Reduced Rare-Earth Flux-Switching Machines for Traction Applications

Abstract

There has been growing interest in electrical machines that reduce or eliminate rare-earth material content. Traction applications are among the key applications where reducing cost and hence reduction of rare-earth materials is a key requirement. This paper will assess the potential of different variants of flux-switching machines that either reduce or eliminate rare-earth materials in the context of traction applications. Two designs use different grades of Dysprosiumfree permanent magnets and the third design is a wound-field variant that does not include permanent magnets at all. Detailed analysis of all three designs in comparison to the required set of specifications will be presented. The key opportunities and challenges will be highlighted. The impact of the high pole-count/frequency of the flux-switching machines will also be evaluated. Experimental results for one of the designs with Dysprosium-free permanent magnets will also be presented.

A topology selection consideration of electrical machines for traction applications: towards the FreedomCar 2020 targets

“…Another advantage of the PMs in the stator is there are no issues with magnet retention. Both PMFS and WFFS machine have been designed for the FreedomCar targets [30]. As would be expected, the PMFS machine produces higher torque than the WFFS machine for the same current supply, but both meet the requirement.…”

“…While the power density of PMFS machines is comparable to that of a PM synchronous machine, the PM utilisation ratio is lower than that of the Toyota Prius IPM machine [11], so a larger PM mass would be needed to obtain this similar power density to an IPM design. However because the PMs would be in the stator, they would be closer to the cooling system, this means less dysprosium (needed to improve the maximum operating temperature of NdFeB) would be required, possibly reducing the overall cost of PM material [30]. Another advantage of the PMs in the stator is there are no issues with magnet retention.…”

“…The WFFS machine has a higher efficiency at maximum speed, but at rated speed fails to meet the target; it has been assumed that this is due to the copper losses from the higher current requirement. Torque ripple is also shown to be an issue, this is somewhat mitigated (from 17% to 7% of average torque) but the targets are still stricter (5%) [30]. A PMFS has also been designed for the same specifications as the Toyota Prius IPM machine, and while the rotor allowed for good mechanical integrity for high speed, and in this case smaller torque ripple, the main drawback was the requirement for a larger magnet mass [11].…”

Design and analysis of a kind of rotor ironless high torque density machine using Alnico magnet

“…Careful optimisation is also presented, and the machine's demagnetisation withstand capability is enhanced by tapering flux barriers towards the airgap. A kind of Dy‐free material is used in a flux switching machine [13]. The special topology eliminates the need for magnet retention, and the novel magnetic circuit makes it less susceptible to demagnetisation.…”

Comparison of traction motors that reduce or eliminate rare‐earth materials

“…Even though several machine topologies have been pursued to accomplish the goal of reducing or eliminating rare‐earth materials, it has been difficult to provide a fair and accurate comparison of all these topologies that takes into consideration the various performance parameters. There have been several machines designed to achieve hybrid vehicle traction requirements of 55 kW peak and 30 kW continuous over a speed range going from 2800–14,000 rpm under the ‘Advanced Non Rare Earth Electrical Machine Program’ funded by the US Department of Energy [25–33]. The specifications are summarised in Fig.…”