On Sun, 12 Feb 2012 01:05:04 +1100 "Phil Allison"
wrote in Message id: :


"JW"

Is there any reason that a 120VAC to 120VAC isolation transformer would
draw 2.54A on its primary when there is no load present on it's secondary?


** Is that 2.54 amps RMS or not ?

Yes. I've measured it with the same result using both a Watts Up meter and
a Fluke 87.

In any case, is it getting rather hot and making a humming noise ?

Making a humming noise, yes. It's not getting hot, the external
temperature rise of the case is only about 10c.

If not, then all is OK.

The magnetising current will drop significantly when a full load is
applied.

Nail. Head.

My test setup and how this all started:

120V in is being fed to a variac, then to the isolation transformer, then
the load. On the secondary of the transformer I added a toggle switch that
would connect the secondary in series or parallel depending whether I
wanted 120V or 240V out. When I first wired up everything as I stated in
my OP, being on the cautious side I placed a 1.5A fuse into the variac so
if I mis-wired something there'd be no damage. Well, on first power up I
slowly raised the voltage on the primary and at a little over 110V the
fuse blew.

The actual wattage when the primary is at 120V is only 42W as measured by
the Watt meter. Here's why - the power factor is really crappy at just
about .15. This would explain everything. I wonder if this is typical of a
1KVA transformer? I never thought to check the wattage being drawn...

BTW:

Adding capacitance in parallel has no effect on the tranny whatsoever -
and it will make the PF worse.

Noted, and thanks.

Also, thanks to everyone else for their input - some very good thoughts.
It would appear that there's no problem after-all.