Magnetic flux as affected by eddy currents in electromagnets

A method has been developed for determining the current time constant τ at which eddy currents become detrimental to electromagnet operation. Plots of flux density versus time, and current versus time were made for various values of τ on electromagnets having a constant air gap and a fixed value of maximum current Imin the coil. Such an electromagnet can be approximated by a linear circuit as long as eddy current effects are negligible. τ was the time required for the current to reach 0.63 Im. This corresponds to the time constant in a linear circuit. Values of were measured at τ and the relationship , where equals 1 and is a constant, was found to be applicable for large values of τ, as expected in a linear circuit. The τ value τcat which began to decrease from 1, determined the onset of eddy current effects since the circuit is no longer linear. Experimental results were compared to a theoretical solution for a ramp field applied to sheets of thickness δ. Calculations were performed of the time by which the flux density lags the applied field. Reasonably good correlation was obtained between and τc. Measurements were made on various soft magnetic materials in various thicknesses. The results provide a guide in selecting material and lamination thickness for a given application.