Evaluation of Mechanical Properties Considering Uncertainties for Hairpin Coils in Electric Vehicle Motor Manufacturing

Abstract

The rising demand for electric vehicles (EVs) underscores the need for enhancing electric motor efficiency. Utilizing hairpin coils with a rectangular cross-section has emerged as an alternative approach to increase conductor density in motor stator slots. However, the manufacturing process for hairpin coil motors presents challenges such as springback, which stems from wire shaping processes such as leveling and bending with plastic deformation. These challenges are closely tied to mechanical properties of the Young’s modulus and yield stress. This study investigates uncertainties in mechanical property evaluation using a combination of tensile tests, analytic solutions, numerical simulations, and a present double-sided four-point bending method. The results show significant variations in the Young’s modulus, which affect the determination of yield stress in tensile tests. It has been revealed that in order to obtain stable mechanical properties of hairpin coils in tensile tests, the conditions should be restricted to a very slow strain rate, slower than $10^{-3}~\text{s}^{-1}$ . To address this, this study introduces an alternative method that combines double-sided four-point bending and tensile tests, allowing for the acquisition of Young’s modulus without prior information on material properties. This approach reduces uncertainties in determination of the mechanical properties with much less time cost.

Keywords

• Hairpin winding motor
• springback
• tensile tests
• double-sided four-point bending
• Young's modulus
• yield stress