Electric machine rotor sleeve

An electric machine comprising:

• a stator;
• a rotor radially inward of the stator and comprising a plurality of surface permanent magnets;
• a rotor sleeve surrounding the rotor, the rotor sleeve comprising:
• an inner layer having a radial thickness between 0.1 mm and 1 mm;
• and
• an outer layer composed of a plurality of layers of fibre reinforced matrix material including:
• a first layer of carbon fibre reinforced polymer comprising carbon fibres oriented substantially 90° to a rotational axis,
• a second layer of fibres having a lower modulus of elasticity than that of the carbon fibres and being oriented between 50° and 75° in either one of a clockwise direction and an anti-clockwise direction relative to the rotational axis, and
• a third layer of the fibres having the lower modulus of elasticity and being oriented between 50° and 75° in the other one of the clockwise direction and the anti-clockwise direction relative to the rotational axis,
• the first layer, the second layer, and the third layer being stacked in this order,
• the outer layer having a radial thickness between 3 mm and 15 mm, wherein:
• a ratio of a radius of the rotor sleeve to a radial thickness of the rotor sleeve including the inner and outer layers is between 20 and 40;
• and
• the electric machine has a maximum rotor rotational speed greater than 15000 RPM.