PSI - Issue 31

Sanjin Braut et al. / Procedia Structural Integrity 31 (2021) 45–50 Sanjin Braut et al. / Structural Integrity Procedia 00 (2019) 000–000

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1. Introduction The electrical traction machines are becoming increasingly important due to nowadays, ecological requirement. The requirements faced by electric traction motors are high power density and high efficiency. The permanent magnet synchronous motor (PMSM) meets the mentioned requirements and it is becoming a trend in traction application, De Santiago et al. (2012) and El-Refaie et al. (2013). To optimize the design of traction motor a multidisciplinary approach, Rølvåg et al. (2020), combining electromagnetic, thermal and mechanical analysis should be applied. Usually, it is done by evaluating the electromagnetic performance as a function of stationary rotational speed of the machine and using simulated electromagnetic losses as heat sources in thermal analysis. The aforementioned design procedure is not optimized for traction motors, since the operating conditions in traction motors vary significantly. According to references: Chai et al. (2016), and Lindh et al. (2016), the centrifugal force acting on the rotor structure is considered as the dominant stress source, while the effect of the thermal loads on the mechanical stresses and fatigue life were neglected. Cazin et sl. (2009) and Baragetti et al. (2020) studied fatigue life of rotating structures due to Bending stresses. According to the published literature, Sikanen et al. (2018) and Huang et al (2019), present one of the first attempts to properly estimate fatigue life due to transient thermos-mechanical analysis of electric machine. In this paper fatigue life of PMSM rotor is estimated according to stress-based approach with mean stress correction and variable amplitude loading obtained from transient thermal – structural analysis. Loads in thermal – structural analysis, performed in ANSYS 2020 R2, were defined based on the measurements performed on electric sports car rear right wheel and electro-magnetic analysis of eddy current losses in permanent magnets. Variable equivalent stress amplitudes were identified with rainflow counting technique for time history of equivalent stresses for critical rotor position. Stress amplitudes were then corrected with three mean stress correction models, namely, modified Goodman, Morrow and Smith-Watson -Topper (SWT). Total damage of the rotor was estimated according to Palmgren-Miner rule and modified S–N curve is a way to include reliability and rotor surface condition. 2. Transient thermal and structural FEM analysis The subject of study is PMSM rotor of a full electric sports car Electric RaceAbout. As the initial mechanical stress analysis shows that critical stresses are well below the rotor material yield strength, a stress-based fatigue approach is selected to estimate the rotor life. To conduct a thermal and mechanical transient analysis, it was necessary to define the loads that occur in the actual exploitation and to characterize the material. However, at the beginning, it is explained what the PMSM motor serves. 2.1. Permanent magnet synchronous motor (PMSM) PMSM traction motor was designed as a three-phase sixteen-pole double-deck embedded permanent magnet configuration. It is originally built for a 4 × 4 full electric sports car Electric RaceAbout (ERA) i.e. one of four similar direct-driven motors. Rotor structure consists of two layers of shaped NdFeB permanent magnets embedded inside the laminated rotor body. Cooling of the motor is carried as a combination of air cooling in the air gap region and liquid cooling in the stator frame. More details can be found in Nerg et al. (2014) and Sikanen et al (2018). 2.2. Transient loads The loads in transient thermal and structural analysis come from tests made on the rear right wheel of the vehicle running on the Nürburgring Nordschleife track. Originally, test results are given in the form of rotational speed, torque, voltage and current, but here in this study, only rotational speed history was used. Other loads were obtained indirectly, i.e., from a combination of measurement results and electromagnetic FE analysis results. The thermal loads (internal heat generation), that is, the eddy current losses in the permanent magnets and the iron losses in the rotor laminations were calculated by 2D-FEM analysis made in Flux2D software by Cedrat. The forced air cooling in the air gap region and the rotor front face region were modeled as variable speed dependent convection.