Physical effects of gross overheating on epoxy-mica insulation systems

[electricpete]

I am interested in learning about what we might expect if a Roebel bar is subject to excess temperature at some unknown value below the melting point of copper.

The few things I know:
Epoxy resin is ‘thermosetting” rather than “thermoplastic”. Therefore, it can get brittle and crack, but should not get soft and flow. Therefore, we would not expect conductors to move during a coil failure event unless the epoxy resin was cracked. Would you agree?

I think there may be a lot of experience here from people that burn coils out of motors. What condition do you find coils in that have been burnt out? Does the coil soften or does it shrink due to off-gassing? (I’m thinking it shrinks).

At what temperature does the epoxy begin to off-gas?


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(2B)+(2B)' ?

 

[TheBlacksmith]

Depending on the coil length, there can be enough thermal expansion of the copper to break the bond between the epoxy and the conductors. Usually in machines with coils exceeding 3 meters in length. Similar to girth cracking in old bituminous class B hydro generators.

 

[Compositepro]

There is a tremendous variation in epoxy formulation and properties. Thermosets do not melt but they can get very soft, like rubber (which is also a thermoset). The best epoxies will not last long at temperatures above 400F, which is far below the melting point of copper. At high temperature, epoxy resins will oxidize and char and will lose their insulating properties.

When it comes to how laminates of materials with differing CTE hold-up to thermocylcling, the layer thickness is the main factor. Thin layers a far more durable that thick layers.