Richard Ferguson wrote:

Someone is offering a Quincy compressor at a reasonable price, but he is
not sure if it is single phase or three phase. I need single phase. The
tag on the motor does not seem to help (he emailed me a close-up photo).
It is dual voltage, 208-230/460. The problem with the tag is that it
has a bunch of numbers, but they are not labeled as to what the numbers
are. :-(

Could I take an ohmmeter and figure it out? I would guess that a three
phase motor would have equal resistance on all three combinations of the
phases, while a single phase with neutral would have different
resistance, probably low between the two hot leads and higher between
hot and neutral. Green should be open relative to all the other wires,
no matter the phase count.

Is there another solution?

Generally you can look at the motor itself. If no bump on the side it's 3 phase,
if there's a bump on the side for start cap; single phase. Another real good
clue is the number of wires in the pigtail. 2 wires - single phase; 4 wires -
3 phase, 3 wires is indiscriminate but you can check to see if any have very low
ohms to the motor case; if so you can assume that it's a ground wire and thus
there are 2 hot wires so single phase.

Looking a slightly different way; if you buy the machine and guess wrong, you
then have to find a motor of the correct size and you may have to upgrade the
controls. Motors are generally pretty available at varying cost (I got a 5hp
Leeson single phase motor brand new for my Quincy compressor) but the real cost
killer is if you have to upgrade the mag switch, that can cost you several
hundred dollars if you aren't careful.

Grant