Discussions about the future of the electric car almost always focus on batteries. But what about motors? What is the current front line of development of mobile e-motors? It is no surprise that electric drives must avoid every single gram of superfluous weight. At the KIT Institute of Electrical Engineering (ETI), Professors Martin Doppelbauer, Marc Hiller, and Michael Braun work on optimizing the power-to-weight ratio. Their motor weighs only 4.5 kg and delivers 30 kW of continuous power. By international standards, this is absolutely top performance.
It does not take an expensive Tesla S for an electric car to outrace even powerful competitors at the traffic light. Fast starts are almost standard with electric motors. That’s one of the major differences relative to internal combustion engines: Full torque is available from scratch. “This is great fun because you can achieve very high acceleration,” explains Martin Doppelbauer. He and his ETI colleagues developed the Karlsruhe motor with its near-record power density of almost 7 kW/kg. Peak levels like these need so called permanently excited synchronous motors (PSM) with magnets in the rotor. However, this solution has the drawback of being relatively expensive. These rotating high-power magnets need neodymium and dysprosium rare earth elements mined mainly in China, a country that is vigorously trying to control exports. This has given rise to extreme price fluctuations recently.
Asynchronous motors, on the other hand, use only copper and steel in their rotors, which makes them less expensive. However, they are unable to achieve peak power densities. Ultimately, like many features in electric cars, decisions about the electric motor under the bonnet are the result of a cost-benefit assessment of the entire system. “All car manufacturers are currently preparing for the use of asynchronous motors in case the costs of rare earths were to explode again,” says Martin Doppelbauer.
In any case, another decisive feature of electric motors is efficient cooling. Cooling of the copper windings in the fixed part, the so-called stator, is indispensable to achieving optimum power densities. Water cooling is the state of the art; alternatives, such as oil cooling, are under development. Another important factor determining power is the magnetic flux density in the magnetic sheet steel. This sheet is currently made out of iron-silicon alloys. “A lot has to be done,” says Marc Hiller, “for these extreme power densities to be achieved in reality.”
Another characteristic of e-motors are high speeds. The higher the speed, the smaller the motor and the higher the power density. Present standard electric motors run at speeds between 10,000 and 14,000 rpm. This is more than twice the speed of internal combustion engines. Racing motors are even able to achieve 20,000 revolutions per minute in continuous operation. However, there are limits in physics. It is impossible to increase ad infinitum the speed of the rotating part. To match the high speeds to the wheel speed, a gearbox must be connected in between. These gearboxes are much simpler in design than today’s standard manual transmissions of internal combustion engines.
Half the cost of an electric drive train is attributable to power electronics. Its size, which is approximately 10 l right now, leaves room for optimization. The central function of this component is conversion of the direct voltage supplied by the battery into three-phase alternating voltage. This is a highly complex problem. The desired speed and the desired torque must be produced despite the varying charging status of the battery. Again, temperature management plays a key role. At present, power electronics achieve efficiencies of 96 to 99 %. 100 kW power, however, still means a thermal load on the order of up to 4 kW. “This must be removed reliably from a relatively small unit,” is Marc Hiller’s description of the challenge posed by temperature management, “and that in an environment characterized by high temperatures.” Power electronics consist of silicon chips of the type also found in computers. The chips must not exceed a temperature of 175°C, levels rarely reached because of the limit temperature of the housing of 110°C. “In the hot environment of the electric motor, a lot of engineering skill is required if these temperature limits are not to be exceeded, plus the fact that this should function in the summer as well as in the winter.” Also the abrupt load changes caused by braking and acceleration constitute a problem to power electronics. As in battery research, new materials might push the power limits. So-called wide band gap semiconductors, such as silicon carbide (SiC) or gallium nitride (GaN), have higher efficiencies and are more resistant to thermal loads.
However, the advantages of new materials are really effective only when power electronics and e-motor are combined. “The present trend is to have only one component. There will then be a positive and a negative pole simply to be connected to the battery. This is all that is needed.” Marc Hiller sees the main advantage of a compact design in the absence of long cable links and plugs and sockets. “The cable between the inverter and the motor experiences high abrupt voltage changes. As a rule, the initial level is not purely sinusoidal. Normally, very many voltage portions are produced which, when aggregated, result in a sinusoidal shape. This gives rise to wave phenomena in the cables and plugs and sockets which, in turn, must be taken into account in motor design.“ The compact design makes temperature management of power electronics even more demanding. For this reason, Martin Doppelbauer is convinced that the entire drive system, from the battery to the motor, needs to be developed and optimized in a holistic, interdisciplinary way. “The power supply system – battery, power electronics – exerts a decisive influence on motor design. On a medium term, this also offers an opportunity of novel technologies being used which are hardly known today.”
