Comparative study of three different radial flux ironless bldc motors

Three different radial flux ironless brushless dc (BLdc) motor types (motors without inner back-iron of the rotor, with inner back-iron of the rotor, and with dual-rotor) are investigated and compared in this paper. The magnetic field distribution, back electromagnetic force (EMF), axial end effect, and torque characteristics are investigated. First, the relationship between the magnetic field distribution and the motor size and pole pairs is investigated. It shows that the two parameters are not independent of each other and the magnetic field distribution is mainly related with the arc length of the permanent magnets. Second, the radial magnetic flux density in the coil regions of the three ironless BLdc motors is calculated and an evaluation factor is proposed to show the quality of the magnetic field distribution. Third, the influence of the winding configuration and the combination of slot and pole number on EMF is analyzed, and the corresponding evaluation factors are calculated and tabulated. Then, the axial end effect of the three ironless BLdc motors considering the fringe effect is investigated and compared. Next, the torque characteristics at full load and rated speed are studied for the three types of motors. Finally, the ironless BLdc motor without back-iron of the rotor is manufactured and tested. The experimental results are in good agreement with the finite-element method.