How to locate the source of common-mode interference in a frequency converter

Locating the source of common-mode interference in a frequency converter requires following a process from the outside in, and from conducted to radiated interference. Common-mode interference mainly manifests as noise current from the motor cable to ground and from the input power line to ground. First, use a high-bandwidth current probe clamped onto the motor cable or input cable to measure the common-mode current spectrum and determine the main frequency band of the interference, typically from tens of kHz to several MHz, corresponding to the IGBT switching frequency and its harmonics. The main internal sources include: common-mode current coupled to the chassis through the parasitic capacitance of the heat sink during IGBT module switching; common-mode current generated by high-frequency fluctuations in the DC bus voltage through the DC bus capacitance to the chassis; and poorly isolated coupling between the primary and secondary sides of the drive power transformer. Use a near-field probe to scan the vicinity of the IGBT module, DC bus capacitor, and drive transformer to find common-mode electric or magnetic field hotspots. Verify by disconnecting or shielding suspected sources one by one. After location, targeted measures include: using shielded insulating pads between the IGBT module and the heatsink, and grounding the shield; optimizing the installation of the DC bus capacitors to reduce their parasitic capacitance to the chassis; and using common-mode current transformers to directly measure and evaluate the effectiveness of different measures on the cables. Eintec Electronics' common-mode filters and dedicated testing solutions can support the accurate location and suppression of common-mode interference in frequency converters.