Operational features (Cogging, field weakening)

Field weakening which was used to extend the speed range of motor drives was discussed. It was found that in Permanent magnet (PM) motors the excitation current was fixed and a reduction in the air-gap field and back emf could be obtained by adjusting the angle in space between the armature and field mmf patterns. The machines with large saliency ratios had the advantage of low permanent magnet flux, reducing the induced emf at high speed and increasing the inverter protection. It was observed that to facilitate the presence of torque there must be both permanence variations due to the stator slotting and harmonics in the space distribution of the excitation.