Simultaneous Measurement System of Differential Mode and Common Mode Currents without Influence for Radiated EMI in Three-Phase Variable Speed Drive System

This paper presents issues and solutions in a simultaneous measurement of the radiated electromagnetic interference (EMI), differential mode (DM) and common mode (CM) currents. The current transformers (CTs) and the oscilloscope (OSC) connected with the coaxial cables are used to evaluate the currents flowing through the power cable selectively. In the high-frequency range above 30 MHz, the current amplitude is small, which reduces measurement accuracy due to the resolution of the measurement system. In addition, couplings between the equipment under test (EUT) and the measurement system are significant in the high-frequency range, which also deteriorates measurement reproducibility of the radiated EMI due to the coupling currents flowing through the coaxial cables. Thus, the simultaneous measurement system is constructed by using high-pass filters (HPFs) and RF amplifiers (AMPs) to increase the measurement sensitivity of the currents and by adding ferrite cores (FCs) and arranging the layout of the measurement equipment to improve measurement reproducibility of the radiated EMI. The effectiveness of the proposed measurement system was experimentally verified using a three-phase AC/DC/AC variable speed drive (VSD) system.