THE MAXIMUM TORQUE OF SYNCHRONOUS AXIAL PERMANENT MAGNETIC COUPLINGS

Axial permanent magnetic couplings are composed of two discs with a small air-gap in-between. Each disc consists of several segments in the shape of slices of cakes. The segments are polarized in axial direction with alternating polarity. In this work the homogeneous magnetization in the segments is replaced by equivalent currents on the surface of the segments (Amperean model). In a simplifled model we consider only radial currents whereas azimuthal currents along the perimeter of the discs are discarded. This corresponds to the arrangement where one of the discs has much larger diameter than the other disc. Compared to the case of two equal discs it leads to a notable error in the magnetic fleld near the perimeter, yet it has only a small efiect on the torque, especially for the case of optimum couplings. This trick allows for summing up the flelds of all segments in closed form. A concise double integral over the radial magnetic fleld component describes the torque. An investigation of this integral reveals many properties of axial magnetic couplings: A diagram is introduced and areas in this diagram are identifled where the torque shows overshoot, rectangular pulse shape or sinusoidal dependence versus twist angle between both discs. The diagram contains also a curve for maximum torque and one point on this curve is of considerable economic signiflcance: It denotes the global maximum of torque over magnet mass.