Tesla boss Elon Musk consistently and strongly criticizes plug-in hybrid systems. Their sophisticated combination of different drive units made them similar to amphibious vehicles “ideal neither in the water nor on the ground.” Marc Hiller also believes that the future belongs to electric-only vehicles, provided that the right technologies are available at reasonable prices. “If occasional long distances are to be covered, hybrid vehicles are still indispensable because of their practical usefulness. At the present time, it is impossible to foresee that charging times of clearly less than one hour will be feasible with today’s battery technologies without considerably damaging batteries in the process,” admits Martin Doppelbauer. So, internal combustion engines are hard to replace for regular long-distance operation.
Marc Hiller also contradicts the common prejudice of Germany having missed the boat when it came to the development of e-mobility. “Especially as far as motors and power electronics are concerned, Germans have not missed the boat. Technically speaking, we are far advanced. However, it is still difficult to get this on the road.” This was due also to the long tradition of large German automotive manufacturers building internal combustion engines, especially Diesel engines. In addition, there was a general lack of willingness on this side of the Atlantic Ocean to make long-term investments in electromobility. ”This is what Tesla is currently doing. That company is building by far the largest battery factory in the world right in the middle of the desert of Nevada. This certainly makes no economic sense today. However, it will clearly reduce prices in the future and improve battery availability.” In the opinion of the KIT scientists, the purchasing bonus currently discussed in politics could have a similar effect. It could help solve the complicated e-mobility problem of what was first, the hen or the egg. “Small volumes mean high costs of components, while expensive vehicles mean low volumes. This vicious circle must be penetrated.” If this could be achieved, also an important contribution to the energiewende would have been made “because a wider distribution of electric vehicles would require faster adaptation of the power grid structure. Although electricity consumption is not going to rise dramatically, grids must become much more decentralized and flexible which, at the same time, advances the possibilities of feeding from renewable sources.”
The text was written by Dr. Stefan Fuchs.
Exzerpt auf Deutsch
“Leicht. Schnell. Belastbar. – Am Elektrotechnischen Institut des KIT entstehen leistungsstarke E-Motoren”
Übersetzung: Ralf Friese
Im Bereich der Elektromotoren wurden in den letzten Jahren große Fortschritte gemacht. Auch hier geht es darum, möglichst hohe Leistungen mit möglichst kleinen und leichten E-Maschinen zu erreichen. Einem Wissenschaftlerteam am Elektrotechnischen Institut (ETI) des KIT um die Professoren Martin Doppelbauer, Marc Hiller und Michael Braun gelang die Entwicklung einer permanenterregten Synchronmaschine (PSM) mit einer bisher unerreichten Leistungsdichte von 7 kW pro Kilogramm Motorgewicht. Es handelt sich dabei um eine Synchronmaschine, bei der im Rotor Hochleistungsmagnete angebracht sind. Voraussetzung für derart effiziente Elektromotoren ist vor allem ein optimales Kühlungssystem für die Kupferwicklungen im feststehenden Teil der Maschine und eine hohe magnetische Flussdichte in den Elektroblechen. Zum Antrieb eines E-Autos gehört neben Batterie und Motor auch die Leistungselektronik. Sie wandelt den von der Batterie kommenden Gleichstrom in dreiphasigen Wechselstrom. Auch hier kommt es zu einer beträchtlichen Temperaturbelastung, die möglichst effizient abgeführt werden muss. Der Trend geht gegenwärtig zur Integration der Leistungselektronik in den Elektromotor selbst. Auf diese Weise entfallen Kabelverbindungen und Stecker, in denen sich störende Wellenphänomene entwickeln können. Gleichzeitig verschärft eine Kompaktbauweise aber die Problematik des Temperaturmanagements. Für das Team des Karlsruher Rekordmotors leistet die E-Mobilität einen wichtigen Beitrag zur Energiewende, weil sie den Umbau zu dezentralen und intelligenten Energienetzen beschleunigen wird.
Photos: Markus Breig