Re: [CAD_CAM_EDM_DRO] high frequency spindle motors

Bob Simon wrote:

>>It might still be possible to rewind these spindles without massive
>>effort, depending on the
>>way they are built.
>>
>>
>
>Now I'm wondering what I really have here. One of the units has a label
>that says 40000rpm at 660hz,120 volts, 8.5 amps. I pulled it apart and
>there was no visible (or olfactory) evidence of a burned winding. The
>windings actually have 4 wires. There is a resistance of 2 ohms on each
>pair of wires. there is no connection between the 2 pairs of wires. I think
>I have a *2 phase* AC induction motor. I did a little reading on polyphase
>motors and discovered that there were actually 2 phase motors used at one
>time. Instead of a 120 degree phasing they require 90 degrees between the
>phases.
>
>I think I may be screwed as far as applying a 3 phase inverter to these
>motors.
>
>

Yup, that does sound like the definition of a 2-phase motor. They are
STILL used. Any
capacitor-run single-phase motor is actually a 2-phase motor, and the
capacitor is being
used to create the shifted phase.

A capacitor doesn't work real well when you vary the frequency, however,
as the value needs
to change for each freq. But, there IS a really neat circuit for
converting 90 degree 2-phase
to 3-phase, anc vice-versa. It is called a Scott-Tee. It is actually
simple to make a Scott-T
transformer setup. You need a 1:1 transformer with one of the windings
center-tapped,
and another 1:1 transformer. I'll try to draw this with ASCII art.

----- --------------------- A
)||(
)||(
X )|| -----
)||( |
)||( |
----- -----|--------------- B
|
----------- |
)||(
)||(
Y )||(
)||(
)||(
----------- |
--------------- C


X and Y are the 90 degree 2-phase connections. A, B and C are the
3-phase connections.
(I hope I've got this right, but I think I do.) You should be able to
run this off a VFD,
but I can't guarantee that it will work. A transformer rated for 120 V
at 60 Hz will probably
work for this application. It will run a bit warm, due to higher eddy
losses at eleven times
the design frequency. But, a transformer with a 50% higher current
rating should give
you enough margin.

You can also generate 90 degree 2-phase power with a large stereo amp
and a simple RC
phase shifter. If you have a big stereo amp and a means of setting the
90 degree phase
shift, you might try it. (Few stereo amps can deliver 120 V RMS output,
but you can
get 40 V out of a modest one, and up to 75 V or so out of the really big
ones.) You won't
be able to do any machining with this, but you could verify the 2-phase
diagnosis and
prove the motors are OK before proceeding with the VFD and Scott-T.

Jon