Effect of Higher-Strength Low-Loss Electrical Steel on the Electric Energy Saving of Traction Motors Considering Thermal Analysis

Higher-strength low-loss electrical steel (ES) grades have shown a noticeable energy saving of interior permanent magnet (IPM) traction motors. This energy saving is achieved by the optimal rotor design enabled by the high mechanical yield strength feature of the utilized ES grade as well as its low loss property. In previous works, the energy saving was explored ignoring the thermal analysis. In practice, thanks to the lower dissipated heat loss, the winding resistance as well as permanent magnet are less prone to degradation in motor designs with higher-strength low-loss ES grades. Therefore, thermal analysis becomes crucial for a better estimation of the energy saving. In order to prove this concept, a coupled electromagnetic-mechanical-thermal finite element (FE) model has been built. Firstly, the optimal bridge thickness of the V-shaped IPM rotor is estimated by evaluating the von Mises stress at the maximum operating speed using a static structural analysis. Then the stator current is optimized by iterative computations of transient electromagnetic and lumped-parameter thermal models at rated operation conditions. This paper illustrates that the utilization of higher-strength low-loss ES grades results in up to 7% temperature reduction which leads to 8% energy saving and/or a significant reduction of rare-earth permanent magnet (up to 5.4%) depending on the designers' main interest.