Ways to utilise ferrite permanent magnets (PMs), in a better way has been in focus the last couple of years since the use of neodymium-iron-boron (NdFeB) PMs has been debated. While ferrite PMs offer a low-cost alternative to rare-earth PMs, it is a trade-off for lower energy density. Depending on the type of PM and if the PMs are surface mounted or buried, the risk of demagnetisation during a fault condition can vary significantly between machines. In this study, the demagnetisation risk of two electrically similar generators with identical stators has been studied during several short-circuit faults at different temperatures. The study is simulation-based, and the results show that the generator with the ferrite rotor will suffer from a small but not significant amount of demagnetisation in the worst, three-phase-neutral, short-circuit case at a temperature of 5°C, whereas the NdFeB PMs will suffer from partial demagnetisation if a fault occurs at 120°C. For operational temperatures between 20 and 60°C both generators will sustain a short-circuit event.

A quasi-3D coupled-field method is introduced and applied on a ventilated dry-type transformer to study temperature rise of windings in this study. A simplified 3D model was first established to calculate energy loss of core and velocity distribution in a plane above the lower yoke. Then two accurate 2D models were built up to figure out energy losses in the windings. With a combination of indirect and sequential coupling, energy losses of both windings and core were used as heat source, and velocities for both 2D models were applied as boundary condition for analysing fluid-thermal field. Final results of temperature rise were calculated with temperature rise of two 2D models. In the end, numerical results were compared with experimental data to prove the effectiveness of this method.

Inter-laminar short-circuit faults between the laminations of electrical machines, and other magnetic devices, have been one of the major challenges for the suppliers and customers of electrical steels. Extra power losses caused by inter-laminar faults depend on many factors including the location of the fault points. In this study, fundamental definitions and concepts of inter-laminar short-circuit faults, effect of inter-laminar faults on configuration of the magnetic cores and FEM verification are presented. Experimental works were performed to study the effect of inter-laminar faults with different configurations on the total power loss to distinguish and locate the critical and destructive faults. In the relevant studies, artificial short circuits of different configurations were applied between laminations of packs of four Epstein size laminations of 3% grain oriented silicon steel. Extra power losses caused by the inter-laminar faults were measured and the results were analysed.

This paper deals with the models elaboration for the asymmetric three-phase transformers. The first model uses the principle of the hysteresis integration in two-dimensional finite-element method (FEM) with magnetic field calculations. This method is based on inverse Jiles–Atherton hysteresis model. The second one is a dynamic electromagnetic model (DEM) based on Tellinen hysteresis model. The DEM and FEM models used in ferromagnetic cores have been modified by replacing the anhysteretic curve by the hysteresis loop. A simulation in time domain is carried out and the simulated results are compared with those obtained experimentally to validate the proposed approach. An error calculation is performed to show the accuracy of results obtained by DEM and FEM models.

A new real-time method able to diagnose multiple semiconductor open-circuit (OC) faults in a three-level neutral-point clamped inverter is introduced in this paper. The proposed diagnostic method is based on the evaluation of the output pole voltages and output currents of the inverter. The proposed method allows a real-time detection and localisation of multiple OC faults in all active power switches and clamp-diodes within one modulation period. Experimental results obtained for different operating conditions of the inverter demonstrate the applicability and performance of the proposed diagnostic approach.

The fault-tolerant ability of a multi-phase doubly salient electromagnetic generator (DSEG) makes it suitable for important applications. However, asymmetric phase windings are a drawback of the traditional five-phase DSEG. This study presents the design of a new five-phase brushless DSEG with symmetrical phases and a robust rotor. The proposed stator poles, rotor poles, and their pole arc criteria form a general design basis for the multiphase DSEG with minimum cogging torque. The proposed new winding configuration overcomes the phase asymmetry drawback of the traditional DSEG. An analysis of the characteristic of the no-load and the loading operation of the five-phase DSEG shows that the five-phase DSEG has fewer harmonics than the three-phase DSEG does. Additionally, the five-phase DSEG has high fault-tolerance ability because adjacent phases will take over the activity of the fault phase if there is an open-circuit fault. Simulated and experimental results of machine performance under the same working conditions offer verification of the design principles presented.

This paper describes the optimisation of the eddy current damping, applied in a passive tuned mass damper (TMD). A semi-analytical model based on scalar potential formulation is extended for different permanent magnet (PM) topologies. Optimal design parameters are acquired by particle swarm optimisation. A 3-DoF mechanical model is presented to predict the quality reduction factor of the TMD. Measurements are performed for validating the semi-analytical model. Furthermore, an experiment is carried out to test a TMD with two optimal PM topologies for sinusoidal excitations.

