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Oil Pressure Controls
Oil pressure controls, oil failure controls, oil safety controls… They’re a pain in the neck when they trip, and diagnosing those problems can really tax even the best of techs.
As semi-hermetic compressors get larger, they can no longer rely on simple splash or “sling” type lubrication strategies where oil is just flung around inside the compressor and is sufficient to provide lubrication. Once we cross that threshold, we need an oil pump to force oil through ‘galleys’ machined into the crankshaft to lubricate all of the bearings. To ensure that the compressor has adequate lubrication at all times, manufacturers require a safety control to prove that there is sufficient oil pressure and shut the compressor off if there is not.
An oil pressure control is, effectively, a differential pressure control with a built-in time delay.
Let’s look a little closer.
The first thing we have to do is look at how we measure oil pressure. The oil that we’re pumping through a compressor starts in the crankcase. It’s already under a certain amount of pressure, depending on the suction pressure of the system. That suction pressure affects the output pressure of the pump. To measure oil pressure properly, we can’t just look at the pump outlet pressure; we have to look at the pump outlet pressure MINUS the crankcase pressure; this is called NET oil pressure.
Measuring crankcase pressure rather than just measuring suction pressure will become important later when we get into troubleshooting. That is why the oil pressure control has two pressure ports and is measuring the differential between those two pressures. The pressure control has a time delay because, on startup, the system needs a period of time to stabilize. This time delay is usually fixed at 90 or 120 seconds. However, it varies depending on the manufacturer, the brand, and the type of control.
Most new controls are electronic, but there are still many mechanical controls out there, so let’s talk about how they work. Once we have a solid understanding of mechanical controls, the electronic ones are pretty easy to understand.
The first thing to understand about oil failure controls is that they require a minimum of three wires. Remember, they’re more than just a pressure control.
So, we’ve got line voltage going to the control, AND we’ve got two wires for the control circuit. Typically, you’re going to see terminals “2” and “M” jumpered together, so everything I say assumes they are connected electrically. Some rare applications will require them not to be jumpered, but that’s beyond our scope here.
Depending on our control voltage, we’ve got line voltage at V1 or V2, one leg of the control circuit at M, and one at L. It’s very important to understand that L also acts as both one leg of the control circuit AND the second leg of the circuit for the heater (H) in the control itself. That means that, unlike most other controls, we have to be careful where we put this control; the leg from L cannot have any other switches in it. This requirement will be made clear in the paperwork included in the control, and now you understand WHY.
Now, let’s look at the switch in the control labeled PC. This switch, pictured as normally closed, will OPEN when there is sufficient pressure differential. So, when we start the compressor, power is applied to V1 (or to V), and once oil pressure builds up to the 9-12 PSIG range, it will open switch PC and de-energize the heater.
Now, if this switch (PC) is closed, and the control is energized (power to V1 or V2), then heater H is energized. Once that heater reaches a certain temperature, switch TD, which is a thermal type switch similar to the one found in an electric heat sequencer, will open, breaking the control circuit.
This may seem complicated, but really stop and take the time to understand this. You’re not really going to be putting a meter across switch PC; it kind of just works in the background. You just need to understand that it’s there and what it does to make the control function.
Electronic controls integrate these functions into an electronic control board and a small differential pressure switch that screws into the oil pump on the compressor. They still have the same basic wiring requirements, having to have the third wire for power and an uninterrupted wire from L to the load. The time delay feature is electronic rather than thermal, but there is still a small differential switch to monitor the net oil pressure.
In operation, an electronic oil failure control works pretty similarly to the mechanical one. Power on the line voltage terminal and L supply the PCB (printed circuit board) with power, while M and L act as the control circuit. The PCB monitors the differential pressure switch, which is typically a brass assembly threaded directly into the oil pump. If oil pressure drops too low, this time, the differential switch will typically open, signaling the PCB to begin the timing that was handled by the heater in the mechanical control.
Electronics have some advantages over mechanical controls. More accurate timing is the first among those. A thermal switch is somewhat dependent on its environment. The same switch in a warm ambient is going to time out faster than it would in a colder ambient. With electronics, timing is repeatable across a wide range of ambient temperatures.
Moving into the future
As electronic controls advance, manufacturers are integrating more features into them. What was once a single-purpose control monitoring the compressor’s oil pressure is now turning into what amounts to a central control unit, taking oil pressure input along with motor current data, high and low-pressure switch inputs, and motor temperature inputs and acts as an integrated safety for the machine. Not only does it provide troubleshooting data to the tech in the form of error codes, but some are acting as data recorders, allowing more detailed troubleshooting if the tech connects to the controller with a laptop or other device and downloads the data.
—Jeremy Smith CM