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.

An ac brushless drive in which Hall-effect sensors are used as rotor position sensors is presented in this paper. Three different methods to obtain a high-resolution position estimation from the low-resolution sensors are described and compared through simulation and experimental testing. The proposed control algorithm's most innovative feature is its adaptability to the entire speed range, including startup, when using any of the three estimation algorithms. The control algorithm has been implemented and tested in order to drive a slotless axial-flux permanent-magnet (PM) machine for domestic appliance applications.

High-resolution position and velocity estimation algorithms based on binary Hall-effect sensors for low-cost vector control drives have been the subject of significant research in recent years. While different estimation algorithms have been proposed and analyzed, no contribution so far has dealt with Hall-effect sensor faults and their repercussion on the drive. Here, single and multiple Hall-effect sensor faults are analyzed, and a method to detect, identify, and mitigate such faults is investigated. The method is general, and it ensures proper operation of the drive and is not dependent on the particular estimation algorithm that is used; by way of example, here it will be applied to a vector-tracking observer. Limitations on the performances of the faulty system are discussed, and experimental results are reported to confirm the theoretical analysis.

In this paper, binary Hall-effect sensor faults are investigated in rectangular-current-fed brushless dc (BLDC) drives and a very effective methodology for their detection, identification, and compensation is explored. It is shown that these faults cause erroneous commutation, generally leading to unstable operation. Using a fault detection and identification technique proposed by the authors in a recent paper on low-cost field-oriented drives, it is possible to pinpoint the faulty sensors. In this paper, it is demonstrated that the destabilizing effect of these faults on motion-state estimation can be compensated for in any position and speed estimation algorithm, as long as it is properly readapted. To this end, it is shown how to incorporate such fault-compensation in three state-of-the-art estimation algorithms: (1) the zeroth-order algorithm (ZOA); (2) the hybrid observer (HO); and (3) the vector-tracking observer (VTO). Comparative experimental tests are performed and it is verified that stable operation is achieved with three, two, or only a single Hall-effect sensor functioning correctly. These results show that the classical BLDC drive with three Hall-effect sensors has an inherent double redundancy to position-sensor faults. With the proposed method, this property can be exploited in systems that require very high reliability, such as in aerospace and automotive applications. Redundancy can be increased, using more than three Hall-effect sensors; reduced using two sensors; or eliminated using a single sensor, in ultra low-cost applications where redundancy is not a requirement.

In slotted axial-flux permanent-magnet (PM) (AFPM) machines, two possible concentrated-winding arrangements can be used: integral slot and fractional slot. Integral-slot concentrated windings (ISCWs) have a number of slots/pole/phase equal to one and have long been a preferred choice with machine designers due to their simple layout. On the other hand, fractional-slot concentrated windings (FSCWs) have a number of slots/pole/phase less than one and have been the focus of a significant amount of international research in the past decade. This paper presents an original comparison between two 10-kW AFPM machine prototypes with concentrated windings: a 20-pole 60-slot ISCW machine and a 20-pole 24-slot FSCW machine. The design of both machines is described, highlighting the constraints that have been set to allow a fair comparison between them. Static and time-stepped finite-element-analysis results are presented. Comparative no-load and load experimental tests are performed on full-scale prototypes, and the results are reported: Important tradeoffs in key aspects such as peak cogging torque, constant power speed range, and additional losses are demonstrated. Finally, conclusions are made regarding the performances and required tradeoffs for both machines.

This paper presents the implementation and evaluation of a high-resolution position estimation system for sinusoidal, surface phase modulation machines based on Hall-effect sensors and a vector-tracking observer. First, the tuning of the observer is presented and a speed-dependent gain scheduling strategy is proposed. Then various harmonic decoupling strategies are investigated to improve the performance of the observer, particularly at low speeds. Stability analysis is performed leading to the definition of local stability limits, within which the actual position is tracked with bounded estimation error. Both simulation and experimental testing illustrate the performance and limitations of the proposed observer topology and of the drive when this observer is used for state feedback.