Variable frequency drives are prolific sources of conducted electromagnetic interference: fast-switching IGBT edges couple noise back into the mains, upsetting sensitive instruments sharing the supply and failing EMC compliance tests. A correctly sized line filter — such as Schaffner’s FN-series three-phase filters — solves this. An incorrectly sized one overheats, trips RCDs, or filters nothing. Here is how to size one properly.
Step 1: Rated current — with margin and derating
Start from the drive’s continuous input current (not motor current — input current runs higher due to rectifier harmonics; use the drive datasheet value). Select a filter rated at or above this figure at your actual ambient temperature: filter current ratings are specified at a reference ambient (often 40–50°C) and must be derated above it, especially inside a warm enclosure. Undersized filters run their chokes hot and fail early; there is no prize for the smallest filter that survives.
Step 2: Voltage and network compatibility
Match rated voltage to your supply (e.g. 3×480/520 V class for EU industrial networks) and check network type: standard filters assume an earthed-neutral TN network. IT (unearthed) networks and corner-earthed deltas need filters specifically approved for them — the Y-capacitors in a standard filter can be stressed or defeat insulation monitoring on an IT supply.
Step 3: Attenuation class — what are you trying to pass?
- Category C3 (industrial, second environment): many drives’ built-in filters suffice; an external filter adds headroom on long motor cables.
- Category C2/C1 (residential/commercial supply, first environment): this is where a quality external filter such as the FN 3100 class earns its place — achieving the tighter conducted-emission limits the built-in filter cannot.
Motor cable length matters more than any other installation variable: longer shielded cables increase capacitive charging currents and conducted emissions, pushing you toward higher-attenuation filters and, above the drive maker’s cable-length limit, output chokes or dV/dt filters as well — a different component solving a different problem (motor insulation stress), often confused with the line-side EMI filter.
Step 4: Leakage current — the RCD trap
EMI filters intentionally conduct small currents to earth through their Y-capacitors. Standard three-phase filters can leak enough to nuisance-trip 30 mA RCDs, and some installations legally cap allowable leakage. If your circuit is RCD-protected, choose a low-leakage filter variant or a type-B RCD rated for drive duty — decide this at selection time, not after the electrician calls.
Step 5: Installation — where filters are won and lost
- Mount the filter on the same bare-metal backplane as the drive, with the shortest possible connection — long filter-to-drive wiring re-couples the noise you just removed.
- 360° shield termination of the motor cable at both ends; the filter cannot compensate for a pigtailed shield.
- Keep filtered (clean) and unfiltered (dirty) wiring physically separated — never in the same trunking.
Worked example
A 15 kW 400 V HVAC drive with ~30 A input current, on a TN network feeding a commercial building (C2 target), 25 m shielded motor cable, RCD upstream: select a three-phase filter in the 36–50 A class (margin + enclosure temperature), C2-capable attenuation, low-leakage variant. That is a textbook FN-series fit.
PLC North stocks tested Schaffner FN-series three-phase filters, including FN 3100 units, across common current ratings. Send us your drive model, cable length, and network type — we will confirm the right filter class from stock. Browse our tested variable frequency drives.