Design criteria for brushless DC motors with hollow rotor of samarium cobalt for applications above 25000 RPM in vacuum

Brushless permanent magnet DC motors have inherent low loss and low input as energy is also provided by the permanent magnet. The design criteria are discussed with the respect to the selection of magnetic materials, large airgaps, bearings, etc., highlighting reduction of mechanical stresses, noise and thermal barriers. The importance of electromagnetic design in relation to electronic controller and Hall sensor commutation are also discussed. Construction, design data and performance for two 2-pole motors are given. It is shown that a large radial airgap over 0.5 mm, which is not at all desirable in induction motors of similar sizes, can be easily achieved with samarium cobalt (SmCo) and neodymium iron boron (NdFeB) magnets