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Axial Flux Permanent Magnet (AFPM) machines with ironless rotors are an attractive and recently studied solution in low-speed applications, due to their potentially high power/weight ratio, high aspect ratio, and high efficiency. Nevertheless, these machines are prone to be affected by manufacturing tolerance during its fabrication process and consequently, the magnets may move freely inside the rotor structure. This work presents a statistical analysis of manufacturing tolerances of an AFPM machine with an ironless rotor, considering several magnet fault types. A computationally efficient superposition method is developed and implemented to obtain both the cogging torque and rated torque considering several tolerance combinations with acceptable accuracy. The results obtained from a statistical analysis of 10,000 designs of a two-stator one rotor tooth coil winding AFPM (TCW-AFPM) machine allowed us to identify the parameters with the most impact on relevant performance indicators and disclosed a substantial increase in cogging and ripple torque when unavoidable combined tolerances are present. [ABSTRACT FROM AUTHOR]

Featured Application: The results of this work can be included in the design stage of modular permanent magnet synchronous machines for fault-tolerant applications. Addressing stator-rotor misalignment, usually called eccentricity, is critical in permanent magnet (PM) machines since significantly high radial forces can be developed on the bearings, which can trigger a major fault and compromise the structural integrity of the machine. In this regard, this paper aims to provide insight into the unaddressed identification and analysis of the impact of eccentric tolerances on relevant performance indices of permanent magnet synchronous machines (PMSMs) with modular stator core. Static and dynamic eccentricity are assessed for different slot/pole combinations through the finite element method (FEM), and the results are compared with those of PMSMs with a conventional stator core. The unbalanced magnetic forces (UMF), cogging torque, back-emf, and mean torque variations are described and related to the eccentricity magnitude and classification. The main findings indicate that severe radial forces and significant additional cogging torque harmonics are generated because of eccentricity. Additionally, it is found that the main differences between modular PMSMs and conventional PMSMs rely on the value of slots per pole per phase. [ABSTRACT FROM AUTHOR]

This paper presents a direct power control strategy for a variable speed axial flux permanent magnet synchronous generator. The method consists on calculating the instantaneous power by measuring the machine currents and estimating the back electromotive force (back-emf). The real-time back-emf estimation is based on tracking the no-load voltage through off-line harmonic analysis, and the mechanical position and speed, obtained with an incremental encoder. A 3 kW laboratory prototype has been built to verify the performance of the proposed strategy. Experimental results are shown and discussed. [ABSTRACT FROM AUTHOR]

This paper presents a methodology for the analytical computation of the slot leakage inductance of tooth-coil winding permanent magnet synchronous machines. Generalized equations for the slot leakage inductance of the whole phase are derived, suitable for several slot-pole combinations, and meant to be used during the machine design stage. For simplicity, self- and mutual-inductance components are formulated using the classical method with detailed permeance determination for both horizontal and vertical distribution of coil sides in the slots, considering a straight-line path for the flux leakage. Several slot-pole combinations were simulated by means of the finite element (FE) method in order to validate the analytical equations in both approaches, FE and analytical, showing adequate concordance. [ABSTRACT FROM AUTHOR]

An introduction is presented in which the editor discusses various articles within the issue on topics including interior permanent magnets, synchronous reluctance machines, and adoption of optimization techniques.

A novel rotor structure for a tooth-coil-winding (concentrated winding) open-slot axial-flux permanent-magnet (PM) machine that reduces the eddy currents and increases the main flux linkage is proposed in this paper. A composite-body-based rotor assembly and a steel-laminated layer inserted on top of the magnets are used to reduce eddy-current losses in the PMs. A prototype of an axial flux generator for a heavy-duty vehicle following this structure is optimized, analyzed and measured. The tooth coil windings produce high-amplitude harmonics in the air gap and the large slot openings reduce the rotor flux. Consequently, a rotor structure that decreases the amount of harmonics travelling through the magnets is required. Steel laminations covering the magnets have positive effects on both phenomena. Two-dimensional and three-dimensional finite-element analysis is used to verify and optimize the geometry of the laminations. Computations are verified by experimental measurements of a 100 kW test machine prototype. [ABSTRACT FROM PUBLISHER]

Armature-reaction-compensated permanent-magnet synchronous generators are a particular type of hybrid excitation synchronous generators, where the main flux is created by the permanent magnets and the field winding is needed only to compensate the armature reaction. This paper studies the feasibility of an armature-reaction-compensated permanent magnet synchronous generator in island operation by comparison with a conventional electrically excited synchronous generator and permanent magnet synchronous generator. The aim of this paper is to demonstrate the performance of an armature-reaction-compensated permanent magnet synchronous generator and its operation principle in island operation. As part of the study, a 55 kW prototype was built and tested. The experimental results of the prototype are presented. [ABSTRACT FROM PUBLISHER]

Traction motor design significantly differs from industrial machine design. The starting point is the load cycle instead of the steady-state rated operation point. The speed of the motor varies from zero to very high speeds. At low speeds, heavy overloading is used for starting, and the field-weakening region also plays an important role. Finding a suitable field-weakening point is one of the important design targets. At the lowest speeds, a high torque output is desired, and all current reserves of the supplying converter unit are used to achieve the torque. In this paper, a 110-kW 2.5-p.u. starting torque and a maximum 2.5-p.u. speed permanent-magnet traction motor will be studied. The field-weakening point is altered by varying the number of winding turns of machine. One design is selected for prototyping. Theoretical results are verified by measurements. [ABSTRACT FROM PUBLISHER]

