| Step | Expected Result/Action |
|---|---|
| 13. Check a single-phase compressor motor for winding-to-winding shorts (partially shorted) windings as follows: Because of the extremely low resistance values being measured, an accurate VOM/DMM must be used to perform this step, otherwise a good compressor may be judged as bad. Set up the VOM/DMM to measure resistance on the R x 1 ohm scale. Moke sure that the meter is zeroed. Connect the VOM/DMM across the leads connected to compressor terminals C and S, then measure and record the resistance of the start winding as shown in Figure SP-9-17. Connect the VOM/DMM across the leads connected to compressor terminals C and R, then measure and record the resistance of the run winding. | VOM/DMM indicates a measurable low resistance for both measurements. As a rule of thumb, the motor windings are usually judged to be good if the resistance measured across the start winding (C to S) is 3 to 6 times greater than the resistance measured across the run winding (C to R). Example 1 Assume that the resistance measured for the start winding is 3.0 ohms and the run winding is 0.5 ohm. In this example?, the motor windings would be judged to be good because the start winding is 6 times the resistance o f the run winding ( 3 . 0 + 0 . 5 = 61 Example 2 Assume that the resistance measured for the start winding is 1.25 ohms and the run winding is 0.5 ohm. In this example> the motor windings would be judged to be partially shorted because the start winding is only 2.5 times the resistance of the run winding (1.25 + 0.5 = 2.5). (Start winding resistance in this example is less than 3 times greater than the run winding resistance.) Example 3 Assume that the resistance measured for the start winding is 4.0 ohms and the run winding is about 0.33 ohm. In this example, the motor windings would be judged to be partially shorted because the start winding is about J 2 times the resistance of the run winding ( 4 .0 + 0 .3 3 = 12). (Start winding resistance in this example is more than 6 times greater than the run winding resistance.) |
