The impact of cable length on radiated EMI of frequency converters

The length of motor cables has a significant impact on radiated EMI in inverter systems. The cable acts like an antenna, and its radiation efficiency is related to both cable length and noise frequency. When the cable length and the wavelength of the noise frequency satisfy a certain relationship (e.g., half a wavelength), radiation is significantly enhanced. Specific effects include: longer cables generally mean a larger common-mode current loop area and higher radiation efficiency. The parasitic parameters of the cable (capacitance to ground, inductance) interact with the inverter output filter and motor impedance, potentially causing resonance at certain frequencies and amplifying radiation at those frequencies.

Long cables can cause voltage reflections and overvoltages at the cable ends, which may exacerbate high-frequency oscillations and radiation. To control radiation, the following should be taken: minimize the length of the motor cable, using the shortest cable possible while meeting application requirements; use shielded motor cables and ensure proper grounding of the shielding; install output filters, such as dv/dt filters or sine wave filters, at the inverter output to smooth the output voltage and reduce high-frequency components. For long cables of fixed length, their resonant frequency can be calculated and considered in the filter design. The impact can be quantified by comparing results at different cable lengths through radiated emission testing. The output filters from Eintech Electronics are designed to address the EMC challenges posed by long cables.