Enhancing motor impedance measurements: broadening the spectrum from low to high frequencies

Three-phase induction motors serve as critical parts in various industrial applications, lauded for their high energy efficiency and notable power density. Obtaining their broadband impedance information is paramount for analyzing conducted emissions, evaluating overvoltage ringing, and assessing motor health status. Nonetheless, conventional methods for motor impedance measurements typically rely on Kelvin clip leads or extension cables, which are effective only in a relatively low-frequency region (i.e. below 1 MHz). This paper presents an improved approach to extend the measurable spectrum from low to high frequencies, up to 120 MHz. The proposed method develops a series of fixture adapters to enable seamless interconnection between the terminals of an induction motor and the coaxial ports of an impedance analyzer. The parasitics introduced by these adapters are identified using boundary-element analysis, and their impacts are minimized based on the de-embedding concept. Experimental results affirm the accuracy and effectiveness of the proposed method for four types of motor impedances (i.e. single-phase, phase-to-ground, common-mode, and differential-mode) across a broad frequency range from 100 Hz to 120 MHz. Moreover, the inaccuracy of motor impedance measurements at high frequencies (i.e. above 1 MHz) using conventional methods, including Kelvin clip leads and extension cables, is also demonstrated.