Finit element modelling of an asynchronous motor with one broken rotor bar, comparison with the data recorded on a prototype and material aspects

This work studies the problem of broken rotor bars in cage induction motors. To this aim, an induction machine from WEG Motors was modelled by means of the finite element method. The motor modelled is a 18.5 kW four-pole induction motor with a straight rotor cage. The field distribution and stator phase currents were computed for a motor without broken rotor bars and for a motor with one broken rotor bar. The computed stator phase currents were compared to the ones recorded on motors respectively no broken rotor bar and with one broken rotor bar. The FE model and an equivalent scheme of the motor permitted to show that the resistance of the rotor cage was higher than expected, considering only the temperature effect on the conductivity of the rotor bars. A metallurgic analysis of the aluminium alloy of two rotor bars revealed the presence of a very high density of silicon and silicon oxide inclusions, as well as the presence of shrinkages and cracks, which have a negative impact on both conductivity and mechanical strength. A corrected value of the rotor bar conductivity was used in the finite element model and led to a good agreement with the experimental results. The field perturbation associated with a broken rotor bar produces a concentration of flux near the bar ahead of the broken one, considering the counterclockwise direction of rotation of the rotor. This flux concentration results in higher current values and hence in higher stresses in the bar ahead of the broken one.