Design of Novel Overhang Structure for Separated Pole-Piece Type Ferrite Magnet Motor

A separated pole-piece type ferrite magnet motor (FMM) is a novel type of motor that can maximize reluctance torque and reduce leakage flux by separating the pole pieces and the web. However, when this type of motor has a short stack length and a large area of pole pieces, the leakage flux in the axial direction has significant influence on the motor's performance. A rotor overhang structure is one of the most crucial design parameters that compensate for the side effects of leakage flux. A conventional overhang structure increases magnetic torque but raises the level of induced voltage. This limits the operation speed range. To overcome such problems, this paper presents a novel multilevel overhang structure. Based on the unique structure of the separated pole-piece type FMM, the overhang length of the pole pieces was set to be shorter than the other parts of the rotor. It was observed that the operating speed range was widened, and more reluctance torque was generated, compared to motors with conventional overhang structures. The proposed overhang structure was optimized by 3-D finite element analysis, and the model was validated by comparing the actual experimental results with its predicted performance.