Asymmetrical Four-Phase Combined Winding Arrangement for Bearingless PM Motors

Conventional bearingless motors typically comprise two sets of independent windings in the stator to produce torque and suspension force, denoted by a separated winding in this study. However, several approaches to combine suspension and rotation windings into one set of windings, known as combined winding, have been investigated for torque density improvement and for increasing the instantaneous suspension force production at start-up. This study proposes a method to convert a typical three-phase brushless dc motor into a four-phase bearingless motor with combined winding, based on the magnetomotive force (MMF) distribution. As an example, a 12-slot/10-pole surface-mounted permanent magnet motor was investigated. The calculation results demonstrate that the suspension force cannot be generated at certain rotational angles when the four phases are symmetrically located. This study introduces torque and force current factors based on the MMF distribution to select several asymmetrical four-phase winding arrangements. The radial force trajectory calculated by the finite element method was evaluated to determine the final model. To verify the proposed method, the prototype was built and tested, and the test results showed that the rotor was levitated successfully and could be driven up to 5000 r/min.