Hermetic motors have three electrical terminals connected through an insulated seal to the motor windings inside the dome. Refer to figure 14-30. Troubleshooting procedures require that these terminals be identified with respect to the winding connected to each. The terminals must be identified as the START TERMINAL, the RUN TERMINAL, and the COMMON TERMINAL. Some manufacturers mark the S-, R-, and C- terminals for start, run, and common, respectively; other manufacturers use different designations.
The terminals can always be identified by using a low-range ohmmeter following the procedure below:
Disconnect all power to the terminals, discharge capacitor where necessary, remove the wires connected to the terminals, and mark the wires so they can be reconnected properly.
Using the ohmmeter, find the two ter- minals across which the greatest resistance occurs. The remaining terminal is the C-terminal. The resistance between the S- and R-terminals is highest because both are being measured in a series circuit.
Identify the S-terminal by placing one meter lead on the C-terminal and then checking the other two terminals to determine which one has the greatest resistance. The S-terminal (starting winding) has windings with many turns of small wire, and therefore has the greatest resistance. The remaining terminal is the connection of the running winding.
Always mark the terminals so they can be identified later.
Figure 14-30. - Identifying motor terminals using an ohmmeter.
Electrical troubleshooting techniques are used on refrigeration and air-conditioning equipment. Electrical troubleshooting is done by a process of elimination. You should begin by checking the most obvious trouble and gradually progress to the more remote possibilities. As a Utilitiesman you cannot troubleshoot an electrical system for an air- conditioner or refrigeration unit unless you understand the function of each component in a system. When you can observe a unit operating and detect what is not functioning properly, you can identify the circuit or circuits that are having trouble. At this point you must be able to test each of the components within a circuit that is not functioning properly. Of course to do all of this. you must also be able to read and interpret electrical diagrams, understand loads, determine paths, and perform electrical testing procedures.Continue Reading