Nanocrystalline foil sensors for local detection of in-plane magnetic flux in laminated machine cores

Abstract

Recently, we presented a novel tangential induction sensor for measurements of local induction values in soft magnetic lamination of machine cores. In contrast to traditional tangential field-coil sensors, it exhibits a soft magnetic ribbon as a nucleus. Sensors with SiFe ribbon were effectively tested for transformer core surfaces, while showing poor reproducibility in the interior. Also tests with thin Fe-based amorphous ribbons proved to lack reproducibility, mechanical stress through core clamping causing magneto-elastic effects. An alternative would be Co-based amorphous ribbons of minimum magnetostriction, however, linked with low saturation magnetization. As an optimum solution, we present here nanocrystalline ribbons of 20 $\mu$ m thickness, combining high permeability, high saturation magnetization and almost zero-magnetostriction. The tendency of saturation due to very high permeability is met by optimized shortening of the nucleus, the demagnetizing effect offering good performance in a wide range of induction. Assembling by a combined 3D/2D printing procedure promises a total sensor thickness of the order of 100 $\mu$ m. Prototypes were effectively tested in the interior of a 3-package transformer core. While earlier sensors were restricted to peak induction measurements, the nanocrystalline ones offer detection of the waveform of local inductions. For the first time, measurements are possible in completely non-destructive way.