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Types of Surface Insulation Resistance and Typical Applications
AISI Insulation Designation Description Typical Applications
C-O The natural oxide surface which occurs on flat-rolled silicon steel which gives a slight but effective insulation layer sufficient for most small cores and will withstand normal stress-relief anneal of finished cores by controlling the atmosphere to be more or less oxidizing to the surface. Fractional horsepower motors, pole pieces and relays, small communication owner transformers and reactors.
C-2 An inorganic insulation which consists of a glass-like film which forms during high temperature hydrogen anneal of grain-oriented silicon steel as the result of the reaction of an applied coating of MgO and silicates in the surface of the steel. This insulation is intended for air-cooled Wound-core, power frequency devices,distribution transformers, saturable rectors and large magnetic amplifiers.
C-3 An enamel or varnish coating intended for air-cooled or oil immersed cores.The interlamination resistance provided by this coating is superior to the C-1typed coating which is primarily utilized as a die lubricant. The C-3 coating will also enhance punchability;is resistant to normal operation temperatures; but will not withstand stress-relief annealing. Air-cooled, medium-sized power and distribution transformers; medium-sized continuous duty, high efficiency rotating machinery. Can be oil-cooled.
C-4 A chemically treated or phosphate surface intended for air-cooled or oil-immersed cores requiring moderate levels of insulation resistance. It will withstand stress-relief annealing and serve to promote punchability. Applications requiring insulation similar to C-3 and a stress-relief anneal. Used extensively for small stamped laminations that require higher resistance than provided by annealing oxides.
C-5 An inorganic insulation similar to C-4 but with ceramic fillers added to enhance the interlaminar resistance. It is typically applied over the C-2 coating on grain-oriented silicon steel. Like C-2, it will withstand stress-relief annealing withstand stress-relief annealing atmosphere. Principally intended for air-cooled or oil-immersed cores which utilize sheared laminations and operate at high volts per turn. Also finds applications in apparatus requiring high levels of interlaminar resistance.
Anti-stick(no AISI Designation) An inorganic insulation similar to C-4 A superior surface treatment providing improved magnetic quality and protection against sticking of laminations during the quality annealing of semi-processed material.
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