Modelling of Eddy-current Losses in a Surface-mounted Ndfeb Permanent-magnet Generator

In a surface-mounted sintered NdFeB permanent-magnet generator, the magnets are usually thin (typically 4 mm), and hence the armature reaction, due to the time harmonics in the stator currents, is high. NdFeB magnets are conducting (resistivity typically 1.6 μΩm) and experience eddy-current losses. Therefore, the combined eddy-current losses in the NdFeB magnets and a solid iron rotor may become significant. An increase in temperature from eddy-current losses can demagnetize the magnets, and high temperature may weaken structural adhesives, leading to the magnets detaching from the rotor backing-iron. A two-dimensional timestepping finite element method is employed to examine the eddy-current losses under a typical stator phase current with a rectifier load. The calculated results are given. It is found that the eddy-current losses are dependent on the magnet thickness, air gap, magnet width, and rotor backing-iron structure (solid or laminated). The modelled generator, with 4 mm magnets and a 1.5 mm air gap, has eddy-current losses of more than 1% of its rated power output. The eddy-current loss in the rotor can be reduced by either increasing the magnet thickness and air gap length simultaneously, or using segmented magnets for each pole and laminated rotor backing-iron. The increase in magnet thickness and air gap length effectively reduces the effect of armature reaction, while segmenting the magnets and laminating the rotor backing-iron effectively reduces the eddycurrent loop areas. Interestingly, laminating the rotor backing-iron on its own leads to a significant increase in eddy-current losses in the magnets and in the total eddy-current loss.