Tip & How-To about Heating & Cooling

Small Compressor Operation

Residential A/C and Refrigeration Hermetic Compressors
A Hermetic Compressor is a sealed, black can with 3 (on rare occasions, 5) copper coated steel tubes and electrical wiring coming out of it. The older models in residential refrigeration were usually about 8 to 10 inches in diameter and 6 to 8 inches high with flat tops. The newer models are smaller in diameter and taller but the refrigerant piping and wiring is basically the same. The refrigerant lines attached to the compressor vary in size according to function. The larger tube is called "The Suction Line" and pulls cold Refrigerant vapor back to the compressor from the "Evaporator". The Evaporator is the coil that gets cold and accomplishes its job of cooling Refrigerators and Freezers by removing the heat in them , not creating cold. That's a subject that requires a large explanation and too much to cover here. One of the remaining smaller tubes pumps out hot gas ( Refrigerant in a gaseous state ), under high pressure to the "Condenser". The condenser is the coil that is always hot and cools the gaseous refrigerant to change it back into a liquid. The third line is "crimped" and "brazed" shut. ( Like solder only harder) . The brazed tube is used by the factory to "Evacuate" ( Remove all the air) and "charge" (Add Refrigerant) the system. It is sometimes called the "Process Line" or "Pigtail". The electrical terminals are normally 3, small, round pins closely grouped together in a triangle and located under a "Bakelite" ( Black, Hard and Brittle, heat resistant Plastic ) cover on the side or top of the compressor. If the Compressor is running normally, it makes a steady hum or higher pitched wining sound. While running, there are several easy tests you can do to see if the system is working the way it should. First, the larger tube from the evaporator should be cool or even cold to the touch. But no ice or snow on it. The smaller tube going to the condenser should be hot and possibly hot enough to burn your forearm. So Be Careful when touching it. The process tube isn't used outside the factory except by one of us service technicians, (possibly), who would install an access valve for checking the refrigerant pressure in the system. The suction pressure determines how cold a refrigerator or freezer will be able to get. The compressor motor and refrigerant compressor are bolted together, mounted in the bottom of the can and then the top half of the can is welded onto the bottom half. (Hermetically sealed). While running, the lower half or so of the can should be cool. The returning refrigerant from the evaporator is used to cool the motor so this temperature is important. The upper half of the compressor should be hot. The cool returning refrigerant from the evaporator is pumped to a high pressure changing it to a gas and then delivered to the condenser. The electrical terminal designations are marked either on the compressor itself under the bakelite terminal cover or on the cover itself. The terminals are marked "R" - "S" and "C". They are the RUN - START and COMMON terminals. The motor needs help to start against the load of the compressor and when the equipment calls for cooling, a relay is used to MOMENTARILY jump the "R" (Run) terminal to the "S" (Start) terminal to give it that extra boost. There are two different types of relays that quickly break the connection between the "R" and "S" terminals when the compressor motor starts. They are called the "Current" and "Potential" relays. The "Current" relay operates from the difference in the amperage pulled by the compressor motor between starting and running. The "Potential" relay operates by the difference in the voltage between starting and running. The current relay pushes directly onto the "R" and "S" terminal pins on the compressor and can often times be seen to have over heated when inspected if the compressor just hums and doesn't start. Also, another quick test is to, "WITH ALL THE POWER OFF !!!!!, pull it off its pins and shake it up and down. It should rattle when shaken. The "Potential" relay is wired to the terminals (Not plugged in) and normally has a start capacitor in the wiring. A start capacitor is round and made of black Bakelite like the electrical terminals. They come in various diameters and heights. When bad, they will sometimes blow out a small hole on its top or show sins of having over heated. The potential relay is more difficult to check and a Technician should be called if you think your problem is in this type of circuit. Trouble shooting from this point on will require us to ask questions in relation to the information above. If you have a question for us, please be as specific as possible using this information as regards the temperatures and look of the things mentioned here. It will help us provide better answers faster with less questions to be answered. I hope this has helped you to understand compressors a little better and how they should operate. Thank you for trusting us to answer your questions. Roger

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