Permanent magnets versus electromagnets

Analysis is based on three factors: the energy per unit volume of the magnetic field which is to be established; the energy available per unit volume of permanent-magnet material; and the volume of copper and iron required for an electromagnet performing the same task. Short gaps of large area are favourable to electromagnets. In electrical machines designed for a high power/weight ratio, permanent magnets may be smaller than electromagnets in ratings up to 1 hp. Permanent magnets offer other advantages such as absence of heat generation.