# Re: Repost RE: Explain 3 Phase

```Original poster: "Alex Crow by way of Terry Fritz <twftesla-at-uswest-dot-net>" <alexcrow-at-blueyonder.co.uk>

Replies below --

On Thursday 03 May 2001 04:00, Tesla list wrote:
> Original poster: "davep by way of Terry Fritz <twftesla-at-uswest-dot-net>"
> <davep-at-quik-dot-com>
>
> Tesla list wrote:
> > Original poster: "Alex Crow by way of Terry Fritz <twftesla-at-uswest-dot-net>"
> > <alexcrow-at-blueyonder.co.uk>
> >
> > Yes, it is 180 degrees out.
>
> 	Measured from the center tap, aka ground, indeed.

No, measured without reference to any 'ground' really - just against any
arbitrary DC level, they are 180 degrees out of phase. If you connect a
voltmeter across two lines that vary *in phase* from 0 to 120V, referring to
the same OV level, you will see *no* voltage, as the difference between the
waves will be zero! We're talking about "potential difference" here, a more
accurate term than just saying "voltage". When the waves are 180 degrees out
of phase, you will see 240V AC. You need to subtract one level from another
to get the P.D, not add!

>
> > Any *electronic* engineer
>
> 	I am one.

I'm not going to enter any ad hominem attack, the phrase was perhaps a little
rashly written. I will leave it to others if they wish to make any
professional (or otherwise) judgements! I just thought you mentioned you were
in the electrical field rather than the electronic.

>
> > will tell you that the two lines are 180 out of phase. The ground
> > reference has *no effect* and can be ignored!
>
> 	If its ignored, hooking up the scope will be interesting.
> 	If its ignored, measurements will be end to end.
> 	Which will show the whole to be In Phase.
>

Hooking the scope end to end does not affect the phase of the two signals in
the least.

> > The waves are a mathematical function of time, but the "time"
> > axis actually represents the "angle" input to the trigonometric
> > functions.
>
> 	Indeed.
>
> > Eg the top wave could be represented as
> >
> > V1=sin(t)
> >
> > and the bottom as
> >
> > V2=sin(t+180)
> >
> > if, of course, you are using degrees to calculate. Most electronic texts
> > use radians by default.
>
> 	OK, measured relative to the ground (center tap) is pi radians...
>
> > The phase angle is unaffected by any static rail between
> > the lines.
>
> 	But the measurement is...
>

OK, yes, the PD between Live1 and GND, and that between GND and Live2, would
appear in phase on the scope.  But for example, let's measure the voltages
against another, DC reference that is independent. applying the probe to L1
and L2 respectively. You will see the traces are out of phase,

What's happening is the two waves are referenced to the same ground - that is
the 'Zero Crossing' for both. One is inverted with respect to the other, so
you will always find 240VAC between them. However, the average voltage over
one cycle will add to zero.

Yes, I agree you could have two waves in-phase with a 240VDC difference
between them, but then you'd be transmitting DC anyway!

> > Of course, I know where you are coming from, ie measuring V1 from
> > Ground to the top live, then V2 from the bottom live up to ground.
> > Yes, then the signals will appear to be in phase, but it's not
> > something you'll find most guys doing here.
>
> 	They really would be best advised to understand, if
> 	contemplating a 240v hookup....

Errm, the 240V hookup is exactly as I've described, from opposite sides of a
transformer, with one winding naturally inverted with respect to the other
(ie center tap to ground).  ie 180 degrees of phase separation.

>
> 	best
> 	dwp

Terry, please feel free to comment on this, especially if you feel it is
getting offtopic!

Alex

```