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Electric motors: These are the development trends

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BMW already in the company name. But not only the people of Munich see their central competence in the engine. Whether that is with the E lektroauto will stay that way in the future is one of the most interesting questions.

Toyota is taking the development of the electric motors into its own hands

Because at least so far Toyota the only manufacturer that is the Prius III has taken the development into its own hands, while Lexus gets its engines from Aisin. Mercedes , for example, uses the electric motor in the Mercedes S 400 Hybrid (der Mercedes S 400 Hybrid in the comparison test ) on the supplier ZF-Sachs and with the Smart ED on Zytek. Suppliers have the opportunity to distinguish themselves in the field of electric motors. Most important today are synchronous motors, which, as external or internal rotors, belong to the category of three-phase motors.

Three-phase motors:
Three-phase motors are common practice for machine tools, as they are based on the three-phase Use alternating current from the grid. In the car, which is powered by the direct current of the battery, three-phase current must first be generated on board - a process that is fraught with losses. The trend towards automotive three-phase motors was driven by the enormous progress made in electronics development. It plays a crucial role in energy management between the battery and the motor: it has to convert the battery voltage into a multi-phase alternating voltage via an inverter and then control the electric motor according to the situation using power electronics. Currents of 150 amperes can quickly arise - almost ten times the current of a normal household socket.

Crucial when designing suchControl devices are that they work with as little loss as possible, i.e. with a high degree of efficiency. And the electronics, which so far have easily reached the size of a ten-liter bucket of water, are to be significantly more compact. At Bosch and Continental, halving it in the next few years is believed to be feasible.

Asynchronous motors:
The advantage of three-phase motors lies in their power density and their universal design. The asynchronous motor is the simplest and therefore inexpensive to manufacture. Its rotor consists of a simple laminated core and a short-circuit winding that is used to generate a magnetic field. Because there is a time delay between the field generation in the rotor and the field in the stator, it runs with a time delay - i.e. asynchronously. Such engines often came in prototypes like the BMW X5 Efficient Dynamics from 2004 in use. Currently only Tesla uses an asynchronous motor in its electric roadster.

Synchronous motors:
The synchronous motor scores with even better efficiency and high power density, the speed of which is the frequency specified in the stator certainly. The rotor moves synchronously with its rotating magnetic field. This requires more control effort. Nevertheless, both with e-models and with hybrids the trend towards synchronous motors, namely to the permanently excited version, the rotor of which is based on high-quality and relatively expensive permanent magnets. From Toyota to Honda to BMW and Mercedes Almost all manufacturers use this principle. Experts therefore see it with a certain concern that the rare earth elements neodymium and dysprosium, which are necessary for the production of magnetic materials, could become scarce and more expensive in an electric car boom.

The way out should be via separately excited synchronous motors lead, in which the magnetization is done by an external power supply. It goes without saying that such an additional power consumption degrades efficiency and costs money.

Types:
Motors for purely electric drive are usually arranged centrally on the drive axle and interlocked with the axle drive. The power reaches the wheels via cardan shafts. With such an (underfloor) arrangement, a compact, block-like design is required above all.

Quite different with the arrangement close to the crankshaft in hybrid models. Manufacturers such as ZF Sachs or Bosch rely on a disc-shaped design that can be integrated between the engine and transmission. Depending on the place of use and installation position, the rotor can run inside or outside the stator fixed to the housing. While in the Lexus LS 600h aIf the internal rotor rotates, the Mercedes S 400 Hybrid relies on an external rotor. The latter design is also very well suited for wheel hub motors - by the way, a design that is recommended for purely electric cars because it requires little space.

This is how electric motors work

Electric motors convert force generated by electromagnetism into rotary motion. They basically have a stator that is firmly connected to the housing. In the example shown in the photo show, electricity flows through copper wire windings and thus generates the necessary magnetism in the iron cores.

The rotary movement is initiated with the help of alternating magnetic fields. The rotor - in this case an external rotor - follows the specified rotation. The rotor then transmits this rotation to an axle. However, there are very different designs of electric motors. With DC motors, for example, the pole change of the magnetic fields is generated by the movement of the rotor itself via sliding contacts. In this case, the magnetic field is created via the rotor winding, through which current of constantly changing polarity flows: sometimes plus, sometimes minus.

Unlike DC motors, three-phase motors can avoid wear caused by sliding contacts. The three-phase current from the socket guarantees a “natural” rotating field. If the stator windings are supplied with the three phases of the three-phase current, magnetic fields which are built up and decayed are created around them, which the rotor then follows.

Lexicon on the subject of electric motors

Torque characteristics - electric motors are characterized by a high starting torque, which only drops in the area of ​​maximum motor power.

Three-phase alternating current, the three phase positions of which are each offset by 120 degrees.

Recuperation - electric motors can be used as generators in overrun mode, thus allowing partial feedback the kinetic energy in the battery. With three-phase motors, practically no additional switching effort is necessary.

Rotor - moving drive element of the electric motor.

Stator - part of an electric motor that is firmly connected to the housing.

Inverter - electronic converter that converts direct current into single or multi-phase alternating current.

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