Torque-ripple minimization in modular permanent-magnet brushless machines

This paper discusses the suitability of four-phase, five-phase, and six-phase modular machines, for use in applications where servo characteristics and fault tolerance are key requirements. It is shown that an optimum slot number and pole number combination exists, for which excellent servo characteristics could be achieved, under healthy operating conditions, with minimum effects on the power density of the machine. To eliminate torque ripple due to residual cogging and various fault conditions, the paper describes a novel optimal torque control strategy for the modular permanent-magnet machines operating in both constant torque and constant power modes. The proposed control strategy enables ripple-free torque operation to be achieved, while minimizing the copper loss under voltage and current constraints. The utility of the proposed strategy is demonstrated by computer simulations on a four-phase fault-tolerant drive system

Optimal torque control of fault-tolerant permanent magnet brushloss machines

Describes a novel optimal torque control strategy for fault-tolerant permanent magnet brushless ac drives operating in both constant torque and constant power modes. The proposed control strategy enables ripple-free torque operation to be achieved while minimizing the copper loss under voltage and current constraints. The utility of the proposed strategy is demonstrated by computer simulations on a five-phase fault-tolerant drive system

Magnetic gear dynamics for servo control

This paper considers the analysis and application of magnetic gearbox and magnetic coupling technologies and issues surrounding their use for motion control servo systems. Analysis of a prototype magnetic gear is used as a basis for demonstrating the underlying nonlinear torque transfer characteristic, nonlinear damping, and `pole-slipping' when subject to over-torque (overload) conditions. It is also shown how `pole-slipping' results in consequential loss of control. A theoretical investigation into the suppression of mechanical torsional resonances in transmission systems encompassing these highly-compliant magnetically-coupled components is included, along with experimental results, from a demonstrator drive-train. The automatic detection of pole-slipping, and recovery scenarios, is also presented

Effect of optimal torque control on rotor loss of fault-tolerant permanent-magnet brushless machines

A faulted phase in a fault-tolerant permanent-magnet brushless machine can result in significant torque ripple. However, this can be minimized by using an appropriate optimal torque control strategy. Inevitably, however, this results in significant time harmonics in the phase current waveforms, which when combined with inherently large space harmonics, can result in a significant eddy-current loss in the permanent magnets on the rotor. This paper describes the optimal torque control strategy which has been adopted, and discusses its effect on the eddy-current loss in the permanent magnets of four-, five-, and six-phase fault-tolerant machines

Powder alignment system for anisotropic bonded NdFeB Halbach cylinders

A Halbach cylinder, fabricated from pre-magnetized sintered NdFeB magnet segments, is proposed for the powder aligning system during the compression or injection moulding of anisotropic bonded Halbach oriented NdFeB ring magnets. The influence of leading design parameters of the powder aligning system, viz. the number of magnet segments per pole, their axial length and radial thickness, and their clearance from the mould, is investigated by finite element analysis, and validated experimentall

Analysis of anisotropic bonded NdFeB Halbach cylinders accounting for partial powder alignment

An analytical technique is developed for predicting the performance of a bonded Halbach oriented anisotropic magnet, with due account of partial alignment of the NdFeB powder during injection molding. The predicted performance of a 12-pole injection molded, Halbach oriented magnet is compared with measuremen

Rotor eddy-current loss in permanent magnet brushless machines

This paper presents an analysis of the rotor eddy-current loss in modular and conventional topologies of permanent magnet brushless machine. The loss is evaluated both analytically and by time-stepped finite-element analysis, and it is shown that it can be significant in both machine topologies. It is also shown that the loss can be reduced significantly by segmenting the magnets

"Pseudo" Direct Drive Electrical Machines With Alternative Winding Configurations

The integration of a magnetic gear and a brushless permanent magnet machine to realize a “pseudo” direct drive (PDD) with alternative winding configurations is investigated. It is shown topologies which facilitate the winding process and achieve a more robust stator construction exist. Comparisons with conventional PDD topologies which are essentially equipped with concentric windings have shown that similar efficiencies can be achieved, albeit at the expense of reduced torque densities. Furthermore, analysis of the magnetic forces exhibited by a pole piece has shown that both the average and dynamic forces are significantly affected by the rated load condition, exacerbating the radial forces

Rotor Eddy-Current loss in permanent-magnet brushless AC machines

This paper analyzes rotor eddy-current loss in permanent-magnet brushless ac machines. It is shown that analytical or finite-element techniques published in literature for predicting rotor eddy-current loss using space harmonic based approaches may not yield correct results in each magnet segment when one magnet-pole is circumferentially segmented into more than two pieces. It is also shown that the eddy-current loss in each equally segmented piece may differ by a large margin, which implies that the temperature distribution in the magnets will be uneven and the risk of demagnetization has to be carefully assessed. The theoretical derivation is validated by time-stepped transient finite-element analysis

Impulse magnetized magnetic screws

This paper is an investigation in the performance of two types of magnetic screws (MSs). The first is the magnet to magnet and consists of permanent magnet (PM) nut and PM screw and the second is a magnet to reluctance and consists of PM nut and a double-start reluctance screw. It is argued that for cost sensitive and long stoke applications, a magnet-to-reluctance MS may be the preferred option. It is also shown that capacitor discharge magnetization techniques can be employed for imprinting helical magnetization distributions on cylindrical PMs, significantly reducing the complexity and facilitating the practical realization of MS systems