The permanent split-phase motor has limited starting torque and is used basically with capillary tube air-conditioning equipment, such as window units. Capillary systems permit high-side and low-side pressure equalization when the compressor is not operating. A run capacitor is wired in series with the starting winding. Both the starting winding and the run capacitor remain in the circuit during operation. No start relay or start capacitor is needed. Figure 1428 is a schematic wiring diagram of the circuits used in this type of motor.
SPLIT-PHASE HERMETIC MOTOR WINDINGS AND TERMINALS
Split-phase motors used in hermetic refrigeration and air-conditioning applications are designed to start under load. These splitphase and capacitor motors use two sets of spiral windings: a starting winding and a running winding. The two sets of windings
differ in their impedance and in their positions in the stator slots.
The starting winding has high resistance and small reactance, whereas the running winding has low resistance and high reactance.
Reactance is the opposition to the flow of alternating current by inductance and capacitance. The running winding has many
turns of large wire and is placed in the bottom of the stator slots. The starting winding is wound of small, high resistance wire and is placed on top of the running winding. Both windings are connected internally at one end to provide a common lead, and when starting, both are energized in parallel. The currents are out of phase with each other and their combined efforts produce a rotating field that starts the motor.
Figure 14-29 shows the starting and running windings of a two-pole motor in their 90-degree out-of-phase positions.
Figure 14-28.-Schematic wiring diagram of a permanent split-phase motor.
Figure 14-29.-View of a two-pole motor having starting and running windings.
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 Cterminals 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.
Clean terminals to provide a good connection.
Using the ohmmeter, find the two terminals across which the greatest resistance occurs. The remaining terminal is the C-terminal. The resistance between the Sand 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. - Odentifying motor terminals using an ohmmeter.