Coupled Electromagnetic and Thermal Analysis of an Axial Flux PM Machine

Abstract

I. Introduction Due to their high torque density and an excellent efficiency, axial flux PM machines are favorable for vehicle propulsion and wind energy conversion. Although many effort was already done to simulate the electromagnetic properties of the machine using full 3D or multilayer-2D simulations, the work towards modelling of the thermal behavior is still very limited. As the disc-shaped rotors will operate as a fan during rotation, convective heat transfer in the air gap will have a major influence on the thermal design of the machine. Rather than performing full 3D combined fluid and thermal analysis, empiric analytical expressions based on CFD are used in this work to define the convective heat coefficient in different parts of the machine. The final thermal simulations are carried out for the stator and rotor individually, where only a segment needs to be modelled due to thermal periodicity. As a result, the use of this coupled modelling technique results in a significant decrease of the overall simulation time. The coupled electromagnetic and thermal modelling techniques are illustrated on a 4kW axial flux PM machine having the yokeless and segmented armature (YASA) topology. Finally, both the electromagnetic and thermal simulation results were validated on a preliminary prototype. II. Coupled Electromagnetic and Thermal Modelling In the first step of the analysis, the electromagnetic behavior of the machine is modelled. The results from the electromagnetic field computations are used to calculate the corresponding losses in post-processing. In a second step, these losses are used as the heat sources for the thermal simulations. As the axial flux PM machine has an inherent 3D structure, multilayer-2D simulations [1] are used to obtain the electromagnetic properties such as flux-linkage, back-emf and (cogging) torque. In postprocessing, the magnetic flux density pattern of the different layers in the core is used to calculated the core losses. The solutions for the air gap magnetic flux density pattern of each layer are combined and used for a time harmonic calculation of the eddy current losses in the permanent magnets [1]. Together with the Joule losses in the machine winding, the iron losses in the stator cores and eddy current losses in the PM's are used as the source terms in the thermal simulations. As the thermal process is particularly slow with respect to the electromagnetic one, the time average values of the losses are chosen as a source in the thermal model. Instead of using full 3D combined fluid and thermal analysis, empirical equations derived in [2] are used to model the convective heat transfer from each of the surfaces near the air gap region. Next to an empirical formula for the convective heat coefficients of each surface, an expression for the reference temperatures is also derived using dimensionless numbers. The reference temperature is the bulk fluid temperature of the domain near each surface which can be expressed as a function of the average stator, rotor and ambient temper...

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“…9). As concerns the use of (3) for our channels, the following remarks are valid: − even if [20] introduced (3) assuming that the channel walls have the same surface temperature T s , also here we can adopt (3), because the wall temperatures of each channel are not too different between them (a few degrees maximum); − considering that the model of Fig. 9 is based on a single wound tooth approach, when solving the thermal network of a single coil, the surface temperature T s is assumed as the one of the coil side faced to the channel.…”

“…Among various types of PMSMs, the thermal study of Axial Flux Machines (AFMs) is quite challenging due to the complex fluid flow regimes resulting from the rotation of the rotor surface facing the stator [1]. AFMs are conventionally studied as a general rotor-stator system by using the developed analytical or empirical correlations for heat convection over the surface of the stator and rotor [1]- [3]. The effect of geometrical parameters including the radius of the surfaces and distance between the surfaces [4]- [5], protrusion of the PMs [6], and the inlet configuration [4] is studied to investigate the fluid flow and heat convection.…”

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“…For instance, a 42 kW water-cooled vehicular permanent magnet synchronous motor was analyzed via multiphysic coupling thermal performance simulation based on a circuit, electromagnetic field, and temperature field intercoupling analytical method [5]. e magnetic-thermal coupling of a 4 kW axial magnetic flux permanent magnet motor was analyzed [6]. e thermalmagnetic coupling of a double-salient-pole and double-rotor permanent magnet motor was analyzed by integrating thermal networks and the finite element method (FEM) [7].…”

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“…Kolondzovski et al (2009) also did the same, for a high speed PM motor. Multiphysics analysis, by coupling electromagnetic and thermal phenomena, is also popular (Kolondzowski, 2010;Fasquelle et al, 2010 andVansompel et al, 2015).…”