DIYbanter - View Single Post - Identifying three phase vs. single phase motor?

In article ,
says...
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. :-(

The voltage ratings indicate in a somewhat roundabout way that this is a
3 phase motor. It'd be possible to have a single phase 460V motor, but
in 30 years working around this sort of stuff I've never run across one.
If it were single phase dual voltage the plate would more likely specify
some variation of 230, 115/230, or 115/208-230, though 5HP 115V motors
are pretty unusual.

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.

A single phase motor will not have a neutral lead. Once the motor leads
are strapped for the appropriate voltage it'll have two power
connections which may be connected hot-hot or hot-neutral, depending on
the power feed.

A better tell-tale of a single phase motor in this size range is the
presence of a start cap. Unless it's a repulsion/induction motor, in
which case it'll be about 4x the volume of a regular induction motor and
have a commutator and brushes on the end opposite the shaft. A 5HP
repulsion/induction motor would be among the unusual 5HP 115V single
phase motors I alluded to above and makes a great compressor motor.

Ned Simmons