Returning to the Fault Zone approach to electric motor troubleshooting we continue this series with Stator. When PdMA refers to the Stator Fault Zone in an electric motor, it is referring to the dielectric properties of the insulation preventing current flow between turns, coils, and phases. It’s important to remember that when an insulation short occurs between turns, coils, or phases that the motor is at the lower part of the PF curve and life expectancy may be short. Very short. This is why we stress that the goal of predictive/condition-based maintenance technology is to identify conditions conducive to these shorts and fix the condition to ward off a reduced life expectancy.
A variety of root causes should be considered when trying to get ahead of these faults. Unfortunately, ground wall insulation, a staple for ground wall insulation trending is not the ideal predictive tool for Stator anomalies. The motor may have experienced a catastrophic turn insulation failure and pass a ground wall insulation Resistance-to-Ground (RTG) test. However, it can be a secondary indication in that if the motor is found to be dirty and contaminated this can lead to localized heating and a conducive environment to turn insulation failure. The insulation is often a final mode of failure as a symptom instead of the root cause. The following are some potential root causes to consider when identifying conducive environments:
- Overload – Excessive Heat
- High Resistance Connection – Circulating Currents
- External Contamination – Moisture/Conductive Material Intrusion
- Design Flaw – Wrong Turns/Inverted Coil – Electrical Imbalance
- Environment – Excessive Ambient/Chemical Attack
- Excessive Vibration – Excessive Friction Between Turns or Coils
- Excessive Starts/Hour – Heating
- Particle Impingement- Rotor Bar, Lamination, Fan Blade, Cooling Slot Separator
Stator fault (End of Life) analysis can be easily performed with the motor running or de-energized (tripped) through Impedance, Current, Inductance, and Winding Resistance. Early identification of conducive environments will include additional tests like Resistance-to-Ground, Rotor Influence Checks, Rotor Evaluation Current Signature, In-Rush/Start-Up, Airgap, and Machine Train Analysis.
For more examples of Stator anomalies and alarm standards applied to your electric motor reliability view https://www.youtube.com/watch?v=uvQNlDdkYq0 on the PdMA YouTube channel.