Effect of Bending-Induced Mechanical Stress on the Dielectric Response of Rectangular PAI/PEEK Insulated Winding Wire

The e-mobility application requires high-efficiency and reliable electric motors. Winding insulation is one of the critical components of machines in terms of reliability. Nowadays, hairpin winding is the most used technology for manufacturing motors for electric cars and aircraft. Although this technology has several advantages, such as a high slot-filling factor and reduced manufacturing costs, the winding wire is exposed to high mechanical strain during the shaping of the hairpins. The authors' previous study revealed that the high mechanical stress resulting from low bending radii causes reduced breakdown voltage of winding wires with layered insulation. This study analyzes the dielectric response of bent PAI/PEEK double-layer-insulated rectangular winding wire. For the measurements, 17 cm long, straight wire samples were prepared and covered with semiconducting tape as an outer electrode. The samples' dielectric response was tested with an OMICRON Dirana dielectric spectrometer. Then, the samples were bent, and their dielectric response was measured again. The results show that the wires' tan delta increases with the bending radius in the kHz and mHz ranges. In these frequency ranges, different polarization processes are most influential on the dielectric response of the samples. At the lower frequency range, the interfacial Maxwell-Wagner process dominates, while at the higher frequency range, orientational polarization affects the dielectric loss of the samples. The increased loss due to the Maxwell-Wagner polarization can be explained by the mechanical stress at the interfacial region of the two (PAI and PEEK) insulating materials. The increased loss in the kHz range probably results from the change in the segmental motion of the polymer chain due to the mechanical stress of the insulation. The study's results revealed that analyzing dielectric response is a potential tool for testing the mechanical stress induced by extreme bending during manufacturing.