A study of the harmonics introduced by soft-starters in the induction motor starting current using continuous time-frequency transforms

The present paper presents an study on the harmonics introduced by thyristor-based soft-starters in the induction motor starting current. It is observed how the use of these starting elements leads to the amplification of the amplitudes of some specific harmonics. The application of continuous time-frequency decomposition tools enables to visualize the full transient evolutions of these harmonics and discern those that are amplified when using such drives. The identification of the current harmonics introduced or amplified by soft-starters is very useful when applying induction motor fault diagnosis techniques based on the analysis of the startup current, since it enables to discriminate between the components amplified by the soft-starters and those amplified by the fault. Soft-started laboratory motors are employed in this work to study the harmonics amplified by a thyristor-based soft-starter under non-ideal supply conditions. The results reveal that these signal processing tools may be very useful to analyze the components introduced by the drive and, at the time, discriminate them versus eventual fault-related harmonics.