Transient finite element simulation of squirrel cage induction machines with respect to fault diagnosis and sensorless position estimation

Abstract

This thesis discusses slotting and eccentricity-related effects disturbing the symmetry of squirrel cage induction machines (IM). Regarding IM slotting, the machine composition features a periodic sequence of basic geometric components, i.e. slot and teeth segments are alternated around the circumference, forming the machine stator / rotor body. These periodic alternations introduce geometric machine saliencies and directional magnetic properties, which are reflected by asymmetric phase currents. Some of these saliencies are constant in time and space (e.g. static air gap eccentricity), others are spatially moving (e.g. turning rotor slotting or dynamic air gap eccentricity). The invariant saliencies introduce a constant offset in the machine current symmetry, whereas the variable parts are causing periodic current modulations. Measuring the phase currents thus can be exploited to deduce information on the related machine saliency. In order to facilitate a measurable current response while reducing the operating point dependency, a dedicated excitation technique is used. The drive’s inverter excites the machine with transient voltage pulses, demanding high flux linkage derivatives. In response, the squirrel cage acts as a barrier hindering the flux to penetrate the rotor and forcing it into the air gap region. Thus, the stator winding is linked to pure leakage flux, composed of slot and zig-zag leakage. A turning rotor modulates these flux components by changing the machine’s magnetic permeances. Since these modulations are linked to rotor slotting and eccentricity, they may be exploited to derive a rotor position signal and monitor the machine’s eccentricity condition. As will be seen, these two saliency sources and their interaction show strong dependence on the machine design. Investigating and describing these dependencies for transient pulse excitation is the aim of this thesis. The results can be used to assess different squirrel cage IM designs with respect to their applicability to sensorless position estimation or condition / fault monitoring. The methodology used includes analytical modeling, finite element simulations and laboratory measurements. The main saliency source investigated is machine slotting and (to some extent) its relation to eccentricity. Investigations are conducted without the presence of fundamental wave excitation. This means, no saturation dependent saliency effects are considered.