windings and prevents backfeed from other elec- trical circuits that would cause a false ohmmeter reading. The first test with the ohmmeter is to determine across which two terminals have the highest resistance. Three separate readings are required to cover all the possible combinations. The two terminals that produce the highest reading (18 Ω in Figure 25‑54) are the start and run terminals, because the meter is reading through both windings (in series). By the process of elimination, the high reading means the other terminal is the common terminal. With the common terminal identified, read from common terminal to each of the other two terminals. Common-to-start shows more resistance than common-to-run. The ohm readings will vary from compressor to compressor and from manu- facturer to manufacturer. The proper reading is the reading of zero occurs, the motor is shorted. If a reading is obtained from any motor lead to ground, the motor is grounded. A reading of infinite resis- tance (full scale deflection on an analog meter) across any two ends of a winding means the winding is open. In each of these cases, the motor must be rewound or replaced. Depending on the size of the motor, resistance readings on three-phase motor windings range from 1 Ω to 50 Ω. The larger the motor, the smaller the resistance. On a single-voltage, three-phase motor, all windings have the same resistance. On a dual-voltage three-phase motor, the three “extra” windings have one-half the resistance of the main windings. 25.8 Identifying Single- Phase Compressor Terminals Most manufacturers of single-phase, hermetic compressors mark the three electrical terminals: C, S, and R (Common, Start, and Run). If these terminals are not marked, they can be identified by checking the resistance of the windings. Electrical power supply to the motor must be disconnected and the connections to the three motor terminals opened. This isolates the motor 1 4 5 2 8 7 9 6 3 (4, 5, and 6) L3 (3 and 9) L2 L1 (1 and 7) Goodheart-Willcox Publisher Figure 25‑53. The low-voltage connection places the windings in parallel. There are three wires at each con- nection, compared to two wires in the series connection for high voltage. 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 18 Start Run Common Goodheart-Willcox Publisher Figure 25‑54. The highest of the paired terminal resis- tance readings is across the start and run terminals, measuring the resistance of both windings. By the process of elimination, the remaining terminal is the common. Chapter 25 Motors 513 Copyright Goodheart-Willcox Co., Inc.