The Professor: Oil Safety Controllers and Their Circuits
February 1, 2010
Oil safety controllers often come in two types: bellows (mechanical) and transducer (electronic).
Remember, net oil pressure (NOP) or sometimes referred to as useful oil pressure, is the difference between the oil pump discharge pressure and the crankcase pressure (as referenced in my column in the Jan. 11 issue). 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. 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 (Figures 1 and 2 illustrate the internal action of the bellows-type controller.)
Transducer-type controllers use a pressure transducer, which 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 (net oil pressure) is accomplished by the transducer mechanically. The pressure transducer is connected to an electronic controller by wires. The pressure transducer 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. Those pressures are crankcase pressure and oil pump discharge pressure.
BELLOWS (MECHANICAL) CONTROLLERThe 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 net oil pressure.
Oil pump discharge pressure - Crankcase pressure = Net oil pressure
So, 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 delay before the heater will warp a bimetallic strip. This warping action will open 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 net oil pressure on start-ups.
Remember, when the compressor is off, the net oil pressure 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 start-up, 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 net oil pressure is developed. As mentioned before, this time delay will prevent nuisance trips of the controller at start-ups.
INTERNAL OVERLOADSIf 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 a lack of net oil pressure. However, the addition of a current relay on one of the legs of the compressor will tell the electronic controller that the compressor is not running and will open a circuit to the safety heater on the oil safety controller (Figure 3).
Publication date: 02/01/2010