Estimation of Suppressed Iron Loss by Stress-Relief Annealing in an Actual Induction Motor Stator Core by Using the Excitation Inner Core Method

It is well known that magnetic properties, such as iron loss of a stator core in a rotating machine like an induction motor, are strongly influenced by residual stress caused by manufacturing processes. In particular, both the residual stress that was generated by the punching and the laminating process greatly influence the iron loss. However, it is not easy to evaluate the magnetic properties of a complex shape like the actual stator core. We are proposing the excitation inner core method to evaluate the change in the magnetic properties of the actual stator core. This method can easily evaluate changes in magnetic properties in an actual stator core. We applied this method to a comparison of the magnetic properties before and after annealing the stator core, which was annealed by the low-frequency induction heating method. In this annealing, the center section of a tooth in the stator core was heated for eight minutes at temperatures up to about 800°, and afterward the stator core was slowly cooled to room temperature. Iron loss of the stator core before annealing decreased by 4.9% (Bex=1.5 T) after annealing.