Re: [CAD_CAM_EDM_DRO] high frequency spindle motors

Bob Simon wrote:

>>>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.
>>>
>>>
>>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.
>>
>>
>
>Interesting! I found an example of a 3-2 phase scott-t on the internet. It
>seems that going from 3 to 2 phase the center tap works, but going from 2
>to 3 phase the tap needs to be offset. The example I found was 480v primary
>and 120v secondary. I wonder if I could use 230v primary and 120v secondary
>to reduce the voltage to the windings and still use a 230 volt VFD?
>
>

Assuming the windings will handle the voltage (not a certainty) you can
always use a VFD
and just program the V/Hz line as 120 V@630 Hz, or whatever the V/Hz is
for that specific
motor. I run a motor where the V/Hz line is at about 75 V@400 Hz, as
high as that VFD will
go in frequency. It is a 230 V VFD, and I just programmed it for 80
V@400 Hz. (Their arithmetic
has a glitch when programmed for 75 V@400 Hz, but it worked fine with 80.)

>Could both transformers be identical? I though about a pair of used 1KVA
>control transformers, but outside tapped 230v primary gets me only 60
>volts. Looks as though I would need really expensive specialty
>transformers...sigh
>
>

No, I don't think so. If you can find some 480/240 V step-down
transformers, they should work fine.
If you put the primary in parallel and the secondary in series, you have
240:240, with a center-tap
on the secondary. You will likely still need series inductors due to
the low inductance of the
motors. I'd try putting the inductors between the VFD and the transformers.

Jon

Jon