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Motor Talk

I wrote a blog in the past recommending the MCC as the best testing location with reasons supporting that recommendation. However, as we know the “best location” is more often than not dictated by other factors. Some of those “other factors” are what I will be discussing today.

Time is a factor that is one of the most compelling dictators as to the location from which data is collected. When time is a factor, the location that allows the fastest data collection MCC vs. In-field disconnects would be the more appropriate location. The MCC is a more compact location without time loss for moving to different locations. Although, if there are VFD’s involved, as in my last job, the In-field disconnects may be a better option. The last job I was on had 30 motors to be tested both On-line and Off-line in 2 ½ days (here is the time part). At first the MCC would seem the best place for the testing. During the morning planning it was revealed that all but 3 of the 26, 480VAC motors were controlled by VFD’s.  In addition there were four 4160V motors.

The time required  for disconnecting, testing, reconnecting, restarting, testing, and moving to the next location, was estimated to be 2 - 2 ½ hours per motor for testing from the MCC. With 30 hrs of available time, testing from the MCC was not going to work. The site disclosed that every 480VAC motor had a local disconnect next to the motor. While this is normally not the “ideal” setup, the motors were grouped together in clumps of 3 to 6 at each location. This allowed us to test from the local disconnect without having to unwire the VFD’s, as the testing was completed from the load side of open disconnect for the offline testing. Subtracting the time needed to unwire and rewire the VFD’s, we were able to complete the required tests in the time allotted. If in your plant or customer site the test locations are set in the procedures, then plant procedures are also a factor. In this case there is no option. One must test from the location cited in the procedure or request a deviation to test from another location. Keep in mind that when requesting a deviation you must have the supporting information ready to present.

Location of the motor is another factor that may change your plans. The motor may be located in a hazardous environment, which will cut down your options. I have in the past identified a ground fault while testing from the MCC. Then, the normal procedure is to test at the motor to isolate the fault in either the motor or the feeder cables. When unable to use an ignition source in the hazardous area, I had the motor removed from the area and tested it in a “safe” area to find the problem was in the feeders, as I had no indication of a problem in the motor.

It is always best to test the entire motor circuit. Consider extending the allotted time, using additional testers or working longer days to be able to complete testing. Another option is to fully test as many assets as possible and test the missed assets at a later date. When the entire circuit is not tested, a problem may be missed.

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