G.W. Jewell

A general framework for the analysis and design of tubular linear permanent magnet machines

A general framework for the analysis and design of a class of tubular linear permanent magnet machines is described. The open-circuit and armature reaction magnetic field distributions are established analytically in terms of a magnetic vector potential and cylindrical coordinate formulation, and the results are validated extensively by comparison with finite element analyses. The analytical field solutions allow the prediction of the thrust force, the winding emf, and the self- and mutual-winding inductances in closed forms. These facilitate the characterization of tubular machine topologies and provide a basis for comparative studies, design optimization, and machine dynamic modeling. Some practical issues, such as the effects of slotting and fringing, have also been accounted for and validated by measurements.

Hybrid-Excited Flux-Switching Permanent-Magnet Machines With Iron Flux Bridges

Hybrid-excited machines utilize the synergies of permanent magnet (PM) and wound field machines. Flux-switching PM machines have emerged as an attractive machine type due to the fact that the excitation sources are located in the stator allowing a simple robust rotor whilst providing high torque density. This paper proposes topologies of hybrid-excited flux-switching PM machines incorporating iron flux bridges to enhance the effectiveness of the field coil excitation. A simple lumped parameter magnetic circuit model is developed to predict the effect of adjusting various parameters. Two-dimensional finite-element analysis has also been used to predict the machine performance, while a prototype machine has been built and tested to validate the predicted results.

Multiphase Flux-Switching Permanent-Magnet Brushless Machine for Aerospace Application

Flux-switching permanent-magnet (FSPM) brushless machines have attracted considerable interest as a candidate machine technology for applications requiring high torque density and robust rotors. To date, published findings have focused exclusively on single- and three-phase FSPM machines. This paper investigates FSPM brushless machines of higher phase numbers by means of a detailed comparison of the electromagnetic performances of three-, four-, five-, and six-phase variants within the specific context of aerospace machine. Machines having both all poles and alternate poles wound are investigated, with the latter offering scope to reduce mutual coupling between phases so as to achieve improved fault tolerance. The finite-element (FE)-predicted electromagnetic performances in both machines, such as electromotive force waveform, winding inductance, cogging torque, and static torque, are validated by the experiments made on a small-scale five-phase FSPM machine. The nature of the machine specification requires that consideration must be given to mechanical stress in the rotor and the tradeoff with electromagnetic design considerations, notably the degree of rotor saliency which can be incorporated. Therefore, a mechanical FE study of the rotor mechanical stresses of multiphase FSPM machines is also comparatively assessed.

Design and control of a novel spherical permanent magnet actuator with three degrees of freedom

The paper describes the design and control of a new version of a spherical permanent magnet actuator, which is capable of three degrees of freedom and a high specific torque. Based on an analytical magnetic field distribution, the torque vector and back-emf are derived in closed forms. An optimal design procedure is proposed to achieve maximum output torque or maximum acceleration for a given payload. The control of the actuator, whose dynamics are similar to those of robotic manipulators, is facilitated by the establishment of a complete actuation system model and the application of the computed torque control law. The validity of the analysis and design techniques, and the effectiveness of the control strategy, are confirmed by measurements.

Analytical Modeling and Finite-Element Computation of Radial Vibration Force in Fractional-Slot Permanent-Magnet Brushless Machines

An analytical model has been developed for analyzing the radial vibration force in fractional-slot permanent-magnet machines. It is compared extensively by finite-element analyses and used to investigate the influence of the following: 1) stator slotting; 2) tangential field component; 3) radius in the air gap for computation; 4) load condition, etc. The major findings include the following: 1) even on an open circuit, the low harmonic component (e.g., the second for a 10-pole/12-slot machine) of the radial force exists due to the slotting effect, although the amplitude is relatively low, while the slotless analytical model cannot predict this phenomenon; 2) on a load, the slotless analytical model is accurate enough for the radial force analysis since the low-order harmonic component of the radial force is mainly due to the interaction between the magnet field and the armature-reaction field and is largely determined by the combination of the pole and slot numbers; 3) it is much more reliable to calculate the radial force in the middle of the air gap rather than close to the stator bore; and 4) the simple formula accounting only for the radial field component in the middle of the air gap is accurate enough for the radial force calculation.

