The sportier side of electric vehicles

Robert Brand, VACUUMSCHMELZE GmbH & Co. KG -June 12, 2013

Performance is a key requirement of electric motors and generators which are used in many applications, most importantly for electric vehicles, and is paramount in all sectors of motor sports: from KERS in Formula 1 to hybrid solutions in Le Mans cars. The main goals are to push performance and innovation to their limits, targets which have been publicly achieved on the race track in recent years.

In precise terms, the aim is to achieve maximum performance at minimum weight - i.e., to optimise power density. Ultimately, the central technological requirements remain virtually identical, despite variations in the economic framework conditions. Innovative optimised drive technologies are the key pillars of eMobility.

The technological challenges of this new form of mobility are clearly demonstrated by the Formula Student championship, a competition in which teams of students from universities all over the world design and build single-seater racing cars and race them at iconic locations such as Silverstone and Hockenheim. However, the winner is not automatically the first to cross the finishing line in an individual race, but the team that accumulates the best overall scores for design and racing performance, based on both economic and environmental considerations.

Tough conditions
A range of dynamic disciplines are designed to push the racing cars to their limits. They must compete in a variety of categories including Acceleration (maximum acceleration and speed in drag racing from a standing start), Skid Pan (maximum lateral acceleration on a circular skid pan) and Autocross & Endurance (maximum traction and agility on the circuit and endurance racing).

From its origins as a race for vehicles with combustion engines, Formula Student was expanded in 2010 to include vehicles with pure electric drives. The resultant rise in interest serves as a testament to the dynamic pace of development in eMobility itself; after only two years, Formula Student Electric now has 85 entrants from 25 countries.

While the design and construction of the vehicles and the participation in the individual races are the work of dedicated interdisciplinary university teams, sponsors contribute the necessary funding and materials. Since the Formula Student Electric was founded, the drive and control unit manufacturer AMK has supported several teams by providing synchronous motors and motor control systems.

A high power-to-weight ratio and lots of torque are the keys to success, delivering the necessary traction control for drag racing and maximising acceleration out of tight bends in circuit racing. Both the vehicle and its drive train must be designed for performance and lightness to combine straight line acceleration with agility.

Optimised power-to-weight ratio achieved by partnership and a new alloy
Since 2011, Vacuumschmelze has partnered with AMK as a sponsor of Formula Student Electric with the aim of optimising electric motors with specific attention to their power-to-weight ratio. In line with the company's slogan, "Advanced Materials – The Key to Progress," Vacuumschmelze produces rotor and stator assemblies from their CoFe alloys VACOFLUX® and VACODUR®. These materials exhibit significantly higher induction than classic electrical steel; for example, at 2.3 T their saturation magnetisation is 13% higher than that of electrical steel.

This year (2012), Vacuumschmelze completed the development of its new alloy, VACODUR 49. Specific heat treatments can be applied to this high-induction CoFe material, increasing its strength in order to handle the requirements of an electric motor or generator.

For example, stator assemblies, which are exposed to lower mechanical stress, can be produced from an alloy with optimum magnetic properties but a relatively low yield strength of 210 MPa. However, higher-strength materials are frequently required for high-speed rotors and an appropriate heat treatment can increase the yield strength to up to 390 MPa, significantly higher than that of electrical steel (see chart 1).

Table 1: Typical material properties of VACODUR 49 compared with electrical steel M270-50A.

Independent of the strength of the material chosen, induction values dramatically outperform those of electrical steel, especially at lower field strengths. M270-50A electrical steel has an induction value of 1.49 T at a magnetic field strength of 2.5 kA/m, but at the same field strength, both variants of VACODUR 49 outperform electrical steel by about 50%, with values of 2.23 and 2.27 T respectively (see Figure 1).

Figure 1: Comparison of static initial magnetisation curves for VACODUR 49 and M270-50A electrical steel.

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