Learned Something Today - Thanks Travis (was: 2 Roof Airs)

[patrick flowers]

Well, it finally became clear to me on the way home. I'm still sure
that there's no phase difference in a 240 volt circuit, but "unloading
the neutral" works. The potentials rise and fall at the same time
across the entire circuit, so you can look at it at any point in time as
two interconnected DC loops. The current from the two 120 circuits is
in opposition in the neutral conductor, so the actual current in the
neutral is the difference between the two. If one circuit is drawing 15
amps and the other 10 amps, then you have 15 amps trying to go one way
and 10 amps trying to go the other - the net is 5 amps on the neutral
conductor.

I've heard the "out of phase" myth for a long time and I think it's
propagated in part due to the term "phase" being applied to the end taps
of single phase transformers even though there's only one waveform
present. I've never had a better explanation until now - Thanks Travis.

Patrick

>
> Travis,
>
> Maybe we're just bashing semantics here, but you're confusing "phase" with
> "potential". There is only one waveform present, therefore it's impossible
> for the taps to be "out of phase". As for "unloading the neutral" that's a
> totally new concept to me - I'll look into it.
>
> Patrick
- --
Patrick Flowers
Mailtoatrick

The GMC Motorhome Page
http://www.gmcmotorhome.com

 

[travism]

> Well, it finally became clear to me on the way home. I'm still sure
> that there's no phase difference in a 240 volt circuit, but "unloading
> the neutral" works. The potentials rise and fall at the same time
> across the entire circuit, so you can look at it at any point in time as
> two interconnected DC loops. The current from the two 120 circuits is
> in opposition in the neutral conductor, so the actual current in the
> neutral is the difference between the two. If one circuit is drawing 15
> amps and the other 10 amps, then you have 15 amps trying to go one way
> and 10 amps trying to go the other - the net is 5 amps on the neutral
> conductor.
>
> I've heard the "out of phase" myth for a long time and I think it's
> propagated in part due to the term "phase" being applied to the end taps
> of single phase transformers even though there's only one waveform
> present. I've never had a better explanation until now - Thanks Travis.
>
> Patrick

Whoa, wait a minute, Patrick; you're nearly there but not quite.

The two "hots" which we'll call the red and the black really ARE out of
phase 180 degrees. This in a conventional 120/240 shore power or residential
power setup that is.

Think of them as two batteries in series with the center tap grounded. One
is positive with relation to ground, the other is negative. But the positive
is also positive with respect to the negative one also, at twice the
potential. If they were 12 volt batteries, you would have 24 volts from the
positive of the one to the negative of the other. Thus they are "out of
phase" by 180 degrees.

That's how you can get 240 or 120 depending on your need. Use the red and
black for 240 or either the red or the black worked against the white
neutral for 120 volts.

Possibly the reason this is confusing comes from comparing it to three phase
power? I'm sure if we were across a booth from each other in a Denny's and I
could sketch on a napkin, I'd have made myself clear before now; I don't
seem to be able to draw "word pictures" very well.

The factory Onan wiring has two 120 volt windings also, but they are wired
IN PHASE. Thus 240 volts can't be had from them. Not a problem in the
motorhome, as has been pointed out several times, since there are no 240
volt loads. But this means that you have to have 80 amps worth of neutral to
carry the return. Think of them as two batteries wired in parallel, or
rather with both negatives grounded. You can get 12 volts from either
battery to ground, but there is no voltage between them since they are in
phase.

You nailed the role of the neutral in the 120/240 system as carrying only
the _difference_ in the loads of the two hots. (I am trying to resist the
urge to refer to them as the "two phases" even though that is a common term
in wiring...it would really confuse things here.)

I hope I'm making some sense; if not, I'll try again or maybe someone else
has a better gift for explaning than I do.

What I was alluding to earlier is the fact that electricians quite commonly
refer to the hot leads as "phases" but this does not really express their
phase relationship to each other, so I'm trying to avoid that by referring
to them as the hot leads.

Best regards,

Travis