Tom Bishop
Technical Support Specialist
Electrical Apparatus Service Association, Inc.
St. Louis, MO
The paper "VPI Processing / VPI vs. Dip-and-Bake" by Tom Bishop, presented at the EASA Convention 2004, provides a detailed comparison between vacuum pressure impregnation (VPI) and the traditional dip-and-bake method for treating rotating electrical machine windings. The discussion includes the benefits and potential pitfalls of both processes, emphasizing that VPI should be considered a system rather than just a process involving resin and vessels.
Dipping and baking have been used for over a century to impregnate and cure windings, providing benefits such as bonding wires to restrict movement, improving heat transfer, increasing dielectric strength, and offering physical protection. VPI, introduced over the past five decades, involves vacuum and pressure cycles during impregnation, offering a more sophisticated approach. The paper explains that varnishes, typically containing up to 50% solids, are suitable for atmospheric dip-and-bake processes, while solventless resins, considered 100% solids, are used in VPI to avoid explosion risks during the vacuum to pressure transition.
The dip-and-bake process involves preheating, dipping, and baking the winding. Preheating drives out moisture and relieves mechanical stress cracks, enhancing fluid penetration during impregnation. The winding is immersed in the impregnating fluid until no air bubbles are seen, then allowed to drain before baking. The curing time depends on the part temperature and the varnish or resin's rated curing temperature. Failure to meet these criteria can result in premature winding failure.
VPI, primarily used for form coil stator windings, involves placing the part in a vessel, drawing a dry vacuum, introducing resin, applying pressure, and then curing the part in an oven. The process ensures complete penetration and retention of the resin, creating a void-free insulation system. VPI-treated windings offer excellent heat transfer, increased dielectric strength, and resistance to contaminants. The process also provides better bond strength and corona resistance, making it suitable for higher voltage machines and motors operated on variable frequency drives.
The paper highlights the benefits of VPI, including improved heat transfer, dielectric strength, and resistance to contamination. However, it also notes the high costs and complexity of the VPI system compared to dip-and-bake. The dip-and-bake process is simpler and more cost-effective, but less effective in penetrating form coil windings and providing complete void-free insulation.
Key Points Covered:
- Comparison of VPI and dip-and-bake processes
- Benefits of impregnation and curing for windings
- Detailed explanation of dip-and-bake process
- Detailed explanation of VPI process
- Benefits of VPI-treated windings
- Costs and complexity of VPI system
- Advantages and limitations of dip-and-bake process
Key Takeaways:
- VPI offers superior heat transfer, dielectric strength, and resistance to contamination.
- Dip-and-bake is simpler and more cost-effective but less effective for form coil windings.
- Proper execution of both processes is crucial to avoid premature winding failure.
- VPI is a complex system requiring significant investment and maintenance.
- Both processes have their unique benefits and are suitable for different applications.
To read the full technical paper, download the PDF below.
ANSI/EASA AR100
More information on this topic can be found in ANSI/EASA AR100
Print
