On Selection of Rotor Support Material for a Ferrite Magnet Spoke-Type Traction Motor

Abstract

Interior permanent magnet motors with ferrite magnets and distributed windings can be a cost effective alternative to rare-earth magnet based motors for demanding applications such as automotive traction. Among different rotor topologies, the spoke type may be preferred, due to its advantages for high flux concentration and resistance to demagnetization, when carefully designed. When high speed operation is required, to increase the power density of the motor, the spoke type rotor must comprise of two sections: a) the ferromagnetic rotor pole to provide the path for the magnetic flux, and b) the non-magnetic rotor support to provide the structural integrity. In this paper, the multiphysics and cost implications of the rotor support material, as part of a high performance ferrite magnet traction motor, are analyzed, and an optimal selection with respect to those criteria is proposed. The performance of the design based on the proposed rotor support material is validated by electromagnetic and structural testing of three sets of customized prototypes. Based on the analysis, the proposed rotor support material may, significantly, boost the cost competitiveness of a low cost ferrite motor for high volume production.

Three-Dimensional Modelling of Demagnetization and Utilization of Poorer Magnet Materials for EV/HEV Applications

“…Even though, several papers such as [15], [17], [18], analytically, or [19], numerically (using FE), have derived and described the impact of 3D parameters on the performance of electrical machines, their modelling is often limited to derivations of airgap flux density, BEMF and torque, while the 3D effect of the stack length variation on the demagnetization In this paper, the 3D effects of the stack length on the demagnetization are, for the first time, investigated. The investigation has been made using 3D FE (assuming nonlinear materials), which is performed on a high power density spoke type ferrite motor, the design details of which have been previously presented in [12], [20]. A lumped magnetic circuit model is derived and used to explain the observations from the 3D FE, based on the variation of the magnet load line against the circuit parameters.…”

On winding design of a high performance ferrite motor for traction application

“…The main requirements of a high speed ferrite motor, specified as part of an all-electric vehicle project, [12], [33], are summarized in Table I. Furthermore, a schematic of the stator and rotor geometry for both a distributed winding (with 2 slots per pole and phase) and a concentrated winding (with 0.5 slot per pole and phase) as well as some major dimensions are included in Fig.…”

“…and Table II. With regards to the design methodology, the following comments should be noted: a) the rotor geometry is a result of a coupled electromagnetic-structural optimization process which has been explained in [12], [33], b) the stator topology with the distributed winding is the result of an optimization to achieve maximum torque, and a minimum ripple torque, while the open rectangular slots were chosen to maximize the flux linkage and simplify coil insertions, c) the stator topology with the concentrated winding is the result of a manual optimization, to achieve a maximum torque while the outer and inner bore diameters were kept fixed and identical to the design with the distributed winding (this was to enable using the same rotor and the same stator housing for the two designs for a simpler comparison). To evaluate the performance of the motor at base and top speed, a 2 dimensional Finite Element (2D FE) was used, where the drive was modelled using a sinusoidal current source with the maximum available line to line voltage fixed to the available DC link voltage of 400 V, Table I (assuming >100% inverter modulation index).…”

“…Furthermore, in terms of the continuous performance, even though the HSFM lags behind the required continuous power at lower speeds (due to the lower than required continuous torque at low speeds), it exceeds the continuous power requirement at the speeds above 6500 rpm, and provides a maximum continuous power of 60 kW, exceeding the requirement by 28%. Based on the results from the dynamic performance testing, it can be realized that the HSFM design with a distributed aluminum winding in the current paper (with 14 liter gross volume identical to the Leaf design), and with the rotor design disclosed in [12] and [33], may provide a competitive maximum torque and power density of 17 Nm/ liter (about 85% of the Leaf motor rating) and 6.4 kW/ liter (about 110% of the Leaf motor rating), and a maximum continuous power density of 4.3 kW/ liter (about 75% of the Leaf motor actual capability tested in [36]).…”

“…In this paper, the performance of a high power density low cost ferrite motor, [12] and [33], with regards to the stator windings will be assessed. In this regard, the distributed wound stator is compared against a concentrated wound alternative, while the rotor in both cases is kept identical to the base line rotor design in [12].…”