Accurate modeling of rotational core losses in ac electrical machines is necessary; these losses occur at the roots of teeth and at the back of the slots in stator yokes of ac machines. In order to achieve a satisfactory model, the experimental measured data have to be as accurate as possible. The purpose of this paper is to propose a novel design of a magnetic circuit based on the electromagnetic Halbach array, which generates a uniform flux density inside the specimen. Particular attention is paid to the test shape to allow accurate measurements.