Self-starting permanent-magnet a.c. motors

Many of the earliest dynamo-electric machines had permanent-magnet excitation. Gradually, however, this form of excitation was replaced by excitation from electromagnets, which seemed to offer the advantage of stronger magnetic fields and so more power from a total volume of material. Since then, many signs have appeared that indicate a reversal of this trend. Small permanent-magnet direct-current motors are commonplace in automobiles. Perhaps most interestingly, new forms of alternating-current machine using permanent magnets have begun to appear, and these show some interesting advantages compared with alternative drive systems. The self-starting permanent-magnet synchronous motor is essentially a new form of motor with a wide variety of applications