Rotor loss prediction in air-cored permanent magnet machines

Despite the cost raise of rare earth materials, low and medium speed permanent magnet (PM) machines have gained big popularity among wind turbine manufacturers due to their high efficiency and reliability. Ironless stator machines have been proposed in the literature due to their reduced manufacturing cost. However, in the modelling process of the aircored machines, the rotor loss calculation is generally neglected. Through Finite Element Analysis (FEA) and experimental testing it has been found that rotor losses can be as high as 40% of the copper losses in an air-cored stator (ironless) low speed machine. This paper describes two methodologies to estimate the rotor losses due to eddy currents using static FEA, and the experimental data from the rotor-locked test. The equivalent resistance to predict the rotor losses can be obtained from the rotor-locked test; paying special attention to space harmonics induced due to the winding topology. Two prototypes have been analyzed: a radial and an axial flux machine. The rotor losses can be predicted with great accuracy applying the superposition principle, when the phase currents are nonsinusoidal (passive rectification).