pavelz@tekig5.PEN.TEK.COM (Pavel R. Zivny) (06/21/89)
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. jat@tc.fluke.COM (J. Andy Tiura) (06/27/89)
In article <4391@tekig5.PEN.TEK.COM> pavelz@tekig5.PEN.TEK.COM (Pavel R. Zivny) writes: > >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 do the phases balance out so 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. 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. You are partially correct. The 120 degree difference between phases would allow a "summing to zero" for the neutral current with LINEAR balanced loads, reactive or not. Your model accurately describes a NON-LINEAR load that creates significant harmonic components in the phase currents. The third harmonic component of the phase currents land right on top of each other on a phasor diagram and seem to add up instead of cancel out. (Yes, I know that's not entirely accurate, but it's easy to mentally picture.) > 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; 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 .... b and c are viable options. Look for power supplies that boast "active power factor correction" or buy an outboard unit to perform the the same task. These are available from several vendors, using both active and passive techniques, to make the non-linear power supply input impedance appear resistive. The line current then becomes essentially sinusoidal, and the neutral current problem you mentioned goes away. (As an additional benefit, you can draw more power from a given branch circuit if the load is power factor corrected.) Look for more power supply vendors to incorporate power factor correction into their OEM supplies now that at least three IC vendors have introduced chips that perform that function. -jat@tc.fluke.com
maa@nbires.nbi.com (Mark Armbrust) (06/27/89)
In article <9355@fluke.COM> jat@tc.fluke.COM (J. Andy Tiura) writes: > >Look for more power supply vendors to incorporate power factor correction >into their OEM supplies now that at least three IC vendors have introduced >chips that perform that function. Especially since this is an IEC requirement effective 1992. There is a reasonable article about the Micro Linear Corp. ML4812 Power Factor Controller in the June 8 issue of _Electronic Design_. This requrement is for supplies with 300+ watt input. -- Mark Armbrust maa@nbires.nbi.com maa@nbires.UUCP