Magnetics Design of Brush and Brushless DC Motors
Through Lumped Reluctance and FEM Simulation Programs

When designing a permanent magnet dc motor, the magnetics design engineer usually faces a myriad of design conditions that are of an interactive nature. He must determine the flux values from the main field (magnets) as well as from the winding. Knowledge of the resultant magnetic field and the armature reaction is also required. He must also keep a continuous check on the iron losses in the different sections of the magnetic circuit. These losses must be established in concert with the copper winding configuration and fill. The outcome of these optimization activities is a magnetic design that integrates both electrical and mechanical performance. This design would define the geometry of all the parts of the magnetic circuit and also the materials involved.

A number of simple lumped reluctance simulation programs that can run on a personal computer have been developed. These programs require a very short computer time and eliminate many of the typical variations on configuration. They can achieve sufficient accuracy for the majority of motor design application. They can be used in place of the slower but more accurate finite element mesh (FEM) simulation programs.

This paper will simulate a set of brush and brushless dc motor designs to demonstrate the development of a "high tech" (rare earth magnet) motor. Some basic formulae used in these programs will also be presented. The intent is to demonstrate that PM motors can be designed with simpler program techniques, as long as the user understands that the resulting boundary conditions will impact simulation accuracy.

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