Effects of Supply Voltage Unbalance on Induction Motors Investigated by Digital Circuit Breaker-Based Motor Current Signature Analysis

Supply voltage unbalance is one of the most common power-quality disturbances affecting electrically driven actuators and rotating machines; yet its interaction with the harmonic indicators used for drive condition monitoring is often overlooked. As an innovative contribution, this article investigates the effects of supply voltage unbalance on Motor Current Signature Analysis indicators in a three-phase induction motor drive, using a digital molded-case circuit breaker as a combined protection and diagnostic sensing device. A dedicated test bench including a three-phase induction motor, a rheostat on the supply line, and a magnetic powder brake is used for testing at various load and supply voltage-unbalance conditions. A preliminary time-domain and phase-angle analysis of the three-phase signals is presented, revealing relevant diagnostic patterns. The proposed method then detects supply voltage unbalance through direct analysis of the frequency-domain signatures. Both voltage and current spectra are examined, with particular focus on their harmonic and interharmonic components, to identify their similarities and differences. Along with the usual 3rd harmonic indicator (already recognized in the literature), the 5th and 7th harmonics are monitored in both current and line-to-line voltage spectra. In the voltage spectra, distinctive interharmonic components are observed around the fundamental components. These interharmonic components also enable localization of the affected phase by exploiting the relationships among the three-phase signals.