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Advances in magnetic materials for electric motors

14 Oct 2015  | Robert Brand

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Designed for high efficiency, permanently excited synchronous motors account for the majority of modern high-performance motors. A commonly found design is the internal rotor motor, in which a rotor embedded with permanent magnets follows a magnetic alternating field generated by the stator. The alternating field between the stator poles, in turn, is generated by alternating current passing through the stator windings. As the use of a solid stator would result in extremely high eddy current losses, core stacks with electrical isolation between the individual lamination layers are used.

VAC's materials and technologies are touted to offer improvements. Although the electrical steel generally used for such motors is low-cost and universally available, it offers extremely limited magnetisation characteristics. While the flux density of the common electrical steel type M270-50A is lower than 1.5 T at a field strength of 1000 A/m, the comparable flux density of VACOFLUX 48 reaches a level of 2.2 T (figure 1).

As the power transmission between the stator and rotor increases by a square law with respect to the induction, high magnetisation is critical for the power density of the motor. By using CoFe materials, motors can be manufactured that have more compact dimensions yet supply the same power output, or alternatively, have the same dimensions yet deliver more power.

Figure 1: Initial magnetisation curve of VACOFLUX 48 compared to conventional electrical steel.

The advantages of this type of material have long been used in generators and motors in aerospace applications. As on-board aircraft systems are progressively electrified to replace older hydraulic systems and aviation fuel costs continue to rise, the weight of the electrical components used must be as low as possible. Lighter-weight engines are also a superior choice for highly dynamic applications, resulting in CoFe alloys also being used in automation technology.

More recently, high-performance hybrid systems have increasingly entered the world of motorsports. It is thus hardly surprising that innovative young developers at universities also rely on these technologies.

Formula Student Electric
A particularly clear example is the Formula Student Electric, an international championship in which students compete to design a formula racing car from scratch. Since the first Formula Student Electric in 2010, the vehicles entered have made enormous technological advancements. Rigorous use of lightweight construction is a key priority for all teams, in addition to aerodynamic packages and an array of diverse chassis technologies.

The AMZ Racing Team from ETH Zürich are currently the world champions with their 'julier' vehicle from the 2013 season. Weighing in at a mere 180kg, the racing car is driven by four M3 motors each delivering 37 kW of power. With a total output of 200 hp, julier can accelerate from 0-100 km/h in just over 2 seconds.

Figure 2: Non-wound stator-rotor system of an M3 motor and FEM simulation.

The vehicle's core stacks, made from VACOFLUX 48, played a central role in boosting the power of the four motors. As FEM calculations show, the motor developed by the Zurich students makes the maximum use of the material's flux concentration, enabling a power density of 7.7 kW/kg to be achieved. Figure 2 shows that the rotor and stator are magnetised at up to 2.3 T to saturation magnetisation.

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