Approach to decrease rotor iron losses of high speed or torque BLDC motors

Abstract

Many industrial applications refer to BLDC motors applied at their thermal limits. Two cases related to the motor configuration lead to high possible iron losses. Referring to the motor synchronous frequency: the first case corresponds to a relative low speed, but a high number of pole pairs (torque motors used in direct drive applications); the second case to a low number of pole pairs, but a high speed (mass storage applications, for example). For these applications, the iron losses must be minimized. Because the excitation flux and the slot geometry introduce high harmonic frequencies, rotor losses are a key issue of the investigation. In this paper, the authors present an original approach to limit the rotor iron losses, which is the main interest of this paper. This approach consists in working in the saturated part of the B-H curve of the rotor material. Different computations using a lumped scheme on one band and 2D FEM simulations, on the other hand, confirm the proposed method of rotor iron loss limitation. The computations of iron losses are validated by measurements obtained with a high speed motor on one hand and a torque motor on the other hand. Finally, using their approach, the authors propose improvements of the motors structure.