Know the difference between VPI & B-stage insulation for your rewinds
You have a 13.8 kV electric motor that you’ve just been informed needs a rewind. The electric motor repair shop has given you two options: VPI or B-stage insulation. You need to understand what the difference is between the two insulation systems, which one fits your budget, and would likely be the best long term solution for your high voltage motor. This article may not answer all your questions for your specific electric motor rewinding, but it will give you the information about how these methods differ and when one is preferred over the other.
When we are referring to high voltage motors, we mean electric motors that have a voltage between 12 kV and 15 kV. Not only do these motors involve higher voltages but they are also large motors in terms of physical size & weight, which can cause them to be a serious challenge for many electric motor shops to work with.
The major issue with high voltage electric motor rewinds involves the rotor/stator coil insulation, which is critical for reliable performance. There are two options when it comes to performing a rewind/insulation process on a high voltage motor: VPI (vacuum pressure impregnation) and B-stage coils. While both methods accomplish the same end goal (I.e. coil insulation), the insulation systems involved couldn’t be more different.
In the VPI process, a preheated stator or rotor is lowered into the VPI pressure chamber and a vacuum is drawn. The vacuum removes all air & draws a solventless epoxy resin into the chamber until the stator or rotor is completely submerged. Then the process is reversed and pressure is applied until the insulation system is completely impregnated with the resin, after which the unit is removed and baked to cure the resin in a curing oven. The result of the VPI process is a 4 to 5 mil build of insulating resin and an almost void-free insulation system for your motor’s windings. It is a somewhat newer process that has been improved & perfected for lower voltage motors, but can still have issues with resin penetration for medium to higher voltage motors.
When it comes to high voltage motors, the alternative to VPI is known as B-stage insulation systems. B-stage coils have been used to insulate motor windings for large generators and motors for the last 100 years. The B-stage process includes the use of resin rich coils that are manufactured with epoxy loaded mica tapes.The slot portion of these coils are “Hot Pressed” and cured while the remaining portions of the coil are uncured and still flexible for the winding process. Eventually the completed winding is cured in a baking oven to finish the process. It is a time tested & durable insulation system for high voltage motors and typically lasts for 50+ years.
Let’s pause for a moment and compare VPI to B-stage coils:
Both VPI and B-stage coils can be used for high voltage electric motor rewinds. The key thing to keep in mind is that both of these methods are available, regularly used, and successful. B-stage coils are the older, more traditional method and probably the best option if you are working with an older motor -- switching from B-stage to VPI coils on an older model motor is not recommended and can be risky. VPI is still a valid option for high voltage motors (as long as performed by a reputable source) and can be more cost effective if your electric motor repair shop has a VPI tank that can fit your motor.
At HECO, we are very knowledgeable and experienced when it comes to insulation systems for electric motors. We’ve been doing VPI since 1981 -- and compiled many years of data, based on in-house testing. We’ve also been performing high voltage motor rewinds (11KV to 13.8KV) for over 25 years and perform both high voltage VPI and high voltage B-stage rewinds. In addition, we have testing capabilities of up to 13.8KV, 4000kVA and lathe machining capabilities up to 96" swing, 20' long, and up to 80,000 lbs, so we can repair the largest of rotating components. Contact us today and let us help you find the best solution for your high voltage electric motor!
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