Temperature limits associated with C3 and C5 core insulation?

I did locate A 976 – 03 - "Standard Classification of Insulating Coatings for Electrical Steels by Composition, Relative Insulating Ability and Application"

It appears it's a lot more complicated than a simple number. As a gross overview I pick out the numbers 350F for C3 and 500-1000F for C5.

Here are some excerpts:

C-3 Organic varnish/enamel coating that is applied to the steel surface and cured by heating. Used for fully processed nonoriented and other electrical steels. It is appropriate to designate a maximum Franklin test current for this type of coating. The required Franklin test current is subject to agreement between the producer and user. This coating generally improves the punchability of the steel, and hence, is quite suitable for stamped laminations. This coating will not withstand typical stress-relief annealing temperatures. The coating normally is suitable for operating temperatures up to about 350°F (180°C)

C-5 Inorganic or mostly inorganic coating similar to C-4, to which ceramic fillers or film-forming inorganic components have been added to increase the insulating ability of the coating. The coating typically is a phosphate, chromate, or silicate coating, or combination thereof. Such coatings are applied to the steel surface and cured by heating. The coatings can be applied to grain-oriented electrical steels, nonoriented electrical steels, and coldrolled motor lamination steels. A C-5 coating may be applied over top of a C-2 coating for applications in which extra surface insulation is required, for example, sheared laminations of grain-oriented electrical steel for cores of power transformers. C-5 coatings are used for applications requiring a high-surface resistivity. It is appropriate to designate a maximum Franklin test current for this type of coating before stress-relief annealing. The required Franklin test current is subject to agreement between the producer and user. The coating will withstand stress-relief annealing up to 1550°F (840°C) in neutral or slightly reducing furnace atmospheres, but some reduction in surface insulation resistivity may occur during the anneal. The coating will withstand burn-off treatments at 600-1000°F (320-540°C) used to remove stator winding insulation during rebuilding of motors. The coating can be used in air-cooled or oil-immersed cores. In some cases, organic components may be added to C-5 coatings to enhance punchability. The applications, use, and properties of such coatings are similar to those of inorganic C-5 coatings. The user should consult the producer if there are particular concerns with coating off-gassing during welding or elevated temperature exposure of the coated steel.


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Hi electricpete.

You have answered it in detail. In reality I have not seen temperature limits, except for C5 enduring annealing temperatures and C3 withstanding operating temperatures.

Normally OEM's do not put to much concern on the winding stripping procedures were the lamination coat could be destroyed while oven heated.