Design and analysis of a novel stator oil-flooded cooling system for aircraft electrical machines with hairpin windings

Abstract

Growing environmental and economic concerns are driving various sectors to improve energy efficiency. In sectors such as aeronautics, this is achieved through electrification and weight reduction, which necessarily involves replacing conventional pneumatic, mechanical and hydraulic systems with electric drives. In addition, electric machines also offer better efficiency and require less maintenance. Then minimizing the weight of electric machines is therefore essential to reduce fuel consumption and permanent magnet machines offer the highest power density. The use of hairpin windings can help reduce weight, due to their high slot filling factor and short winding ends, and have already demonstrated their potential in sectors such as automotive. Another key factor in improving power density are cooling systems. Among all the liquid cooling systems, stator oil-flooded solutions are the most promising, both for their cooling capacity and for the improved electrical isolation between conductors that make them suitable for aeronautical applications. This dissertation deals with the combination of these two aspects, hairpin winding and flooded stator cooling for aircraft applications, and presents a novel cooling system that has been patented. For this purpose, the proposed solution is described and the advantages of this cooling arrangement are detailed. The equations defining its behavior are presented, its potential is experimentally evaluated by using a motorette and design criteria are provided. Subsequently, the presented concept is applied on an industrialized and field-tested machine for heavy-duty off-road vehicles in substitution of a water-jacket cooling arrangement, ending with a prototype in order to assess gains in power density. Next, the novel cooling arrangement is applied on a conceptual design for an aeronautical application. Finally, the main results of this dissertation are summarized and the main future research lines are outlined.