Performance evaluation of flux reversal machines with rare-earth and non-rare earth excitations for micro wind energy

Cogging torque in flux-reversal machines (FRM) is relatively high compared with other types of stator active PM machines, due to their unique double salient topology. This research develops a hybrid novel skew with rotor pole pairing (SKCpp) method and analyses it using 2D finite element analysis (FEA), comparing its performance with two distinct excitation topologies, i.e., rare earth (RE-FRM) and ferrite (NRE) flux reversal machines (FRMs). The 3-phase, 6/8 pole FRM is modeled, and electromagnetic performances are compared with RE excited FRM (Machine A) and NRE excited FRM (Machine B), which have similar performance requirements for wind generator applications. These various excited machines are evaluated based on their power generation performance, demonstrating exceptional overload and speed capabilities through 2D FEA. The efficiency of Machine A is somewhat higher than that of Machine B, but cogging torque and torque ripples are at least 40 % higher than those of the former. It is found that the torque density of Machine B is only 54 % of that of Machine A, but by employing this structure, the cost saving is achieved by 47 %, since the machine is 1.8 times heavier. Further, the demagnetization risk analysis is performed up to 150 °C. At higher temperatures, Machine A is prone to deeper demagnetization risk, due to temperature susceptibility, which degrades the power factor and thereby limits the performance of the generator. Unlike Machine A and Machine B, which possess the minimum demagnetization risk, the occurrence is recorded below room temperature. Overall summary, it is found that the NRE excited proposed Machine B is the best alternative to the RE excited proposed Machine A for medium speed wind turbine generator applications.