Electric Power Principles, 2nd Edition

16Permanent Magnets in Electric Machines

Of all changes in materials technology over the last few decades, advances in permanent magnets have had arguably the largest impact on electric machines. Permanent magnets are often suitable as replacements for the field windings in machines, that is, they can produce the fundamental interaction field. This does four things:

1. Since the permanent magnet is lossless it eliminates the energy required for excitation, usually improving the efficiency of the machine.
2. Since eliminating the excitation loss reduces the heat load it is often possible to make permanent magnet machines more compact.
3. Because the excitation is lossless, it is possible to build machines with high pole count. That reduces the amount of back iron required and so can be used to make the machine substantially lighter.
4. Modern permanent magnets have very large coercive force densities which permit larger air gaps than conventional field windings; this in turn permits design flexibility, which can result in even better electric machines.

These advantages come not without cost. Permanent magnet materials have special characteristics which must be taken into account in machine design. The highest performance permanent magnets are brittle, some have chemical sensitivities, all are sensitive to high temperatures, most have sensitivity to demagnetizing fields, and proper machine design requires understanding the materials well. This chapter will not make the reader into a seasoned ...