A Cost Effective Brushless D.C. Motor

The design and performance of a brushless d.c. motor with high power density using ferrite magnets and a two transistor drive is described.

The motor utilizes a single-phase operating principle which permits the use of an electronic drive circuit utilizing only two power switching devices, making it cost effective with a conventional d.c. commutator motor using P.W.M. speed control. High power density is obtained by the use of an external rotor construction and a multi-pole format with a concentrated winding. The starting problems normally associated with single-phase motors is addressed and an experimentally proven solution is described. Significant reduction in starting current is achieved as a no cost bonus and significant improvements in overall operating efficiency can be obtained if the power density is reduced to normal auto industry levels. This motor system is ideal for accessory drives requiring variable speed, long life and high power density at a cost level similar to existing automotive variable speed techniques.