Determination of critical thermal operations for high-speed permanent magnet electrical machines

Purpose - The thermal design of high-speed electrical machines is a greater challenge in comparison with conventional electrical machines. When designing the machine, the calculated temperatures in all parts should be lower than their critical temperatures. This paper aims to perform thermal analysis for different rotor types according to the level of shield from eddy currents in order to achieve a safe thermal design of the machine. Design/methodology/approach - The machine under study in the paper is a high-speed permanent magnet (PM) motor designed for speed n = 31,500 rpm and power P = 130 kW. A thermal-network method was used for thermal analysis of the machine. Findings - The minimum value of the coolant flow in the air gap that provides an effective cooling of the machine was estimated. The coolant itself is not able to provide an effective cooling of the magnets if they are not shielded from eddy currents. Research limitations/implications - The results are obtained only by the thermal-network method. Numerical techniques and practical measurements for comparison and validation of the existing results should be implemented in future. Practical implications - The paper offers useful practical information when a safe thermal design of a high-speed PM electrical machine should be performed. Originality/value - The paper demonstrates how three different design types of a high-speed PM electrical machine are thermally analysed in order to find out which type fulfils the rigorous thermal criteria. The practical significance of the paper is beneficial for the designers of high-speed PM electrical machines.