A/C Breaker Tripping / Contactor Buzzing

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busman

busman

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Location
Northern Virginia
Occupation
Master Electrician / Electrical Engineer
I have an A/C compressor. Wired with 10/2, 40 foot run, two JB's from panel to disconnect. All splices seem fine. Unit is very old, but was recently checked twice by A/C company with nothing found wrong. Unit trips the CB about 5-10 seconds after starting (sometimes) and sometimes while running. Nameplate is FLA=18.6, MCA=24, Max Fuse/Breaker=40, LRA=109. It is on a 40 amp breaker with 40A time delay fuses in disconnect. The highest inrush current I measured was 210A and it quickly settled to about 16A.

The only problem I could find was a very old contactor that was buzzing. Is it possible for the contactor to create enough voltage drop to increase the running current to trip the breaker? Any ideas would be greatly appreciated.

Thanks as always.

Mark
 
A contactor is MUCH cheaper and easier to replace than a compressor. I have seen worn contactors cause many problems more than a tripping breaker. Change the contactor.
 
I'd start with a clamp-on ammeter. Voltage drop will reduce current, not increase it.
Click to expand...

With motors, a lower input voltage will actually increase current. The motor is not a static load like a resistor. As the input voltage drops, the motor tends to slow down. That reduction in speed causes more current to flow to keep the motor going.

Or think of it this way. Since the motor is turning a fixed load (the compressor) it basically has a constant output power. If you reduce the input voltage, to keep the same output power the current goes up.

Lighting ballasts can work the same way. Less voltage in, the ballast compensates by drawing more current.

Steve
 
I hate being on jobs like that. Possibilities are many: bad motor, bad contactor, bad breaker and who knows what else. Maybe you are missing something, I don't know what it would be though. Have you tried checking voltage across each contact to make sure you're not dropping voltage across contacts? One of the troubles with a lot of meters is that you don't pick up momentary glitches.

Please let us know how problem gets resolved.
 
I checked the voltage on the secondary side of the contactor under starting. 246 volts before starting. Lowest voltage during starting 229V. I have a fast response Amprobe min/max/avg/peak meter. Thats how I measured over 200 amps of inrush current.

I did a bunch of reading on the internet this morning. It seems that as compressors get older, they become harder to start. It is also possible that the run capacitor is bad. Many websites recommend an "hard start kit" which is a start capacitor/relay combo that goes in parallel with the run capacitor. Aparently some compressors have start caps and some don't. I only saw one large cap on this compressor.

Sounds like a job for the A/C guys. I have changed the breaker. The max voltage drop on starting appears to be 7%.

Any more thoughts?

Thanks,

Mark
 
Mark if contactor is not the problem (if replaced) the refrigerant levels being low etc. will cause a trip. I know the HVAC techs checked it out but did it leak when they left..........."just checkin" This has happened to Me before so I started carrying a set of gauges.
 
You guys are reading my mind. I picked up a hard start kit at the A/C supply yesterday. That's next if it keeps tripping.

Thanks again.

Mark
 
Technical question---------Does the unit have a hard start kit, either a start capacitor with a potential relay or an all in one boost kit installed? This usually gives the old compressors a kick.

Contactor buzzing, not usually an issue since the unit is 240 single phase, if the was a contact problem the unit wouldn't start.

Jim
 
Jim,

Yep!... EPA certified and trained used to be Electrical Maintainence Tech for a large Florida Housing Authority.... heck it was free... even the gauges. I was just offering a thought... Why You ask?
 
Jim said:
Contactor buzzing, not usually an issue since the unit is 240 single phase, if the was a contact problem the unit wouldn't start.
Click to expand...

You can have bad contacts and still start but sometimes drop voltage across arcing contacts. Somewhat unusual but it does happen. I agree the buzzing is not normally an issue other than the irritation of the noise. :)
 
Triphase, I just ask because I'm the same. 13 years as an HVAC service mech, then a manager where I acquired my electrical contractor?s license. Now I'm a manager of a 19 man maintenance crew providing commercial office space maintenance.

Jim
 
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