[AT] More On Electric Motors

[Larry D. Goss]

LOL!  My Gentle Giant machinist friend has his whole shop set up for
3-phase operation and he has the sort of converter you're describing.
I've never talked with him about the design of it though.  I haven't
stopped in to have coffee with him for a while.  Maybe I can find some
time this weekend to go over to his place and discuss it a bit.  I do
know that it growls like crazy until at least one of the other 3-phase
motors in the shop is started, and he says that's inherent in the design
of the converter.

Stay tuned.

Larry

-----Original Message-----
From: at-bounces at lists.antique-tractor.com
[mailto:at-bounces at lists.antique-tractor.com] On Behalf Of
Bustedtractor at aol.com
Sent: Thursday, January 26, 2006 11:25 PM
To: at at lists.antique-tractor.com
Subject: Re: [AT] More On Electric Motors

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!)
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