Hybrid excitation synchronous generators are a special class of electrical machines in which the total rotor current linkage is produced by the simultaneous action of two different excitation sources: permanent-magnet (PM) excitation and field winding excitation. In the island operation, there is a need to compensate the armature reaction to keep the generator terminal voltage constant. The permanent magnet excitation is used to create the no-load excitation to the generator and the rotor winding is used as an armature reaction compensating winding. This paper studies the applicability of an armature-reaction-compensated PM synchronous generator to a marine diesel genset. This paper presents an analytical approach for the electromagnetic design of the proposed topology. Finite-element analysis and experimental measurements are performed to verify the results of the analytical approach. [ABSTRACT FROM PUBLISHER]

Hybrid and full electric technologies are fast emerging in vehicles and mobile working machines, where electric machines and internal combustion engines are used together to power the systems. Permanent magnet (PM) technology plays an important role here despite the high magnet prices. This paper theoretically and empirically studies the design principles of PM synchronous machines (PMSMs) for hybrid applications, where a high starting torque and a wide field weakening range are needed. Several embedded-magnet PMSM magnetic circuit topologies are considered as possible candidates. A 10-kW PMSM prototype was built and tested. Experimental results verify the theoretical considerations well. [ABSTRACT FROM PUBLISHER]

High power machine has become a large market for wind power and ship propulsion electric, among other applications. Since the size of these machines is much larger than conventional industrial ones, optimum design must be considered in order to reduce the material cost and increase profitability. In this paper, a simple analytic optimization algorithm is used to maximize the apparent airgap power transferred under tangential stress constraint. In this approach, close related expressions between the main design variables, operational restrictions, and external dimensions are derived to build the mathematical structure of the optimization process. To improve the torque capacity estimation of the designed machine, a correction procedure, based on the previous result, is used to remove the idealizations considered for the initial design. Close agreement with the finite element analysis results are found with this approach, which is based on analytical method. [ABSTRACT FROM AUTHOR]

In this paper, a negative-saliency permanent-magnet (PM) synchronous machine analysis is presented. This particular saliency feature is achieved by replacing a portion of the magnet material by a soft iron piece over the rotor pole. In this manner, the d-axis inductance is increased, whereas the q-axis inductance is almost not affected, leading to the condition that Ld is higher than Lq (negative saliency) corresponding to the inverse condition of typical PM machines. An expression for the optimum pole configuration is derived. It is shown that, with appropriate control of the stator current based on the machine's saliency, the unfavorable effects of magnet reduction on torque production can be compensated. It is also shown that the machine saliency affects the location of the operating points when it operates under vector control. Finally, the theoretical analysis is validated with experimental results obtained from a prototype axial-flux PM machine that exhibits negative saliency. [ABSTRACT FROM AUTHOR]

We designed and tested a new axial flux permanent-magnet machine (AFPM) with field weakening capability. This paper provides a summary of key design features, optimization, and test results of an AFPM prototype. The proposed field weakening method is based on the control of the d-axis component of the armature reaction flux. The paper focuses on the air-gap flux control capability of the topology. [ABSTRACT FROM AUTHOR]

In designing new topologies for permanent-magnet machines based on rare earth magnets, it is necessary to diminish the undesired cogging torque. This paper presents a 3-D finite-element analysis to evaluate the effect of magnet shape and stator displacement on cogging torque reduction, for axial flux machines. It analyzes the final electromagnetic torque for the proposed configurations. Finally, it presents the resultant cogging torque waveform for a 5.0 kW prototype, based on our optimization techniques. [ABSTRACT FROM AUTHOR]

In this paper a description and operating principles of a consequent-pole permanent-magnet machine are presented. In addition, a sizing analysis, finite-element analysis, and experimental results for a prototype machine are addressed. Due to its particular configuration, this machine allows for a wide range of control of the air-gap flux with minimum field ampere-turn requirements and without brushes or slip rings. Two components of the field flux are produced. One, which is almost constant, is produced by the permanent magnet located on the rotor surface. The other, which is variable, is produced by a field winding positioned circumferentially in the center of the stator. These two flux components converge in the air gap. The excitation level of the machine is manipulated by controlling the dc field current. Three-dimensional finite-element analysis and experimental results demonstrate that ft is possible to vary the flux over a wide range to keep the terminal voltage constant as the speed increases. A 3-kW 1000-3000-r/min eight-pole and 32-Vac generator using this configuration is tested to verify the flux control capability of this structure. [ABSTRACT FROM AUTHOR]

This paper analyzes the impact of manufacturing tolerances on the cogging torque of a 24-slot 28-pole tooth-coil-winding permanent magnet synchronous machine with a modular stator core (TCW-MPMSM). Dimensional tolerances and asymmetries associated with the modular topology are studied by means of finite element simulations in order to identify key parameters that increase the cogging torque above the expected values of a faultless machine. Among five selected dimensional parameters, it was found that angular displacement, radial displacement, and tooth–tip width deviations of the stator segments have the most significant impact on the cogging torque. Considering these three key parameters, a full-range tolerance analysis is carried out by means of a proposed superposition-based approach, evaluating all possible combinations of typical deviation values. It is concluded that the cogging torque increment, generated by tolerances, is relatively independent of the faultless tooth–tip width of the stator segments and the arc-to-pole ratio. Robustness of the TCW-MPMSM, regarding cogging torque generation, depends on both the tightness of the tolerances handled in the manufacturing process and the rated cogging torque: the lower the cogging torque of the ideal machine, the less robust is the machine and, therefore, manufacturing imperfections will be required to be tightened. [ABSTRACT FROM AUTHOR]