Steve Kraus wrote:

I'm trying to figure power factor for a large transformer-type DC power
supply. This is pretty much just for my own amusement (so I can figure
power draw from current and voltage readings) so there's nothing critical
here.

This device runs on 208V 3-phase (as in each leg is 120V from neutral but
there's no neutral connection). The 3 transformer primaries are wye
connected but there's no other connection to the wye point. It draws about
18A per leg.

I decided to compare current and voltage waveforms using a dual trace
scope.

To look at current, rather than put a small resistance shunt in one line I
decided to take advantage of the small voltage drop that occurs between the
breaker panel and the cutoff switch for the device. So I had one channel
of the scope looking at the voltage difference between one leg at the
cutoff switch and the "hot" side of a 120V outlet on the same leg.

For voltage I just looked at one leg vs. neutral.

I'll skip over boring details about having to float the common side of the
scope in order to do this without creating a short circuit. Anyway, it
seemed to work. Emphasis on "seemed."

What I saw looked like voltage vs. current was out of phase by about 90°.
Is that what one would reasonably expect going into a transformer? Seems
kind of extreme so I wonder if my entire setup was bogus in some way.

Part of the reason for the question is that in retrospect I should have
probably been looking at voltage as measured from one input leg to the wye
point, not neutral since that's what a transformer primary sees. But I
would think the wye would be similar to neutral.
what you're seeing in the xformer is normal. the xformer is returning
the current phase from the current applied source which comes back out
of phase with it, thus, the current you see is lagging behind from which
it was created from the voltage you now see going the other way...

Power Factor (F) is the difference between True power (resistive) and
Apparent power (reactive)...