[Beowulf] UPS & power supply instability

Bruce Allen ballen at gravity.phys.uwm.edu
Wed Sep 28 16:29:49 PDT 2005

> It is also a True Fact that the shared neutral current on three lines
> that only draw in the middle third of the half cycle DO NOT OVERLAP --
> the period is broken up into sixths and in each sixth cycle one and only
> one line is drawing and dumping current.  Furthermore, it is drawing a
> peak current that is LARGER than you'd expect from the rms power
> consumption.  So instead of cancelling, you have a line current on the
> neutral that is something like I_0 sin(3\omega t), or the third harmonic
> of the base frequency \omega.  As Jim noted, EACH cycle of this current
> would carry as much as any single line, so your rms current (think
> "average" for the purpose of determining e.g. line heating, voltage drop
> on the neutral, and so on) is 3x as great.
> Line heating is given by I^2 R, so 3x the current is 9x the joule
> heating of the neutral line and will ALSO produce overheating in your
> primary three phase transformer, reducing its lifetime if nothing worse.
> Carrying 60 amps (give or take) on a line that's safe at 20 amps is a
> Bad Idea.


This is a very clear explanation.

In addition to having separate neutrals, it's also helpful to have a K13 
or K20 rated transformer (typically 480v to 208v one) in your power 
distribution system.  This has an oversized neutral which can safely 
absorb and dissipate some or all of this power.

[PS: since your're a physicist, it should be sin(120 pi t/sec)  not 
sin(120 pi t).]


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