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B-Stage vs. VPI Insulation Systems for 13.2KV/13.8KV Large Electric Motors


DSCN2738.jpgHigher voltage (Medium voltage) Insulation systems have been developed over many years, with a recent interest in newer vacuum pressure impregnation (VPI) insulation systems that have a perception of being newer and better but not with an improvement in longevity or quality. The newer VPI insulation systems today are promoted by some manufacturers and vendors as being better than the older systems that have lasted for the last 50+ years with great success. The main reason for the VPI processing of newer design large motors is for economics. The newer motors are smaller in physical size and have higher temperature rises. The use of VPI in these motors allows the manufacturer to reduce costs for manufacturing purposes… but not necessarily for longevity. This trend has now been incorporated into older machines with the same reasons: for lower costs to process but not necessarily for longer lasting service. 

HECO was one of the first service companies in the Midwest (1981) to have a VPI system and process utilizing VPI coil insulation that was manufactured at HECO (with our HECO Coil MFG division). The main point here was we developed our coil insulation system to be compatible with the VPI resin we researched to be the best alternative for use in VPI systems (A 100% solids solventless Epoxy Resin). It took us several years to perfect the VPI process and the tape systems used to provide full penetration of the insulation. This was perfected through 7200 volts. HECO is a believer in VPI processing with compatible tested insulation systems that have a proven success in voltages up to 7.2KV. However, in the higher voltages up to 13.8 KV these VPI/coil systems are not always perfected and there are many documented failure stories where service centers tried to switch a B-stage insulation system that ran for 50+ years over to a VPI insulation system. Why switch if the original insulation system outlasted its expected life?

B-Stage coils are resin rich coils manufactured with epoxy loaded tapes that have been used predominantly for the last 50 years. This use was mainly in Large Generators and any large motor in the 12KV to 15KV range. These systems have been perfected over many years with proven success…. That success being measured in long term service life of the windings. These motors are in every facet of modern industry and to this day provide many benefits that continue to provide longer lasting service life. 

Comparisons and points: 

