This paper reviews the progress that has been made in the analysis and design of axial-flux permanent-magnet machines over the past decade, with particular attention to aspects such as electromagnetic and thermal modeling, materials, manufacturing, pulsating torque, and extended speed range. Comparisons with other machine types and applications are also reviewed.

Recent Advances in Axial-Flux Permanent-Magnet Machine Technology

A design for a passive magnetic bearing system that can stably levitate a rotor in all directions is described. The bearing system consists of levitation magnets coupled with a Halbach array stabilizer, which induces currents in stabilization coils, in order to overcome the inherent instability of a system composed only of permanent magnets. The levitation magnet system consists of two pairs of annular ring magnets which provide an upward magnetic levitation force to counteract the downward gravitational force of the rotor. The Halbach array stabilizer consists of two stabilization coils shifted in angular position with respect to one another and centered in the vertical direction between two rotating Halbach arrays. Magnetic fields from permanent magnets are calculated using superposition of fields due to patches of magnetization charge at surfaces where the magnetization is discontinuous. Induced currents in the stabilization coils are calculated by computing the time derivative of the magnetic flux through those coils. Magnetic forces on the rotor are computed using a superposition of forces on each patch of magnetization charge. The entire magnetic bearing system, consisting of both the levitation magnets and the Halbach array stabilizer, is stable to both vertical and lateral displacements. Results are compared with a simpler straightened approximation of the Halbach array stabilizer.

Stable Levitation of a Passive Magnetic Bearing

A winding design approach is proposed to create a single motor winding, which is able to produce both radial force and torque This approach can be used to design new bearingless motors, as well as to transform conventional motor designs into bearingless motors by simply modifying the winding end connections The resulting winding has two sets of terminal connections, one for torque and one for suspension The suspension terminals experience no motional-electromotive force when the rotor is centered, which means that the suspension drive can have a low voltage rating and that rotor vibrations can be passively dampened by simply short-circuiting the suspension terminals Bearingless motors that use these so-called dual purpose no voltage windings can be designed to have higher torque density and lower losses associated with the magnetic suspension operation than traditional bearingless motors, which utilize separate torque and suspension windings It will be shown that many popular winding designs, including fractional-slot and concentrated windings, can be realized as dual purpose no-voltage windings The proposed approach applies to traditional $p\pm 1$ pole-pair bearingless motors, as well as the bearingless consequent-pole and ac homopolar motors Fractional-slot motor winding theory is used to derive the new winding requirements and a generalized design procedure; example designs are explored through finite element analysis and experimental results from a hardware prototype of a bearingless ac homopolar motor

Design of Dual Purpose No-Voltage Combined Windings for Bearingless Motors

A novel winding structure without separated suspension winding has been proposed for bearingless motors. The proposed winding structure basically has two parallel paths in three-phase motor windings. One three-phase winding path has a neutral point in general, but another three-phase winding path is connected to a suspension inverter, which injects currents for suspension force generation. Additional suspension windings are not needed in the stator slots, because the two parallel path windings are used for both torque and suspension force generation. In the proposed winding structure, speed induced voltages are cancelled each other so that no speed induced voltage is induced at suspension terminals.

A Novel Parallel Motor Winding Structure for Bearingless Motors

This paper reviews recent developments in power loss analysis applicable, but not limited to, the thermal design of permanent-magnet machines. Accurate and computationally efficient loss prediction is an essential element in the thermal analysis of electrical machines and has become an increasingly important part of the machine design process. The continuous drive toward “more electric” technologies has resulted in a need for a more comprehensive and detailed design approach, where various multiphysics and multidisciplinary effects are accounted for. This “design for application” methodology relies strongly on the advancements and evolution of the existing theoretical and experimental design techniques to satisfy the evermore-demanding machine design requirements. The thermal behavior and efficiency of the power conversion are essential performance measures in the “design for application” approach. An overview of the challenges and limitations regarding power loss analysis in the context of the thermal design of electrical machines is provided in this paper. All of the major loss components associated with the active parts of a machine assembly are discussed.

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Power Loss Analysis in Thermal Design of Permanent-Magnet Machines—A Review

It is now evident that the value of endowing rotating machines with the capability to produce transverse forces in addition to their normal torque-producing function can be extremely high. The bridge configured winding is a cost-saving connection scheme for electrical machines to exploit transverse magnetic forces on the rotors. The winding scheme retains the machine's three-phase terminals such that ordinary motor inverters can be employed for the normal torque-producing function while providing separate terminals for transverse force action. This paper describes the practical implementation of such a winding connection in a conventional electric machine and demonstrates that controllable transverse forces can be produced.

Practical Implementation of the Bridge Configured Winding for Producing Controllable Transverse Forces in Electrical Machines

The maximum force capability of conventional active magnetic bearings is limited since the magnetic flux density in the iron is limited due to saturation. We introduce in this paper a new concept of active magnetic bearings that exploits high bearing force from a number of parallel ironless stator and permanent-magnet rotor discs. We optimized the design of the bearing by solving tens of thousands of finite-element models in nested loops. We commissioned an experimental rig comprising a set of interleaved bearing discs and a 3 degrees-of-freedom mechanical platform to verify its force production capability.

Active Axial-Magnetomotive Force Parallel-Airgap Serial Flux Magnetic Bearings

Conventional magnetic bearings accomplish a specific load capacity, defined as the ratio of maximum sustainable weight to the total self-weight, of up to 35:1. In this paper, the authors introduce a class of passive magnetic bearings that comprise a large number of parallel airgaps and discs and can deliver specific load capacities substantially higher than 35:1. Two-dimensional planar, two-dimensional axi-symmetric, and three-dimensional finite-element analysis (FEA) have been undertaken to predict the force capability of the bearings. An unoptimized prototype passive magnetic bearing is constructed to demonstrate the concept and its force-carrying capacity. The experimental results are then compared with those obtained from the FEA. Further optimization of the bearings is done across the whole design space comprising tens of thousands of models using an automatic mesh generator in conjunction with solving the FE models in nested loops.

W K S Khoo

Papers

Practical Implementation of the Bridge Configured Winding for Producing Controllable Transverse Forces in Electrical Machines

Active Axial-Magnetomotive Force Parallel-Airgap Serial Flux Magnetic Bearings

Passive Contra-Magnetized Parallel-Airgap Serial Flux Magnetic Bearings: