Application of Frequency Converter for Examining Induction Motors by Means of Stator Current Spectral Analysis

Nowadays, one of the most promising methoda for monitoring damage occurring in the rotor windings of induction motors consists in the spectral analysis of stator current in steady-state and starting modes of operation. However, almost all such studies have been conducted without taking into account frequency converters that are commonly used in drive systems. Frequency converters can be used to provide an increase in the starting time of an induction motor and a change in the voltage and frequency, which should make it possible to achieve much more reliable diagnostic results. Therefore, it is relevant to study the potentialities in applying a frequency converter for to monitor the condition of the rotor windings of motors according to the stator current. A testing bench has been developed for conducting the investigations. The studies have been conducted based on an AIR71A6-type induction motor having a fault-free rotor winding and one broken bar. A Vesper E4-8400-010N frequency converter was used as a power converter. An Ethernet Network Merging Unit was used for to register the stator current taken from the current transformers. The signal of stator currents were processed based on a short-time Fourier transform with the use of a Flat-top window function. The frequency-time spectra of the signal of stator currents have been obtained and analyzed with and without a breakage in the rotor winding bar of the induction motor. The frequency converter has been used for to provide an increase in the start-up duration and a change in the operating parameters in the course of the motor operation. A harmonic component characteristic of damage has been detected in the case of a break, which confirms the fact that the converter can be used for motor diagnostics. It is shown that frequency converters in the diagnostic process need to be given more attention not only as a possible source of interference and distortion of diagnostic information, but also as a tool for obtaining more reliable results concerning the condition of the rotor windings of induction motors.