On Fri, 07 Sep 2007 02:10:27 GMT, Ken Sterling (Ken Sterling) wrote:

Bruce (or anyone qualified).....
Three phase power ----- isn't each leg 120 degrees from the others?

Right. Figuring the sine wave overlap, it's more like six-phase
power - every 60 degrees of an electric motor rotation one of the
phases is at peak voltage, either negative or positive. That's why
it's so darned efficient for running motors, since there's always
smooth forward motion. It's the difference betweeen a one-lung hit
and miss Johnny Popper and a Packard Merlin.

And vibration sensitive things like surface grinders should only run
on real 3-phase, for the same reason. Try running it on a phase
converter and the surface finish on the products can go straight to
heck.

Single phase power - 220v - isn't each leg 180 degrees from the other?

Residential style 120/240 single phase is one center-tapped
transformer winding, with the center tap grounded for the Neutral.

The '180 degrees' is a convenient way to explain it because the
polarity /appears/ to be opposite when viewed/measured from the center
tap. In reality there's no differential at all.

If the above is correct (and I'm hoping I understand it correctly) it
would stand to reason, that if you needed to hook up a 220v single
phase machine and you had 3 phase available at that location you
could, (and I am not saying it's correct) connect to two of the three
phases and run the machine, even tho the sine waves wouldn't be 180
out, it probably wouldn't hurt anything. Am I correct in this
assumption???

Absolutely - you are going across one secondary core winding of the
utility distribution transformer, and there's no worry about phase
angle there.

Two times to worry: One is if you have 120/208V Wye power, because
it is only 208V phase to phase. Most small motors are dual-voltage
rated 208V/240V and will gladly run on 208V but at a higher current
draw. But there ARE pieces of gear that do not take kindly to running
on 208V - this is when you connect a simple 16V/32V Buck-Boost
transformer between the utility and the load to kick 208V up to 240V.

The other: If you have 120/240 Open Delta or "High Leg" power - the
High Leg (usually coded Orange and connected as B phase, but not
always) is 208V to ground, not 120V.

When you run across Open Delta panels in the field, it is
considerate to put a big note on the breaker panel to warn the less
enlightened among us that 'The B Phase is 208V to ground, and
connecting 120V loads to the B Phase is a Very Bad Idea.'

You can connect 240V loads between any two phases on Open Delta, but
you can only connect 120V light loads between A-N and C-N. Try
placing them on B-N and they won't live very long - but they glow
really bright before they blow...

-- Bruce --