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Fraunhofer IWU: Precision-fit windings – innovative forming technology makes electric motors more powerful and efficient

June 8, 2023. Electric motors play a key role in the mobility revolution: Their operation does not cause any emissions that are harmful to the environment or the climate. Nevertheless, weight, required installation space and, in particular, energy consumption contribute significantly to the ecological footprint of a vehicle. An important factor for the efficiency of an e-machine is the degree of utilization of the available installation space for the copper windings in the stator. Fraunhofer IWU is now relying on forming production processes to increase the so-called slot filling factor from the current average of 50 percent to over 80 percent by means of position-matched windings.

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A groove in the stator could be filled to over 80 percent by a position-adapted wire geometry. State of the art for today's windings: 50 percent. (Photo: Fraunhofer IWU)

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The automotive industry usually relies on the so-called permanently excited synchronous motor for electric drives. This consists of the essential components stator and rotor. The rotor is the rotating inner part of the motor, which contains a set of permanent-magnet poles that interact with the magnetic field of the stator. The stator is the stationary outer part of the motor, which contains a set of coils that are supplied with electric current to generate an alternating magnetic field. As a result, the rotor rotates at a certain speed, depending in particular on the intensity and frequency of the magnetic field.

The use of concentrated windings, also known as toothed coils, is particularly suitable for the simple construction of a stator while at the same time making targeted use of materials. Here, the coils of copper wire each wrap around one tooth of the stator. Due to the circular outer geometry of the stator, trapezoidal grooves are formed between the teeth. However, according to the current state of the art, automated winding processes with round wire can usually only fill about half of these grooves. Grooves filled to more than 80 percent would further improve the efficiency and power output of the motor due to lower electrical resistance, better current flow and resulting higher magnetic field strength; alternatively, a motor could be dimensioned smaller for the same power output, and the weight and required installation space would be reduced.

The Fraunhofer IWU is researching forming process chains for the production of electromagnetic tooth coils in order to increase the groove filling factor. The special feature here is that, depending on the position within the slots, the wire is to be designed geometrically in such a way that the space available at the respective position is filled in the best possible way (position-adapted winding geometry). For example, at the level of the wider base side in the trapezoidal groove, a flat and wide design of the winding makes sense, while on the short base side a narrow wire geometry stands for the best possible utilization of space. The conductivity of the wire in the respective geometry must also be taken into account.

For the production of such a winding, in which the wire and windings require different geometries depending on their position, Fraunhofer IWU relies on forming technology, as this approach has a number of advantages. During production, the material is utilized in the best possible way, and there is no waste material that would have to be recycled. Compared to additive manufacturing processes, for example, the energy required for production is low; simple tools are all that is needed to produce high volumes in short cycle times and thus ensure economical manufacturing.

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Photo: Fraunhofer IWU

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