Why a Detuned Reactor Is Not a Bigger Line Reactor
A variable frequency drive on a weak grid draws a current spectrum rich in 5th, 7th, 11th and 13th harmonics. A passive harmonic filter uses a reactor in series with a capacitor bank, tuned so that the series resonance sits below the dominant harmonic — typically at 189 Hz for a 7 % reactor or 134 Hz for a 14 % reactor when the fundamental is 50 Hz. That is what “detuned” means, and it is the reason the reactor is a filter component, not merely an impedance.
Buyers often specify “a 7 % reactor” without the capacitor, the drive rating, or the grid short-circuit ratio, and receive a component whose inductance is right but whose saturation, thermal class, and insulation coordination do not match the filter it will be installed in.
Key Sizing Parameters
| Parameter | Typical value | Why it matters |
|---|---|---|
| Detuning factor p | 5.67 % / 7 % / 14 % | Sets resonance below the 5th harmonic; 7 % is common |
| Inductance tolerance | ±3 % typical | Resonance shifts with tolerance; a wide band can move resonance onto a harmonic |
| Rated current | Drive input current + harmonic allowance (often 1.1–1.3×) | Undersized reactors overheat within hours |
| Linear current vs overload | 1.0× continuous, 1.5× for 1 minute | Must match drive overload rating |
| Insulation class | Class H common, Class F on smaller units | Enclosure ambient plus harmonic heating |
| Thermal rise | ≤ 70 K at rated current | Directly affects capacitor life in the same cubicle |
Inductance Tolerance Is the Hidden Risk
A 7 % reactor with a ±10 % tolerance can present effective detuning anywhere from 6.3 % to 7.7 %, which moves the series resonance by several hertz. If the resonance drifts close to the 5th harmonic, the filter amplifies instead of attenuating, and both reactor and capacitor can fail. Specify ±3 % and require a measured inductance value on the nameplate, not just a nominal.
Gap stability matters too: a gapped laminated reactor whose gap closes under vibration or thermal cycling loses inductance silently. Laminated cores with clamped, varnished gaps hold tolerance better than loosely stacked ones.
Thermal Behaviour in a Filter Cubicle
The reactor and its capacitor share a cubicle, and the capacitor is the more temperature-sensitive of the two. A reactor running at 90 °C surface temperature heats the air around the capacitor and shortens its life by roughly a factor of two for every 10 K above rated ambient. Practical measures: mount the reactor above or beside the capacitors rather than below, use Class H insulation so the reactor can tolerate its own heat, and specify the temperature rise at the actual cubicle ambient rather than at 40 °C open air.
What to Put in the RFQ
- Fundamental frequency, drive rating, and expected harmonic spectrum.
- Detuning factor and inductance tolerance with a measurement requirement.
- Continuous and overload current, plus the ambient inside the cubicle.
- Insulation class, temperature rise limit, and hipot level.
- Mechanical: mounting orientation, terminal layout, and whether a thermal switch is required.
- Standards referenced: IEC 61558-2-20 for reactors, plus local grid code requirements.
Related reading
- Air Gap Design for Inductors and Transformers
- Motor Drive EMI Filter Design
- Thermal Design of Transformers and Inductors
- SMC vs Laminated Steel Core
We build detuned and line reactors with measured inductance on the nameplate, Class H insulation, and temperature rise reported at your cubicle ambient. Send the drive rating, detuning factor, and harmonic spectrum to sales@sirui-tech.com and the Magnetic Core team will return a reactor and capacitor pairing recommendation.