You know the drill: you’re on a warm refrigerator service call. Your first troubleshooting move is to half split the problem between the sealed system and everything else. In other words, “Am I dealing with a sealed system problem?” You want to answer this question first because everything else you do flows from this differential diagnosis.
So you break out your gauges, install line piercing valves and get your high and low side pressures, right? If you don’t understand sealed system thermodynamics and how it moves heat around the system, then, yes, this is probably what you would do. But techs who actually understand how sealed systems work roll smarter than that. Here’s a quick example of diagnosing a sealed system problem non-invasively using an amp clamp and a temperature measurement.
Why does this work? To understand why this simple method works, you need to understand two things:
- Sealed system thermodynamics. In other words, how does a sealed system move heat around?
- Basic electric motor operation
Sealed system knowledge nuggets
The sealed systems we work on (fractional horsepower, cap tube systems) have two saturation zones: the middle of the evaporator and the middle of the condenser. Everywhere else, the refrigerant is either superheated or subcooled. But in the middle of the evap and condenser, the refrigerant is saturated. This is key because at saturation– and only at saturation– refrigerant pressure and temperature are directly related to each other. This means that if you know one, you know the other.
So, for example, I could get a temperature shoot of the middle of the evaporator and now I know the low side pressure by simple table look up. That’s what the PT tables are for. (Actually, I use the Danfoss Ref Tools app on my phone– it’s a digitized version of the printed PT tables with some added perks.) Similarly, I could measure the temperature at the middle of the condenser, like I show in the video, and I know the high side pressure by table lookup.
All that cool info without needing valves or gauges. BUT I had to have a technical understanding of how sealed systems work already in my head in order to know that I could even do this. Troubleshooting is all about what’s in our heads which we then implement with our hands.
Ever hear of “condenser temperature split”? That’s the difference in temperature between the mid-point of the condenser and the ambient temperature. In residential refrigerators, engineers design for a 30F condenser temperature split. That means that in a normally running sealed system carrying a significant heat load, you will read a 30F MAX split at the condenser. If the compartments are just keeping things cool, the split will be less.
In a warm box, like the service call we’re on, the compressor should be running straight out trying to move more refrigerant molecules around the system. But if the system has leaked all the refrigerant or if the compressor ain’t compressin’ any more, then there are no molecules to move or compress. No molecules moving means no heat is moving because heat is carried on refrigerant molecules. With no heat being picked up inside the box, no heat will be rejected at the condenser cuz there ain’t no heat to reject! This will manifest to the astute technician by a condenser at room temperature (condenser split = 0F). That’s what I showed in the video.
As simple as that looked (and was) to do, it took a whole lotta knowin’ to know to do what I did. If you want to learn how to troubleshoot like I do, I’ll teach you. You can also download our FREE Guide to Advanced Refrigeration Repair.
Basic electric motor operation
When we talk about a hermetic compressor, we’re really talking about two parts: the electric motor and the vapor pump. The motor is directly connected to the vapor pump by a drive shaft.
One of the basic operating principles of electric motors is that the more torque they must supply (in other words, the harder they have to work), the more amps they require. Amps, of course, are a direct proxy for watts because watts = amps X volts.
Watts are a unit of work as is horsepower. Both are just different units describing the same physical thing: something tangible being done in the real world like making a motor shaft spin.
You can think of horsepower as a measure of the mechanical output power of the motor at the shaft. You can think of watts as a measure of the electrical input power required by the motor to make the shaft spin. Remember: nothing is free in this world so we can’t get something for nothing. So to get work out (horsepower) we have to put work in (watts).
Oh, but it gets worse! Everytime we convert work or energy from one form to another, we incur a penalty from losses like friction, heat loss, hysteresis, etc. So it’s not just putting in work to get work out. No, instead we have to put MORE work in than we’ll get out because of these losses. These losses can be as high as 40% so they are not trivial. Engineers figure out how much these losses are and account for them in the design.
Suppose you were to set up an electric motor with a power supply and get it running. Suppose also that you had an amp clamp around one of the power supply wires for the motor. If we were to somehow apply a braking friction to the spinning shaft, you would see the amps climb on your meter. The more pressure you apply to the shaft, the higher the amps would go. If you keep braking all the way to stopping the shaft from rotating at all, you would be looking at what’s called “locked rotor amps” on your amp clamp. This very high amperage will quickly overheat the motor stator and the internal thermal overload will open.
Now lets’ apply this to the motor inside a hermetic compressor ball. The big job of the compressor is to compress vapor. Sounds obvious but this is very important. Remember we talked about heat being carried around a sealed system on refrigerant molecules? And with no molecules, no heat is being moved? Well, no molecules also means there’s nothing to compress so the vapor pump is just spinning la-dee-dah getting a free ride on the motor shaft. The motor feels this, too, because it’s not having to get the vapor pump piston to compress refrigerant molecules. In other words, low torque requirement. This is will be reflected in a low amp reading exactly as in the example above.
Amp measurements are simple to do and give a ton of information IF you understand amps, watts, and electric motors. We teach all this and much more in our Core Appliance Repair Training course.