Development and evaluation of an axial gap motor with neodymium bonded magnet

In general, radial gap motors employing neodymium sintered permanent magnet (Nd sintered PM) are used to achieve high torque density in many applications. However, the motors are not suited to a flat, disk-like shape because the dead space, such as the coil ends, occupies most of the motor volume. Therefore, axial gap motors are frequently used for flat shape instead of radial gap motors. Nd sintered PM is a well-known high-performance magnet that has high residual magnetic flux density, but eddy current loss easily occurs in the magnet because of its high conductivity. In axial gap motors for industrial applications, it is difficult to make measure against eddy current loss of Nd sintered PM in terms of cost. Therefore, general axial gap motors employing Nd sintered PM often have the unsatisfactory characteristics such as low efficiency, even though the motor produces high torque. Accordingly, this paper discusses an axial gap motor employing neodymium bonded permanent magnet (Nd bonded PM) for flat shape. Compared with Nd sintered PM, Nd bonded PM has lower residual magnetic flux density, but also lower cost. In addition, Nd bonded PM has extremely low eddy current loss due to its low conductivity. It is found from 3D-FEA and experimental results that the axial gap motor employing Nd bonded PM can achieve higher torque and higher efficiency compared with the radial gap motor employing Nd sintered PM with the same PM weight and a flat shape.