BLDC motor stator and rotor iron losses and thermal behavior based on lumped schemes and 3-D FEM analysis

Many industrial applications refer to brushless DC (BLDC) motors applied at their thermal limits. Two cases related to the motor configuration lead to high possible iron losses and, consequently, to the necessary knowledge of the thermal behavior of the motor. 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). The iron losses are a key issue since the considered frequencies are over the usual range of the known field of magnetic material vendor characteristics. Because the excitation flux and the slot geometry introduce high harmonic frequencies, rotor losses are a key issue of the investigation. This paper considers a torque motor with a high number of slots and high number of poles. The authors present two different methods to model the iron losses and the thermal behavior. The first method uses two lumped schemes: the first scheme permits us to determine the flux distribution as well as the stator, magnet, and rotor yoke iron losses. The temperature gradient distribution is computed using an original lumped thermal scheme of the motor. The second method uses finite-element method (FEM) analysis with a two-dimensional model for the magnetic behavior and a three-dimensional (3-D) model for the thermal behavior. A testing bench for a BLDC torque motor has permitted us to measure the iron losses at no-load generating operation and the temperature using thermal probes and infrared camera. Then, simulations and measurements results are compared to characterize the accuracy of the two methods. The two main interests of the presented work consist of: (1) the computation of the whole iron losses in the stator as well as in the rotor yoke and the magnets; and (2) the comparison of the thermal behaviors obtained with a thermal lumped scheme and 3-D FEM simulations.