Design and analysis of high-speed brushless permanent magnet motors

The paper reports on the design of a 20000 rpm, 3-phase brushless permanent magnet DC motor for use in a friction welding unit, in which studs up to 3 mm diameter are welded by coordinating the rotational speed of the motor with the force applied by a linear permanent magnet servo-actuator. The motor consists of a stator having 3 teeth, which carry nonoverlapping windings, and a 2-pole diametrically magnetised sintered NdFeB magnet rotor. The air gap flux density distribution is essentially sinusoidal. The advantages and disadvantages of such a motor topology for this and other high speed applications are discussed, and an optimal air gap diameter is derived. The effect of the stator tooth tip geometry on the waveform of the induced EMF is investigated by finite element analysis, and validated by measurements, whilst the merits of laminated silicon iron and soft magnetic composite materials for the stator core are considered