FlexiCoil – Development of an Economical Series Production Process for Preformed Coils in Electrical Drives


According to the state of the art, stator coils of an electric machine are wound from conventional enameled copper wire with a round conductor cross-section. This process limits the maximum achievable copper fill factor and thus the efficiency of electrical machines. The aim of the research project FlexiCoil was therefore to develop a process and tool technology for the economical production of electric motors with a high copper fill factor. To this end, coils with variable conductor cross-sections produced using forming technology were investigated and manufactured.

  Cross-section Flexicoil Copyright: © IEM Cross-section of a stator segment with the FlexiCoil winding.

The target geometry of the coil was determined at the start of the project. Preliminary tests showed that multi-stage forming achieves the best compromise between manufacturability of the coil and near-series, economical production. The target geometry could be realized with five forming stages. The formed coils were insulated with an epoxy-based varnish and installed in the prototype. To enable assembly, the stator was designed without tooth tips. Additionally, a segmented stator was used. The efficiency and thermal behavior of the test carrier were determined by measurement and compared with the behavior of a reference machine with round wire winding.

The copper fill factor achieved by FlexiCoil coils exceeds that of a conventional winding by 33%. At high torque, this results in a reduction of copper losses of up to 17%. The innovative production of the coil leads to an increase in the production costs of the complete drive of 10 to 15%. The molded coils are still in the early stages of development. Future research is expected to further reduce the cost of the coils. Already, the FlexiCoil project shows a way to increase the copper fill factor of an e-machine and thus the power and torque density as well as the efficiency.