If you want to discuss clearly the role of insulating paint, you must first understand what is eddy current?

When a piece of metal is placed in a time-varying magnetic field or allowed to move in a magnetic field, an induced current will be generated in the metal piece. This current forms a closed loop in the metal piece, much like a vortex of water, so it is called an eddy current , Referred to as vortex.

The outside of the iron core of the transformer is a coil. When the alternating current passes through the coil, the magnetic flux passing through the iron core continuously changes with time. It generates an induced electromotive force in the iron core, thereby generating eddy currents. These eddy currents cause the iron core to generate a lot of heat, waste a lot of electrical energy from the sliding contact line, and may damage the electrical appliances. In severe cases, the iron core may even be heated and melted locally, causing the iron core to "fire". Therefore, we must find a way to reduce the eddy current, one of the ways is to increase the resistivity of the core material.

               

      Toroidal Cores Power Transformer Core 

               

                     Instrument Transformer Cores

If we look closely at generators, motors, and transformers, we can see that their iron cores are not a single piece of metal, but are made of many thin silicon steel sheets coated with insulating paint. Why use thin silicon steel sheets, and why do they need to be painted with insulating paint? What are their roles?

The role of silicon steel sheet

Due to the large resistivity of this sheet material (the eddy current loss of silicon steel is only 1/5 to 1/4 of that of ordinary steel), the eddy current loss is greatly reduced.

The role of insulating paint

In AC motors and electrical appliances, iron cores made of laminated thin silicon steel sheets coated with a thin layer of insulating paint or insulating oxide are widely used. After the transformer core is coated with insulating varnish, it is divided into a lot of small cross-section conductors, so that its resistance becomes larger. The eddy current is limited in the narrow slice. When the magnetic flux passes through the narrow cross section of the slice , The net electromotive force in these loops is small, the length of the loop is large, the resistance of the loop is large, and the eddy current is greatly weakened. Therefore, although the iron core coated with insulating paint also has eddy current loss (part of the iron loss), it is also much smaller. In this way, safety and high efficiency can be achieved by applying insulating paint.