Re: [CAD_CAM_EDM_DRO] 3 phase converter and odd behavior on CNC mill

writetoalan wrote:

>A friend of mine recently wanted to show me a part cutting on his CNC
>mill (a BPort) that he had running with a phase converter. The
>machine was behaving very badly when I showed up, however. He said
>that it was running earlier in the day, but when I popped the back to
>look at the voltages I got the following (when referenced to ground):
>120V, 60V, 60V. This seems *very* wrong to me as I would have
>expected at least two 120V legs (the two from the Power company).
>Anyhow, I am planning to look at the thing before he blows up the
>CNC. My question is pretty simple. The converter is a Cedarburg and
>it has a bunch of wires coming out of it; a "bunch" because I don't
>know if it is a Wye or Delta wound motor. What is the best way of
>determining if a coil is burned out.... and I don't have a megger.
>
>

GASP! A Cedarburg! That is NOT a phase converter, despite the false
name on the box! It is a starting capacitor and a pair of relays! Stop him
from using it immediately, it sounds like he may already have damaged the
machine. (Since it WAS working, and now isn't.)

I don't think Cedarburg makes a rotary converter. Anyway, it would not
matter if it was wye or delta.

You should have 240 V between 2 lines (the supplied single phase) and
somewhere between 200 and 270 V or so between the other two pairings.
Don't measure to ground, as there IS no reference to real ground (or
neutral)
in a phase-converter setup. If you have a wye-connected rotary converter,
you can attempt to set up a neutral for small control loads, but it is not
going to work terribly well. You can't connect the generated neutral in
this case to the power mains neutral.

The 120 - 60 - 60 sounds suspiciously like this was being run from 120 V
line-line, not 240 V L-L. This is approximately what you would get,
60 V from each supplied main to building neutral. And, the generated
leg would be 104 V from building neutral. If the phase converter
was running normally (but at half voltage) it would generate 120 V
line-line on any of the 3 lines. It is possible that ONE of the mains
fuses has blown, or that a faulty circuit breaker or wiring problem
has opened one of the supplied legs. But, if everything was wired right,
that should completely shut it down, as there SHOULD be no other path
for curren to flow, but in from one of the 240 lines and out the other.
You might check for grounds on the lines. But, that would have
given you zero volts on one of the lines (to neutral), so apparently that
is not it. (A line grounds out, blows the fuse, and the equipment continues
to run on half voltage. This is a classic electrical mystery story, and it
happens all the time. It often ends with somebody getting a shock!
So, be careful, something is really wrong there!)

Anyway, it is best to ALWAYS measure 3-phase systems line to line,
as all sorts of odd systems that are not balanced around neutral exist.

Jon