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The coupling factor or the core sensitivity (SC) is a coefficient used in order to characterize the proportional relation between the magnetic field measurement and the current going through the conductor. However, this coupling factor is only constant over a limited range of current and frequency. Any change of this coefficient leads to input current measurement error. Eddy currents are a direct effect of Lenz’s law which states that the direction and magnitude of the current induced in a conductor by a varying magnetic field is such that it creates a magnetic field that opposes the change that produced it. In an AC current sensor application using a ferromagnetic core, eddy currents are induced inside the core as a response of the tangential varying magnetic field. These eddy currents generate an induced magnetic field, Heddy, opposite to the exciting magnetic field, Hexc. This is measured at the sensor level as a reduced core sensitivity or otherwise said, a current measurement error. In order to reduce eddy currents, it is necessary to cut the current paths in the core. This is achieved by using a laminated core with thin sheets. These sheets must be electrically isolated from each other. The lamination can be done in the Y direction by rolling or in the Z direction by stacking sheets. Eddy currents still flow, but with a reduced magnitude.
