Small compressors normally incorporate some type of splash lubrication system. Larger refrigeration compressors, however, will typically incorporate a forced oil lubricating system to pump oil to the bearings. These larger compressors also have a safety control to monitor the oil pressure delivered by the oil pump. These oil pressure controls are designed to shut down the compressor if the delivered oil pressure is inadequte.
These controls do not simply monitor the outlet pressure of the oil pump. They are designed to monitor the net oil pressure within the compressor. If the net oil pressure is below a predetermined value for a specific period of time, a set of contacts opens, shutting down the compressor. Normally if the net oil pressure is below 9 psig for 90-120 seconds, the oil pressure control will stop the compressor’s operation. Once this occurs, the control needs to be manually reset in order to operate the compressor again. This prevents the compressor from cycling repeatedly due to lack of proper oil pressure. It also alerts someone that the oil pressure within the compressor is not adequate and some corrective action needs to be performed.
The net oil pressure of a compressor is the outlet oil pump pressure, minus the pressure of the refrigerant in the compressor crankcase. For example, if the outlet oil pressure is 60 psig and the refrigerant pressure in the crankcase is 20 psig, the net oil pressure will be 40 psig (60-20 psig).
TWO TYPESTwo types of oil pressure controls are commonly used: mechanical and electronic.
A typical mechanical oil pressure control uses two bellows to control a linkage that operates a set of contacts. One bellow is exposed to the outlet oil pressure of the compressor; the other is exposed to the refrigerant’s crankcase pressure. When a difference between the two bellows falls below 9 psig, a set of contacts closes.
These contacts now energize a set of heaters located close to a bimetal switch. The heaters warm the bimetal switch and after 90-120 seconds, the switch opens. The bimetal switch is typically wired in the control circuit of the compressor and when it opens, it shuts down the compressor.
An electronic oil pressure control performs the same function as a mechanical oil pressure control, but instead of using bellows and a mechanical switch, a pressure transducer is used to measure the net oil pressure. The transducer sends a signal to an electronic control board, which opens the contact of a relay after a 90-120 second time delay if the net oil pressure is not adequate. The contacts of the relay are wired in series with the controls operating the compressor, so when the relay’s contacts open, the compressor shuts down.
Whenever an oil pressure control shuts down a compressor, the cause of the shutdown must be determined. Simply resetting the control does not solve the problem. More than likely the oil pressure will trip again, shutting down the compressor and the system again.
The cause of the oil pressure failure is not always apparent. At times a technician must monitor the system over a period of time to determine the true cause of the problem.