Finding Faults and Failings - Using Motor Diagnostics to Detect Cable Faults

Field Testing
During a routine predictive maintenance route in a New York City commercial building, we identified unusual MCA test results in a 600 HP, 1150 RPM, 4160 Vac electric motor-driven a pump. This motor is shown in figure #1 below. The phase angle and current/frequency ratio (I/F) test results should be +/- 1 digit between phases if there are no problems. As shown in Table #1, the phase angle and I/F were significantly off on one phase. The instrument software identified it as a "Shorted Winding: Recommend check at motor if tested from MCC."

Figure #1 - 600 HP, 4160 Vac, 1150 RPM Motor

T 1-2         T 1-3         T 2-3  Resistance                0.6554    0.6563    0.6544  Impedance                   252        232        235  Inductance                   100         92          93  Phase Angle                  82          86          80  I/F                              -47         -46         -50 Table #1

Discovery
Before testing at the motor, we recognized that the cable from the motor control to the motor was excessively long. As a result, it was forced into the cabinet and up against one of the sides. We decided to inspect the cable prior to going through the difficulty of taking apart the connection box and connections. During the inspection, the electrician pulled the cable away from the side of the cabinet. Discoloration (shown in figures #2) was identified as partial discharges between the cable and cabinet, which would have lead to a catastrophic cable to ground failure at some point in the future. After moving the cable a way from the side of the cabinet, MCA test were performed which identified good test results, which are shown in Table #2. Test were repeated to confirm the discovery.

Figure #2 - Discoloration shows partial discharges between the cable and cabinet

T 1-2         T 1-3         T 2-3  Resistance                0.6549    0.6547    0.6541  Impedance                   255        233        236  Inductance                   101         92          93  Phase Angle                  85          85         88  I/F                              -47         -47         -47 Table #2

Conclusion
Cable faults are far more common in commercial and industrial applications than many realize. The use of MCA diagnostic tools allow the field service technician to identify insulation issues in both motor and cabling that feeds the motor. We have been able to trend winding degradation, winding contamination issues, and identify broken conductors and rotor faults well in advance of the motor ultimately failing.

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