Investigation of ferrofluid cooling in modular permanent magnet machines

Compared with conventional non-modular machines, the flux gaps in alternate stator teeth of the modular machines can increase the winding factor and provide flux focusing effect, and hence can increase the torque/power density. In addition, the flux gaps can also be used as cooling channels to improve the thermal performance. This paper investigates an advanced cooling approach using ferrofluid as coolant to fill in the cavity around end-windings and flux gaps. The multiphysics modelling in this paper has shown that the influence of the flux gaps on machine thermal performances comes from two aspects: (1) the gravitational effect, the flux gaps allow more self-circulating coolant to transfer heat to the housing. This helps to reduce machine temperature by around 5°C for a flux gap width of 2mm; (2) the magnetic body force, i.e., the thermomagnetic effect. This is very effective in non-modular machine cooling. But its efficiency slightly deteriorates in the modular machines. This is because the self-circulation of ferrofluid in the flux gaps due to the gravitational effect has been negatively affected by the thermomagnetic effect.