The core of a transformer forms the magnetic circuit and also serves as the structural framework for the main body. Transformer cores are generally laminated from silicon steel sheets that are 0.35mm thick and coated with insulating varnish on the surface. The main purpose of coating the core with insulating varnish is to reduce core losses:
Silicon steel sheets themselves are conductors. Under the effect of alternating magnetic flux, induced currents are also generated within the core.
If the core were not coated with insulating varnish, it would become a solid whole, equivalent to a conductor with a very large cross-sectional area. Since its electrical resistance would be low, even though the induced voltage is not high, the induced current would be extremely large. This would generate massive eddy current (circulating current) losses. In severe cases, it would cause the core to overheat or even melt, a phenomenon known as core "burnout" or "fire."
After each sheet is coated with insulating varnish, the core is divided into many conductors with small cross-sections. This significantly increases the overall resistance. Although eddy current losses (which are part of the iron loss) still exist in a varnished core, they are greatly reduced.
Therefore, silicon steel sheets are made relatively thin and must be coated with insulating varnish.
