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RE: [datacenter] balancing phases on 3phase UPS?


Excessively unbalanced three-phase generator currents may cause overheating of the generator rotor if the generator is operated close to its kVA rating. Generator manufacturers generally specify the maximum allowable current unbalance at rated load.

The (unfortunately rather technical) explanation of this overheating mechanism follows below.

Unbalanced 60 Hz, three-phase stator currents can be split in socalled symmetrical components, namely a positive sequence set, a negative sequence set and a zero sequence set (see for instance http://www.cadickcorp.com/tech006.htm, note that in the negative sequence set the b-phase and c-phase have been swapped relative to the positive sequence set). Each set consists of three pc. balanced currents, respectively spaced at 120 degrees in the positive sequence set, -120 degrees in the negative sequence set and 0 degrees in the zero sequence set. These sets of stator currents each produce electromagnetic fields in the stator bore that are experienced differently by the rotor:
The positive sequence EM field rotates in the stator bore at the same speed, namely 60 Hz, and in the same direction as the generator rotor (to the rotor this appears as a DC field).
The negative sequence EM field rotates in the stator bore in the opposite direction of the rotor at a speed of 60 Hz, which the rotor sees as a 120 Hz revolving field. This field generates eddy currents in the damper windings and the (solid) rotor body. These eddy currents generate additional heating of the rotor, over and above normal heating by the current flowing in the rotor windings.
The zero sequence EM field does not rotate in the stator bore (it only alternates) but is seen by the rotor as a 60 Hz revolving field, which also generates eddy currents in the damper windings and rotor body.

As was pointed out by Dan Lawrence, current unbalance may be caused by harmonics. Harmonic stator currents will create revolving EM fields at various speeds in the stator bore. These fields will also generate heat in the rotor. However, generators with socalled 2/3 pitch stator windings are insensitive to third harmonics, which generally are the dominant ones. The origin of these harmonics may be double conversion UPS's or rectifiers. Since UPS's and rectifiers should normally be equipped with harmonic filters at their input side, it would seem that the 35A-35A-15A unbalance is mainly caused by load unbalance rather than harmonics.

Aad van Rooijen
Senior Engineer
Interxion HQ

Jeff Wasilko wrote:

> Hi:
> I just started at a new job and I noticed that in one of our
> datacenters 1 out of the 3 phases is about 20A less loaded than
> the other 2 (35A vs 15A). In the past I was told that if the
> phases weren't similarly loaded that it could cause problems
> during transitions to/from generator. Is this worth worrying
> about?
> Thanks!
> -j
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