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Title: Fast and accurate hot-spot estimation in electrical machines
Author: Romanazzi, Pietro
ISNI:       0000 0004 6500 6737
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2017
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Temperature is one of the parameters that limits the output torque and reduces the lifespan of electrical machines. Models that can provide accurate estimation of the temperature field in the most critical components (e.g. windings) at lower computational effort can be useful to improve the design process and reduce the time to market. Depending on the application, engineers usually rely on hi-fidelity models, e.g. based on the finite elements method (FEM), or lower order models, e.g. thermal equivalent circuits (TECs). The aim of the present work is to provide new tools and methodologies to obtain the temperature distribution within the windings using reduced order hi-fidelity models or improved TEC that could account for any working condition, including AC effects. A new methodology, based on the multiple scales method (MSM), is introduced which homogenises the complex windings domain and allows for the estimation of its effective thermal properties. The homogenisation through the MSM is performed solving a single elementary cell. The MSM also allows for the reconstruction of the actual thermal field. Extensive numerical and experimental validation is provided, in particular for the case of electrical windings encapsulated with epoxy. The thermal homogenisation is then combined with an electromagnetic homogenisation technique to estimate winding losses including AC effects, such as proximity and skin effects. The coupled analysis is validated numerically on reference test problems, and experimentally, on a suitably built "motorette". The method is proven to correctly predict losses including thermal effects and to estimate magnitude and location of the temperature hotspot within the winding domain. This work also introduces a new approach for building thermal equivalent circuits that represents the most commonly employed modelling technique for electrical machine thermal analysis. Here the TEC approaches are thoroughly analysed, highlighting limitations. The proposed new technique extends the range of numerical accuracy, accounting for high Biot numbers (up to Bi = 2) and internal heat generation. The result of this approach is higher spatial resolution about the temperature field within the winding domain and thus enables improved information on hotspot location and magnitude. The method is experimentally validated and also applied to model an electrical machine for full-electric in-wheel vehicle propulsion.
Supervisor: Howey, David ; Bruna, Maria Sponsor: ADEPT
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Mechanical engineering ; Electrical engineering ; Multiple scales ; Electrical machines ; homogenisation ; hot-spot ; Thermal analysis