Summary: When it comes to insulating high-voltage electrical machines, Vacuum Pressure Impregnation (VPI) is often hailed as the gold standard. Some repair shops insist that VPI is the only way to ensure reliability, performance, and longevity. But is that really the case? The truth is, while VPI has its advantages, it is not the only option [1]—and in many cases, it may not even be the best one.
Firstly, what is VPI?
VPI involves placing the stator with its windings in a vacuum chamber to remove air, then applying low-viscosity, heated resin under pressure to fill all gaps. The resin is then cured at high temperatures, creating a solid insulation layer.
How it works:
- Air is evacuated from the windings in the impregnation chamber, creating vacuum.
- Resin is injected into the chamber under pressure, ensuring penetration into the windings, and stator is allowed to stay submerged for a period of time.
- The wet stator is moved to an oven to be cured at high temperatures to form a hardened insulation structure.
Resin-Rich Insulation
Resin-Rich technology uses mica tapes with impregnated high-viscosity resin on individual coils when they are manufactured. These coils are then hot-pressed in a hydraulic molding machine to form a solid bar, ensuring a consistent and durable insulation layer.
- The coil’s slot portion, manufactured as a solid structure with a flexible end-winding, are directly inserted into the stator, followed by their bracing and connection.
- The stator is further cured in the oven before it is ready for final testing.
www.irispower.com
, accessed on 20th Mar 2025)
www.preformedwindings.com accessed on 20th Mar 2025)
VPI vs. Resin-Rich: A Fair Comparison
VPI is widely used by motor OEM manufacturers because it is cost-effective at high volumes while improving mechanical strength and thermal performance. However, there are several key challenges for their use in motor repair shops:
- No guarantee of slot fill: For one-off repairs, there is no guarantee of complete resin fill in stator slots [2].
- High upfront and maintenance cost: A VPI system requires a vacuum chamber, curing oven, and strict process control—upfront investments that cost millions of dollars, on top of the need for regular maintenance and high resin turnover.
- Operator dependency: Achieving high-quality repairs depends heavily on resin used, expertise and process management. For example, retaining resin during curing requires rotating the stator which is difficult to do unless the stator core lamination is removed from its frame.
On the other hand, Resin-Rich technology offers distinct advantages [3]:
- Guaranteed slot fill: Because the insulation is applied at the coil manufacturing stage, there is no risk of incomplete resin penetration.
- Enhanced durability: The insulation structure is more robust, improving dielectric integrity and mechanical strength.
- Reliable quality: The process is standardized and controlled by specialized coil manufacturers, ensuring consistency.
Why Resin-Rich May Be the Better Choice
Many repair shops promote VPI for a return of their investment and to keep turning over their resin. But for critical HV machine repairs, Resin-Rich technology offers a more predictable and reliable alternative. By partnering with top-tier coil manufacturers, we can focus on delivering high-quality coil insertion, bracing, and connections—ensuring maximum value for our customers.
The bottom line? VPI is not the only solution, and in many cases, Resin-Rich is often the smarter, more reliable choice for high-voltage machine repairs.
References
- “Progress in Rotating-Machine Insulation Systems and Processing” by G. C. Stone and G. H. Miller, DEIS Feature Article (July/August – Vol. 29, No.4)
- “Resin-rich vs. VPI” by Michael Mitten, Electrical Apparatus Magazine (April 2025)
- “Resin Rich vs VPI” by Preformed windings