Investigation of Ferrofluid Cooling for High Power Density Permanent Magnet Machines

This article investigates an advanced thermal management method adopting ferrofluid (FF) for improving the end-winding cooling of permanent magnet (PM) machines. An oil-based liquid with nano-sized ferromagnetic particles (which is known as FF) is used to fill in the cavity around the end windings. This is to establish an effective heat flux path between the end winding that is often regarded as hot spot in electrical machines and the external cooling system, i.e., water jacket, to improve the cooling performance of the PM machines. This improvement does not only result from the higher thermal conductivity and thermal expansion of the nanofluid with metal particles but also from strong thermomagnetic convection generated by the magnetic body force of the ferromagnetic particles within the FF. Multiphysics models considering the interaction between the electromagnetic field, the heat transfer, and the fluid dynamics have been built to study the thermal performances of a PM machine under different load conditions. Several factors affecting the thermomagnetic convection, such as the temperature-dependent magnetization curve of the FF, the concentration, and different ferromagnetic materials as well as different current densities, have been investigated to analyze their influences on cooling performance. One major finding is that, compared with other coolant without magnetic body force, the FF can significantly reduce machine peak temperature, e.g., by around <inline-formula> <tex-math notation="LaTeX">${36.4}~^{\circ }\text{C}$ </tex-math></inline-formula> when the current density is <inline-formula> <tex-math notation="LaTeX">$\boldsymbol {22.1}~\text {A} / {\text {mm}}^{ \boldsymbol {2}}$ </tex-math></inline-formula>.