There are two primary technologies of magnet retention in high speed permanent magnet machines, namely high-strength non-magnetic metal sleeve and high strength composite sleeve. Each offers unique advantages to the system and motor/generator performance. System designers select the optimum approach to meet system performance goals for rotor's tip speeds up to 360 meters/second and operating temperatures up to 230oC.
The metal sleeve can be designed to provide some stiffness to the rotor structure. It also acts to effectively shield the magnets from stator's harmonic currents. Eddy currents generated in the metal sleeve due to stator's harmonic currents and stator slotting impede high frequency fields from penetrating the magnets and generate losses. Most of the absorbed energy in the metal sleeve readily dissipates to the cooling medium in the airgap and the rest is conducted to the magnets and/or end supports. Carbon fiber sleeves are significantly stronger and lower density than their metal counterparts thus allow the use of more magnet mass or thinner sleeve for similar magnet volume. The result is smaller magnetic gap and better magnetic performance with carbon fiber sleeves. However, they do not provide any harmonic filtering. Moreover, due to their low thermal conductivity they act as thermal barriers to heat generated in the magnets. Rotor loss reduction and management techniques such as segmenting magnets or conductive layer shielding are frequently employed to enhance system performance when using carbon fiber sleeves.