Analysis of a Magnetic Screw for High Force Density Linear Electromagnetic Actuators

This paper describes a high force density linear electromagnetic actuator based on the concept of magnetic screw-nut, in which helically disposed, radially magnetised permanent magnets are placed on both the screw and the nut. Magnetic force and torque can be developed between the two parts without direct mechanical contact. Analytical and numerical analysis has been carried to predict the performance of the magnetic screw-nut assembly. It has been shown that a thrust force density in excess of 10 MN/m³ can be achieved for airgap lengths varying from 0.4 mm to 0.8 mm and a lead λ greater than 7 mm. When combined with a naturally cooled permanent magnet brushless machine having a torque density of ~15 kNm/m³, the resulting thrust force density of the combined system, i.e. electrical machine and magnetic screw-nut assembly, is an order of magnitude higher than that of a liquid-cooled tubular permanent magnet brushless machine.