VPI Systems 1

[petronila]

Dear All,

Merry Christmas and Happy New 2011

Best Wishes for every body and many thanks for the received help.

I'm starting to analyze the VPI process and resins. Probably we are going to build our VPI system

Based in your experiences is better epoxy or polyester resins?

Pro and Cons?

Regards

Carlos

 

[DickDV]

Our shop always used epoxy resins with great results. While I am no expert on the chemistry of this stuff, it seems that epoxy has better heat transfer and mechanical strength characteristics.

 

[electricpete]

From what I have heard and read: Epoxy has historically been viewed as "better", particularly for resistance to chemical attack and moisture. But with recent advances in both types they now perform almost the same. Almost all OEM's use Epoxy. Repair sShops are split. Epoxy is not economically suited to a low through-put process because of shelf life considerations.... that is the reason many shops opt for epoxy. As a customer I would prefer epoxy all other things being equal, but we don't specify it.

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(2B)+(2B)' ?

 

[electricpete]

An article from a customer who claims to have found better performance from epoxy based rewinds based on a large population:

http://www.mt-online.com/component/...motor-refurbishment-project.html?directory=90

Lessons from a Major Motor Refurbishment Project

The operating histories and conditions found in JEA motors with polyester insulating systems were consistently worse than those found in systems using epoxy materials. For that reason, the decision was made to prohibit the use of polyester based insulation materials in JEA motors requiring full winding replacement.

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(2B)+(2B)' ?

 

[electricpete]

Correction in bold:

Epoxy is not economically suited to a low through-put process because of shelf life considerations.... that is the reason many shops opt for epoxy. As a customer I would prefer epoxy all other things being equal, but we don't specify it.

should have been:

Epoxy is not economically suited to a low through-put process because of shelf life considerations.... that is the reason many shops opt for polyester. As a customer I would prefer epoxy all other things being equal, but we don't specify it.

Also here is a quote from EPRI 5036 Volume 17 (Insulation Systems) highlighting some desirable properties of epoxy

Since the early 1950s, major efforts were made in the development of high-voltage groundwall insulation. These efforts improved the quality of mica paper and backing materials, the mechanical
properties of the resin, and the processes by which the synthetic binder is applied and cured. Significant advances in this area since that time are:
· The introduction of a post-VPI process for small- and medium-sized machines, i.e., the coils are insulated, wound, and connected prior to VPI bonding with a thermosetting resin and cured.
· Almost exclusive use of epoxy resins as the bonding agent. Compared to polyesters, epoxy resins were found to have superior mechanical properties and resistance to moisture and chemical attack. Also, they do not shrink as much after curing. Solventless grades of epoxy, suitable for use in a VPI process, were developed.

The solventless are the so-called 100% solids resins. I think those are the cadillac of resins. One could argue there is less chance of void in a resin which does not give off gases during curing.


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[electricpete]

Here is a quote from EPRI 5037 V6 that provides some more info on 100% solids / solventless resin:

An alternative available for these windings is the
100% solids msin, often epoxy-based, containing
no solvent to be driven off during cure. Rather
than simply drying, this material cures through
polymerization induced by heat. Special processing
of the winding is mquired, however, because such
resins tend to have short pot life. Keeping a large
tank of the material on hand is seldom practical,
because it would cure too rapidly within the tank.

So perhaps the shelf-life issue I was referring to early is more associated with 100% solids resins. 100% solids resins are typically epoxies. I don't know if there are any commonly used epoxy resins which are not 100% solids, but I think most Polyester resins are not 100% solids.

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(2B)+(2B)' ?

 

[wolf39]

Stator winding bars impregnated with polyester resin were suffering from a certain expansion, this resulting in internal discharge activities. Modern VPI insulation based on epoxy resin practically doesn't expand, thus increasing service life of stator windings considerably.

Wolf

http://www.hydropower-consult.com