On Fri, 24 Dec 2004 22:47:19 GMT, "toller" wrote:


I think one of the problems in this thread is the confusion of using
"phase" to refer to both true phases as in 3-phase versus the 180-deg
phase difference between the two hot legs of a single-phase AC circuit.

I don't know when it became prevelant or popular (if it actually is or
whether it's still fairly isolated) to refer to the two hots as two
"phases"

As I understand it, everything is pretty much the same whether you have true
240v (one 240v hot and a 0v neutral)...

In the U.S. I don't believe there is any such thing. Household supply
is obtained from a single phase of a three phase system. For all
intents and purposes, when the power grid appears in your
neighborhood, forget about two of those phases. You will never be
involved with them. Only single phase is getting into your house.

The single phase current is transformed from a fairly high voltage off
the feeders to 240V from the output of a center tapped transformer up
on the pole. With regard to that 240V, there is no neutral. The 240V
comes from the two secondaries of the transformer. And there is no +
or -. This is AC, not DC. It is 240V across the two poles.

Our well known 120V supply comes, as you know, from one hot leg and
the center tap of the transformer. Either hot leg will work.

Here is the most important thing: ALL of this; the 240V, and each 120V
is developed from the same single phase of the three phase system. It
is single phase power. There is no "two phase" power.

...or "2 phase" 240v (two 120v hots 180 degrees out of phase).

There is no "two phase" power. The two legs or poles are 180° out of
phase, as you say, because they come off the opposite legs of the
secondary of the transformer. They are NOT two phases of the three
phase system. It is single phase. All of the current in your house
comes from the same, SINGLE phase of the three phase system.

However 3 phase electricity is very different. Or so I have heard.

Yes and no, but I will leave that discussion to those with experience
with it. I have none.

I think that is why 240v is not called "2 phase"; it can be thought of as 1
phase,

It IS single phase. There's no "thinking" about it.

but is completely different than 3 phase.

Sigh. Yes, you have stated a tautology.


- -
LRod

Master Woodbutcher and seasoned termite

Shamelessly whoring my website since 1999

http://www.woodbutcher.net