Starting cap failure?

[et...@whidbey.com]

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

[Ralph Mowery]

In article <5ujupclqvbqv63fcg...@4ax.com>,
et...@whidbey.com says...

>
>
> 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

There are many ways to check the capacitors. You can find capacitor
"meters" and testers from about $ 15 up. You may be able to find a shop
that works on air conditioners and take the capacitors to them and have
them checked. Usually they will have a voltmeter device that just hooks
across the capacitor. Takes longer to find the meter than to check the
capacitor.

[Rheilly Phoull]

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 ?

[tabb...@gmail.com]

Or as a quick test clip some additional C across it.


NT

[Phil Allison]

tabb...@gmail.com wrote:

-----------------------------

>
> Or as a quick test clip some additional C across it.
>
>
>

** That reminds me of what was the common practice with suspect electros BEFORE we all got ESR meters.

How quick we do forget.



.... Phil

[et...@whidbey.com]

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

[et...@whidbey.com]

<SNIP>

>>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.

<SNIP>

> 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

Whoops! I was wrong about the starting device. I just assumed it was
some sort of potential relay like I had seen in other motors. It's a
bad thing to assume sometimes. In reality the starting device in an
adjustable delay solid state relay. The on time, once energized, is
from .25 to 5 seconds. It resets when power is removed. Supossedly
good for 100,00,000 operations. But the original timer failed after
only approximately 600 operations.
Eric

[gghe...@gmail.com]

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.

[et...@whidbey.com]

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