Optimum Magnetic Circuit Configurations for Permanent Magnet Aerospace Generators

Abstract

In the design of high-speed low-power electrical generators for unmanned aircraft and spacecraft, maximization of specific output (power/weight) is of prime importance. Several magnetic circuit configurations (radial-field, axial-field, flux-squeezing, homopolar) have been proposed, and in this paper the relative merits of these configurations are subjected to a quantitative investigation over the speed range 10 000-100 000 rev/min and power range 250 W-10 kW. The advantages of incorporating new high energy-density magnetic materials are described. Part 1 deals with establishing an equivalent circuit for permanent-magnet generators. For each configuration the equivalent circuit parameters are related to the physical dimensions of the generator components and an optimization procedure produces a minimum volume design at discrete output powers and operating speeds. The technique is illustrated by a quantitative comparison of the specific outputs of conventional radial-field generators with samarium cobalt and alnico magnets. In Part II the specific outputs of conventional, flux-squeezing, and claw-rotor magnetic circuit configurations are compared. The flux-squeezing configuration is shown to produce the highest specifilc output for small sizes whereas the conventional configuration is best at large sizes. For all sizes the claw-rotor configuration is significantly inferior. In Part I1l the power densities available from axial-field and flux-switching magnetic circuit configurations are maximized, over the power range 0.25-10 kW and speed range 10 000-100 000 rpm, and compared to the results of Parts I & II. For the axial-field configuration the power density is always less than that of the conventional and flux-squeezing radial-field configurations.