Effect of a shielding cylinder an the rotor losses in a rectifier-loaded PM machine

The objective of the research project is the development of a high-speed, high-efficiency generator system, consisting of a permanent-magnet (PM) generator loaded with a six-pulse controlled rectifier. In this paper, the effect of a shielding cylinder surrounding the rotor of the PM machine on the rotor losses is investigated. A machine model including the losses in copper, iron, magnets and shielding cylinder is derived and validated by means of measurements. A shielding cylinder is generally not necessary to obtain a sufficiently small angle of overlap, nor effective to reduce the reactive power. It introduces extra rotor losses in the cylinder, but it shields the rest of the rotor, thus avoiding losses in the rotor iron and the magnets. At high speeds, a shielding cylinder reduces the rotor losses; at low speeds, segmenting the magnets is better. The distance between the shielding cylinder and the stator should be large enough to obtain acceptable rotor losses