This paper deals with the losses caused by the welding to fix laminated cores of a traction motor. Although a number of studies have been performed on the motor design to increase the motor efficiency, there are few technical studies to implement the effect of manufacturing process, such as welding, on the motor design. However, the losses may have a significant effect on the motor performances. Thus, in this paper, welding loss models considering flux densities, frequencies, stack length and the number of welding passes are proposed from the data measured directly by welded ring cores. Finally, the effect of these welding loss models are verified through a comparison with measured motor losses.