Chuck Yung
Senior Technical Support Specialist
Electrical Apparatus Service Association
Repairers often face the challenge of determining the cause of motor failures without complete information from the motor user. This paper explores methods to differentiate between damage caused by lightning and other transient conditions such as inadequate ground fault protection or multiple start attempts after an initial failure. Transient conditions, rapid bus transfer, and ungrounded systems can mislead repairers into attributing motor failures to lightning. Other transient events include re-closure, improperly located power factor correction capacitors, accidental connection of a new dual-voltage motor to the wrong voltage, and lightning. Correctly identifying the root cause of a winding fault and using appropriate protection can prevent repeat failures, saving downtime costs that often exceed the motor cost.
The paper discusses lightning arrestors and surge protection, emphasizing proper selection and installation. It includes meteorological sites that plot lightning strikes in North America and root cause failure analysis techniques for evaluating winding failures. Electrical stresses, ranging from low-voltage turn stresses to high-speed, high-voltage transients, can lead to insulation breakdown. Mechanical, thermal, and environmental stresses also contribute to insulation system degradation. Differentiating between cause and effect in winding failures associated with electrical stress can be challenging, often requiring systematic elimination of other possible explanations.
Transient voltages, defined as unexpected changes in voltage, can be destructive to electrical equipment, causing premature failures such as turn-to-turn shorts or turn-to-ground faults. These voltages can reach up to 3.5 times their normal peak values with extremely short rise times. Various sources of transient voltages include repetitive striking, rapid restrike (bus transfer), opening and closing of circuit breakers, capacitor switching, current limiting fuses, partial discharge (corona), and lightning. Each source has unique characteristics that can lead to motor winding damage.
Partial discharge occurs when the voltage is high enough to break down pockets of air within the insulation, causing arcing and releasing ozone that chemically attacks the insulation. This is most destructive when coils are loosely fitted in the slots. Machines rated over 6 kV typically use gradient tape to control partial discharge. Lightning can cause voltage surges through direct contact or induction, with surges propagating along the line. Variable-frequency drives can also introduce voltage spikes during starting and stopping.
Post-mortem analysis is crucial for identifying the root cause of motor failures. Analysts must avoid preconceived ideas and let the evidence guide the investigation. Effective methodologies include the Five Why Methodology, fishbone diagrams, 8D Problem Solving Process, statistical analysis, and Failure Mode Effects Analysis (FMEA). Proper documentation and photo evidence are essential for thorough analysis.
Surge and lightning arrestors should be used to protect motors susceptible to lightning damage. The leads should be as short as possible with minimal bends to reduce impedance. The paper provides guidance on arrestor selection based on system voltage and grounding configuration.
Key Points Covered:
- Differentiating between lightning damage and other transient conditions
- Importance of correct root cause identification to prevent repeat failures
- Electrical stresses and their impact on insulation systems
- Sources and characteristics of transient voltages
- Partial discharge and its destructive effects
- Post-mortem analysis techniques for motor failures
- Selection and installation of surge and lightning arrestors
Key Takeaways:
- Accurate identification of motor failure causes is crucial for effective protection and prevention of repeat failures.
- Transient voltages can significantly reduce winding life and cause premature motor failures.
- Systematic analysis and elimination of other possible causes are necessary to isolate the electrical stress responsible for the failure.
- Proper surge and lightning protection can mitigate the risk of damage from transient voltage events.
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