Development of Variable Stress Applying System for Shrink Fitting of Stator Housing in IPM Motor

High efficiency electrical machines such as rotating machines and transformers are expected to be realized for solving the environmental problem. It is well known that the iron loss of stator core for rotating machines increases by a mechanical stress caused by the shrink fitting of the stator housing. In order to clarify the influence of the mechanical stress to the characteristics of rotating machines, several papers [1]– [6] describe the simulation technic such as the combined analysis of a mecahnical stress and an electromagnetic field. In this paper, we develops the variable applying stress system with the hydraulic unit to clarify the influence of the shrink fitting to the motor characteristics. Fig. 1 shows the schematic of the variable applying stress system. We designed these system following three requirement. (a) The circumferential compressive stress of the stator core caused by these system is more than 100 MPa. (b) These system can adjusts the circumferential compressive stress of the stator core at an interval of 1 MPa. (c) These system uniformly applies the circumferential compressive stress of the stator core in the axial and circumferential direction. In order to realize the concept of these three points, these system is adopted the hydraulic mechanism. When the hydraulic unit applies the oil pressure to the oil room, the pressure bulk head is deformed by the oil pressure and consequently the circumferential compressive stress is generated in the stator core. These system can adjusts the circumferential compressive stress of the stator core by the oil pressure of the hydraulic unit. The circumferential compressive stress, which is measured by eight biaxial strain gages installed on the surface of the back-iron for the stator core, is changed linearly with respect to the applied oil pressure by the hydraulic unit, and the maximum circumferential compressive stress is generated more than 100 MPa under the oil pressure 15 MPa. Fig. 2 shows the measurement results of the iron loss in no-load with respect to the applied oil pressure by the hydraulic unit. The load motor rotates the test motor, which is adopted the interior permanent magnet motor with concentrated winding, at a constant rotating speed. Then, the loss of the test motor with the unmagnetized and magnetized permanent magnet rotor in no-load is measured by the torque detector. The iron loss of the test motor is calculated by a difference between the loss of the test motor with the magnetized permanent magnet rotor ant it with the unmagnetized permanent magnet rotor, which is included only the mechanical loss without the iron loss. As the applied oil pressure increases, the iron loss is gradually increased and the iron loss under the circumferential compressive stress of 100 MPa (applied oil pressure 15 MPa) in the stator core is increased by 2 times compared with the non-stress. As explained above, the proposed system can measures the iron loss of the actual motor under mechanical stress with various operating point. The more measurement results will be included in the full paper.