Rafal Wrobel

Design Considerations of a Direct Drive Brushless Machine With Concentrated Windings

This paper discusses the performance attributes of the open-slot, modular-wound, external-rotor, topology of electrical machine. Combinations of pole and slot numbers are presented for which the winding factor is maximal and torque ripple is minimal. An optimization of the magnetic circuit design of six promising pole-slot configurations is undertaken using a parametric finite element model (FEM) combined with a genetic algorithm (GA). These designs are benchmarked against a conventional 1.5 slots/pole external-rotor brushless dc machine. These candidate electrical machine versions are characterized by having the same external-rotor diameter, total slot area available for the winding and by equal volumes of permanent magnet (PM). Based on the analysis, the most promising motor structure was selected and a prototype wheel-hub motor has been built for application in a small electrical vehicle. Test data from the prototype is used to validate the findings of the initial analyses and practically demonstrate the attributes of the topology.

Optimization of permanent magnet shape for minimum cogging torque using a genetic algorithm

The paper presents an approach to minimization of the cogging torque in permanent magnet (PM) machines using surface-mounted magnets with discrete skew angle. For the purpose of determining the proper arrangement of PM-pole slices, an optimization procedure based on a genetic algorithm is applied. The torque and objective function are determined from a simplified model for torque calculation only partially supported by three-dimensional (3-D) field solution. The results are validated against the 3-D finite-element model as well as experimental data obtained from a prototype machine. A new outer-rotor brushless dc motor motor for an electric fan is considered as a sample model.

Contribution of End-Winding Proximity Losses to Temperature Variation in Electromagnetic Devices

This paper presents an investigation into proximity losses in end-windings informed from 3-D finite-element analysis of an ac power inductor. The proximity effect in winding conductors is a result of circulating ac currents caused by magnetic fields generated by nearby conductors. The calculated results confirm that the effects within the end-winding while significant are lower compared to those with the active length of the conductors. The common approach of predicting proximity losses using 2-D field analyses accounts for active length of the conductors only, and therefore, an end-winding correction factor is needed to obtain a more accurate estimate of loss. The theoretical prediction of losses within the inductor has been validated experimentally on a prototype inductor. A simple method to account for the interdependence of ac loss and temperature is presented and is shown to differ significantly from the well-known dc variation of resistance with temperature.

Investigation of Proximity Losses in a High Speed Brushless Permanent Magnet Motor

This paper presents a finite element investigation into the proximity losses in brushless AC permanent magnet motors used in hybrid/electric vehicle applications. The proximity effect in winding conductors is as a result of eddy-currents caused by magnetic fields generated by nearby conductors. This paper considers the influence of the conductor shape and disposition on the losses for a given stator lamination. Several structures of the winding are analysed and compared in respect to the loss and AC resistance. The analysis shows that the proximity losses can be significantly reduced through an appropriate choice of conductor shape and winding technique. The calculated results have been validated experimentally on the machine prototype for three different winding arrangements

Estimation of Equivalent Thermal Parameters of Impregnated Electrical Windings

It is common practice to represent a composite electrical winding as an equivalent lumped anisotropic material as this greatly simplifies a thermal model and reduces computation times. Existing techniques for estimating the bulk thermal properties of such composite materials use either analytical, numerical, or experimental approaches; however, these methods exhibit a number of drawbacks and limitations regarding their applicability. In this paper, a numerical thermal conductivity and analytical specific heat capacity estimation technique is proposed. The method is validated experimentally against three winding samples with differing configuration. A procedure is presented which enables bulk thermal properties to be estimated with a minimal need for experimental measurement, thereby accelerating the thermal modeling process. The proposed procedure is illustrated by the modeling of three coil exemplars with differing windings. Experimental thermal transients obtained by dc test of the coils show close agreement with a lumped-parameter thermal model utilizing estimated material data.

