Nick Simpson

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.

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.

AC losses in high frequency electrical machine windings formed from large section conductors

In the design of compact, power dense electrical machines found in automotive and aerospace applications it may be preferable to accept a degree of eddy current loss within the winding to realise a low cost high integrity winding. Commonly adopted techniques for analysing AC effects in electrical machine windings tend only to be applicable where the conductor dimensions are smaller than the skin depth and for ideally transposed windings. Further the temperature variation of AC losses is often overlooked. In this paper a more general approach is proposed based on an established analytical model and is equally applicable to windings where the conductor size greatly exceeds the skin depth. The model is validated against test measurements and FE analysis on representative single layer winding arrangements. A case study is used to illustrate the application of the proposed analytical method to the coupled electromagnetic and thermal modelling of a typical slot winding.

An Accurate Mesh-Based Equivalent Circuit Approach to Thermal Modeling

This paper presents a method of automatically constructing and parameterizing accurate lumped parameter thermal equivalent circuits (TECs) with nodes arranged in a regular mesh pattern. The approach exhibits a number of advantages over conventional lumped parameter TECs including superior thermal field resolution, reduced model construction and setup times and more accurate identification of hot-spot temperatures and their location. The method serves as a desirable compromise between the fine detail and high computational cost of a full finite element analysis and the coarse detail and short solution times afforded by lumped parameter TECs. The benefits of the method are demonstrated by the analysis and experimental test of a demonstration linear actuator.

Impact of slot shape on loss and thermal behaviour of open-slot modular stator windings

This paper presents results from an investigation into the optimal design of an open-slot, modular stator winding. The impact of the stator slot shape on the winding temperature rise is explored, taking account the distribution of loss that occurs in the open slot winding, including ac effects, and the heat transfer characteristics from the winding assembly into the stator core pack. The application focus is a single-layer, concentrated wound brushless PM machine, however the work is applicable to other machine formats. Alternative stator lamination profiles are compared; the commonly used parallel sided tooth with a trapezoidal slot, and a parallel sided slot with a trapezoidal tooth. The investigation includes the development of a reduced order thermal model representation of the stator. This model is employed to undertake coupled loss and thermal analyses to provide more accurate comparisons of the designs accounting for ac and temperature effects. The experimental and theoretical findings indicate the parallel sided slot design will result in a 37°C lower winding temperature or an 11% increase in torque at the intended machine operation point.

A multi-physics design methodology applied to a high-force-density short-duty linear actuator

This paper presents an iterative coupled electromagnetic and thermal design methodology applied to a short-duty high-force-density permanent magnet tubular linear actuator. A difficulty with such coupled methodologies is balancing the accuracy of the modelling methods with the computation time. This problem is often addressed by employing a relatively detailed electromagnetic model and a coarse low computational cost thermal model. The thermal model typically requires calibration or is evolved from previous validated designs. In this paper, a two-dimensional electromagnetic finite element model is coupled with a thermal equivalent circuit model which is automatically constructed and parameterised using geometric and material data. A numerical method of estimating the equivalent thermal properties of the winding amalgam is used along with published empirically derived convection and radiation heat transfer correlations. The relatively high number of network nodes and more accurate thermal material properties minimise the need for thermal model calibration and allows for improved temperature prediction, including winding hot-spots, whilst maintaining a low computational cost for steady-state and transient analyses. Thereby allowing the actuator to be designed to operate with a peak temperature close to the thermal limit of the electrical insulation system which is difficult to achieve with more traditional lumped parameter models containing few nodes. The effectiveness of the design methodology is demonstrated by the design and experimental test of a prototype actuator.

A General Arc-Segment Element for Three-Dimensional Thermal Modeling

This paper demonstrates the benefits of a generalized arc-segment lumped parameter thermal equivalent circuit representation that can be used in the 3-D thermal modeling of transformers, wound passive components, and electrical machines. The arc-segment is particularly suited to modeling stator/rotor teeth and end-windings. Commonly used two-resistor lumped parameter thermal networks do not accurately account for internal heat generation, which is essential for accurate temperature prediction. In contrast, the use of a T-network formulation can accurately cater for internal heat generation along with material anisotropy. The element formulation is verified numerically and experimentally using 2-D and 3-D finite element analysis and through the thermal analysis of a power inductor.

Multi-physics design of high-energy-density wound components

In this paper a computationally efficient high-fidelity multi-physics design tool applicable to E-core power inductors is developed. The tool is composed of 2-D electromagnetic and 3-D thermal finite analyses coupled to models for inductor core and winding loss. The models are fully parametrically defined and appear as a black-box problem which can be used to perform parameter studies or design optimisation. For example, the influence of strip or edge wound rectangular conductors on ac loss generation and thermal performance can be evaluated or inductor designs which satisfy a given specification can be identified. The tool is demonstrated by the design and experimental test of a high-energy-density filter inductor for an automotive application.

A Multiphysics Design Methodology Applied to a High-Force-Density Short-Duty Linear Actuator

This paper presents a coupled electromagnetic and thermal design methodology which addresses the problem of balancing accuracy against computation time. A case study on a short-duty permanent-magnet (PM) linear actuator is used to illustrate the approach. The proposed method employs a two-dimensional electromagnetic finite-element (FE) model coupled with a detailed thermal equivalent circuit (TEC) model which is automatically constructed and parameterized using geometric and material data. A numerical method of estimating the equivalent thermal properties of the winding amalgam is used along with published empirically derived convection and radiation heat transfer correlations. The relatively high number of network nodes and more accurate thermal material properties minimizes model calibration and allows improved temperature prediction, including winding hot spots, while maintaining a low computational cost for both steady-state and transient analyses. A comparison between experimental and theoretical actuator performance shows that the design methodology provides good accuracy electromagnetic and transient thermal performance predictions without the need for direct model calibration and can yield an optimized design within an acceptable time frame.

Design of a brushless PM starter-generator for low-cost manufacture and a high-aspect-ratio mechanical space envelope

This paper presents the results of a brushless permanent magnet (PM) starter-generator design, which caters for low-cost manufacture and a highly constrained mechanical space envelope. The starter-generator design addresses the low-cost requirement through the use of aluminium winding conductors and ferrite PM. This presents several challenges which include, but are not limited to, the selection of an appropriate machine topology to realise a high specific output with the lower performance materials, minimizing the power losses associated with the higher resistivity of aluminium, and the enhancement of thermal performance. The problem is further exacerbated by the demanding space envelope, operating requirements, and the necessity of “design for manufacture.” The selection of an appropriate machine topology is paramount in the present application as the limited mechanical space-envelope results in a “pancake” like geometry in which the aspect ratio of the stator outer diameter to the machine active length is high. To provide a solution satisfying all these challenging design requirements, an approach combining the theoretical electromagnetic and thermal analyses together with tests on machine subassemblies has been employed here. Such a method allows for a more informed design process, where the manufacture and assembly nuances affecting the starter-generators performance are identified and accounted for prior to prototyping of the complete machine assembly. This paper discusses the employed design methodology in detail. A number of machine designs with alternative winding constructions have been considered providing an insight into challenges and limitations for the cost-effective winding construction utilizing aluminium conductors. The results from analysis of the starter generator suggest that the proposed machine design is capable of achieving the design requirements for both continuous and transient operating modes.