1. Open winding
2. Grounded winding
3. Shorted winding
An open winding is one in which the wire that forms the winding has separated or broken. A grounded motor is one in which the insulation around the winding wire has broken down and the copper wire itself is making electrical contact with some part of the motor casing. A shorted winding is one in which the winding insulation has failed, and one part of the winding is making unintended contact with another part of the winding. These conditions are illustrated in Figure 11-1.
If all show continuity (as they should), then take another ohm measurement between any pin and the compressor casing. It should read "infinite."
The pin-to-pin readings should be something like two, six and 8 ohms, or four, 11 and 15 ohms. The highest reading should equal the sum of the other two because it is actually a reading of the two other windings in series.
The second-highest reading should be two to four times higher than the lowest reading (the start winding normally has two to four times as much resistance as the run winding). If one winding is open, two of the readings will be infinite. If any reading is zero, it indicates that a winding is shorted.
If the winding resistances are all reasonable, then you must take an ohm reading between any one of the pins and the casing ground. You'll need to scrape off a bit of paint on the casing to ensure that you're making good electrical contact between your ohmmeter probe and the metal of the casing. You'll also need to make sure that your ohmmeter is set on the highest ohm range available.
If you get an infinite reading, the motor is not grounded to the casing. It is not necessary to check the other pins, because you have already determined that the pins are continuous with each other. If any winding is grounded, all three pins will show a less-than-infinite reading to the casing.
Checking compressor motor windings (three phase)"Ohming" out the windings on a three-phase motor is even easier than on a single-phase motor. The pin-to-pin resistances should all be equal. They may be quite low (maybe one-half ohm or less). Be careful not to jump to the conclusion that just because the winding resistance is very low, that the windings are shorted. The three-phase motor windings are checked to the casing ground in the same way as a single-phase motor.
Checking motor winding insulationIn critical applications where a motor failure would cause catastrophic loss (usually the shutdown of a facility) to the owner, as a preventative measure, maintenance personnel routinely check for an impending failure of motor winding insulation.
As you know, if you measure resistance between a motor winding and the casing of the motor, it should be infinite. Well, that's not quite true. In fact, the resistance of the insulation that separates the motor winding from the casing is extremely high, but it's not quite infinite, even though most ohmmeters will indicate that it is. However, if a meter with a high-enough range is used, the actual less-than-infinite resistance of the motor winding insulation can be read. A recording of the readings is kept and compared from year to year.
If the resistance is lower than the historical resistance, the owner may want to consider buying a replacement motor so that it will be available in the event of a future failure.
(To order Understanding Electricity and Wiring Diagrams for HVAC/R, contact the ARI at 4100 N. Fairfax Drive, Suite 200, Arlington, VA 22203; call (703) 524-8800; fax (703) 528-3816; see www.ari.org on the Internet.)