Why is it Important to Test High Resistance Connections?
High Resistance Connections are often the cause of premature motor failure. They develop slowly and over time begin affecting the applied voltage and current balance at the motor, resulting in numerous problems. These connections can be in the junction box, at the starter, or even at the internal connections of the winding itself.
High resistance connections can result from a number of causes. A poorly compressed lug, loose bolts, splitbolts (yes, they are still out there) or even corrosion can cause them. So the reality is that something as inexpensive as a lug or a washer could cost you thousands in a motor winding – plus downtime. But there a reliable method to conduct motor testing for high resistance connections.
PdMA motor testing for high resistance connections
Recently our test team was conducting motor testing for a Paper Mill in Virginia. This Mill has a top notch motor program in place, and incorporates our maintenance and testing services into their regular outage schedule. During the outage a Pulper Pump motor was reported to be running hot during normal operation. The motor had been recently changed. Using the PdMA we performed an offline test from the starter and discovered a high phase to phase resistance imbalance.
Upon inspection of the connections at the motor, the feeder cable lugs were corroded and in poor condition. As shown by the connection photo, there are broken strands and evidence of corrosion.
As in the case of every motor we typically test, this one was running prior to the outage. The question is, how much longer would it run with a connection failing? What was the cost of a day of testing, versus a potential rewind of this motor? It is amazing that some still question the value of motor testing, as this one issue provided an ROI for the entire testing outage.
Thoroughly examining the circuit
There is more to the circuit than simply the connection, and it would be incorrect to ever assume a single point of failure. A motor circuit is a combination of many components, from the power transformer, through the controlling device and ultimately to a turning shaft. As part of our standard procedure, a second test was performed at the motor leads with excellent results. The leads were then re-lugged and repaired, and a final test was performed at the starter - again with excellent results.
It is always good practice to have a written procedure in place for making connections. Everything from proper compression tools, quality lugs and fasteners, as well as detailed instructions for insulation should be readily available to anyone responsible for making reliable motor connections. And don’t forget the anti-corrosive connection compound.
