Randy wrote:

Hello,

I have been running a small shop for several years and would now like
to get my first CNC machine. To do this I will need to upgrade my
phase convertor or get 3 phase from the power co.

The power co has agreed to hook me to an exsisting 120/240 open delta
supply ,there are two transfromers on the pole and the third leg is
wild, not sure of the exact voltage.

Will a CNC machine run off this? I know I have to make sure none of
the controls are hooked to the wild leg, but other than that will it
be OK?

I'm looking at a Cincinnati Arrow 750. 17 HP spindle which is more
than I need but the price and travels look good. However the 17 HP
will exceed my current phase convertor, which is not CNC rated anyway.

Randy
Thank You,
Randy

Remove 333 from email address to reply.

Hi Randy

Your message deals with two separate issues sort of rolled into one.
'Open delta' is just a transformer connection scheme to deliver a 3 ph
delta service with only two transformers. The 'wild leg' you refer to is
the product of a center tapped delta service, which allows the more
standard 120/240 V system to coexist with 240 3ph, all in the same
service.

Open delta can have stability problems compared to the same service done
with three transformers. This could be an issue with CNC machines and
their associated electronics.

The wild leg will probably not be a consideration as most likely the
machine you're looking at is 3ph and has no neutral connection back to
the panel. Any low voltage control the machine requires is most likely
done at the machine with control power transformers. It's my experience
that machine connections tend to be 3ph or 1ph, not both. I'd be
suspicious of a machine that required a 3ph connection and a separate
120 V 1ph connection. A work light on a mill might be an example, but
certainly not the machine controls themselves.

The only problem with center tapped delta services, other than using the
wild leg with a neutral which is just carelessness, is the fact that if
your service panel contains many 120v 1ph branch circuits you'll tend to
have unused slots in the panel. Careful placement of 3 ph and 2 pole 1ph
(anything with no neutral) branch circuits can minimize this trait.

Regards
Paul
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