Tolerance induced rotor protrusion and resulting torque fluctuation of a synchronous machine

Tolerances in electrical machines can influence their characteristics. These influences can lead to torque fluctuation and are therefore unwanted. Neglecting tolerance effects can lead to the loss of the certification of conformity of industrial systems. Tolerance induced rotor protrusion occurs usually through uneven tolerance ranges for stator and rotor core. This paper studies the probable appearance of rotor protrusion by measuring a significant number of mass produced rotors. Based on these measurements rotor protrusion is likely to happen. To assess the influence of this tolerance on torque output a three dimensional electromagnetic finite element model is built. The model represents an internal permanent magnet machine in the 100 kW range used for hybrid drive applications. The machine is studied in an unskewed and a skewed variant. The results show up to 3 % reduction of maximum torque output. The unskewed machine additionally exhibits a nonlinear influence. This nonlinearity results from flux concentration effects on the protruding end which are further discussed. Protrusion also intensifies axial forces by almost 50 %. In conclusion two measures to reduce the impact or size of this tolerance range are discussed.