Fig. 1
Oil safety controllers often come in two types: bellows (mechanical) and transducer (electronic). Remember, net oil pressure (NOP), sometimes referred to as useful oil pressure, is the difference between the oil pump discharge pressure and the crankcase pressure. Bellows-type controllers sense both crankcase pressure and oil pump discharge pressure usually through some type of tubing and then transmit the pressure to flexible bellows (Fig. 1).

The tubing connected to the bellows is usually copper capillary tubes, high-pressure rubber hose, or a type of high-pressure plastic with a braided aluminum shrouding. Fig. 2 illustrates the internal action of the bellows-type controller.

Transducer-type controllers use a pressure transducer that senses a combination of oil pump discharge pressure and crankcase pressure. The pressure transducer has two separate ports to sense both crankcase pressure and oil pump discharge pressure. The subtraction or difference between these two pressures (NOP) is accomplished by the transducer mechanically. The pressure transducer is connected to an electronic controller by wires. It then transforms a pressure signal to an electrical signal for the electronic controller to process.

Both types of oil safety controllers are referred to as differential-type controllers. The nomenclature comes from the fact that they sense two different pressures; crankcase pressure and oil pump discharge pressure.

Fig. 2


Notice in Fig. 2 that the oil pump discharge pressure acts to open the differential pressure switch. Conversely, the crankcase pressure acts to close the switch. One has to remember that the difference between these two pressures is the NOP as shown in the equation: Oil pump discharge pressure minus crankcase pressure equals NOP.

If there is a fall in net oil pressure below 9 pounds per square inch differential (psid), the pressure differential switch will close and a heater in series with the pressure differential switch will be energized. There is usually a two-minute (120-second) delay before the heater will warp a bimetallic strip.

This warping action opens the timing switch contacts, which are in series with the motor starter or contactor coil. This action takes the motor out of service and must be manually reset on most controls.

Notice that manually pushing the reset button will reset (close) the timing switch contacts once the bimetal strip cools down. The reason for the two-minute time delay is to prevent nuisance trips of the oil safety controller.

Often there are times when the crankcase may have liquid refrigerant in it from an imperfect system. The two-minute delay gives the crankcase time to clear any unwanted refrigerant during periods when refrigerant migration or flooding has occurred. It also avoids shutdowns during short fluctuations in NOP on startups.

Remember, when the compressor is off, the NOP is 0 psi and the differential pressure switch contacts are closed. The heater in the oil safety controller will not be energized during the off-cycle because it is wired to the line side of the motor starter contacts.

When the motor starter contacts are opened, this action takes L2 out of the heater circuit. At startup, when the motor starter contacts close and the compressor starts, the differential pressure switch contacts will stay closed and the heater will be energized until at least 9 psid of NOP is developed.


If a motor is equipped with both an internal inherent motor protector and an oil safety controller, the oil safety controller may trip due to a motor overheating or overloading problem on some systems.

When the internal overload opens, the motor is shut off but the motor starter coil remains energized with contacts closed. This will trip the oil safety controller in a matter of two minutes because of lack of NOP.

The addition of a current relay on one of the legs of the compressor will tell the electronic controller, however, that the compressor is not running and will open a circuit to the safety heater on the oil safety controller.

John Tomczyk is a professor of HVACR at Ferris State University, Big Rapids, Mich. He can be reached by e-mail at

Publication date: 05/01/2006