Check the windings for short circuiting to the frame. M […]
Check the windings for short circuiting to the frame. Most household appliance motors with a shorted winding will not run and will probably open the fuse or trip the circuit breaker instantly (600 volt systems are "ungrounded," so a 600 volt motor with a shorted winding may run and not trip a fuse or circuit breaker).
Use an ohmmeter to check resistance value. With an ohmmeter set to the Resistance or Ohms test setting, place test probes into the appropriate jacks, usually the "Common" and "Ohms" jacks. (Check the meter's operation manual if necessary) Choose the highest scale (R X 1000 or similar) and zero the meter by touching both probes against each other. Adjust the needle to 0 if possible. Locate a ground screw (often a green, hex head type) or any metal part of the frame (scrape away paint if needed to make good contact with metal) and press a test probe to this spot and the other test probe to each of the motor leads, one at a time. Ideally, the meter should barely move off the highest resistance indication. Make sure your hands are not touching the metal probe tips, as doing so will cause the reading to be inaccurate.
It may move a fair amount, but the meter should always indicate a resistance value in the millions of ohms (or "megohms"). Occasionally, values as low as several hundred thousand ohms (500,000 or so), *may* be acceptable, but a higher number is more desirable.
Many digital meters do not offer the ability to zero, so skip the "zeroing" information above if yours is a digital meter.
Check that the windings are not open or blown. Many simple "across the line" single-phase and 3-phase motors (used in household appliances and industry respectively) can be checked simply by changing the range of the ohm meter to the lowest offered (R X 1), zeroing the meter again, and measuring the resistance between the leads of the motor. In this case, consult the wiring diagram of the motor to be sure that the meter is measuring across each winding.
Expect to see a very low value of resistance in ohms. Low, single digit resistance values are expected. Make sure your hands are not touching the metal probe tips, as doing so will cause the reading to be inaccurate. Values greater than this indicate a potential problem and values significantly greater than this indicate the winding has failed opened. A motor with high resistance will not run - or not run with speed control (as is the case when a 3-phase motor winding opens while running).