Many years ago, when it was found that the existing mechanical motor protection devices were not always fast enough, several manufacturers began developing a solid-state motor protection device. Most work on the same principle—sensors located in the windings of a compressor are brought out to a module that senses their resistance. On three-phase machines, there are three sensors. On single-phase machines there is just one. The sensors, which are embedded directly in the motor’s windings as it is wound, are actually electronic thermistors. Thermistors are solid-state temperature-sensitive resistors. Essentially, the thermistor’s resistance increases as the temperature in the winding increases.
When testing these sensors, always refer to the module manufacturer’s resistance ratings since they vary among different brands.
A number of newer hermetic compressors use solid-state motor protection to prevent burnouts and other compressor failures. Such failures may be related to overheating, or to conditions of overvoltage and undervoltage. Solid-state compressor protection modules may be checked in or out of the circuitry. However, they cannot be repaired or adjusted. If inoperative, they must be replaced. Internal compressor sensors are always checked as part of the procedure. If the sensors are determined to be faulty, then the compressor must be replaced. A multimeter can be used to determine any faults in this device.