High Frequency Characterization of Magnetic Rod Cores with Anisotropic Power Loss

Here we describe a core test fixture which properly aligns the field within a low aspect ratio rod core sample without introducing nonlinear power loss mechanisms. The magnetization at the center of a long solenoid inductor is air -ONA, where ix, is the relative permeability of the core, N is the number of turns, / is the coil current, and L is the length of the coil. The end portions of the magnetic core can be emulated with additional solenoid windings, as shown in Fig. 1. In order to maintain constant flux through the core test sample, the emulation coils must generate a field equal to the magnetization of the core: (1,-1)N, IjL = N,114 (1). Where the x subscript indicates parameters for the outer excitation coil and the e subscript indicates parameters for the core emulation coils on each side of the test sample. This arrangement is beneficial because the portions of the core which experience flux perpendicular to the intended measurement direction are now replaced with windings which have isotropic, linear, and predictable power loss. The excitation and core emulation coils should be connected in series to ensure the coil currents have the same phase. Since 4 = I, the currents drop out of (1). We then define an optimal turns ratio, a, between the core emulation and excitation coils as crop, = NJAc= (1.4-1) (414) (2) Fig. 2 shows finite element simulation results for different values of the turns ratio a, where the vertical axis is the amount of flux that is perpendicular to the intended measurement direction divided by the total amount of flux. The a = a op, line assumes that the future's turns ratio is changed for each permeability, according to (2). The perpendicular flux ratio reaches up to 0.5% because a nonzero gap between the core and emulation coils was assumed for the simulation.