Hairpin-winding motors are regarded as an emerging technology in electric vehicle (EV) motor technology owing to their high efficiency and compact design. While there have been some studies on the design factors that can increase the efficiency of hairpin motors, studies on the efficiency decrease of motors caused by the manufacturing process have not been adequately reported, despite the importance of this issue. This study aimed to investigate the interaction between plastic deformation and electromagnetic response during the straightening process of hairpin coils to minimize copper loss by optimizing process parameters. The increase in electrical resistivity caused by plastic deformation was quantified using integrated modeling and experimental validation, as well as its subsequent impact on copper loss. The results demonstrate that an appropriate design of the wire-straightening process can reduce resistivity increase and minimize copper loss by about 1%, which is a significant value for motor efficiency. This integrated approach provides a pathway for improving motor efficiency, thus contributing to the development of more energy-efficient EVs.