Chuck Yung
Technical Support Specialist
Electrical Apparatus Service Association
St. Louis, MO
The paper "Stator Core Repair and Testing" by Chuck Yung, presented at the EASA Convention 2007, provides detailed guidance on the repair and testing of stator cores in electric motors. The paper aims to ensure quality restacks, identify key areas for quality control, and offer useful tips for special cases. It also debunks myths about alternative core repair methods and introduces a new effective alternative for repairing shorted laminations.
When a motor failure includes damage to the laminated core, the decision to repair or replace the motor depends on factors such as price and availability. If a replacement motor is not available, a labor-intensive restack of the stator core may be necessary. The goal of a restack is to dismantle the core, clean and recoat the laminations to minimize heating from eddy currents in shorted laminations, and reassemble the core with the correct geometry.
Core losses are categorized into hysteresis losses, notching stresses, and eddy-current losses. Hysteresis losses are minimized by the annealing process, while notching stresses are relieved during manufacturing. Eddy-current losses occur when adjacent laminations are shorted together, behaving as a single thicker lamination. The stacking factor, which describes the amount of actual lamination steel content over the core length, is crucial for minimizing core losses.
Proper stacking pressure, maintaining the original core length, and using low-loss silicon steel for replacement laminations are essential for reducing eddy-current losses. The paper emphasizes the importance of using laminations of the same thickness as the original to avoid increased losses. Eddy-current losses vary with the square of the frequency, so motors operating at higher frequencies require thinner laminations.
The paper debunks several myths about core repair methods, such as using water to create iron oxide coatings, sodium silicate, and acids. These methods are ineffective and can cause further damage. Instead, the paper recommends using suitable coreplate materials that can withstand burnout oven cycles.
The restack procedure involves drawing a detailed diagram of the stator core, sanding each lamination to remove burrs and residue, coating the cleaned laminations with coreplate, and restacking them to the original configuration. The paper highlights the importance of maintaining stack geometry and using a stacking fixture to minimize irregular lamination surfaces.
An alternative method for repairing cores with localized missing sections of teeth involves using magnetic wedge material to reconstruct the tooth geometry. This method is limited by the reduced ferrous material in magnetic wedges.
The paper introduces a new process called Cure-a-Core, which uses an aqueous solution of phosphoric acid with zinc, manganese, and iron in suspension to etch shorted laminations and form a durable coreplate. This process has successfully reduced core losses and improved power factor in several service centers.
Key Points Covered:
- Factors influencing the repair-replace decision for damaged stator cores
- Core losses: hysteresis, notching stresses, and eddy-current losses
- Importance of stacking factor and proper stacking pressure
- Myths about alternative core repair methods
- Detailed restack procedure
- Alternative repair methods using magnetic wedge material
- Introduction of Cure-a-Core process for reducing core losses
Key Takeaways:
- Repairing stator cores is labor-intensive but necessary when replacements are unavailable.
- Core losses must be minimized through proper stacking and use of suitable materials.
- Myths about core repair methods are debunked, emphasizing the need for effective techniques.
- The restack procedure requires careful attention to detail and maintaining stack geometry.
- Magnetic wedge material can be used for localized repairs but has limitations.
- The Cure-a-Core process offers a new, effective method for reducing core losses and improving power factor.
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