Check your oil safety switches!

Many commercial refrigeration compressors (i.e. Copeland) and essentially all industrial reciprocating compressors, like Vilter 450, 440 & 350 series, have force fed lubrication systems.  The oil that is in the crankcase is pumped to various journals and bearings to lubricate these internal parts.  The oil pump is located at the end of the crankshaft and is driven by the rotating shaft.  The oil is picked up from the crankcase at compressor suction pressure, and the pressure is increased by the pumping action to force it throughout the compressor.  If we compare the pressure at the pump inlet (suction pressure) to the pressure at the pump outlet (oil pressure), we can determine if the pump is working properly and if the compressor is getting good lubrication.  This function is provided by the Oil Failure Safety Switch (OFSS).

There are several components built into the OFSS:

  • The control contact - generally between terminals L & M.  These contacts are normally closed and open only when the OFSS indicates that the compressor is not receiving lubrication.  This switch is temperature activated by the heat produced from the resistance heater.
  • The differential pressure switch - This switch opens and closes in response to the difference between suction pressure and oil pressure.  The differential switch is a closed contact when the pressure difference is below approximately 12 psi and an open contact above 12 psi.
  • The electrical resistance heater - This heater is producing heat when the differential switch is closed and no heat when the switch is open.  The heat from this heater opens the contact between L & M.  It requires from 90 – 120 seconds of heat to open L & M.
  • The dropping resistor - allows the OFSS to operate on either 115v or 230v.

When a compressor is OFF and sitting idle, the crankshaft is not turning and the oil pump is not generating any differential pressure.  The differential pressure switch internal to the OFSS is therefore a closed contact.  As soon as the compressor control circuitry tells the compressor to START, the crankshaft and the oil pump begin to rotate.  The OFSS allows the compressor 90 – 120 seconds to develop oil pressure that is 12 psi greater than suction pressure.  If this differential pressure is not achieved within the time period, the heat from the resistance heater will be sufficient to open the L & M contact and shut down the compressor.  If sufficient differential pressure is developed then the compressor is allowed to continue to run.  However, if the differential pressure ever drops below 12 psi and remains low for 90 – 120 seconds, the compressor will shut down. 

The OFSS has a manual reset button.  If you ever need to push the small button projecting through the switch cover, you know that the compressor ran for 90 – 120 seconds without proper lubrication and the reason should be determined.

These OFSS need to be checked periodically to ensure its proper operation.  Here are a couple of methods to check the operation and proper wiring:

  • If the compressor is externally driven (direct drive or belt drive) and the motor is disconnected from the compressor, you have a perfect opportunity to check the OFSS.  Turn ON the compressor motor by engaging its starter either in a Hand or Auto position.  The motor should start but the compressor will not operate (the belts are off).  Within 90 – 120 seconds the motor should shut off.
  • If the motor is operated on 460/3/60 power with a 115v control circuit you can shut off the main 460 power and then engage the starter.  The control circuitry and the OFSS are in use, but with no main power, the compressor will not run. You will hear the starter initially close and in 90 – 120 seconds it should open.
  • While the compressor is in normal operation, you can hold the differential switch in the open position and the OFSS should shut down the compressor.

Make sure that your Oil Failure Safety Switch is operating as designed! Compressors and overhauls get expensive!