The future wound in a coil
Why an electric motor?
After the internal combustion engine provided practically unrivaled propulsion in personal mobility over the past 130 years, a paradigm change is currently emerging. To curb climate change and to noticeably reduce the high levels of noise and air pollutant emissions in big cities, not only new mobility concepts are needed. Needed as well are drive systems that don’t use fossil fuels and operate with zero emissions, at least locally. This puts the focus on a propulsion system which the German automotive industry ever since the construction of the Flocken car in 1888 had lost sight of to some extent: the electric motor. On a small scale, it’s been operating in billions of household appliances of all kinds, be it in washing machines or blenders. On a large scale, it’s been propelling trains, moving people in elevators and lifting loads in cranes. Now it’s about to see a revival in automotive engineering.
How an electric motor works
Our physics teacher used to explain it like this: Simply put, an electric motor consists of three components: a stator, a rotor and a commutator. The stator is a magnet that’s permanently connected to the housing. The rotor is located between the north and south sides of this magnet. It sits on the motor axis and consists of iron around which copper wire is wound. When current flows through the wire the rotor becomes an electromagnet that also has a north and south pole. Because the same magnetic poles repel each other the rotor rotates as soon as the same poles of the rotor and stator are in proximity of each other. However, after half a rotation, the mutually attractive north and south poles of the two magnets would be next to each other and the rotation would instantly stop. That’s why, after half a rotation, the direction of the electric current in the rotor’s copper wire is simply reversed. This reversal of polarity is achieved by means of the sliding contacts in the commutator. In the case of AC motors, the polarity of the magnetic field automatically reverses in sync with the cycle of the grid frequency, which at 50 hertz occurs 50 times per second.
Which motors are used in electric cars?
DC electric motors no longer play any part as traction motors in today’s passenger cars. They’re only used as servo motors, for instance in power windows. Electric vehicles use AC motors for propulsion. In spite of their greater technical complexity, they offer a range of advantages: long life and low maintenance and, above all, high levels of efficiency and power density. Unlike those of DC motors, the stators of AC motors consist of at least three coils. Together, they generate a rotating magnetic field corresponding to that of the rotor.
Are all AC motors created equal?
No, AC motors are classified either as synchronous or asynchronous motors. In a synchronous motor, the magnetic field moves in synchronicity with the rotor speed. The excitation of the rotor may emanate from permanent magnets. In this case, the motor is referred to as a permanently excited synchronous motor (PSM). Or the excitation is caused by currents induced in the windings, in which case the motor is a separately excited synchronous motor (SSM). The magnetic rotation force is generated by the current flowing through the windings of the stator. In the case of the asynchronous motor (ASM), the magnetic field moves in interaction with the excitation field of the rotor, i. e. asynchronously. In this case, the current in the rotor windings generates the magnetic rotation force with the excitation field.
Asynchronous or permanently excited synchronous motor?
It depends on the purpose for which it’s used. A permanently excited synchronous motor (PSM) is basically superior to an asynchronous (ASM) unit in terms of efficiency and power density. At higher rotational speeds, the efficiency of an ASM comes closer to that of a PSM. However, a well-designed PSM will always win out as a traction motor, although an ASM has advantages, too: It doesn’t require expensive magnets and has no drag torque. The latter means that it doesn’t produce any counter torque when it’s not energized – and thus won’t slow down propulsion. Consequently, an ASM is ideally suited for use as an additional drive motor, for instance in an electrical all-wheel system, because the vehicle in large part is driven only with the main drive axle and the ASM just runs in parallel. Now the benefits of both sides, i. e. high-level power density, good efficiency, no magnets and no drag torque, need to be combined. “In this direction, we can expect considerable potential of the separately excited synchronous motor. There’s a lot happening in this area at the moment,” says Thomas Pfund, who leads Schaeffler’s E-Systems business unit
How can the electric motor still be improved?
Ever since the auto industry has rediscovered the electric motor as a propulsion machine, vehicle manufacturers and suppliers have been busy exploring ways to optimize this technology. Expert Pfund outlines the challenges: “Efficiency and power density must be enhanced, production costs reduced by new manufacturing technologies and optimized material utilization, and noise emissions minimized” – after all, an electric automobile should not be humming like a streetcar. Even more important, he adds, is another effect: “Due to the growing efficiency of the motor, future electric vehicles will require less energy and, as a result, deliver much greater range on a single battery charge.”
Is Formula E helping to optimize the electric motor?
In motor racing, the auto industry tests the limits of what’s technically feasible. This applies to Formula E as well. The PSM electric motors in the race cars of the Audi Sport ABT Schaeffler team are among the most efficient power-plants in the entire field, but also extreme units from a detailed perspective: The steel panels making up the steel packages of the Audi e-tron FE06 have a mere thickness of 0.05 millimeters (0.002 inches) in order to minimize losses induced by “dirty air.” Obviously, this comes at a price. The steel sheets currently used in production cars are about five times as thick. Such examples show where materials and production processes reach their limits and result in important findings that Schaeffler contributes to mass production of e-motors. Another topic with production relevance is the fact that SiC semiconductors operate with clearly higher cycle frequencies and therefore also have effects on electric motors. Formula E started using this technology and now SiC inverters have found their way into production, too. Clever designs, e. g. for cooling systems, have great potential for adoption by mass production as well. Schaeffler’s expert Pfund is sure that “the race has just begun.”
What’s the role of coil winding?
Traction motors for electric cars operate at high rotational speeds. The rotor may be spinning around its own axis more than 18,000 times per minute. To keep the motor from overheating while continuously delivering high power density, the resistors of the energized coils must be minimized. The winding of the copper wires is a science unto itself and highly sophisticated also in terms of manufacturing technology. Bar wave winding, an innovative method developed by Schaeffler’s subsidiary Elmotec Statomat, promises a development leap in this area. A braided band of flat wires is inserted into the steel sheet package of the stator that generates the magnetic field. This principle allows a very high fill level of energized copper to be achieved. Thomas Pfund is expecting initial permanently excited synchronous machines using bar wave winding to appear in production vehicles in 2022.
Is there still potential in terms of environmental protection and cost reductions?
High field strengths like those required for traction motors can only be generated with rare-earth magnets. A commonly used light-rare-earth element is neodymium, which, in the right combination ratios with other elements such as iron or boron, becomes a magnet. However, a pure neodymium magnet loses its magnetization at temperatures of 80 degrees centigrade (176 °F). To prevent this, heavy-rare-earth elements such as dysprosium and terbium are added. These are truly rare and can be extracted only by a complex process. This makes them “brutally expensive,” according to expert Thomas Pfund. However, a clever design of the electric motor and an intelligent cooling concept has clearly reduced the proportion of these critical materials. Also, work to achieve new material combinations is in progress – although rare earths cannot be completely dispensed with.