like, what's your phase? [RE-wrenches]

Dan Rice danrice at scinternet.net
Fri May 13 18:27:59 PDT 2005


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Drake and Don,

It seems, from your two messages, that you're referring to two different
things. 120/240 volt "split phase," is what we grid connected folks have in
most of our houses, and what Don has described -essentially, it's single
phase AC. 2-phase AC, which is now archaic and rarely found, may be what
you're referring to, Drake, when you refer to two phases which are 90
degrees out of phase. I don't know the history of two phase AC, but from
what I understand, it is related to the out-of-phase currents creating a
"rotating" magnetic field to develop starting torque in AC induction motors.
Capacitor start windings in single phase motors accomplish the same thing.
Once started, induction motors work fine on single phase power, and two
phase power pulses are not ideal. Three phase power works much better to
drive motors, and two phase has been discontinued. I had an instructor for
an NEC class talk about two-phase power systems -said that some are still in
use in old industrial installations in the midwest.

I hope that helps.

Dan Rice
Abundant Sun, LLC.
----- Original Message ----- 
From: <i2p at aol.com>
To: <RE-wrenches at topica.com>
Sent: Thursday, May 12, 2005 10:30 AM
Subject: Re: like, what's your phase? [RE-wrenches]


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In a message dated 5/12/2005 8:52:32 AM Pacific Standard Time,
solar at ecoisp.com writes:
I've always wondered what the point of the "two phase system" is.  Where is
it grounded?  How is it configured to put a balanced load on the
generator?  Would it be set up so as to get two, two phase systems from one
source?  This has always been a brain teaser.  I've never seen a two phase
system.

Drake
Hi Drake, this is how I think of it.

Considering a 120/240 setup, we can think of it as two 120 vac phases (180
deg out of phase). Since they are out of phase we can represent them as
V1=120
sinwt and
V2=(-)120 sinwt. The voltage difference (V1-V2)= 240 sin wt (remember the
minus a minus from algerbra = a positive). Now the difference (as measured
from
L1 to L2) is single phase 240 vac. It is derived from two 120 vac " phases".
sharing a common neutral ( center tap of the service).

Don

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