Thriving Under Pressure: The VPI Treatment for Motor Windings

Electric motors, although relied upon for their durability in aggressive operating environments, can be surprisingly delicate. David Bevilacqua, Technical Manager for HV Motors and Coils @ERIKS_UK & Ireland, explains how opting for Vacuum Pressure Impregnation (VPI) could be the solution to ensure they are protected and perform as they should.

In aggressive environments where electric motors are regularly exposed to high temperatures alongside wet and corrosive substances, the insulation materials they are constructed from can quickly degrade over time.

Therefore, in order to keep these critical components operational, contamination and moisture ingress should be prevented where possible. Both contamination and moisture can create tracking paths through the winding, allowing electrical leakage to earth and, eventually, a motor to trip. For a motor to stand the test of time then, the insulation resistance must be maintained.

Thankfully though, there are some simple solutions whereby motors can be treated to resist moisture. These include dipping the winding in a polyester or epoxy resin to create a moisture-resistant coating, or a process called Vacuum Pressure Impregnation (VPI). Our experience suggests the latter is the most effective.

Pressuring performance

VPI works by forcing resin into the depths of a winding using vacuum and pressure, in order to fill air pockets which, if left unresolved, can make a motor winding far more susceptible to future wear and tear. This method also binds together any loose lamination in the core: not only helping to keep moisture out but also enhancing the winding’s mechanical strength and heat dissipation properties.

The advantage of VPI over dipping is that moisture resistance is integrated into the winding structure itself, rather than simply forming an outer coating.

Although VPI has been an available solution for many years, many treatments use resins optimised for low-cost processing rather than performance. Instead, it is best to opt for a more effective 100% solid epoxy resin which is optimised for performance and protection.

VPI and beyond

Although VPI is an important technique for improving the longevity of a motor, it isn’t a one-size-fits-all solution and, in some scenarios, additional measures will be required to ensure a motor’s protection, such as a complete rewind. In this situation, it is best to consult the experts who will be able to advise on the best plan of action.

For example, if a motor is in good condition apart from its porosity and moisture absorption, then VPI alone has the potential to increase its service life by several years. If, however, the winding has deteriorated too far, often the best course of action is a complete rewind using highly absorbent VPI-compatible material, followed by the VPI process. The compatible material will increase the uptake of the epoxy resin to help form a solid, homogenous mass, providing maximum moisture resistance.

The bigger picture

With any equipment failure, it is important to look at the bigger picture in order to determine the most appropriate solution. For example, ERIKS was involved in a project where a certain type of AC freight train’s traction motors were proving unreliable due to the damp and dirty conditions they were operating in. In this scenario, it would have been easy to recommend VPI as a quick-fix solution as, after all, the damp and dirty operating conditions suggested that contaminant and moisture ingress to the motor were the main problems. However, after further investigation we were able to determine that the failure mode was due to a breakdown between phases on the stator winding connections.

Realising that VPI would only partly solve the problem, we not only stripped the connection rings and reinsulated them using VPI, but also redesigned them. The original high temperature silicone insulation system within the coils was retained, to provide higher-temperature protection where it was most needed. Meanwhile, the redesigned rings were sealed with Class H 180°C epoxy VPI, providing greater protection against environmental conditions.

The result was a complete elimination of the motor failure mode, and an ongoing programme to modify all the customer’s other traction motors in the same way.

Ultimately, electric motors should be durable and relied upon to stand the test of time, even in the most aggressive operating environments. Although VPI alone won’t always be the most suitable solution, it should certainly be considered as a first option when it comes to repairing damaged motor windings to improve their longevity.