On Mon, 27 Feb 2012, Luke S. Crawford wrote:
On Mon, Feb 27, 2012 at 04:02:44PM -0800, [email protected] wrote:
the math in sine-wave AC circuits is odd, but a 208v circuit is exactly 2
120v circuits (if the circuit is wired correctly, you could take either
leg of the 208v circuit and use ground as your neutral and you would
measure 120v on each half of it)
My understanding was that a 3 phase circuit was 3 'hot' lines, each
one peaking 120 degrees after the next one. If you want 120v, you
peel off one of those 'hot' lines and pair it with a 'neutral' line
(which is a 'return' or something? I do not actually understand the
details of that bit. I should find a book.)
When the voltage is at peak or valley, it's 120v out of phase with the
neutral line.
My understanding of split-phase 208v is that you take two of those 120v
lines and use one for the 'hot' and one for what was the 'neutral' wire
in the 120v example.
Now, if these lines were 180 degrees out of phase, when the first 'hot'
conductor was at 120v the other hot (that I've put on the 'neutral')
would be at -120v, and the difference (what my single phase PSU would see)
would be 240v, and I wouldn't be confused.
But they aren't 180 degrees out of phase. they are 120 degrees out
of phase, and that means that when one hot is at 120v, the other hot is
at -88 or something? is that right? this is the part I don't have the
math for. But I do understand that because they are not completely out
of phase, my equipment doesn't see as much voltage as it would otherwise.
by doubling the voltage, and cutting the current in half, you cut the
power lost by wiring resistance to 1/4. This also cuts the heat generated
by the wiring.
I'm pretty sure that's wrong. If it was power loss in the wiring that
was taking it from 240v to 208V then it would vary based on conductor
length and thickness. I monitor this sort of thing pretty closely,
and my 120v circuits are solidly 120v, and my 208v circuits are solidly
208v.
I think it's the phase difference.
I wasn't saying that resistance was causing the voltage drop, I was
explaining why 208v is better to use than 120v
David Lang
but if your equipment will run at 208v, you are best off using that
voltage. If you put a meter on the equipment, you will see it talk exactly
half the current in Amps as the same equipment would draw running at 120v
(the gains from the higher voltage are not going to be large enough to
read on the metter). It's not a big difference on an individual server
basis.
Hm. Well, I should try it. I now have the almost exact same PDU model,
one wired for 120v single-phase and one wired for 208v split-phase, both
have a 'true rms' current read and read voltage. Man, I need a 208v
line in my test lab.
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