مرکز منطقه ای اطلاع رساني علوم و فناوري - Magnetic flux as affected by eddy currents in electromagnets

Abstract :

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 $B$ versus time, and current $I$ 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 $RL$ 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 $RL$ circuit. Values of $dB/dt$ were measured at τ and the relationship $dB/dt = C/\\tau ^{m}$ , where $m$ equals 1 and $C$ is a constant, was found to be applicable for large values of τ, as expected in a linear $RL$ circuit. The τ value τcat which $m$ 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 $\\Delta t$ by which the flux density lags the applied field. Reasonably good correlation was obtained between $\\Delta t$ 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.