This article is courtesy of Coldtronics Inc. manufacturers of HVACR alarm systems.
When diagnosing a problem on a compressor, measuring its amperage draw is part of determining if the compressor is performing properly. How does a service technician know the correct amperage draw? The best way to determine the correct amperage draw for a particular compressor is by referring to the rating chart for that compressor.
Compressor manufacturers will publish a rating chart for each compressor they manufacture. These charts can be published in either a table or graphic format. The compressor chart will list the correct amperage draw for the compressor under its various operating conditions. To use the chart the service technician must know the evaporating temperature, condensing temperature and voltage applied to the compressor. By using the chart, a service technician can determine the correct amperage draw for the compressor and use that information to accurately troubleshoot it and the rest of the system.
Obtaining these charts can be a little time consuming, but the extra time involved is worth knowing the correct amperage. With the use of the Internet this information is now a little easier to obtain. Most compressor manufacturers will stamp an amperage rating on their compressor. They will usually stamp the rated load amps (RLA) of the compressor. However, the service technician cannot use this value to determine the correct operating amperage. RLA is a mathematical calculation required to meet Underwriters Laboratories Inc. (UL) approval only.
The compressor manufacturer must run a series of tests to determine the Maximum Continuous Amps before the overload trips. Once that has been determined, UL says divide the MCC by 1.56 to determine the RLA.
Some compressor manufacturers, such as Copeland, use a different factor. They divide the MCC by 1.44. If the RLA has any value, it is to determine at what amperage draw the compressor overload will trip and to determine the fuse/circuit breaker size and the wire size.
Trying to determine if a compressor is good or bad using RLA is not correct. It has nothing to do with what the correct amperage draw should be under its various load conditions.
When measuring the amperage draw of a compressor make sure to read only the amperage drawn by the compressor. Choose a location on the wiring diagram that will isolate the compressor from any other loads — such as the condensing fan motor. It is important to measure the amperage draw of the compressor only when trying to determine if it is operating properly.
Diagnosing a defective compressor can be tricky at times. Often times a system problem may seem to be the result of a defective compressor when in fact it is due to some other cause. Using the amperage draw of the compressor will help any technician determine if the compressor is truly defective. However the technician must know what the correct amperage should be in order to make a decision on the condition of the compressor.
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