How to locate the common-mode noise source of the motion controller MC

To identify common-mode noise sources in a motion controller (MC), an external-to-internal, frequency-domain analysis approach is needed. First, check external ports, especially the power input, motor drive output, and communication interfaces, as these are the main injection points for common-mode noise. Use a spectrum analyzer with a current probe or near-field probe to perform a frequency sweep test on the cables of these ports to identify the frequency distribution of noise. The switching frequency and harmonics of the switching power supply are common common-mode sources, such as the 100kHz to 1MHz band. The high dv/dt of the PWM output of the motor driver generates common-mode current through parasitic capacitance, with a frequency consistent with the switching frequency.

Communication interfaces such as EtherCAT differential signals also generate common-mode noise due to unbalanced conversion. Internal noise sources, such as clock circuits and high-speed digital buses, can be filtered at the source using common-mode inductors and ferrite beads. For example, CMZ series common-mode chokes can be used to suppress common-mode noise at the power supply end, and PBZ series ferrite beads can suppress high-frequency noise on the board. During positioning, it is important to distinguish between common-mode and differential-mode noise. Common-mode noise has the same phase on the L and N lines and should be measured relative to ground. By disconnecting or blocking suspected sources one by one and observing changes in the noise spectrum, the source can be accurately located.