Impact of the embedding of magnets on a sensorless drive method

Abstract

Various sensorless control methods such as "Direct Flux Control" (DFC) make use of machine anisotropies to detect the rotor position. Such anisotropies lead to rotor-position-dependent phase inductances. It is known in principle that the ratio between Ldd and Lqq in a machine can be influenced, for example, by embedding the permanent magnets or by designing the magnetic circuit in a way that the soft magnetic material is driven into the strongly non-linear saturation region of the BH curve. This paper is intended to quantify these effects and their impact on the DFC signals. After a brief explanation of the physical effects which lead to magnetic anisotropies, the impact of embedding the magnets on the phase inductances and their influence on the DFC signals is discussed for a selected motor. The motor specific quantities and the DFC signals are calculated via finite element calculations (FEM). Concerning the soft magnetic materials nonlinear and hysteresis effects are taken into account. The results are reinforced by measurements and transferred to similar motor designs with different slot-pole combinations.