Design and Comparison of Permanent Magnet Machine Topologies for Aerospace Actuators

In aerospace actuator applications electric machines are required to produce large static and dynamic torque while being constrained with stringent weight and volume restrictions. Among various types of electrical machines, Permanent Magnet (PM) Machines with strong rare earth magnets offer the highest torque and power density with low rotor inertia. This paper presents the design and study of three high-density PM machine topologies from the perspective of aerospace actuator applications. The machine topologies are compared with three key metrics for the same current density: torque per unit mass, torque per unit squared rotational inertia, and torque per unit volume. Such thorough comparison of these three candidates PM machine topologies for aerospace actuator application is not reported in available publications. Key requirements of specific actuation applications, such as torque density and active length, are taken into account for the design. An inner-rotor radial field PM machine topology is chosen as the baseline candidate. An outer rotor radial field PM machine is studied as the second topology as it can offer higher torque density in the same volume. Finally, the axial field machine topology is studied for their suitability for applications with low profile requirements. For design and comparison of the machine topologies, at first the electrical and magnetic loading are determined for each motor topology for a given actuator profile to maximize torque. The winding and magnetic circuit parameters are then calculated to achieve the desired electrical and magnetic loading. Analytical design software is used to validate the design calculations. A finite element analysis tool is used to accurately determine the performance of each designed motor. Helpful design guidelines and a comparison of the key parameters are presented. The results show an improvement in the torque per unit mass and torque per unit volume metrics for outer-rotor radial and axial field electric machines over inner-rotor machine.