Does a ferrite ring really work, and where is it fitted?
Yes; a ferrite ring is one of the cheapest and fastest interventions for reducing common mode noise. It is fitted at the inverter output, on the receiver supply cable and on sensitive control cables. To increase its effect, wind the cable a few turns through the ring and position the ring as close as possible to the device being protected.
A ferrite ring presents a high impedance to common mode currents in the cable passing through it, while barely touching the differential mode signal. In other words, it suppresses noise without degrading the useful signal. That is why it is among the first remedies applied to inverter-induced interference.
Where it is fitted:
- At the inverter output (the three conductors going to the motor are passed through the ring together; the earth conductor stays outside the ring).
- On the receiver's supply cable, as close as possible to the receiver input.
- On control and signal cables, on the side of the device being protected.
Three details determine the effect:
- Position: the ring must be close to the device being protected. Fitting it in the middle of the cable is ineffective.
- Number of turns: winding the cable a few turns through the ring increases the impedance with the square of the number of turns; a single pass is often not enough.
- Choice of conductors: pass the outgoing and returning conductors through together. Passing only one conductor also chokes the differential signal and disrupts operation.
A ferrite alone is not magic; it gives a lasting result when applied together with separation, shielding and correct earthing.
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