Tom Bishop, PE
Senior Technical Support Specialist
Electrical Apparatus Service Association
St. Louis, MO
The paper "Root Cause Failure Analysis: Stator Windings" by Tom Bishop, presented at the EASA Convention 2010, provides a detailed examination of the various causes and patterns of stator winding failures in electric motors. The analysis aims to help technicians identify the root causes of failures and implement effective winding protection to prevent future occurrences.
The paper begins by outlining the importance of identifying both external and internal causes of winding failures. External causes include transient voltages, unbalanced voltage, and under-/over-voltage, while internal causes involve defects in winding methods and materials. The author emphasizes the need to analyze the failure mode and pattern to accurately diagnose the cause of failure.
Failure modes are categorized into five groups: turn-to-turn, coil-to-coil, phase-to-phase, coil-to-ground, and open circuit. These modes can occur in various combinations, making it challenging to determine the initial problem. The paper provides examples of failure patterns, such as symmetrical damage with all phases overheated, symmetrical damage with some phases overheated, asymmetrical damage with grounding, and asymmetrical damage excluding grounds. Each pattern is associated with specific causes, such as excessive loading, single-phasing, damaged insulation, and transient voltages.
Thermal stress is a significant factor in winding failures, with eight basic types identified: thermal aging, overloading, voltage variation, voltage unbalance, ambient temperature, load cycling, poor ventilation, and circulating currents. The paper explains how these stresses impact winding temperature and insulation life, providing guidelines for managing them.
Electrical stress, including dielectric aging, transient voltages, and partial discharge (corona), also contributes to winding failures. The paper discusses the effects of transient voltages from various sources, such as lightning, capacitor switching, and variable-frequency drives, and highlights the importance of surge protection.
Environmental stress, or contamination, is another critical factor. Moisture, abrasion, poor ventilation, and chemical damage can all compromise the insulation system. The paper provides examples of how these stresses manifest and offers solutions for mitigating their impact.
The integrity of the stator winding is closely tied to the quality of insulating materials. The paper identifies common insulation inadequacies and defects, such as improper cell wall or slot insulation, inadequate phase insulation, poor coil bracing, and damaged lead wire. It emphasizes the importance of selecting appropriate materials based on the motor's application and environment.
Winding temperature protection is essential for preventing excessive temperature and permanent damage. The paper outlines various methods of protection, including current-sensitive devices, combination current- and temperature-sensitive devices, and temperature-sensitive devices like RTDs, thermocouples, and thermistors. It provides guidelines for selecting and installing these devices to ensure effective protection.
Key Points Covered:
- Identifying external and internal causes of winding failures
- Analyzing failure modes and patterns
- Managing thermal stress and its impact on winding temperature
- Addressing electrical stress and transient voltages
- Mitigating environmental stress and contamination
- Ensuring the integrity of insulating materials
- Implementing winding temperature protection
Key Takeaways:
- Accurate diagnosis of winding failures requires analyzing both failure modes and patterns.
- Managing thermal and electrical stress is crucial for maintaining winding integrity.
- Environmental stress can significantly impact insulation performance and should be mitigated.
- Selecting appropriate insulating materials based on the motor's application is essential.
- Effective winding temperature protection can prevent excessive temperature and permanent damage.
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