Common-mode voltage or current is the part that both conductors of a pair share in the same direction with respect to a reference, as opposed to differential-mode, the wanted signal between them. Common-mode noise usually comes from ground potential differences or external fields and returns through stray capacitance, so cables radiate it efficiently. It is suppressed with common-mode chokes, balanced circuits, isolation and receivers with a high common-mode rejection ratio.
Drive cable is radiating common-mode noise off the screen, put a ferrite ring over all three phases together.
A clamp-on ferrite only acts on common-mode current because the differential currents cancel inside it, which is why every conductor goes through the ring together.
Also heard as
- CM
- longitudinal
Related terms
EMI
Electromagnetic interference: unwanted electrical noise that disturbs the operation of a circuit or system.
Ground loop
Unintended current path formed when connected equipment is earthed at points of different potential.
EMC
Electromagnetic compatibility: a product neither upsets nearby equipment with its noise nor gets upset by theirs.