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 densityBversus time, and currentIversus 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 linearRLcircuit 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 linearRLcircuit. Values ofdB/dtwere measured at τ and the relationshipdB/dt = C/tau^{m}, wheremequals 1 andCis a constant, was found to be applicable for large values of τ, as expected in a linearRLcircuit. The τ value τcat whichmbegan 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 timeDeltatby which the flux density lags the applied field. Reasonably good correlation was obtained betweenDeltatand 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.