In this paper, a complex vector model is provided for an integrated three-phase shell-type rotating transformer. The equivalent magnetic circuit is used to obtain the transformer parameters. Furthermore, using vector representation of the dq transformation, the voltage equations of the rotating transformer in double-reference synchronous frame are obtained. Thanks to the developed model, the transformer design parameters are selected such that the voltage equations in double-reference frames are decoupled. For the first time, the machine complex vector representation is exploited to design a symmetrical rotating three-phase transformer. For this purpose, two separated equivalent circuits in double-reference frames or two decoupled sequences networks are developed. The decoupled circuits simplify the analysis of the rotating transformer in the unbalanced conditions as well as balanced circuits. Being symmetrical is an important issue for the rotating transformer connected to the rotor winding of a doubly-fed induction generator. Moreover, the steady-state performance and sequence analysis of the rotating transformer are discussed. Finally, the validity of the obtained model and accuracy of the presented analysis are verified by finite element analysis and experimental tests.

Scaling laws are used when the size of a certain machine design with known performance needs to be adjusted for a new application with known requirements, or when a machine design is geometrically scaled and one needs to determine its performance. Scaling laws derived in this study allow one to quickly and accurately recalculate parameters of a geometrically scaled permanent magnet machine. They basically consist of two separate important scaling procedures: axial scaling and radial scaling. The third and inevitable scaling procedure is rewinding, which is used to adjust the winding for a required voltage level. Exact but simple analytical equations for the various parameters (torque, power, losses, mass, resistance, inductance, efficiency etc.) of the machine are derived using three independent scaling factors, one for each scaling procedure. Special attention is given to the inclusion of end-winding influence and three-dimensional permanent magnet loss effects. Algorithms for fast determination of winding parameters for a given voltage and fast determination of scaling factors for scaling based on the torque requirement with stack length limitation are presented. All derived scaling equations are numerically validated using two state-of-the-art motor design software packages with automated extraction of parameters based on finite-element calculations.

Modular linear doubly salient permanent magnet motors are well adapted to linear propulsion systems because of their distinct characteristics, such as high efficiency and power density, reduced maintenance and initial cost, low noise and permanent magnet (PM) leakage flux, and fault tolerance capability. However, such motors suffer from high cogging thrust. In this study, various techniques based on previously proposed methods for PM machines are applied on the studied motor and evaluated by using non-linear three-dimensional time-stepping finite element analysis; three novel, optimised techniques are then presented. The techniques presented are based on the minimisation of the variation in air-gap reluctance relative to the displacement. The PM volume and average air-gap length are kept constant in all optimisations. Since the average thrust and thrust ripples under load condition may be affected by cogging reduction techniques, these values are presented for all given techniques. The results show that the proposed techniques not only maintained the average thrust under various loads but also significantly suppressed the thrust ripples for different average thrusts and cogging forces.

This paper investigated the influence of 2, 6-di-tert-butyl-p-cresol (DBPC) on the long-term ageing characteristics of oil–paper insulation and the deposition and migration of copper sulphide induced by the reaction of copper and dibenzyl disulphide (DBDS) in oil-immersed transformers. A thermal ageing experiment at 130°C were designed and conducted by adding different concentrations of DBPC to the naphthenic mineral oil and after the experiments a set of characteristic parameters were tested. The result indicates that the excessive concentration (≥0.6%) of DBPC will slightly increase the ageing of insulating oil and increase the carboxyl group in the mineral oil, affecting the reaction of DBDS and copper, and the insulating paper wrapped on the copper strip was also aged seriously accordingly. Meanwhile, the deposition of copper sulphide on the insulating paper is found to be slightly increased with the increase of the concentration of DBPC. When insulating oil contains DBPC and DBDS, due to migration of DBDS–Cu and DBPC–Cu in oil, the copper ions in the oil would be fluctuating at a certain period of time. Although the addition of a small concentration of DBPC can improve the antioxygenic property of the insulting oil to some extent, the transformers that contain excessive concentration of DBPC may be more harmful than beneficial.

In many applications, interior permanent magnet synchronous machines (IPMSMs) with fractional slot concentrated windings (FSCWs) are considered promising candidates in terms of higher power density and efficiency. In addition, employing a multiphase stator winding improves the drive train availability and increases reliability. This study investigates the effect of applying stator shifting to five-phase FSCW winding IPMSMs to suppress the effect of the slot harmonics by doubling the number of slots. In this case, the winding coil pitch will be two, which stands as a compromise between single-tooth and distributed winding topologies. This highly improves the air gap flux distribution, significantly reduces both rotor core and magnet eddy current losses, and increases saliency ratio and reluctance torque component. Moreover, an improved performance under fault conditions, in terms of lower torque ripple, and core and magnet losses, adds to the main advantages of this technique. Various slot/pole combinations suitable for five-phase machines are investigated. A full simulation case study based on two-dimensional finite element analysis is applied to the 20-slot/18-pole stator with single-tooth winding under both healthy and open-circuit phase fault cases.