Abstract
Litz wires are employed in high-frequency electrical machines due to their advantages of reducing the ac losses, including minimizing the skin effect and the proximity effect. In order to improve the reliability of such machines, and enable accurate thermal predictions at the design stage, accurate calculation of the thermal conductivity of litz wire is important. In this paper, a calculation method based on the Gasar porous metal materials model is put forward. In this method, a cell model is extracted from the litz wire, and a thermal resistance network is used to calculate the equivalent thermal conductivity (ETC). Following this, two finite-element analysis (FEA) models for the same litz wire are built, one with actual thermal conductivities for the different constituent materials and another with the calculated equivalent thermal conductivity for an equivalent material, with the two models showing similar thermal characteristics. Finally, an experimental setup is built for measuring the steady-state ETC of litz wire. The apparatus structure and characteristics are described in detail, and the experiment uncertainty and measurement errors are analyzed. Three types of litz wire are measured in the experimental, and the results from experiment and calculation are consistent.
Highlights
High speed and high pole number electrical machines are increasingly popular due to favorable characteristics such as high power density, compact size, and reduced mass[1]–[3]
Where RLitz is the thermal resistance of the Litz wire, Rm is the thermal resistance of the whole sample, Rab is the thermal resistance of one aluminum block and Rcontact is the contact thermal resistance between the aluminum block and the hot/cold meter bar, as shown in Fig. 11 (c)
The measurement assumes that the thickness of all Al blocks are constant at 2.25 mm, where the bulk thermal resistance and contact resistance of 2x aluminum blocks are calculated to be 407.75 K/W
Summary
High speed and high pole number electrical machines are increasingly popular due to favorable characteristics such as high power density, compact size, and reduced mass[1]–[3]. If the real structure with different materials is considered when establishing the thermal resistance network or FEA model of slot, it will be very complicated, or in many cases impossible due to the aforesaid random-nature in wire placement. This paper is organized as follows: Section II describes the process of representing a real Litz wire into a representative cell model with a corresponding thermal resistance network for calculating the ETC.
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