Two Components, Two Jobs
Both a line reactor and a DC link choke reduce harmonics and protect a drive, and they are frequently specified as alternatives. They are not interchangeable: the line reactor sits in the AC input and attenuates the current harmonics the drive injects into the grid, while the DC link choke sits between the rectifier and the DC bus and smooths the bus current, reducing capacitor ripple and rectifier stress.
Choosing between them — or using both — is a function of drive size, grid strength, and the harmonic limits the installation must meet.
Functional Comparison
| Item | AC line reactor | DC link choke |
|---|---|---|
| Location | Drive input, in series with mains | Between rectifier and DC bus |
| Primary effect | Reduces input current THD, limits inrush and commutation notches | Smooths DC bus current, reduces capacitor ripple current |
| Typical impedance | 2 % to 4 % (sometimes 5 %) | Expressed as inductance sized to hold ripple below a target |
| Current waveform | AC, with harmonic content | DC with ripple; unidirectional in a diode front end |
| Core | Laminated steel, gapped; three-phase versions have three limbs | Laminated core, gapped, sometimes with a winding tap |
| Effect on THD | Typical reduction from ~110 % to 65–80 % current THD | Indirect: improves power factor and reduces capacitor heating |
When a Line Reactor Is the Right Answer
Use a line reactor when the problem is at the grid interface: high current THD, a weak supply with significant voltage distortion, multiple drives on one feeder with resonance risk, or a need to limit inrush and protect the drive’s input rectifier from transients. A 3 % impedance reactor is the common default for drives above roughly 10 kW; below that, DC link chokes are often built into the drive already.
Line reactors also help when the feeder is shared with sensitive loads, because they attenuate the notches the rectifier produces, which otherwise propagate to other equipment.
When a DC Link Choke Is the Right Answer
Use a DC link choke when the objective is capacitor life and bus stability: it reduces the ripple current the electrolytic capacitors must absorb, which is often their dominant aging factor, and it lowers the peak current through the rectifier diodes. Many medium-power drives include one internally; the specification matters mainly when the drive is supplied without it or when the application has high cyclic load.
A DC choke does less for grid-side THD than a line reactor of comparable cost, because the mains current is still a pulse train. Expect harmonic improvement, but not the 30–40 % THD reduction a line reactor typically provides.
Sizing and Specification Notes
- Line reactor impedance is specified as a percentage of the drive’s rated input impedance at rated current; a 4 % reactor has more inductance than a 2 % one, but also more voltage drop and heat.
- Voltage drop matters: a 4 % reactor at full load costs roughly 4 % of the input voltage, which must be inside the drive’s tolerance.
- Insulation class should be Class H for enclosed drives; ambient inside a cabinet can exceed 50 °C.
- Saturation must be specified for the inrush and fault current, not only rated current.
- Three-phase construction should keep phase inductance matched within a few percent to avoid circulating currents.
Related reading
- Detuned Reactor and Harmonic Filter Design
- SMC vs Laminated Steel Core
- Air Gap Design
- Motor Drive EMI Filter Design
We supply both AC line reactors and DC link chokes with measured inductance, matched phases, and Class H insulation, and we can quote both options for the same drive rating so the harmonic and thermal effects are comparable. Send the drive rating, grid short-circuit ratio, and harmonic target to sales@sirui-tech.com.