Oil pressure trips on semi-hermetic compressors aren't a problem you'll run into every day,however when you do they can be a real pain in the neck to diagnose and repair. Here are a few things to check for when troubleshooting an oil safety trip.
The obvious place to start is to check that there's oil in the compressor. Check the level in the sight glass. If the oil level seems low but the compressor stays running, watch to see if the level increases after 10 or 15 minutes of run time:
* If the level doesn't increase, add enough oil to maintain about a half a sight glass.
* If the level increases, then the oil was logged out in the system and is now returning. A flooding expansion valve can cause this problem.
Monitor the superheat at the compressor, and watch for foaming oil in the sight glass.
Freezers without a pressure limiting expansion valve can have this problem when coming out of defrost. Use a crankcase pressure regulating valve.
A bad three-phase contactor can cause oil safety trips. The oil safety is wired in series with the compressor contactor, so if the compressor intermittently single-phases due to a voltage drop across a burnt contact, it won't start. In these cases, it's common for the compressor windings to overheat, and for the internal overloads to open.
The oil safety can still be energized and time out. Without a pressure differential, this will trip the oil safety after 90 to 120 seconds. The contactor will open, and the compressor will cool down. Then you show up, reset it, and it runs. Because you find no oil pressure problems, you leave only to return in a couple days to the same problem.
Oil pumps are protected by a filtering screen located in the oil sump of the compressor. If the screen become plugged with debris picked up from the system by the returning oil (anything small in the system, such as solder scale, dirt, copper burrs, etc. will eventually end up in the compressor) it can give you the illusion that the oil pump may be failing.
Before you change the oil pump, check and clean the screen.
Use a good suction line drier as well as a liquid line drier
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