Copyright Goodheart-Willcox Co., Inc. Chapter 13 Electric Motors 269 13.9 Hermetic Compressor Applications Motors used inside hermetic compressors are surrounded by refrigerant and oil. The winding insulation must be compatible with the refrigerant and oil used. Three ter- minals connect the run and start windings for a single-phase motor, or the three run windings of a three-phase motor. The terminals are not insulated on the inside of the compressor and contact the refrigerant oil. The oil is not conductive, providing it is not contaminated with acid. While the terminals should be labeled, this is not always reliable. A technician can check the terminals with an ohmmeter to determine the common, run, and start terminals. Figure 13-31 shows a single-phase motor diagram as an example. The numbers are used to temporarily assign a name to the unknown terminals. Resistance between terminals 1 and 2, 1 and 3, and 2 and 3 are taken. The highest reading between 2 and 3 indicates that terminal 1 is the common terminal where both windings are connected. This is evident in the winding illustration since the meter is connected to the run and start windings in series, resulting in the high- est reading. The second highest reading is between terminals 1 and 2, indicating that terminal 2 is the start winding. The remaining terminal, terminal 3, is the run winding. Since three-phase motors have only run windings, all three readings should be approximately the same. There should be infinite resistance between winding terminals to ground. 13.10 Motor Protection Fuses and circuits breakers do not always protect the motor. These devices pro- tect the wiring from overheating to protect against fire. Motors use internal and external overloads that react to heat or current. One device is the snap disc bimetal. The snap disc is automatically resettable when it cools down. It can be attached to motor windings, or it can be external, such as on the hermetic compressor housing. Some bimetal snap discs contain a low-resistance heat- ing element to sense overcurrent. The overcurrent generates heat and activates the snap disc. Thus, this type protects against ambient heat and overcurrent. Thermistors can also be used to sense temperature and provide a signal to a controller. Other motor protection methods, specifications, and motor troubleshooting will be covered in Chapter 15, Troubleshooting Overview. S R C 1 2 3 1 and 2 12 Ω 1 and 3 3 Ω 2 and 3 15 Ω Goodheart-Willcox Publisher Figure 13-31. Single-phase motor diagram used to determine terminal identification.