Introduction

Permanent magnets have been employed in a wide range of electrical apparatus for a great many years, and it is well beyond the scope of this text to discuss their design for all current applications. However, the dramatic improvements in material properties that accompanied the evolution of rare earth magnets have focussed interest on certain electromechanical and electronic devices, in which these materials may be applied to advantage. With this in mind, we discuss a wide variety of applications, which reflect the scope of new design activity today. This includes devices whose extremely high production quantities continue to make low cost ceramic ferrite the dominant material in today's market, and high added value products, which exhibit significant performance benefits using high energy rare earth magnets.

The most important application for permanent magnet materials is in direct current (d.c.) rotating electric motors. Ceramic ferrites have long been used in these machines to provide a steady magnetic field from their stators, but more recently rare earth magnets have been employed to particular advantage to promote the evolution of electronically commutated brushless d.c. motors, in which the permanent magnet assembly usually becomes the rotating component. The high energy of rare earth magnets is often used to produce a greater air gap flux density in a d.c. motor, which yields a corresponding improvement in the motor's output torque. The magnet's high coercivity is also attractive, because this improves its resistance to demagnetization from the motor's own armature winding.