Modeling of Equivalent Circuit Considering Welding Losses on Electrical Steel and Characteristic Analysis for a Traction Motor

Abstract

The interior permanent magnet synchronous motor (IPMSM) has been widely used for the traction motor in the HEV/EV due to high efficiency, high torque and power, wide driving range for the fuel economy and GHG reduction. For the HEV/EV, the fuel economy is strongly related with traction motor efficiency, therefore, researches on various fields such as electromagnetic design, control, process and material have been performed to increase the motor efficiency. Since electrical steel that is used for the stator core is dependent on the mechanical stress and strain, the motor performance can be affected by the stamping, interlocking, welding and shrink fitting during the manufacturing process. For the stator core lamination process, especially for the traction motor, Tungsten Inert Gas (TIG) welding is widely used. This TIG process affect the additional losses due to the formation of the short circuits and thermal residual stress. However, there is a definite lack of literature to implement the welding loss on the motor design. Instead, electromagnetic design, control strategy and processing effects on electrical steel have been proposed to increase the motor efficiency and power density only. This research addresses the effects of welding losses on the traction motor output as well as efficiency. Since the losses due to welding are composed of the eddy current losses and hysteresis losses due to the thermal stress so that is it not feasible to calculate the losses exactly. Therefore, in this study, the welding losses were measured directly by the welded ring cores with 200 mm diameters obtaining 0, 4 and 6 welding passes respectively. After that, magnetization and iron losses were measured in terms of the magnetic flux density and frequency. Separating the welding loss data from the experimental loss result, the general quadratic equations were delivered in terms of the welding passes and stacking lengths. In addition, the equivalent loss resistivity was calculated from the measurement result, and proposed novel equivalent circuit tacking account of the welding losses. For verification of the proposed method, the losses were measured and compared by the load test with a 80 kW traction motor for EV. Analysis results indicate that the welding loss increases about 10 % of the iron loss for 6 passes decreasing over 1 % of motor efficiency at low torque and 5000 rpm, whereas high torque region where the winding losses are dominant or high speed region at the flux weakening point, the welding losses are decreased relatively lower than the winding losses resulting in little effect on the motor efficiency and output. With 8 pass welding, it is shown that 0.1 % of efficiency decrease at overall area including 0.5 % efficiency reduction at the maximum efficiency. This analysis result implies the welding loss influence could be varied in terms of the welding number of passes indicating that we can minimize the welding losses by the optimal determination of the welding passes. Furthermore, this deterioration may affect the fuel economy especially, at the city mode driving since there are most frequent driving points within this range which is affected significantly.