A Novel Technique to Quantify Micro Insulation Defects on Grain-Oriented Electrical Steel Coating Using the Scanning Vibrating Electrode Technique

Abstract

Electrical steels are a specialised group of low carbon silicon steel alloys widely used in transformers, generators and motors. It has been previously shown that reducing eddy currents by insulating laminations in an iron core significantly increases efficiency. During production electrical steels are initially coated with magnesium oxide (MgO) slurry that reacts with the silicon in the upper surface of the substrate during annealing to produce a magnesium silicate called forsterite. The material receives a phosphate based secondary coating which helps to ensure the material is fully insulated and improves magnetic properties by adding tension to the material. Until now industrial measurements of resistance have been conducted using copper probes on a Franklin tester to give an average resistance of the area but small defects such as pin holes commonly average out to produce infinite resistance insulation. A novel method of using a Scanning Vibrating Electrode Technique (SVET) has enabled high resolution results of over 6500 times greater than the conventional industrial method used. The SVET is an electrochemical method traditionally used to resolve and quantify localised corrosion currents at the surface of a metal submerged in an electrolyte. This novel technique works by submerging samples with defects into 0.1% NaCl electrolyte. A 1µA current is passed through a sample that has an insulation coating. The SVET can detect regions where insulation has failed or is thin. Testing has been conducted on samples High permeability substrate with just forsterite coating, just phosphate coating, both coatings and both coatings with defects.

Electrochemical Scanning Techniques for the Examination of Bi-Metallic Coins

“…The SVET is also predominantly used as a tool for the investigation of corrosion and inhibition effectiveness, although other more novel applications have been reported [17]. Previously the SVET has been restricted to planar or relatively flat surfaces due to the height dependent nature of the technique, however over the last decade a combination of enhanced theoretical understanding [18] and equipment modifications now allows the topographical profile of non-planar -e.g.…”