Impact of Asymmetric and Symmetric Overhangs on Torque Quality and Axial Magnetic Force Computations in Surface Mounted PM Synchronous Motors

Abstract

I. Permanent magnet (PM) motors have become great alternatives to traditional motors. Overhang, either on the rotor or the stator, is commonly used is these motors. In this study, various alternatives of stator or rotor overhang in PM synchronous motors are investigated in detail. Both asymmetric and symmetric overhang cases are considered in the paper. Impact of overhang on the torque quality and torque densities are investigated using 3D Finite Element Analysis (3D-FEA). Axial magnetic forces during assembly with and without overhang are also simulated and variation of axial forces for different cases of overhangs are explored. A prototype PM motor with rotor overhang is used to validate the analyses experimentally.II. INTRODUCTIONRotor overhang is commonly used in different types of permanent magnet motors for numerous reasons. There are various studies in literature covering rotor overhang including magnetic modelling, 2D and 3D-FEA mostly for linear permanent magnet and induction motors, axial flux motors and brushless DC motors. Very limited studies exists on radial flux PM synchronous motors. Most studies are focused on analytical modelling of overhang and 2D/3D simulations [1-4]. However, no detailed investigation on both symmetric and asymmetric overhangs on the motor performance are conducted. Influence of torque quality, cogging and back-EMF harmonics are not covered especially for asymmetric overhang cases since some applications do require asymmetric placement of the rotor. In this paper, both asymmetric and symmetric overhang cases will be investigated in detail. Impact of such cases on the motor performance and torque quality are covered. Axial magnetic forces are also be explored for PM motors since it is a crucial issue in assembly of framed and frameless PM motors.III. INVESTIGATED PM MOTOR CASES AND FEA SIMULATIONS AND IV. EXPERIMENTAL VERIFICATIONDetailed 3D-FEA is performed for different cases of overhang. The motor is selected as a low speed surface mounted permanent magnet synchronous motor as shown in Fig. 1. The prototype motor in a test system is also shown in Fig. 2. Various cases of overhang is investigated in great detail using 3D-FEA. Cases-1 does not have any overhang. Case-2 has rotor overhang and Case-3 has stator overhang. In each case, rotor stack is kept the same and torque quality as well as motor performance is explored. Axial magnetic forces acting on the rotor are also covered in 3D-FEA as seen in Fig3, 4 and 5. A prototype motor with rotor overhang is also manufactured and tested for different cases and some of the 3D-FEA simulations including axial magnetic forces are verified. The test results and comparison with the FEA simulations will also be given in the final version of the paper. **