Magnetic field analysis of an ironless brushless DC machine

This paper presents an analytical solution for the magnetic field inside an unconventional ironless brushless (ILBL) dc motor design. We discuss the unique characteristics of the design, which adopts an "inside-out" construction with an internal ironless stator. With the intention of obtaining an analytical solution for the magnetic field, we propose a model based on a magnetic pole concept for representing the magnetic circuit. The magnetic field inside the motor, which has no iron for guiding magnetic flux paths, is obtained by solving Laplace's equation for magnetic scalar potential. We present both analytical and numerical solutions for the magnetic field and compare them with measured results of a 20-pole prototype IL design to show the validity of the model. The IL design is simple, easy to manufacture and, as indicated by the results, has an optimum number of magnets, for which its performance becomes maximum.