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Re: 110, 115, 117, 120 or 125 VAC ??? (208 vs 230)
From: Jim Lux[SMTP:jimlux-at-earthlink-dot-net]
Sent: Sunday, January 04, 1998 12:49 PM
To: Tesla List
Subject: Re: 110, 115, 117, 120 or 125 VAC ??? (208 vs 230)
In re: 208 vs 230
For three phase power, a common configuration for light industrial and
office type loads is a Wye connected three phase system, with 115 Volts
from the phase to the neutral. This works out to 208 volts between phases.
In an office building, where the vast majority of the loads are 115 Volt,
this works out well, providing the phase loads are balanced so you don't
get much neutral current.
230 Volts, is, of course, the voltage at twice the standard (!) 115 Volts.
You'll see this either in residential service (to run stoves, a/c, and the
like), and in factories, where the motors run on 230V delta connected (i.e.
230 V between phases). And, of course, you'll see 460V motors/service as
well (it is easy to make a dual voltage 230/460V motor with split
windings).
To supply the ubiquitous 115V loads in such an industrial facility, they
either use a transformer from 230/460 down to 115, or one of the phase
transformers has a grounded centertap. This last isn't very common, because
it makes the neutral not at the same potential as ground, but, if you don't
have a neutral (i.e. everything is delta wires) who cares. Also, the single
phase loads would have to be a small fraction of the overall facility, or
the phase load unbalance would cause problems.
Small "dry" transformers, in the 1-5 kVA range, are very common for the
step down from 460 to 115. You see them surplus all the time..
With respect to 208/230: it is important for motors. Motors which can run
at 208 as well as 230 have heavier windings, to carry the inevitable higher
current (to get the same HP out). They run at higher slip and dissipate
more power, sort of the perpetual brownout condition. For experimental use,
who cares, but if you are buying a hundred motors for a factory, and are
sizing them right to the load with no margin, it makes a difference.