A/C Unit Tripping Breaker

I have a Lennox HS24 central a/c unit. The outside unit has tripped the breaker in the house panel twice in about the last 4 weeks (the fan in the attic kept running, but blew hot air in the house). I reset the breaker and the outside unit came back on and the a/c was fine for about 2 weeks, then the breaker tripped again. This time when I reset the breaker, the fan on the outside unit came on, but I could not hear the compressor start up. I turned the breaker off, waited a few minutes, then turned it back on. This time the compressor started and the unit has been cooling fine ever since (about two weeks). I'm in Texas, so it runs a lot. I might add that I've noticed the compressor seems louder over the past few weeks than it used to (whining sound). I had an a/c service company look at it after the first tripped breaker. The put their pressure gauge on it and declared the problem is with the breaker. I asked if they had a meter so the could test the electrical system and see if it is over-amping and the gave me a puzzled look. They pointed out some charring around the contactor, but didn't think anything should be done about it. Not having much confidence in their troubleshooting abilities, I let them go and haven't called anyone in yet. Can you tell from the symptoms what might be causing the breaker to trip periodically? When the unit is running, the breaker feels warm, but that may be normal?

 

You were right in letting that serviceman go. The breaker tripps from either too much amprage or overheating.
Your problem is on an off.
First lets look at the nameplate on the condenser. This will tell you what size breaker or fuse you need. If it says minumum fuse you need to have a fuse either in the house or at the disconnect.
Is the breaker they installed too small?
A loose wire at any connection (ie breaker, disconnect, contactor or the compressor can cause higher amprages.
When the compressor is first started it can draw very high amprages for a moment, this amprage should not exceed the LRA(locked rotor amprage) on the name plate.
If the Air Conditioner is turned off, LEAVE IT OFF FOR 3 MINUTES so the freon pressures in the system can balance.If the electric co or someone at the thermostat does this quick on and off the breaker will trip.
Digital Thermostats have a built in timer to help prevent this problem.
You could lower the starting amperage by installing a Hard Start Booster Capacitor.
Your contactor is showing burn marks, could be a poor conductor and cause the amperage to go up.
If the condensor fan motor were to stop,(bearings starting to go bad and tripping the thermal overload(resets by itself)the compressor would work harder and trip the breaker.
Another problem could be the compressor is starting to ground out( electric from the motor shorting to the frame). If it tests a ground out I would recommend replacement of the compressor if under 5 years old the manufactor warranty covers the compressor. Otherwise relace the condensor you would get all new motors outside and a new contactor.
If the compressor or condensor is changed do have a liquid line filter installed.
GOOD LUCK