Eddy-Current Losses in Laminated and Solid Steel Stator Back Iron in a Small Rotary Brushless Permanent-Magnet Actuator

The ever-increasing necessity to miniaturize smooth torque rotary actuators has placed severe constraints on the current slotless permanent-magnet technology. These constraints are mainly the consequence of the relatively large effective magnetic airgap which limits the achievable magnetic loading and, therefore, the torque density of slotless permanent-magnet rotary actuators. Further, the miniaturization has to be achieved at ever-decreasing costs and, therefore, it would be advantageous to implement solid back irons. However this, even at reduced magnetic loading, still results in considerable eddy-current losses. Therefore, a careful selection of the various actuator dimensions and most appropriate soft magnetic laminated or solid material is a prerequisite for applications that require smooth speed and torque characteristic. This paper will identify the influence of various parameters of the slotless permanent-magnet actuator on the eddy-current loss in the back iron using both 2-D and 3-D finite-element analysis.