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The power density of electric machines is a critical factor in various applications, i.e. like the power train. A major factor to improve the power density is boosting the electric current density, which increases the losses in the limited volume of the electric machine. This results in a need for an optimized thermal design and efficient cooling. The dissipation of heat can be achieved in a multitude of ways, ranging from air cooling to highly integrated cooling solutions. In this paper, this variety is shown and analyzed with a focus on water cooling. Further various structures in electric machines are presented.
A planar testbench is built to systematically analyze water cooling geometries. The focus lies in providing different power loss distributions along cooling channels, accurate temperature readings in a multitude of locations, as well as the pressure drop across the channel. The test bench results are aligned with simulations and simplified analytical evaluation to support the development process.
The main goal in this paper is to determine temperature gradients in the material close to the stator to quantize the potential for future cooling jacket designs. One question ,to answer is: How large the gradient is considering a realistic power loss distribution. Another sensible point are the different thermal expansions of aluminum used in cooling jackets and the steel core of the stator. This can be bypassed by using a steel cooling jacket. In this case, the performance of a steel cooling jacket compared to an aluminum version is investigated and also if light weight construction can compensate the lower thermal conductivity of steel.
After the analysis, an outlook about future changes of the measurement methods are given and first potentials for future cooling jackets are proposed.
Distribution of Cooling Structures in Water Cooled Electrical Machines using Localized Loss Profiles
(2023)
Cooling is a critical factor for improving power density in electrical appliances, especially in integrated drives for mobile applications. However, the issue of distributed losses in electric machines can lead to hotspots and temperature gradients within the electric drive. Traditional cooling jackets use unidirectional flow without or with evenly distributed cooling structures. This often aggravates the issue of hotspots, resulting in thermal derating and thus limiting the operation range. As well, a non-demand oriented distribution of cooling structures leads to unnecessary pressure losses.
This problem is addressed with a newly elaborated method for distributing cooling elements, i.e., pin fins with varying density distribution inside the cooling channel. Results from previous work, numerical simulations, and measurement data from a planar test bench are used. The approach segments the cooling channel by using a loss profile. This profile and analytic heat transfer calculations are used to determine the required density of cooling elements for dissipating the locally induced losses. For a linear channel with uniformly distributed losses, this results in an increasing number of cooling elements within the channel in fluid flow direction. With localized losses, this will result in an increased density distribution in the respective areas. The method is evaluated by applying it to a planar test channel and investigating the temperature distribution on a test bench. First results indicate that the newly developed cooling element distribution provides an advantageous temperature distribution. The temperature gradient along the cooling channel shows a reduction from 23 K to 9 K with the distributed cooling elements.
The method, previously tested in the linear planar channel, then is applied to the construction of a cooling jacket with a specifically designed two-layer cooling channel. This design is analyzed using CFD, a prototype is currently under production. Tests on the prototype will follow in further investigations.
