25.9.1 Grounded Windings Each motor terminal must be checked to determine whether it is shorted to ground. A grounded winding is one that has developed an internal electrical short to the device’s frame. Use one of the ohmmeter probes to scratch through paint or oxidation on the steel shell of the hermetic math that adds up. When the measurement across the common-to-run terminals and the measurement across the common-to-start terminals is added together, it should equal the measurement across the run-to-start terminals. See Figure 25‑55. The approximate start winding and run winding resistance readings for fractional horsepower single-phase motors are presented in Figure 25‑56. 25.9 Identifying Motor Problems The ohmmeter is used to check motor windings, which is a process called “ringing out” a motor. All electrical connections to the motor must be removed to prevent false readings and possible damage to the meter. 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 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 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 18 Ω 6 Ω 12 Ω C C C S S S R R R C-R + C-S = S-R 6 Ω + 12 Ω = 18 Ω A B C Goodheart-Willcox Publisher Figure 25‑55. Identifying terminals by measuring resistance. A—The common terminal is the one remaining when the highest terminal pair (run-to-start) reading is obtained. B—A reading from common-to-run yields the lowest reading. C—The common-to-start reading falls between the other two. Goodheart-Willcox Publisher Figure 25‑56. Run and start winding resistances, in ohms, for common sizes of fractional horsepower single- phase motors. Winding Resistances Motor hp Run Start 1/8 1/6 1/5 1/4 4.5 4.0 2.5 2.0 16 16 13 17 514 Heating and Cooling Essentials Copyright Goodheart-Willcox Co., Inc.