The airgap magnetic field distribution and back-emf waveform are analyzed from the 3-D finite element models of the permanent magnet brushless DC (BLDC) motor with air-cored stator. The influence of magnet thickness, length of airgap and outer diameter of the rotor on the performance is investigated. It can be found that waveform of back-emf can be improved to reach flat roof width with 120° electrical degrees by means of increasing the airgap diameter. Analytical results are verified by the experiments of the prototype motor.
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