Analysis and design optimization of an improved axially magnetized tubular permanent-magnet machine

This paper describes the analysis and design optimization of an improved axially magnetized tubular permanent-magnet machine. Compared with a conventional axially magnetized tubular machine, it has a higher specific force capability and requires less permanent-magnet material. The magnetic field distribution is established analytically in the cylindrical coordinate system, and the results are validated by finite-element analyses. The analytical field solution allows the analytical prediction of the thrust force and back-electromotive force (emf) in closed forms, which, in turn, facilitates the characterization of a machine, and provides a basis for design optimization and system dynamic modeling.

Alternate Poles Wound Flux-Switching Permanent-Magnet Brushless AC Machines

Flux-switching permanent-magnet (FSPM) brushless machines have emerged as an attractive machine type by virtue of their high torque densities, simple and robust rotor structure, and the fact that permanent magnets and coils are both located on the stator. Both 2-D and 3-D finite element analyses are employed to compare the performance of a conventional all poles wound (double-layer winding) topology with that of three modular alternate poles wound (single-layer winding) topologies, in terms of output torque, flux-linkage, back EMF, and inductances. It is shown that the FSPM machine can be designed in this way without incurring a significant performance penalty, but that some degree of rotor skewing or a variation in stator and rotor pole combination may be required in order to maintain a sinusoidal back-EMF waveform and reduce the torque ripple. Experimental validation is reported for both conventional all poles wound and alternate poles wound FSPM machine topologies.

Influence of slot and pole number combination on radial force and vibration modes in fractional slot PM brushless machines having single- and double-layer windings

This paper systematically investigates the radial force and vibration modes in fractional slot surface-mounted permanent magnet (PM) brushless machines having different slot and pole numbers, viz. 2 p = Ns ± 2 , 2 p = Ns ±1, q=0.5, respectively, with either single- or double-layer winding. The dominant vibration mode can be determined from the lowest order of radial force harmonic. It shows that for integer slot PM machines, q=1, the dominant radial force is mainly caused by the fundamental magnet field itself, and the dominant vibration mode order is equal to pole number which is usually high. However, in all fractional slot PM machines, the dominant radial force is mainly produced by the interaction between the harmonic in magnet field and the harmonic in armature reaction field, while the noise and vibration can be significantly higher since the dominant vibration mode order can be as low as 1 or 2, which strongly depends on the slot and pole number combinations, as well as the winding topologies, as confirmed by both developed analytical model and finite element (FE) analyses.

Multi-Phase Flux-Switching Permanent Magnet Brushless Machine for Aerospace Application

Flux-switching permanent-magnet (FSPM) brushless machines have attracted considerable interest as a candidate machine technology for applications requiring high torque density and robust rotors. To date, published findings have focused exclusively on single- and three-phase FSPM machines. This paper investigates FSPM brushless machines of higher phase numbers by means of a detailed comparison of the electromagnetic performances of three-, four-, five-, and six-phase variants within the specific context of aerospace machine. Machines having both all poles and alternate poles wound are investigated, with the latter offering scope to reduce mutual coupling between phases so as to achieve improved fault tolerance. The finite-element (FE)-predicted electromagnetic performances in both machines, such as electromotive force waveform, winding inductance, cogging torque, and static torque, are validated by the experiments made on a small-scale five-phase FSPM machine. The nature of the machine specification requires that consideration must be given to mechanical stress in the rotor and the tradeoff with electromagnetic design considerations, notably the degree of rotor saliency which can be incorporated. Therefore, a mechanical FE study of the rotor mechanical stresses of multiphase FSPM machines is also comparatively assessed.

Cogging Torque in Flux-Switching Permanent Magnet Machines

The influence of manufacturing tolerances on the cogging torque waveforms, as well as the back-EMF waveforms, in 3- and 5-phase flux-switching permanent magnet (FSPM) machines having different numbers of stator/rotor poles is investigated by finite element analyses and experiments. The cause of excessive cogging torque in the prototype FSPM machines has been identified. It is due to modular stator structure which makes it mechanically weak, leading to the intrusion of C-core into the airgap and an uneven inner stator bore. The analyses presented in the paper should be useful to the design and analysis of modular PM machines in general.