Refrigeration control of the compressor oil pressure is very important!
Understanding how the control works, allows you to properly monitor the oil pressure.
Most semi-hermetic compressors above 5 hp have refrigerant oil pumps. The pump lubricates the compressor during operation.
As the pump runs, it generates pressure. This pressure must remain above a certain level to assure correct lubrication.
If the oil pump fails to deliver the oil as needed, the compressor will seize. Metal rubbing on metal will not last long!
An oil pressure control must be installed to guard against loss of oil pressure.
How do they work?
The control monitors two different pressures, the pump pressure and the crankcase pressure.
The difference between them is the compressor oil pressure.
An example: If the oil pump is generating 115 psi and the suction pressure is 70 psi, the oil pressure is 45 psi.
Most compressors need a minimum 10psi to lubricate all moving parts.
If the oil pressure falls below the minimum for a predetermined time, the control contacts open, stopping the refrigeration compressor.
Why not stop the compressor immediately if the pressure drops too low?
Why have a time delay built into the control?
When the compressor starts, often the pump doesn't generate the needed oil pressure immediately.
Refrigerant mixed in the oil can boil out and the pump slowly build the necessary pressure.
Without the time delay, the compressor would stop prematurely creating nuisance service calls.
I prefer a 120 second delay on the oil pressure control.
It gives the condensing unit sufficient time to generate oil pressure without damaging the compressor.
What could cause an oil pressure refrigeration control to open?
Let's start with the obvious, lack of refrigerant oil! Check the oil level sight glass.
If you know you have enough oil?
1. Liquid Refrigerant: Liquid refrigerant in the crankcase can cause loss of oil pressure.
2. Crankcase heater: Having a working crankcase heater is extremely important.
3. Defective Low Pressure Control: If the low pressure control fails to open and the compressor runs in a vacuum, the oil control will eventually trip.
4. Defective Oil Pump: Sometimes the pump itself quits pumping.
5. Defective oil pressure control: The control itself can stop working. I've watched oil pressure controls open, when the oil pressure was 55 psi!
How do I check an oil pressure control?
Simply turn off a running compressor and remove the compressor leads from the contactor. Turn to power back on.
The contactor should close but the compressor, of course, will not start.
The oil pressure control senses no oil pressure and should open the control circuit after the time delay.
If it doesn't, replace the control.
An oil pressure refrigeration control is vital. Low or no oil pressure will eventually destroy the compressor!
Whew!! A lot of information on this page. Hope it helps!
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