A General Cuboidal Element for Three-Dimensional Thermal Modelling

This paper presents a lumped parameter approach for three-dimensional (3D) thermal modelling based upon the use of general 3D elements which are formulated to accurately cater for internal heat generation. Commonly used thermal networks tend not to account for an internal heat generation that is essential for accurate temperature predictions. In contrast, the general element proposed in this paper includes the internal heat generation as well as a material thermal anisotropy. To validate the technique, thermal models of an inductor using the equivalent circuit method based around a general cuboidal element and a full 3D finite element analysis was constructed and analyzed. The calculated results from the cuboidal element method show good agreement with the FEM predictions.

Thermal Modeling of a Segmented Stator Winding Design

This paper presents a thermal analysis of a segmented stator winding design. As the thermal performance is one of the main factors limiting a machines output capability, a thermal test on a complete prototype machine is an essential part of the design process. However, for the segmented stator winding design, a test-informed thermal analysis on a single stator tooth can be performed prior to the manufacture of the full machine. This approach allows for a rapid and inexpensive assessment of the thermal performance of the complete machine and early identification of design modifications needed. The research has been applied to the design of a highly efficient and compact permanent-magnet traction motor. A thermal model for a single tooth was developed and supported by tests to identify key heat transfer coefficients. A number of winding assemblies were compared, and the most promising was selected for the final motor prototype. The results from the approach are compared with thermal test results from the complete machine.

Derivation and Scaling of AC Copper Loss in Thermal Modeling of Electrical Machines

Accurate prediction of temperature-dependent ac winding loss effects is crucial in the design of electrical machinery. Average ac winding loss as a function of operating frequency is commonly characterized by the ratio of the equivalent ac and dc resistances (Rac/Rdc). However, as the ac and dc components of the winding loss scale differently with temperature, a single value of Rac/Rdc derived for one temperature can be inadequate when used in thermal modeling. In this paper, methods of deriving the Rac/Rdc ratio, together with scaling techniques of the ac winding loss accounting for thermal effects, are discussed. As an alternative to computationally intensive 3-D finite-element analysis, an experimental approach based on tests on full-scale stator assemblies is proposed. A previously proposed scaling technique of the ac winding loss is discussed and developed further. The proposed techniques of deriving the ac winding loss accounting for temperature variation are illustrated using both theoretical analysis and experimental data.

Design study of a three-phase brushless exciter for aircraft starter/generator

This paper presents a design study of a 3-phase AC main exciter (ME) for an aircraft starter-generator. A computationally efficient methodology for optimizing the design of the ME is presented. The optimisation is carried out using coupled two-dimensional (2D) magnetostatic finite element solver and particle swarm optimisation procedure (PSO). The ME design is then analysed using 3D FE to account for the end-winding effects, and the results are fed into a lumped-parameter circuit model of the ME. The circuit model allows for the operating modes of the ME being analysed in a computationally efficient manner also accounting for non-linearities. The theoretical findings are experimentally validated on a prototype generator.

Design and Characterization of a Three-Phase Brushless Exciter for Aircraft Starter/Generator

This paper evaluates a three-phase ac main exciter (ME) for a wound-field synchronous aircraft starter-generator capable of operating in both starting and generating modes. The research has been based around a 225-kVA generator utilizing the existing single-phase ME hardware. Initially, a reconfiguration of the existing single-phase unit into a three-phase one has been carried out. Subsequently, an optimization of the ME stator has been performed. The hardware evaluations have been performed in parallel to the development of design methodologies suitable for future starter/generator design. The three-phase ac ME design needs to satisfy the required current output at both start and generate modes with minimum input voltampere rating of the inverter required to drive the ME. The MEs output capabilities have been predicted in all operating conditions using a computationally efficient combination of 3-D finite-element models together with a lumped-parameter circuit approach. The theoretical findings from the analyses have been validated on a prototype ME, confirming that the three-phase ac ME is capable of generating the required current output in both operating modes. The developed modeling system allowed for the identification of the MEs optimal operating points with regard to the minimum input apparent power (in voltamperes) at the required output power and a methodology for the accurate and computationally efficient evaluation of future starter/generator designs.