I picked up a medical grade pot-bellied 2hp Copeland head air compressor a few weeks ago because it would run. I had to put a new start capacitor on it to get her going. After she got going, however, the contactor would chatter right at start up for a few seconds. Now, two weeks after starting to use the machine she's popped the start capacitor again.

Of course now I'm looking at replacing the chatterbox contactor AND the start capacitor again. I'm going to replace the run capacitor as well. The motor is only pulling about 7amps while running. I'm just curious if I could be looking at any other possible culprit?

Machine is 220v and is running on pretty stable current.

 

First concern...Is the head unloader valve operating properly ? This causes the head to drop pressure when high reservior tank pressure has been achieved...

The compressor will not easily overcome residual head pressure to start the next time low reservior pressure commands it to. That would be the short " Pffffft "
of air after set pressure has been attained...

Secondly, when the motor operates "normally" coming up to speed, is the start switch kicking out as it is supposed to. Check this without the compressor attached - no belts - and take note of amps at rated speed w/ no load.

The compressor should be up to speed in a very few seconds, without sounding as though it is laboring to achieve it. If the motor has to repeatedly labor past residual pressure, the start cap will not last long...

 

Didn't dawn on me to check head relief.

The motor and compressor are a sealed unit, though. Spool is almost nonexistent. It's off, the it's on.

 

Chase down that chatter. I've never seen a contactor chatter unless the control voltage was bad. You will be looking for connections that are intermittent during vibration.

 

Also be suspicious of "crimped" type connectors which are most likely used on the wires. Over size, under size, oxidized, or otherwise less than perfect appearing connections should be re-placed.

I prefer soldered lugs for connections that cannot be easily accessed or otherwise have a hard service life(this would include vibration) But that's just me.

 

I use the crimp-ons, then solder them after I crimp them.

 

If the contactor (relay) electrodes could be pitted / oxidized / aged from arc induced erosion, then the contacts may chatter. I have seen this on an air conditioning unit. Replaced the contactor and the unit ran perfectly.

 

One of my students brought one into school with similliar sympthoms.
Turned out, the pressure switch is wired in series with the contactor coil, and the pressure switch was flaky. With every surge of air from the compressor, the contacts would open causing the chatter.We jumped out the pressure switch, briefly, to verify this before ordering a replacement.

 

On Motor control units, the culprit is usually under voltage, corroded contacts, a weak coil, or sticky slides in need of lube.