The compressor motor is a capacitor-type, split-phase motor that has two windings. The starting winding has the capacitor in series with it to give great starting torque, with the winding composed of a smaller-size wire than the running winding. By using a capillary tube method of refrigeration, the starting load is normally rather low. At the start, the running winding is energized, drawing a larger starting current, which causes the relay to close the starting winding contacts, energizing the starting winding almost immediately.
As the motor comes up to speed, the current value drops. At about 65 percent of full speed, the current drops low enough for the starting relay to open the circuit to the starting winding and the full load is then carried by the running winding. If the load is too heavy, the current draw will be large and the overload protective device will open and stop the motor until the overload protection cools, when it will again attempt to start the motor.
To check the continuity of the compressor motor winding, disconnect the three motor leads C, S, and R (Figure 5-28), making sure the entire unit is unplugged from the source of electrical supply. The ohmmeter should show continuity between any two of the three terminals, with a larger resistance between terminals C and S than between C and R. If the ohmmeter shows no resistance on any of the tests, an open winding is indicated.
While making the preceding test, you should test the compressor motor for grounding. This is done by putting the ohmmeter on its highest scale and touching one lead to the compressor housing and the other to each of the motor terminals, one at a time. If the winding is not grounded, the ohmmeter should read full scale (or the maximum resistance) in ohms.
It must be remembered that faults other than motor trouble may be the cause of compressor failure or of a motor drawing high current. Some of the compressor failure causes are a stuck compressor, high head pressure, low voltage, or a plugged capillary tube.