What is the impact of cable length on EMI radiation from industrial servo systems?

Cable length is a significant factor affecting radiated EMI in servo systems, especially for motor power cables and unterminated signal cables. Cable length is directly related to radiation efficiency: when the cable length approaches 1/4 of the wavelength of the interference frequency or an integer multiple thereof, it becomes a highly efficient antenna, resulting in a dramatic increase in radiated interference. For example, for 30MHz noise with a wavelength of 10m, a 2.5m cable (λ/4) has the highest radiation efficiency. Echotronics' countermeasures include:

1) Minimizing cable length without compromising installation.

2) For motor cables, if length cannot be reduced, ensure proper shielding and grounding at both ends, and use output reactors or dv/dt filters.

3) For signal cables, use impedance-matched cables and terminate them correctly to avoid reflections and standing waves caused by open circuits or short circuits.

4) Connect a common-mode choke (CML) or ferrite core in series at the driver end of long cables.

Through optimization, even with longer cables, radiated interference can be controlled within the EN 55032 limits, typically requiring a combination of pre-testing and targeted filtering.