Jim Phillips writes:

> What I really meant to say is that when the system is on line power,
>  I can get 220 across two legs, and 110 from each leg to ground.  BUT,
>  when the UPS kicks in I lose the 110; that is, what used to 110
>  (the hot-to-neutral/ground) is now 220.  In fact, there is no way
>  to get 110 out of the UPS (at least as far as I can determine).  I've
>  tried each leg to ground and there is no 110.

Jim,

In order to get the situation that you describe when the system is on line
power, there must be a transformer with a center tap. The reason is that 110
is one-half of 220. That simple ratio implies that the power taps are in
phase with each other.

That's not the normal situation in three-phase power.

There are two common three-phase transformer/motor/generator systems in use
in the United States, called wye and delta. Both depend on how the
transformer/motor/generator is wound.

The wye configuration is wired so that three transformer windings meet at a
central (neutral) point. Each of these windings are 120 degrees apart in
phase space, thus you can't simply add the voltages of two arms if you wanted
to double the voltage of any arm.

The most common voltage in the US for any one of the internal arms of the wye
is 120VAC. The difference between the outer taps, due to the 120 degree phase
difference, becomes the square-root of 3 times 120VAC, which equals 208VAC.

The wye configuration is very popular for motors or other situations where an
unbalanced load may occur. A wire can be run between the two remote ends to
connect the two center taps of the "sending" and "receiving"
transformers/motors/generators. Any unbalanced load will thus form a residual
current in the neutral wire -- and thereby somewhat protect the whole
circuit.

A three-phase delta configuration is generally wired so that 240VAC appears
between the taps. That voltage difference (a single phase of the three-phase)
is then stepped down using a center-tapped transformer to provide two 120VAC,
in-phase single-phase lines.

The wiring diagrams for these two configurations are on the web at:

     http://www.ktc.net/Powertronics/wiring.htm

Many of the world's power systems are listed on:

     http://www.clary.com/power.html

for their three-phase, single-phase voltages and the configurations of the
generator wiring for those voltages. Note that for single-phase systems, the
lower voltage (e.g., 120V/240VAC) is always just half of the full tap voltage
when wired as a delta.

And I think that's what's wrong with your UPS. A single-phase UPS system
generally works by generating a near-sinusoid current by switching at least
two large transistors at 60Hz. That switched current is fed through an
inductor (in this case, a transformer) to generate the desired voltage. If
that transformer has a center-tap, it most likely has two settings
(480VAC/240VAC) and (240VAC/120VAC). That setting (if available) may simply
be in the wrong position.

Once a UPS kicks in, it disconnects its output terminals from the incoming
line and switches those terminals over to its internal "generator". In
essence, you have two different power sources appearing at the same terminals
-- but at different times (when line power is on or off). It is my guess (and
it is only a guess) that the internal voltage generator is set to generate
twice as much voltage as you wish.

A second (backup) guess would be that your UPS simply doesn't have a
center-tap on its internally generated power transformer -- and all you're
going to get is 220VAC. But if you're measuring 220V tap-to-ground, from both
taps, and you are able to draw substantial power from either tap-to-ground
configuration, then this guess is not as likely as the first.

Wirt Atmar