Advanced cooling concepts for ultra-high-speed machines

High-speed electrical machines are gaining an increasing attention as they enable higher power densities in several applications such as micro-machining spindles and turbo compressors. This brings along an important challenge in thermal management due to the higher loss densities in the machine. Therefore, a careful thermal analysis is required along with the electromagnetic and mechanical analysis during the design phase of the machines. In this paper, five different forced cooling options for a slotless-type ultra-high-speed permanent-magnet machine are compared. Fast and sufficiently accurate thermal models are derived for analyzing these cooling concepts such that the cooling system design could be integrated in the machine optimization procedure, which would not be feasible when using computationally very intensive methods such as 3-D Finite-Element-Method (FEM) or Computational-Fluid-Dynamics (CFD). Measurements are carried out on the stator of an off-the-shelf 1 kW, 280 000 r/min machine to experimentally compare different cooling methods and to show the validity of the thermal models. A simplified case study shows that the power density can be more than doubled by the selection of a proper cooling system.