On Fri, 25 Aug 2017 11:05:34 -0700 (PDT), wrote:

On Thursday, August 24, 2017 at 6:31:18 PM UTC-4, wrote:
I run my machine shop off af a rotary phase converter. It has
starting caps that are switched out of circuit after the motor is
started.
Lately the device sometimes has a hard time starting. I switch it
on and the motor may not come up to speed practically instantly, the
way it is supposed to. If this happens I turn off the breaker, wait a
minute, and then try again. It always starts fine on the second
attempt. Except this morning it took three tries for the thing to
start. The motor did actually spin up some this morning on the first
try but it was really slow starting so I shut the breaker off.
I suspect starting caps but wonder why the second attempt always
worked until today. And then today it started fine on the third
attempt.
The phase converter has been started almost daily for at least
fifteen years using the same breaker in the main breaker panel.
I thought that maybe the breaker might be making a bad contact on
one leg of the single phase 250 volt input. Or maybe a contactor
inside the converter is not making good contact. Or maybe, and I think
most likely, the starting cap(s) is(are) the problem. I just wonder
why, if the thing doesn't start spinning right away on the first try
it does on the second or third.
This weekend I'll have time to look inside the phase converter and
I would like to know if there is a way to diagnose the starting caps.
The contactor contacts I know how to check. And I could buy a new
breaker. But I would like to not just buy stuff until the thing works
properly.
Advice?
Thanks,
Eric

Knowing little about converters, it sounds like it could be
mechanical? Is it anything like a starter motor with a 'bad'
spot? It's hard for me to see how a cap would need two tries
to do it's job right.

George H.
Yeah, I'm starting to think the contactor contacts are the problem.
Just so you know, a Rotary Phase Converter (RPC) is just a 3 phase
AC induction motor that is run from single phase power. Once the
motor is spinning it generates power in the third winding, AKA leg,
that would normally be connected to a three phase supply. They are
started the way many single phase motors are started. A typical type
of single phase induction motor has a starting winding that is powered
only while the motor is starting. This winding is shifted in phase
about 90 degrees from the main winding. Both physically as well as
electrically. The electrical phase shifting can be done using
resistance, capacitance, or a combination of the two. There are other
schemes as well but the capacitance method is common on motors that
need lots of torque when starting, like a motor on an air compressor.
A simple RPC can be made using just a 3 phase motor. A rope can be
wrapped around the motor shaft and then pulled fast to get the motor
spinning. Once it is spinning single phase power is supplied and the
motor will spin up to operating speed if the shaft was spinning fast
enough from the rope pull. Another motor can be used to spin up the
RPC motor as well. This motor is then either disconnected mechanically
or just turned off once the RPC motor is running. Capacitors can also
be used to supply phase shifted current just like a single phase
motor. Again, once the motor is up to speed the starting capacitance
is disconnected. Even though the RPCs described above will supply 3
phase power the generated power will not be perfectly in phase with
the other two phases. And depending on the load the phase shift will
vary as will the voltage. So more sophisticated RPCs will use
capacitors across the windings to balance the power so that the phases
are very close to 120 degrees apart and the voltage in the generated
leg is close to the voltage supplied to the motor by the single phase
line.
That's my simple explanation of RPCs.
Eric