Fault diagnosis of squirrel cage induction motor analysing line neutral volrage

Abstract

This paper describes the modeling, determination of parameters, measurement and analysis of cage asynchronous motor failures, such as inter-turn short circuit, rotor rod rupture and ring rupture. The mathematical model of an asynchronous motor with a non-sinusoidal field in the air gap is based on the calculation of chained fluxes with the help of the winding function, taking into account the spatial harmonics of the magnetomotive force. All parameters are calculated from the actual winding geometry and standard parameters obtained from classic short-circuit and no-load tests. Special attention is paid to the stator and rotor differential leakage reactance and the influence of beveling of the rotor rods. A standard mathematical model was used to verify the non-sinusoidal mathematical model. The parameters of both models were confirmed by measurements on a laboratory model. The most common fault is an inter-turn short circuit, and it is modeled by introducing a fourth phase whose number of turns depends on the location of the short circuit. The new phase is galvanically connected to the phase in which the fault occurred, however they belong to different electrical and magnetic circuits. The mathematical model for simulating rotor faults remains unchanged compared to the model of a healthy motor. Since the dimensions of the rotor cage do not change during these faults, all parameters, except for the rotor bar resistance, remain the same in both models. In order to investigate the star point voltage, computer simulations of non-sinusoidal models with different numbers of rotor bars were performed. Three laboratory models were built for the purpose of experimental verification of the results. The first laboratory model was realized for the purpose of measuring the voltage star point of a healthy motor. The voltage is measured between the star point of the motor and the star point of a well-balanced load. For the harmonic analysis of the star point voltage, a fast Fourier transform algorithm was applied. In addition, the frequency of the interharmonic measured in the star point depends on the rotation speed, so it was used to measure the rotation speed. Other laboratory The model was implemented for measuring the star point voltage of a motor with an inter-turn short circuit. On the stator winding specially made for this purpose, five disconnections were performed on the winding of the same phase. By connecting the external resistance between the taps, the resistance of the inter-turn insulation is simulated. At the moments when the resistance is connected, pulses appear in the constellation voltage, and a wavelet transformation was used to detect them. The third laboratory model was realized in order to measure the voltage of the motor constellation with a broken rotor rod. The frequency spectrum of the constellation voltage shows that, compared to a healthy motor, the interharmonic near the third harmonic increases more than fifty times, which is a clear sign of rotor rod breakage.

Goran Petrović

Full Professor | Department of Electrical Measurements

Prof. dr. sc. Goran Petrović is a full professor at the Faculty of Electrical Engineering, Mechanical Engineering and Architecture in Split. His research interests include measurement of electrical and process quantities, analysis of geoelectrical and geothermal features of the soil, instrumentation for smart grids, measurement and application of synchrophasors. He is the author of numerous papers published in top-tier scientific journals and contributed to valuable international and national scientific projects.