  • The VPI systems used today in 12KV to 15KV systems must be matched in insulation and resin compatibility. This is done through years of research to provide a system that allows for full penetration and proper bonding of materials for heat transfer. Keep in mind that the manufacturing and processing is economically cheaper so it is favored in today’s manufacturing of motors for that reason. Trying to match insulation systems of coils manufactured by one supplier and then having the VPI process performed by another is a recipe for disaster unless this has been proven for many years with successful service. (Most companies that offer this type of system have only been producing 12KV to 15KV VPI coils/systems for 5 to 15 years, and also have a compatible VPI resin system in their facility to eliminate process variances and compatibility issues….But mainly the life expectancy is not known nor proven as they have not been in service long enough to be proven.
  • Electrical testing of “Green” VPI coils must be performed at lower voltage levels because the mica tapes are not cured/finalized without the particular VPI resin penetration and oven baking. This can cause quality issues as the coils cannot be safely tested at the full higher Hi-potential test voltage levels without damage to the insulation. Instead the higher Hipot voltage levels must be performed after the motor is wound and VPI processed & Oven cured. At this point any issues necessitate the need to oven strip (Burn out) and start over again with a complete set of new coils. With the risk of damaging the stator lamination steel in the burn out process (which was already done at this point prior to winding the first time). 
  • VPI insulation systems can be hard to control the coil size due to insulation materials relaxing (before use) during the winding process. They are in a green uncured state so the coils must be wound in the slot with more space to allow for coil insertion. This will allow the coils to not sit vertical in the slot and cause angular top to bottom coil issues. This then lends to more air voids between the slot and coil sides, whereby the VPI resin is now left to fill these voids. The problem being the resins are now a much thinner resin (to be able to penetrate 13 to 14 layers of mica tapes) and do not have the mil build and thickness to fill these voids completely. So potential voids are left, which cause PD activity and corona discharge = shorter winding life. 
  • In retrospect, B-Stage (Resin Rich Epoxy) coil systems have been the dominant coil type for over half a century. This is primarily due to the systems being engineered and proven with machines lasting 30, 40 and 50 years with the same winding. Additionally, the B-Stage systems have proven reliability and longer lasting life. 
  • B-Stage resin rich coils are manufactured with epoxy resin rich mica tapes engineered to be finished at specific height and widths. They achieve this by “Hot Pressing” the slot sections to form a strand consolidated, cured coil that fits the slot for a given machine (with minimal designed mechanical clearance to allow for coil insertion in to the slots but strand consolidated finished to high tolerances/sizes.)… This is achieved by utilizing hydraulic presses that compress the insulation (Width & Height) to specific sizes while introducing heat to cure the epoxy resin in the process. 
  • With the slot section being fully cured with B-Stage, electrical testing can be performed at the maximum Hipot (IEEE approved) voltage levels for a finished machine. This system also allows for partial discharge testing and tan delta testing which proves resin penetration in the coil slot section and confirms low levels of partial discharge (PD activity) within the coil. If one coil does not meet the standards of testing then another coil is MFG that does. (100% coil tested before shipment to a particular service company). 
  • Other concerns are if a coil/winding issue becomes present in the future for VPI vs. B-Stage. With VPI there is little to no coil repair or patching. The coils are impregnated in the slots with resin binding them to the slot, so no coil removal is possible. Any repairs become almost impossible. With B-Stage partial repairs are practical. Failed coils have a better chance of being repaired or replaced. (Because the coils are not bonded or glued to the slot) which allows easier jumping out of bad coils, etc. 
  • With VPI insulation systems for High voltage (12 to 15KV), when a winding eventually fails, it has to be burn out oven processed and stripped. Burn out ovens with VPI (12 to 15 KV) systems are more apt to damage the stator lamination steel because longer burning and exposure to higher heat levels are necessary to remove the resins and allow for coil removal. Thus causing a stator restack in many cases. 
  • With B-Stage the stripping is easier because the burn oven temperatures can be set at lower temperatures and gradually brought up to control the heat and provide less damage to lamination steel. Thus keeping the core iron in good condition. In many cases the coils can be cold stripped without the need for higher oven temperatures. 
  • With B-Stage systems there is no dependence on needing a larger VPI tank to handle the large motor sizes. The coils are resin (epoxy loaded) and therefore do not require anything beyond an epoxy/polyester spray coating or treatment of end turns and blocking (for added protection) and then oven baking. No concerns are present for insulation compatibility nor resin penetration. There is no concern for resin cleanliness or aging as you have with VPI resin systems. No concerns for proper process control to ensure the best possible penetration levels as with VPI.



Although there are a variety of points listed in this article that favor B-Stage insulation systems, the trend is surely moving towards VPI for these higher voltage large electric motors. Let's be clear, this article is not meant to discourage end users or engineers from pursuing the use of VPI in 12KV to 15KV machines, it is a readily available and regularly used process in today's marketplace, and we can provide this option to our clients.

However, more and more often, we are seeing specifications that call for a switch from B-Stage to VPI insulation systems in older, vintage, machines. The question to ask is "why?". Are they being switched so that a preferred vendor gets the repair? Are they being switched to VPI because its the "newest and best" technology? Is it because VPI can be more cost effective? Is it an attempt to fix a recurring problem?

Whatever the reason, we just want industry professionals to know and understand the differences between the two insulation system options and that B-Stage is still a viable, quality, option to be considered.


Todd A. Hatfield, VP Engineering & Repair

HECO – All Systems Go



Download What To Look For In An Electric Motor Repair Shop


 About the author:

Todd Hatfield is a co-owner of HECO and is the Vice President of Engineering and Repair. He has over 35 years' experience in generator and electric motor repair and engineering. Todd has a BS in Electrical Engineering, and his areas of expertise are: electrical and mechanical motor redesigns and engineering, root cause of failure analysis, and quality electric motor rebuilding.

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