Optimal split ratio for high-speed permanent magnet brushless DC motors

The ratio of rotor to stator diameter, i.e. split ratio, is one of the most important design parameters for permanent magnet machines, as it can significantly influence their power density and efficiency. Previous work has shown that an optimal split ratio exists for minimum copper loss and can be obtained by using a simple formula. However, this expression neglects the stator iron loss which for high-speed and/or high pole permanent magnet machines can become the dominant motor loss. This paper investigates the influence of iron loss on the optimal split ratio for maximum efficiency, with particular emphasis on high-speed machines.