Something's Missing On Your Annual PM's

Once a year, we usually do a little more on a PM (preventative maintenance) visit than the rest of our visits. Some call this PM an annual PM. And most are done in the spring of each year to prepare equipment for summer operation. But one of the most overlooked preventative things we can do is overlooked, and this task can both increase sales and prevent an unplanned failure for your customer. What I'm talking about requires an investment of up to a few hundred dollars for the test equipment, but pays dividends back! It's a megohmeter. It measures in millions of ohms. If you don't own one, you really should consider the investment. Meggers, as they are called, measure the electrical resistance of motor windings to ground just like an ohmmeter with one very large exception. Meggers do this while applying line voltage to the windings, thus checking for leakage in conditions more similar to when a motor is operating. Most have 500 and 100 volt settings. 500 volts is usually sufficient in most cases. Winding insulation can break down due to acid formation in the system eating away at windings, winding movement during motor startup or with temperature expansion and contraction, and moisture. As moisture or contaminants increase, winding resistance typically decreases. We can detect this early with regular (annual) checks of each motor and logging the results on the units maintenance log card. You have one in every one of your units, right? Now to run the test, make sure you have one probe connected to a good chassis ground in the unit. This probe usually has an alligator style clip on it. The other probe is used to touch the compressor terminals or fan motor wiring. Be careful here-500 volts doesn't tickle! Note the results of your findings on the card and remember that readings can vary by quite a bit from one reading to another, one day to another, morning vs. afternoon, etc. You are looking for a trend, not a one-time reading. When megging motors, winding temperature and especially moisture will greatly affect the reading. Expect to see lower readings after rain or on damp, foggy days. My recommendation is to run the motor for 30-60 minutes prior to testing so your "trend" readings are not so skewed by these influences. TIP: Never use this type of meter while a compressor motor is subjected to a vacuum. You can easily destroy the insulation of the winding under this condition. TIP: Using a meter capable of 1000 volts - Use this scale sparingly. Higher voltage induces more stress on the insulation and is almost never needed anyway. TIP: If your company sells "full maintenance" agreements where the company repairs equipment for a preset flat fee, meg out the motors prior to beginning the service. Any motor not passing should be excluded from the agreement/contract until the customer pays to have the issue corrected. This is the same as you would hopefully do for any full maintenance agreement item...You exclude defective items from the contract until they are repaired at the customers cost so you don't end up becoming financially liable to repair the issues at your companies expense. TIP: A chart to use as a reference... Reading             Condition                             Suggested                                               Percent of windings in megohms       indicated                       maintenance / tasks                                                  in field         100+                 Excellent                      None                                                                          30% 50-100              Fair/Good                      If noted on a warm compressor,                                    35%                                                             think about replacing drier (moisture) 20-50               Moisture or                     Replace drier, possibly more than once                         20%                        contamination                 Change oil if acid is detected 0-20                 Severe                           Consider oversized filter/drier, replace oil                        15%                        contamination                 and possibly refrigerant, perform proper & deep                                                             vacuum, System burnout procedures should be                                                             followed. Prepare yourself that motor may still                                                             fail. Consider preparing customer for replacement. Note: This is my scale, but you can find other people or companies have a scale a bit varied from this one. Readings are on motors that are warm and had been operating at least 30 minutes prior to reading.