Why did these caps fail?
 

I bought a used air compressor of unknown vintage. I checked
everything mechanical, wired it up and it started and ran well. I
expected this because I had seen it running before I bought it.
Anyway, after several starts over maybe 5 days the motor acted like it
was straining to start. Just before I got to the braeker panel the
breaker popped. I first checked the pump and it turned over nicely so
I took off the capacitor cover on the side of the motor and both caps
looked bad. There are two starting caps on this motor connected in
parallel. No run caps. One cap failed to the point that one spade lug
had fallen off due to heating at the point where it was riveted to the
cap. Both caps have vent holes. The cap with the melted off lug had
white stuff coming out of the vent. The other cap did too but there
was less of it. I know that motor start caps fail over time but I
don't know what the fail mechanisms are. This motor did sit idle for
several years before I bought it. Do motor start caps need to be
"re-formed" like the electrolytics found in electronics?
Thanks,
Eric

Why did these caps fail?
 

Why did these caps fail?
 

Jon Elson scribbled thus:

wrote:

I bought a used air compressor of unknown vintage. I checked
everything mechanical, wired it up and it started and ran well. I
expected this because I had seen it running before I bought it.
Anyway, after several starts over maybe 5 days the motor acted like
it was straining to start. Just before I got to the braeker panel the
breaker popped. I first checked the pump and it turned over nicely so
I took off the capacitor cover on the side of the motor and both caps
looked bad. There are two starting caps on this motor connected in
parallel. No run caps. One cap failed to the point that one spade lug
had fallen off due to heating at the point where it was riveted to
the cap. Both caps have vent holes. The cap with the melted off lug
had white stuff coming out of the vent. The other cap did too but
there was less of it. I know that motor start caps fail over time but
I don't know what the fail mechanisms are. This motor did sit idle
for several years before I bought it. Do motor start caps need to be
"re-formed" like the electrolytics found in electronics?
Thanks,
Eric
Check that the starting switch is opening when the motor is up
to speed. With the start caps disconnected and the belt off
(no load) you should be able to start the motor by kicking the
pulley and turning the breaker on. Then shut it off, and
see if it makes a sharp click-scuff-scuff-scuff when stopping.
That will prove the centrifugal mechanism is working, but you
probably will have to disassemble the motor to check that the
contacts are not welded. You MAY be able to do a continuity
check while poking the start switch through a vent hole, depending
on the motor construction.

And, of course, it could easily just be bad capacitors, they
do go bad eventually.

Jon

Whilst I agree the centrifugal switch could be sticking or otherwise at
fault, those caps have to handle an awful lot of current during start
up until the switch opens and disconnects them.

--
Best Regards:
Baron.

Why did these caps fail?
 

On 04/04/2014 05:18 PM, wrote:
I bought a used air compressor of unknown vintage. I checked
everything mechanical, wired it up and it started and ran well. I
expected this because I had seen it running before I bought it.
Anyway, after several starts over maybe 5 days the motor acted like it
was straining to start. Just before I got to the braeker panel the
breaker popped. I first checked the pump and it turned over nicely so
I took off the capacitor cover on the side of the motor and both caps
looked bad. There are two starting caps on this motor connected in
parallel. No run caps. One cap failed to the point that one spade lug
had fallen off due to heating at the point where it was riveted to the
cap. Both caps have vent holes. The cap with the melted off lug had
white stuff coming out of the vent. The other cap did too but there
was less of it. I know that motor start caps fail over time but I
don't know what the fail mechanisms are. This motor did sit idle for
several years before I bought it. Do motor start caps need to be
"re-formed" like the electrolytics found in electronics?
Thanks,
Eric


"Sounds like the goddam Spanish Inquisition!" -Bones

Do you have a Grainger catalog or its local equivalent?

Why did these caps fail?
 

On Fri, 04 Apr 2014 23:54:18 -0500, Jon Elson
wrote:

wrote:

I bought a used air compressor of unknown vintage. I checked
everything mechanical, wired it up and it started and ran well. I
expected this because I had seen it running before I bought it.
Anyway, after several starts over maybe 5 days the motor acted like it
was straining to start. Just before I got to the braeker panel the
breaker popped. I first checked the pump and it turned over nicely so
I took off the capacitor cover on the side of the motor and both caps
looked bad. There are two starting caps on this motor connected in
parallel. No run caps. One cap failed to the point that one spade lug
had fallen off due to heating at the point where it was riveted to the
cap. Both caps have vent holes. The cap with the melted off lug had
white stuff coming out of the vent. The other cap did too but there
was less of it. I know that motor start caps fail over time but I
don't know what the fail mechanisms are. This motor did sit idle for
several years before I bought it. Do motor start caps need to be
"re-formed" like the electrolytics found in electronics?
Thanks,
Eric
Check that the starting switch is opening when the motor is up
to speed. With the start caps disconnected and the belt off
(no load) you should be able to start the motor by kicking the
pulley and turning the breaker on. Then shut it off, and
see if it makes a sharp click-scuff-scuff-scuff when stopping.
That will prove the centrifugal mechanism is working, but you
probably will have to disassemble the motor to check that the
contacts are not welded. You MAY be able to do a continuity
check while poking the start switch through a vent hole, depending
on the motor construction.

And, of course, it could easily just be bad capacitors, they
do go bad eventually.

Jon
Greetings Jon,
I suppose it could be the starting switch but I doubt it. Every motor
I have seen that had a stuck starting switch overheated quickly. This
motor has not been running hot. Nevertheless, I'll run the air down
until the compressor starts and once the motor is up to speed I'll
turn it off and listen for the switch closing.
Eric