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

I’ve never before experienced such a fast product development in my 17 years at OCAS. The technology is not new, but industrialisation is never a given fact.

Nathalie Van den Bossche

core plate processing (like slitting and punching). (2)Once stacked, the individual sheets are positioned in a heated press. The varnish softens, cures and bonds to the adjacent steel sheets. Thanks to the self-bonding technology, the key material properties of electrical steel strip remain intact.

As a significant share of the automotive volume is expected with bonding varnish in the near future, ArcelorMittal decided to extend their product range with the doublesided coated bonding varnish. Triggered by automotive customers who are investigating the usage of bonding varnish for the next generation motors, the first industrial trials were done. Meanwhile, the first customers report good adhesion results, matching their expectations. Although bonding varnish is a promising technology, the biggest obstacle for its large-scale application is the concern about mechanical failure of the adhesion under periodic load conditions at an elevated temperature during the operation of the motor. This will need further investigation.