Figure 1. A bellows controller.

Figure 2. The internal action of a bellows controller. (Courtesy of Prentice Hall.)
My Jan. 8 article titled “Net Oil Pressure vs. Discharge Pressure” covered calculating net oil pressure for a compressor. This month’s focus will be on two types of oil safety controllers and their circuitry.

Oil safety controllers often come in two types: bellows (mechanical) and transducer (electronic).

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 (Figure 1). The tubing connected to the bellows usually consists of copper capillary tubes or a type of high-pressure plastic with a braided aluminum shrouding. Figure 2 illustrates the internal action of the bellows-type controller.

Transducer-type controllers use a pressure transducer to sense the combination of oil pump discharge pressure and crankcase pressure. The pressure transducer has two separate ports to sense crankcase pressure and oil pump discharge pressure (Figure 3). The transducer accomplishes the subtraction — or difference — between these two pressures to arrive at the net oil pressure. The pressure transducer is connected to an electronic controller by wires (Figure 4). The transducer is actually mounted directly into the oil pump. The pressure transducer then transforms a pressure signal to an electrical signal for the electronic controller to process.

The advantage of an electronic oil safety control is that it eliminates capillary tubes. Thus there is less of a chance for leaks. Also, the electronic clock and circuitry are much more accurate and reliable.

Both types of oil safety control-lers 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.

Figure 3. A pressure transducer.


Notice in Figure 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 net oil pressure, as illustrated in the equation below:

Oil pump discharge pressure – Crankcase pressure = Net oil pressure

So, if there is a fall in net oil pressure below 9 psid (pounds per square inch differential), the pressure differential switch will close and a heater in series with the pressure differential switch will be energized. There is usually a 2-minute (120-second) time delay before the heater will warp a bimetallic strip. This warping action will open the timing switch contacts that are in series with the motor starter or contactor coil (Figure 2). This action takes the motor out of service, and its mechanism 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 2-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 2-minute delay gives the crankcase time to clear any unwanted refrigerant during periods when refrigerant migration or flooding has occurred. It also avoids nuisance shutdowns during short fluctuations in net oil pressure on start-ups.

Note: The resistor in series with the heater and the differential timing switch is simply a voltage drop resistor used to make the control versatile. Depending on how the resistor is wired, the control can be used on 230- and/or 115-volt systems.

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 (Figure 2). When the motor starter contacts are opened, it opens the heater circuit by opening Line 2 (L2). 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 120-second time delay prevents nuisance trips of the controller at start-ups.

Figure 4. A pressure transducer connected to an electronic controller.


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 an overloading problem. When the internal overload opens, the motor is shut off but the motor starter coil remains energized with its contacts closed. The 2-minute timing circuit would be activated due to a lack of net oil pressure. This will trip the oil safety controller in a matter of 2 minutes because the heater will still be energized through the line side of the motor starter. Remember that there is no net oil pressure when the compressor is shut off on its internal overload protector. In time, the compressor will cool off and the internal overload will reset (close). But the compressor will not be able to be restarted until the oil safety controller is manually reset by pushing the reset button. This condition can be prevented by the use of a current sensing relay.

Figure 5. The inductive coil controls a normally open set of contacts in series with the time delay heater. (Courtesy of Copeland Corp.)


The use of a current sensing relay allows the compressor to cycle on the internal overload while not affecting the operation of the oil safety control. The current sensing relay is wired to the load side of the contactor. It acts like an inductive-type ammeter. One leg of the load side of the compressor’s motor is passed through the relay’s inductive coil. The inductive coil controls a normally open set of contacts in series with the time delay heater (Figure 5). The contacts are closed when normal motor current is sensed. When the current drops quite low, or reduces to zero because of the motor being off, the contacts of the current relay will open and take power away from the time delay heater. This prevents a tripping of the oil safety controller in case the internal overload opens (compressor off), and the motor starter coil is still energized.

Note: It is often necessary to incorporate another field-wired relay when dealing with an electronic oil safety controller in these situations. The reason for this is that the electronic oil safety module requires power to the module on an oil safety trip for a proper reset. The added relay simply provides this power to the module during a trip. Always consult with the compressor and control manufacturer before wiring any of these controls or devices.

Tomczyk is a professor of hvacr at Ferris State University, Big Rapids, MI.

Publication date: 02/05/2001