Air gap magnetic field – key parameter for synchronous and asynchronous machine fault detection

Abstract

Calculations and measurements of a magnetic field at specific places in a machine air gap have been performed in order to determine magnetic field sensitivity to machine geometry and fault occurrence. Installation of measuring coils and a Hall sensor on and around the stator tooth, around stator yoke and the pole shoe has been considered. Firstly, a series of numerical simulations based on the FEM model were conducted for various positions of the embedded sensors. In order to confirm results of the FEM calculation, measurements on a real machine were performed. There was a good agreement between the results obtained by the measurement and the results obtained by the FEM. Calculations and measurements were conducted for various operating states: idling and rated load, as well as for the normal and faulty state. In this paper it is shown that the machine air gap is a key place for monitoring the magnetic field and a main source of reliable information about a machine condition. It is also demonstrated that the measurement sensitivity to the machine construction effects becomes lower as a distance from the air gap increases. The best measurement sensitivity has been obtained by monitoring a flux density distribution from the stator tooth. On the other hand, the lowest sensitivity has been obtained with the measuring coil embedded around the stator yoke. In the case of inter-turn short circuit of the armature winding, results obtained by measuring magnetic field in the air gap from the rotor side were more sensitive to the fault occurrence than those obtained from the stator side