Method for measuring and characterizing core loss in a motor

Because of the complex shape and variable flux densities of the toothed cores of motors, characterization of their core losses has been difficult. Manufacturers' material specification sheets usually refer only to Epstein tests, based on sinusoidally varying flux densities in simple geometries. Here, a more accurate and direct method for measuring core loss is presented. The method spins the motor core in a magnetic field and then measures the spin speed and drag torque imposed on the core. This test was used to successfully characterize the core loss in a small dc brushless spindle motor. Among the parameters examined were the turning speed, maximum gap flux, lamination material, lamination thickness, and the effects of stress-relief annealing. Mathematical models of the core loss were constructed for specific core geometries based upon an assumed form for the empirical data. The actual core losses were higher than would have been predicted from conventional Epstein tests.