How to locate the source of common-mode interference in industrial servo systems?

Servo common-mode interference is mainly coupled to ground through parasitic capacitance. Source location requires a combination of time-domain and frequency-domain analysis. First, a current probe is used to measure the common-mode current spectrum on the power line and motor cable to identify the main interference frequencies, such as switching frequency harmonics. The impedance characteristics of the CML series common-mode inductors (e.g., CML4532A-510T, 51μH) from Audiotech are then analyzed to determine the matching degree between their insertion loss curve and the measured interference frequencies.

Simultaneously, using a separation test method, the motor line, encoder line, and communication line are disconnected sequentially, and the changes in conducted interference test results are observed to locate the main coupling path. For high-frequency interference, a near-field probe can be used to scan hotspot areas on the PCB, such as the IGBT drive circuit and DC bus capacitors. A typical solution includes installing a CMZ3225A-201T common-mode inductor (200μH) near the interference source and adding a PBZ1608A-102Z0T ferrite bead (1kΩ@100MHz) at the motor output. This method can accurately locate the source of interference and reduce the common-mode conducted emission of the system by 10-15 dB, meeting the requirements of the IEC 61800-3 standard.