Influence of shaft relative permeability on rotor losses in 2-pole slotless high-speed motor

High-speed permanent-magnet (PM) machines play an important role in high-power density applications, where demands on high efficiency and low weight are elevated. However, high-speed PM machines present a high proportion of harmonic losses, resulting in requirements on large and costly filters, unless considered at the design stage. The high-speed machine in this paper is a 2-pole slotless design. Since the airgap flux density created by the stator windings is nearly sinusoidal, the rotor losses are pre-dominantly generated by time harmonics from the stator windings [1]. The phase-to-phase voltage harmonics from a 2-level regular-sampled pulse-width modulation (PWM) process (fig 1a) is illustrated in figure 1b. Harmonic sideband groups appear around multiples of the switching frequency n⋅fsw, where n is an integer. The two first harmonic sideband groups are illustrated in figure 1b. The sideband groups generate harmonic losses in the motor