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

Abstract

High performance electric motor designs with ferrite magnets have recently gained interest due to the high and volatile price of rare earth magnets. However, due to the relatively poor coercivity of ferrite magnets, these designs are highly susceptible to demagnetization, as a result of which accurate modelling and better understanding of this phenomenon is particularly important. In this paper, the impact of the motor stack length and level of magnetic saturation on the demagnetization risk are studied based on 3-Dimensional Finite Element (3D FE) simulations and a proposed lumped circuit model. It is shown that reducing the stack length can significantly enhance the demagnetization resistance, with the effect being more pronounced for designs with a higher level of magnetic saturation. To benchmark the practicality of the concept, a previously presented high performance ferrite based design is modified by using a 30% weaker grade of ferrite magnet whilst shortening the stack length. It is shown that the demagnetization withstand capability of the design was significantly enhanced and exceeded the short circuit requirement with a good safety margin. The theoretical findings have been supported by prototype testing.

Comparative Assessment of Single Piece and Fir-Tree-Based Spoke Type Rotor Designs for Low-Cost Electric Vehicle Application

“…From Fig. 16 and confirming the findings in [19], it is realized that due to 3D effects (for both rotor designs) the demagnetization performance of the 39 mm stack model is, significantly, superior to that of the nominal design with 195 mm stack length. Furthermore and confirming the predictions in Section III.…”

“…To meet the high power density requirement, designs with high contribution of reluctance torque, as well as flux concentration effect, such as Interior Permanent Magnet (IPM) with U-shape [10], [11], V shape [12], LC shape [13], and spoke magnet topologies [7], [8], [14], [15], and [16] can be employed; furthermore, few high torque density dual stator topologies and axial flux designs have been reported [17], [18]. To meet the demagnetization resistance criterion several approaches can be followed, such as: a) burying the magnets deep in the rotor, while tapering the flux barriers toward the airgap [10], or extending the rotor pole tips to bypass the demagnetization field [8], b) reducing the number of turns and thickening the magnets in the direction of magnetization [7], [8], c) increasing the end winding leakage ratio by shortening the stack length and increasing the magnetic saturation, [19], and d) applying analytical techniques to diagnose and prevent demagnetization during the motor operation [20]. In terms of the stator windings, both distributed configuration [7], [8], and a concentrated type [14], [15] can be employed.…”

Demagnetization investigation of a partitioned rotor flux switching machine with hybrid permanent magnet

“…[4][5][6] Especially when traction machines operate under extreme conditions, such as overload and deep flux-weakening, the PMs suffer from high irreversible demagnetization risk. [7][8][9][10] In this condition, the output torque capability will be reduced to some extent, which impacts the continuous high-reliability operation of the vehicles. However, during the design process of the conventional PM machines, the anti-demagnetization investigations of the PMs are often ignored.…”

“…[7][8][9][10] In the Ref. 6, the demagnetization characteristics of the PM-assisted synchronous reluctance motor are investigated under different current loads, revealing that the PM is demagnetized seriously when 2.5 times rated current is fed.…”

“…6, the length to diameter ratio of PM motor is optimized in the Ref. 7, thus making the PM can remain relatively high operating points even under 4 times the rated current. Meanwhile, under the flux-weakening condition, partial irreversible demagnetization risk of Recently, the limitations of rare-earth PMs have emerged due to its high price and unstable supply.…”