In an electric drive, magnetic fields have to be created in order to transform electric energy into kinetic energy. The magnetic properties of the motor’s main components, referred to as electrical steel sheets, are the decisive factor in the efficiency of the electric motor. Scientists at the Technical University of Munich (TUM) have investigated the way these steel sheets are processed and have concluded that using blunt cutting tools deteriorates the magnetic properties of the steel sheets significantly.
Current criticism of electro-mobility usually focuses on long battery charging times and inadequate range. Researchers are working to increase the efficiency of electric drives in order to reduce the energy requirements of electric vehicles. This involves a large number of individual components, in particular electrical steel sheets: These sheets are important since the magnetic fields that move the motor using attractive and repulsive forces are generated within them.
Depending on the motor’s design, a variety of different holes have to be cut in the steel sheets, for example to make room for copper coils built into the motor. Each steel sheet is “stamped” individually in a press, using the same principle as a hole punch. Special cutting tools are used to ensure that the specified geometries are created in the steel sheets. Finally, the steel sheets are put together in order to form the desired shape.
Researchers from the TUM Chair of Metal Forming and Casting have investigated this process in detail in their workshop. “We wanted to examine how manufacturing the electrical steel sheets by blanking influences their magnetic properties,” says project director Hannes Weiss.
The scientists found out that the sharpness of the cutting tools used has a very significant impact on the magnetic properties of the steel sheets. The effect can be compared to a pair of scissors which dulls over time: More energy is needed to cut paper with the scissors. Regarding blanking, worn cutting edges result in higher tension in the steel sheets themselves – the material is bent and thus subject to increased mechanical stress. The resulting stress has a major impact on magnetic properties. “In some cases as much as four times the amount of electricity is needed to achieve the same degree of magnetization,” Weiss explains.
Another factor also has a major influence, the distance between the cutting edges, referred to as the cutting clearance. Once again, the process can be illustrated using the example of scissors: When the screw which holds the scissor blades together loosens, the distance between the blades becomes too large and the paper frays when cut. “Sharp cutting edges and a very small cutting clearance are optimum in achieving the best magnetic properties and thus a high level of efficiency.”
Weiss and his team have formulated recommendations for the production process. However, economic factors also have to be taken into account, the engineer explains: “When the cutting tools and their maintenance incur additional costs, the final price of the electric drives produced rises as well.”
Processing electrical steel sheets is not only an important issue in the context of electric motors. The sheets are also used in transformers, for example in mobile phone chargers and computer power supplies. The researchers also want to focus on the processing methods in this context as well. Weiss: “If we think of the enormous number of transformers in use, then even a small increase in efficiency can save a large amount of energy.”