Data Centers

GD/Astronautics 

Dynamics Group 595; Analog Stations and Operators 

We’re starting to look at three-phase circuits. If the circuit is balanced (and for now, we’ll generally assume that it is), you have three voltages which are equal in magnitude and frequency but are out of phase with each other by 120 degrees. These are usually referenced to a neutral voltage, so you have the following present in the system:

When a three-phase system is balanced - that is, there is equal current drawn through each of the branches due to equal loads - there are two possible equivalent ways for the load to be connected to the source. The loads can be connected either in a wye formation or a delta, which you may remember from way, way back.

Note that there is no neutral line in the delta configuration. If the loads are truly balanced in the wye configuration, the neutral line carries no current in any case and for the purposes of analysis, you can ignore it.

There are good reasons to use both the wye and delta formations. The wye configuration gives you a nice convenient place to ground, which makes it a good choice for a situation where you’re worried about momentary surges in voltage. It also lets you access both line-to-line voltage and line-to-neutral voltage. A delta configuration does a better job of staying stable if the loads become unbalanced.

On-Call Welcome again to On-Call, our regular Friday tale in which we ease you into the weekend with readers’ tales of the ridiculous things they’ve been asked to do on evenings, weekends and the other odd times techie folk are asked to go out and fix things up.

This week’s contribution comes from reader “JF”, who tells us that “About eight years back, I was co-ordinating a data center build-out.”

The existing facility sported a trio of 220KvA UPSes, but at the new bit barn it was decided to run with just two.

“The decision was made to save costs by relocating one of them to the new building rather than buying all new equipment.”

JF says he “begged the business to call a complete shutdown to remove the UPS. They asked me what the odds of something going wrong, and I made the error of trying to provide an accurate estimate of the risk by sayin there was about a one in 100 chance of problems.”

JF thought a one per cent risk of power failure across 25,000 square feet packed full of server racks, live, in production, would scare off the bean counters.