Bonding varnish, a universal solution In recent years, the electric motor has been increasingly used to replace the conventional combustion engine in order to reduce carbon emissions. Now, the electrification of vehicles is urgently demanding high-performance motors. Electrical steel, a high silicon containing and thin sheet steel, is the soft magnetic material for the motor’s stator and rotor. These are composed of hundreds of stacked magnetic steel sheets. Insulation coatings on both sides of the electrical steel sheet obstruct the interlaminar eddy current, excited by the alternating magnetic field. Conventionally, welding/interlocking/clamping techniques are used to fix the magnetic core. These conventional fixation techniques have a huge impact on the magnetic properties of the electrical steel.
With interlocking, a mechanical connection is made between adjacent laminations by partially punching a tab through the thickness of the lamination and later pressfitting this tab into a matching depression on the top surface of the lamination stamped immediately prior. This approach works well for thicker steels, but becomes more challenging the thinner the steel. Interlocking and welding also introduce short circuit paths between the laminations, as they penetrate through, or bridge across, the insulating core plate coating on the steel surface, creating eddy current paths. Though this effect has been known for some
time, more recent work has shown that, with the use of thinner steels and higher fundamental core excitation frequencies, the adverse effects of interlocking and welding on unitised core magnetic performance are increased.
DOUBLE FUNCTIONALITY To overcome this drawback, an approach has been developed over the last 10-15 years, whereby the laminations are bonded together using an adhesive. Several competing approaches have been developed using a glue in the stamping process or a mill-applied bondable coating. The risk in
applying a glue between the laminations is incompatibility between adhesive and core plate coating on the steel to be bonded, which can vary between steel suppliers. The bonding varnish is a more universal solution: directly applied on the electrical strip, which has a double functionality, it acts as an insulating material, as well as to bond laminations when thermally activated under pressure. The curing process comprises of 2 stages: (1) The first stage is the drying phase in which a dry, non-sticky, flexible and still reactive coating is formed, withstanding