25.9.3 Shorted Windings A shorted winding occurs when two wires on the same winding short out or touch each other shortening the path of the current flow through that winding. This causes the winding to pull more amperage causing the fuse or breaker to trip. With power disconnected from the motor, use an ohmmeter to check the winding resistance as discussed earlier. If the math of the resistance measurements does not add up, there is a problem. If the meter shows an infinite resistance reading, there is an open winding. If the meter shows a reading of zero resistance or a one that does not add up, the winding is shorted. 25.10 Motor Bearings and Lubrication All motors have a bearing (lubricated support for a rotating shaft) inside the end bell (cover) at each end of the rotor. The rotor shaft is inserted through the bearing, which holds the shaft in perfect alignment. Alignment is critical because the air gap between the rotor and the stator is only a few thousandths of an inch, and the gap must be equal on all sides. The end bells are held in position by four bolts that extend through the stator shell from end to end. unit to provide a good electrical connection. Connect the other ohmmeter lead to one of the motor terminals (C, S, or R). The ohmmeter should read infinity or OL (unlimited resistance). A reading of zero resistance, or less than infinite resistance, indicates a winding has become shorted to ground. Repeat the test for each motor terminal. A “grounded” hermetic is not repairable and must be replaced. See Figure 25‑57. A motor with a winding shorted to ground is indicated by a blown fuse or breaker. 25.9.2 Open Windings An open winding refers to a broken wire in the motor winding that is not touching the motor shell. To locate or determine an open winding, the motor must be checked with an ohmmeter. Check each winding by touching the meter probes to two terminals and recording the reading. Repeat for each pair of terminals until all combinations have been checked. Since there is no complete path (circuit) for current flow if the circuit is open, the resistance indicated by the meter would be infinity (OL). See Figure 25‑58. A broken wire can occur in either winding or in the common connection. The condition is not repairable in the field, so the hermetic must be replaced. HOLD ON OFF OHM DCV DCA A C V ACA COM VΩ A mAμA m 20 m 2 200 1k 750 20 200 2 200 200m 2m 20m 2 10 10 2 100m 20m 2m 200m 0 200 2k 20k 200k 2M 20M DIGITAL MULTIMETER 0 C R S Goodheart-Willcox Publisher Figure 25‑57. A zero resistance reading when the ohmmeter probes are touching the hermetic case and a motor terminal indicates a short to ground. The hermetic must be replaced. HOLD ON OFF OHM DCV DCA A C V ACA COM VΩ A mAμA m 20 m 2 200 1k 750 20 200 2 200 200m 2m 20m 2 1010 2 100m 20m 2m 200m 0 200 2k 20k 200k 2M 20M DIGITAL MULTIMETER C R OL Infinite resistance Open winding S Goodheart-Willcox Publisher Figure 25‑58. An infinite resistance reading when the meter probes are touching two motor terminals indicates an open winding. This hermetic compressor must be replaced. Chapter 25 Motors 515 Copyright Goodheart-Willcox Co., Inc.