Newsgroups: comp.sys.sgi
Path: utzoo!henry
From: henry@utzoo.uucp (Henry Spencer)
Subject: Re:  Power for Iris 220S
Message-ID: <1990Feb23.215503.17868@utzoo.uucp>
Organization: U of Toronto Zoology
References: <9002192231.AA15320@>
Date: Fri, 23 Feb 90 21:55:03 GMT

In article <9002192231.AA15320@> art@lsr-vax.UUCP (Art Hays - PSTAFF) writes:
>The typical three phase wye transformer has the same guage wire for
>the phases and neutral.  The assumption is that if the loads on
>all the phases are equal, there will be very little current flow in the
>neutral...	The current waveform of a switching power supply is far from
>a sine wave.  It draws current in brief periods near the peak of the
>voltage waveform.  For various reasons (which I dont claim to fully understand)
>this creates currents in the neutral...

It's not hard to understand.  If the current waveforms are sine waves
and all lead/lag the voltage waveforms by the same amount so they are
120 degrees apart like the voltages, then the sum of the current waveforms
is zero.  For example, when one of them is at maximum positive current,
the others are mildly negative (one has recently gone negative, the other
is about to go positive).  However, this doesn't work for an arbitrary
waveform.  In particular, in the example, if the loads draw current only
in the middle of the cycle, then when load #1 is at max positive current,
the other two will be at zero instead of being mildly negative.

Help is on the way, because the way to get maximum power while staying
within a given current limit is to draw current over the entire sine
wave rather than just the peaks, and the power-supply manufacturers
see a considerable market for supplies that get the most out of an
ordinary 15-amp circuit in particular.  But for a while this will be
premium technology applied only in things that really need it.
-- 
"The N in NFS stands for Not, |     Henry Spencer at U of Toronto Zoology
or Need, or perhaps Nightmare"| uunet!attcan!utzoo!henry henry@zoo.toronto.edu