Many safety switches are used to avoid any accident to an air compressor. Out of that, the most important safety switch is “Low oil pressure switch” in short LOPS.
As the name indicates; this switch will operate in condition when the oil pressure in the crankcase is lower than set limit & stops the compressor.
So, The details of LOPS is as below
INDFOS make IPS-70 switch is commonly used & the function of this switch is to monitor the oil pressure in the crankcase of an air compressor.
This switch is connected to the oil pressure sensing line coming from the crankcase to the oil pressure gauge, through Tee connection.
The main spring can be set to balance different pressure on the bellows.
When the pressure increases, the bellow is affected. So that the main spindle is moved upwards until the spring pressure balance the bellow pressure.
The spindle has guiding knob and differential adjusting nut which jointly transmit the spindle movement to the switch.
The differential pressure has to be set at zero (Indicator scale at extreme left, facing to LOPS). And the main scale is set to required pressure. (Minimum oil pressure required by the compressor. i.e. 14 PSIG for reciprocating air compressor & 35 PSIG for rotary air compressor).
The pressure on the main scale can be set by rotating the knob given at top of the switch.
How to set & test Low Oil Pressure Switch (LOPS)
Connect an Ohmmeter across normal open contact of the switch to observe the switch operation.
Start the compressor and watch the oil pressure at which contacts close, then stop the compressor to see at what oil pressure contacts open.
By adjusting the knob further fine adjustment can be done to the exact pressure required. So confirm the setting by starting & stopping the compressor.
Since the switch remains open during starting, a normal close contact of the timer is connected to the normally open contact of LOPS.
The timer is set for 60 sec. As the compressor starts, a timer starts its timing cycle and the compressor oil pressure also start building up in 60 sec. timer contacts will become open.
Due to developed oil pressure in sensing line, LOPS contacts closes & continued the healthy running condition of an air compressor.
In case the oil pressure does not build due to any reason, this contact remains open and the compressor will trip in 60 sec. time span.