The oil pressure switch acts as a function of the differential pressure, which is obtained by using two diaphragms or bellows in their physical constitution. One bellows is connected to the crankcase reflecting the pressure thereof, while the other is connected to the discharge of the oil pump, the pressure difference between the two bellows is the differential pressure, used to drive a differential pressure switch in the pressure switch.
An operating relay incorporated into the pressure switch allows the compressor to operate from 90 to 120 seconds with the pressure below the minimum safety level. This allows the compressor to start at zero oil pressure and also prevents unnecessary shutdowns in case of momentary pressure drops.
However, if the pressure does not rise within the preset time, the pressure switch will turn off the compressor and before the compressor can be restarted the pressure switch must be manually adjusted.
The operating relay consists of an operating switch and a thermal element. The switch is connected in series with the coil of the contactor that drives the compressor and the thermal element is connected in parallel with the coil of this contactor. The pressure differential switch is connected in series with the thermal element, controlling its operation. The resistor in series with the thermal element of the relay limits the flow current over the thermal element and makes the pressure-failure control adaptable to 115V and 230V circuits.
The oil pump operates only when the compressor operates, the total oil pressure will be exactly equal to the sump pressure during a rest period of the compressor, ie when the compressor is switched off the sump pressure equalizes with the discharge pressure the pump.
When the compressor drive signal is released it will pass through the oil pressure switch contacts L and M and feed the contactor coil, when the contactor coil is energized, the contacts 13 and 14 are closed and consequently the supply of the operation relay, ie the thermal element and the relay will be energized.
If, after the compressor starts, the differential pressure of the oil should increase to the pressure of the safety control, the pressure differential switch will open by turning off the thermal element of the relay, allowing the compressor to continue its normal operation. On the other hand, if the useful oil pressure does not increase to the connection value within the set time, the differential pressure switch will not open and the thermal element will be turned on. Continuous operation of the thermal element of the relay will cause the bimetallic element to deform and cause the operating contacts to open. This turns off the contactor coil that drives the compressor by stopping it.
