The electric motor is 150 years old. Siemens is now redesigning it from the ground up for use in electric vehicles. The company’s focus is on increased power density.
It sounds like an inventor’s dream: A motor that converts nearly 100 percent of energy input into motion, accelerates a vehicle smoothly and evenly, and is so compact that it doesn’t require an engine compartment but is instead mounted on an axle or wheel. This is no dream, however; it’s already close to reality in cars powered by electric motors rather than internal combustion engines.
“Electric motors are the perfect drive system,” says Dr. Karsten Michels, Head of Development at the Siemens Industry Sector’s Inside e-Car unit. Nevertheless, developing electric motors is not straightforward. Although Siemens has been building electric motors for around 150 years now, it’s simply not possible to pick an industrial motor off the shelf and install it in a production car. Automakers are very demanding, and the motors have to be suitable for installation in tens of thousands of vehicles by a highly automated process. Such motors need to have a high output yet weigh as little as possible. They also have to operate as efficiently as possible and function properly even during extreme temperature fluctuations.
Determining which development goal should be given top priority depends on an automaker’s unique mix of requirements. Just as there are gasoline and diesel engines, there are also many variations of electric motors. Almost every electric motor used in vehicles today is either an electrically excited asynchronous machine or a permanently excited synchronous motor. An asynchronous machine is around 10–15 percent heavier and larger than a synchronous motor with the same output, and its power density is lower. Nevertheless, it does offer a strategic advantage in that it doesn’t require permanent magnets because the flow of current itself creates a magnetic field. The lack of permanent magnets eliminates the need for rare earth metals such as neodymium. As a result, companies that manufacture asynchronous machines don’t have to rely on China, the main supplier of rare earth metals, and they also don’t need to worry about sharply rising prices for key materials.
A characteristic feature of asynchronous machines is that their torque diminishes more strongly at higher rotation speeds than is the case with synchronous motors. The main objective with regard to future asynchronous machines is to further increase their power density. For example, researchers are working on using new sheet metal with improved magnetic and mechanical properties, and on optimizing cooling systems. Unlike a combustion engine, an electric motor has a maximum output that’s much higher than its average output. For example, a 50-kilowatt motor can easily produce 120 kilowatts of power for a short time. The length of time this “electric turbo” feature can remain on depends mostly on the type of cooling system used. Today’s electric motors are surrounded by a water jacket. A better cooling approach would be to extract additional heat directly from the inside of the motor. But the idea of snaking water lines through a rotating electric machine isn’t as risky as it sounds. “Motors that we’ve put on our test rig show that this can be done in electric vehicles,” says Michels. Asynchronous machines would benefit from this approach because heat losses are higher in their current-carrying rotors.
Close to 97 Percent Efficiency. There’s no doubt that permanently excited synchronous motors are the best-performing electric machines. They are already close to achieving efficiencies of 97 percent. In a synchronous motor, the rotor always moves precisely in line with the magnetic field created by the stator, which means there’s no slip. Their electric losses are also lower than those of asynchronous machines. Development engineers are therefore very interested in them if they can minimize their use of rare earth metals. One way of achieving this would be to utilize a hybrid motor that operates like a permanent magnet under partial load but can also cover higher output requirements using separate excitation of the magnetic field.
The team at Inside e-Car is not only looking to develop new motor concepts but also to optimize the overall drive system. The objective is to carry out customer projects by adapting proven industrial drive system technology to the requirements of automobiles. This applies especially to the inverter, which converts direct current from the battery into the alternating current the electric motor runs on. The inverter also makes it possible to store the electricity from braking energy that the electric motor produces when operating in the generator mode. “Here, the difference in requirements between industrial and automotive applications is even greater than it is with motors,” says Michels.
For one thing, the heavy batteries in electric cars make every kilo of weight and every liter of volume a major issue. An automotive inverter’s service life of around 8,000 hours is lower than in industry, but the inverter also has to withstand both bitter cold temperatures and extreme heat. Industrial inverters are often air cooled, but the power density needed for an automotive application can only be achieved with water cooling. “Industrial inverters are nevertheless a good foundation for us to build on, especially in terms of control accuracy and overall precision,” Michels explains. “We can also utilize the broad range of motor-control expertise Siemens has accumulated over many years with the regulation of multi-axis machine tools.”.
While the Inside e-Car team develops drive systems for mass use in electric vehicles, Dr. Tilo Moser from Corporate Technology (CT) in Munich is testing the limits of drive system technology. Moser was one of the developers of the Furtive eGT’s drive system. The results of his work have already been impressively demonstrated on a racetrack, where this electric sports car, which was built by the French company Exagon, achieved an output of 300 kilowatts and torque in excess of 500 Newton-meters. These numbers are similar to what a large eight-cylinder gasoline engine can deliver, but they were produced by two electric motors mounted on the rear axle. The power pack is limited to a speed of 250 km/h, not because it can’t propel the vehicle any faster but because this extends the battery’s service life.
Moser and his colleagues at Corporate Technology initially equipped a prototype with their complete electric drive system. The car’s high power density of 2.6 kilowatts per kilogram impressed Exagon so much that the company asked Siemens to develop the components for the production vehicle, which will be launched at the end of 2013. Inside e-Car and Corporate Technology are now working together closely, with Moser serving as Technical Project Manager and Dr. Franz Wagner supervising the overall project. “Our advantage here is that this project gives us the opportunity to push today’s technology to the limit even as we’re able to ensure production-vehicle quality,” Wagner explains. This applies not only to the permanent magnet’s power density but also to the drive system’s efficiency, which will be about 96 percent. Siemens developers achieved this record value by combining several measures, including the use of a special magnetic material and the adaptation of its shape and arrangement in a manner that ensures minimal energy losses.
Inside e-Car will use other new ideas in future projects. For example, it’s now working on wireless solutions for inductive battery charging like those used today with electric toothbrushes. The foundation for these solutions was developed by CT, which passed the technology on to Inside e-Car after success-fully testing it. Power electronics and electric motors will also merge in the future to create compact motor/drive system units. “There are many options here, and we’re looking at all of them,” says Michels.
Anything that makes electric drives lighter, more efficient, and more comfortable can speed up the market success of electric cars. According to International Energy Agency forecasts, more than 120 million vehicles will be manufactured in 2030, and half of them will be electric models. That would correspond to a global market volume of at least 60 million electric motors, although many vehicles will actually be equipped with more than one drive system motor.