When performing offline (MCE) testing from the Motor Control Center (MCC) it is important to remember the resistance of the cable run is measured along with the stator winding. If there is a resistive imbalance it can be caused by the circuit and/or the windings. To find the “real” imbalance of the motor windings take another test at the motor terminals, thus eliminating the circuit. If the imbalance is in fact caused by the windings, then it will show up as a higher imbalance than when tested from the MCC. For example, a test from the MCC with three resistance readings of .1700, .1600, and .1755 will give us a 5.2% imbalance. After taking a perfectly balanced cable run out of the measurement, and testing from the motor leads, we would get .0700, .0600, and .0750 which will give us a 12.2%.
In summary, when confronted with a resistive imbalance from the MCC, it is important to perform further testing before identifying the motor as the faulty component.
To learn more about resistive imbalance view our MCEMAX Fault Zone – Power Circuit case study that can be found on our website
(http://www.pdma.com/webinars/Power_Circuit_Fault_Zone/powercircuit.html)or YouTube channel (https://www.youtube.com/watch?v=F4vrpo7DZRs).