Path: utzoo!attcan!uunet!zephyr!tektronix!tekig5!pavelz
From: pavelz@tekig5.PEN.TEK.COM (Pavel R. Zivny)
Newsgroups: sci.electronics
Subject: PCs and line power
Keywords: 3 phase balance, diode bridge supply
Message-ID: <4391@tekig5.PEN.TEK.COM>
Date: 21 Jun 89 00:05:04 GMT
Distribution: na
Organization: Tektronix Inc., Beaverton, Or.
Lines: 38


I just witnessed a little problem with power distribution in certain computer 
'lab'; this is 3 phase, 208/120 V, 100A circuit loaded mainly with computers
and such. It turned out to draw about 60 60 45 A from respective phases. AND,
IT WAS TAKING ABOUT 100 A from Neutral wire.

This is a problem, since the neutral is a 100 Amps wire only. 

	Why the phases balance out sooo poorly ?

	Well, I have a guess. All these "diode bridge - filter cap"
pwr supplies (most of them switching, but that makes no difference here) take
most of the power at the moment of voltage peak on the line, right.  (yes, it
depends on the L & R in the way, but roughly I think this is true). And since
the peaks on different lines happen at different time (120 degrees shift) -
they can't balance out - poor thing neutral has to carry every each of these
peaks out of the door.

	I guess it really takes a bunch of small computers, monitors, and
small electronic gear; big mainframes probably have cleaner supplies ( there
the problem was obvious, therefore taken care off).

	Well, I see three possible solutions:  
a) Thicker neutral (not very elegant solution, since re-wiring of an existing
    system is going to be a pain) 
b) Better supplies (questionable; when you go shopping for PC clones next time,
    how much are willing to spend on clean supply ? How are you even going to
    know which ones are the better ones ?) 
c) _Some_ filter (balancer ?) or what. Like fly-wheel loaded, 3 Phase
    asynchronous motor; or some cleverly switched bank of caps; or ....

	QUESTION:  Has anybody better idea(s) ? Experience to share ?
		


		  Pavel Zivny		 pavelz@tekig5.PEN.TEK.COM	 
Disclaimer: I don't know what am I talking about, and my employer has nothing 
	to do with it.