Ferrite Ring (Ferrite Ring / Common-Mode Choke)
A ferromagnetic ring slipped over a cable. When all the conductors in the cable pass through the ring together, the magnetic effect of the useful signal cancels out; only common mode noise flowing in the same direction meets a high impedance and is suppressed.
The ring behaves like a series inductance for common mode current, and its impedance rises with frequency. For differential (useful) current the net magnetic flux seen by the ring is close to zero, so the signal is not degraded. This is why it is a cheap and often effective solution.
- Where it is fitted: on the motor cable at the inverter output (the three phases together, with the earth conductor left outside the ring) and on sensitive control cables.
- How many turns: passing the cable through the ring more than once increases the impedance noticeably; if there is room, try 2-3 turns.
- Position: place the ring as close as possible to the noise source.
- Limit: a ferrite does not compensate for poor earthing and wrong cable routing; apply the separation and shielding rules first.
If you are experiencing an intermittent response problem on the receiver's supply and control cables, a ferrite is one of the early steps in diagnosis because it is cheap to try.
Context in which this term is used: Wireless Crane Remote Control Selection Guide