Brushless DC Motors Using Unsymmetrical Field Magnetization

The prevalent types of brushless dc motors have evolved from the classical synchronous machine structures, using two-phase, three-phase, or four-phase stator windings and symmetrical N-S field magnetization patterns. The popular two-phase two-pulse brushless motor requires auxiliary means for starting and for ensuring relatively constant torque with rotor position. Three methods are used: 1) skewed stator poles, which introduce unsymmetrical air-gap reluctance and the resultant unsymmetrical air-gap magnetic field, 2) the Wessels design, which uses a rotor pole pattern of north-south-zero (N-S-0) and auxiliary stator poles, and 3) a combination of N-S-0 rotor pattern and 1200 electrical (el) pitch stator coils to produce an unsymmetrical air-gap magnetic field. The electromagnetic torque produced by motors using each of these methods is calculated using the coenergy method.