Rotor dynamic analysis of asynchronous machines including the finite-element-method for engineering low vibration motors

In addition to the mechanical excitations, such as unbalance, also electromagnetic forces due to eccentricity cause vibrations in asynchronous machines. For slip-ring induction machines without parallel windings in the rotor and stator the magnetic spring, which describes the magnetic pull due to these eccentricity fields, is independent of the kind of eccentricity - static and dynamic eccentricity - and independent of the orbit of the rotor. For squirrel cage rotors the static and dynamic eccentricity as well as the orbit of the rotor influence the value of the magnetic spring. A theoretical rotor model is presented to describe the orbits of the rotor for three different kinds of dynamic eccentricity excitation- magnetic eccentricity, eccentricity of the rotor mass and a bent shaft. A finite-element calculation of the undamped magnetic spring value is shown as well as a finite-element rotor dynamic calculation of a 2-pole asynchronous machine with a single sided rotor core eccentricity. With the separation of the calculated shaft displacements the influence of each excitation on the shaft vibration can be evaluated, and the rotor design can be optimized for the operating speed range