On Fri, 25 Aug 2017 09:25:37 +0800, Rheilly Phoull
wrote:

On 25/08/2017 6:31 AM, 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


After 15 years I reckon I would just get another cap and see if that
fixed it. What sort of device takes out the cap, centifugal ?
There is an electronic device that's got its innards potted. It senses
the starting current and when the current drops to the set level it
drops out the contactor that connects the starting caps.
There 10 starting caps connected in parallel so I don't think it's
such a good idea to replace them all just in case that's the problem.
They are all stuck in place with some pretty good double sided foam
tape so removing them would be a hassle too.
I cannot see how many mfd the caps are rated for but I can see on
one cap that they are made for a phase converter.
I just remembered that years ago the device mentioned above failed.
When I tried to start the converter it would just hum. There is a
button inside that I can press that bypasses the starting device and
energizes the contactor coil. When the starting device failed the
first time I was told by the phase converter maker to press this
button and see if it starts OK. It did which is how I diagnosed the
problem.
The company that made the converter has gone out of business so I
can't call them for advice. But I'm gonna try the button thing again
and if it starts fine then I guess I know the problem. What I don't
understand is why the converter sometimes starts to spin up slowly and
at other times just sits there and hums loudly. And then starts just
fine on the next attempt. Could it be that when it starts to spin up
slowly that the rotor was in just the right position that the
balancing caps in the converter provide enough phase shift to start
spinning the rotor, albiet slowly?
Thanks,
Eric