[Beowulf] UPS system for Linux cluster

[Lux, James P]

> -----Original Message-----
> From: beowulf-bounces at beowulf.org 
> [mailto:beowulf-bounces at beowulf.org] On Behalf Of Tom Pierce
> Sent: Wednesday, April 29, 2009 9:22 AM
> To: vlad at geociencias.unam.mx
> Cc: beowulf at beowulf.org
> Subject: Re: [Beowulf] UPS system for Linux cluster
> 
> Dear Vlad,
> 
> I did a similar project last fall. The only unpleasant 
> surprise is that most UPS are capable of running continuously 
> at 80% of their kVA
> value.   So you may want a larger system.


Actually, this isn't particularly surprising to folks who deal with electrical power. It *is* surprising to folks who see volts*amps (VA) and think it means watts.

Most power conversion equipment has a higher VA rating than Watt rating (and, so do most loads, so it works out ok in practice)  PF (Power Factor) = Watts/VA, and a typical PF is around 0.85.

The other spec you have to watch out for is "inrush current".  There's actually two different kinds of inrush:
A) when AC power is first connected (or the switch is turned on)
B) when the power supply is "electrically enabled" 

The first one can easily be tens of amps, as the filter capacitors in the input stage charge up.  Fortunately, this is a short duration surge, but it can lead to nuisance trips of things like GFCIs, and if you plug in a whole rack full of power supplies at once, it can be pretty exciting (hmm, 20 power supplies, each drawing 50A..)

The other thing to watch out for on three phase supplies (this has been in the electrical code for at least 10 years) is that the neutral current on a 3phase Y(star) connection can be very, very high.