In this paper, we propose a simple driving method that uses only one three-phase inverter for an actively controlled 2-DOF bearingless motor. This motor has a six-slot/six-pole structure, including two separated windings for the single-phase six-pole motor and the three-phase four-pole suspension. The stator core has a staggered-tooth structure to produce starting torque. The single-phase motor winding is connected between the neutral point of the three-phase suspension winding and the midpoint of the dc power sources. The radial suspension force is calculated analytically and verified using the finite-element method. In addition, we introduce a ripple compensation method for the suspension force. A prototype of a bearingless permanent magnet motor with 2-DOF active suspension was built and tested. The results of experiments demonstrated that the rotor, which was driven at speeds up to 8000 r/min, could be levitated successfully, using only one three-phase inverter.

This study evaluates the performance of a homopolar bearingless motor driven at 100 000 r/min with a mechanical output power of 120 W. The rotor has two salient iron core parts assembled into the shaft in tandem. The stator comprises two units located axially in tandem and an axially magnetized permanent magnet that is sandwiched between these two stator units. This magnet provides three-dimensional magnetic bias flux to polarize the rotor core parts, resulting in single polarity of one rotor core; thus, this type of motor is called homopolar motor. Each stator has two separated three-phase windings for the motor rotation and magnetic suspension, and the bearingless motor examined in this study has active suspension control with four-degrees-of-freedom (radial and tilting motions). The rotating part consists of only iron cores, and hence, it has the advantage of a rigid robust structure. Additionally, no angle detection is needed for the active magnetic suspension control. Therefore, the homopolar bearingless motor is suitable for high-speed applications. The conventional homopolar bearingless motors have an eight-pole rotor, but the proposed structure has a six-pole rotor to reduce the driving electrical frequency. Although this six-pole homopolar bearingless motor has 20% suspension force variation and 4.5° of force error angle, the rotor is driven up to 100 000 r/min without touchdown. This study presents the structure and principle of the proposed homopolar bearingless motor and the experimental results of the load test.

This paper focuses on core loss reduction of a consequent-pole bearingless disk motor using a C-shaped stator core made of soft magnetic composites (SMC). The core loss of the previously built test machine was significantly high, because the C-core has a three-dimensional flux path; therefore, conventional laminated steel cannot be used. In this paper, the solid iron core is replaced by the SMC, and its effectiveness is experimentally demonstrated. The core loss with solid iron, when the bearingless motor is driven at 3000 r/min under no load condition, is 3.8 W, but that with the SMC core is reduced to only 0.6 W. The maximum efficiency with solid iron is 27%, but that with the SMC core is improved to 61%. It is experimentally verified that the SMC core is effective to reduce the core loss and improve the efficiency of the bearingless disk motor with C-shaped stator cores.

This study describes a simple combined winding configuration for a two-degrees-of-freedom (DOF) controlled bearingless motor with a six-slot/six-pole permanent magnet (PM) structure. This bearingless PM motor has only three concentrated coils that are wound alternately on six teeth of the stator and employs only one three-phase inverter for both the two-DOF radial magnetic suspension and single-phase motor drive. Theoretical calculation and finite element method (FEM) analysis demonstrated that the proposed three-coil combined winding configuration generates a four-pole rotating field and six-pole alternating field for generating radial suspension force and torque, respectively. To verify the proposed configuration, we built and tested an outer rotor bearingless PM motor with a diameter of 58 mm and a height of 5 mm. The rotor was successfully levitated and driven up to 7500 rpm. The calculated and measured results were compared for both the proposed combined winding configuration and the previously developed separated winding configuration. The measured motor efficiency of the prototype with the combined winding at 5000 rpm with 20 mNm output is improved by 5% due to the copper loss reduction when compared to that with the separated winding.