phase motor are connected 
to a 230 volt source. A bank of electrolitic capacitors are connected between 
one of these lines and the third lead of the motor. There will also be a bank 
of oil capacitors connected in parallel with the electrolitic capacitors. The 
electrolitics are disconnected when the motor comes up to speed. The oil 
capacitors stay in the circuit. The 3 phase power is not very even, but motors can 
be quite forgiving. Sizing both types of capcitors is mostly a trial and error 
method. I don't know of any formulas to determine the value in microfarads of 
the capacitors needed for either starting, or running. Electrolitic 
capacitors are usually in a black plastic case. Oil capcitors are typically in aluminum 
cases. Oil capacitors are built for continuous duty and electrolitics are for 
starting purposes only.

Will in MN

(PS: I just told you more than I know on this subject!)