Design of permanent magnet brushless motors for high speed applications

Abstract

High speed motors that develop rotational speeds in excess of 5000 rpm are necessary for centrifugal and screw compressors, blowers, grinding machines, mixers, pumps, machine tools, textile machines, drills, handpieces, aerospace, flywheel energy storages, etc. The actual trend in high speed electromechanical drives technology is to use permanent magnet (PM) brushless motors, solid rotor induction motors or switched reluctance motors. The highest efficiency and highest power density is achieved with PM brushless motors. The following issues, which are essential in electromagnetic, mechanical and thermal design of high speed PM brushless motors, have been discussed: (a) volume and mass: the higher the speed, the higher the power density, (b) power losses and efficiency: special attention must be given to windage and core losses, (c) laminations: cobalt alloy, non-oriented silicon steel or amorphous alloy laminations, (d) stator windings: slotted or slotless windings, (e) stator conductors: small diameter stranded conductors or Litz wires, (f) higher harmonics generated by the solid state converter: parasitical effect as losses, vibration and noise depend on the harmonic content, (g) cooling system: intensive air or liquid cooling system, (h) PM excitation system: if the temperature of the rotor or shaft exceeds 150°C, SmCo PMs must be used, (h) rotor tensile hoop stresses: properly selected rotor diameter, rotor diameter-to-length ratio and rotor retaining sleeve (material and thickness), (i) thermal compatibility of rotor materials, (j) rotor dynamics.