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Line Reactor vs Harmonic Filter: Which One Do You Need?
Line reactor vs harmonic filter comes down to what you are trying to fix. A line reactor is a series inductor on the input side of a drive. It cuts harmonic current by roughly 30 to 50 percent and protects the rectifier from transients. A harmonic filter is either a tuned LC network or an active power-electronic device that targets specific harmonic orders until distortion meets a limit. One is a low-cost protective component. The other is a compliance investment that costs several times more.
Here is the mistake that costs the most money, and it is not choosing the wrong device. It is fitting the right device on the wrong side of the drive.
Owen runs maintenance at a bottling plant in Leeds. In early 2025 his team fitted a line reactor to a 75 kW conveyor drive that kept tripping on overvoltage. The trips continued.
The reactor was doing exactly what a reactor does, but the problem lived on the output side, in a 140 m motor cable reflecting voltage spikes back into the DC bus. The part was correct. The location was wrong.
If you have already compared the broad options, our guide to harmonic mitigation strategies compared covers the strategy view including multi-pulse drives and active front ends. This article stays at the component level: which device, and where it goes.
Mid-specification and want a second opinion before you buy? Talk to our engineers with your drive rating and cable length. That is faster than working it out from a catalogue.
Key Takeaways
- A line reactor and a harmonic filter sit on the same side of the drive but solve different problems. The reactor reduces harmonics and protects the rectifier. The filter drives distortion down to a compliance limit.
- Line reactors typically cost 1.05 to 1.15 times a standard 6-pulse drive. Passive filters run 1.35 to 1.70 times, and active filters 1.75 to 2.5 times.
- Most installations need a reactor sized at 3 percent impedance. Move to 5 percent when the available short-circuit current is below about 10 times the drive’s full-load current.
- A reactor alone will not meet IEEE 519 at the point of common coupling. It reduces distortion, it does not guarantee compliance.
- Fitting a 3 percent reactor ahead of an active filter can shrink the filter you need and cut its cost by roughly 20 percent.
Line Reactor vs Harmonic Filter: The Short Answer
Both devices attach to the input side of a VFD. That is where the similarity ends.
| Line reactor | Harmonic filter | |
|---|---|---|
| What it is | Series inductor, passive | Tuned LC network, or active power-electronics |
| Where it sits | Drive input, in series | Drive input, in parallel or series |
| What it does | Adds impedance, smooths current, blocks transients | Cancels or absorbs specific harmonic orders |
| Typical result | Cuts input current THD roughly 30 to 50 percent | Brings THD to 5 percent or below at the PCC |
| Relative cost | 1.05 to 1.15x a standard drive | 1.35 to 1.70x passive, 1.75 to 2.5x active |
| Maintenance | None, it is a coil | Passive: capacitor aging. Active: electronics and cooling |
The one-sentence rule: fit a reactor to protect the drive and take the edge off distortion, and fit a filter when a specific distortion limit has to be met.
That is the whole decision in most cases. The rest of this article is about the cases where it is not.
One note on scope. Everything here applies to low voltage drives up to 690 V. Above that, the rectifier topologies and the harmonic behaviour change, and you can read about that in our article on harmonics on high voltage drives.
They Are Not the Same Kind of Device
Treating these as two versions of the same product is where a lot of specifying errors start. They are not close relatives. One is a passive component with nothing to fail. The other is a subsystem.
A line reactor is an inductor
It is a coil of copper wound around a magnetic core, wired in series with each input phase. Its impedance rises with frequency, which is the entire mechanism. At 50 Hz the impedance is small and the drive gets nearly full voltage. At the 5th and 7th harmonic frequencies it is five to seven times higher, so those currents are opposed far more strongly than the fundamental.
That is why a reactor reduces distortion at all, and also why it cannot eliminate it. A reactor changes how much harmonic current flows, not whether it is generated.
A note on power factor, because vendors often state this loosely. Reducing harmonic current lowers the kVA the drive draws while the kW stays the same, so a reactor does improve true power factor. It does not improve displacement power factor, and its inductive reactance can nudge that figure slightly worse. Both halves matter if you are reporting power factor to a utility.
A harmonic filter is a network or an active device
Passive filters are tuned LC branches. Each branch targets a specific harmonic order, usually the 5th or 7th, and presents a low impedance path so that current circulates inside the filter instead of flowing back to the supply. They work well, but only for the orders they are tuned to and only near the load they were designed for.
Active filters are inverters in their own right. They measure the harmonic current the drive is drawing and inject an equal and opposite current to cancel it. That makes them adaptive, effective across a wide order range, and considerably more expensive.
The practical consequence: a reactor is a component you buy once and forget, while a filter is a system sized to your load profile and, in the active case, maintained.
Input Side vs Output Side: The Mistake That Costs the Most
This is the single most common error in this whole subject area, and it has nothing to do with choosing between a reactor and a filter. The line reactor vs load reactor distinction is about which end of the drive you are working on.
Three different series inductors exist in a VFD installation, and they are not interchangeable.
Line reactor: input side
Wired between the supply and the drive’s input terminals. It protects the rectifier bridge from supply transients and voltage notching, reduces the harmonic current the drive draws, and adds a little impedance that softens the effect of other loads on the same bus.
It does nothing for the motor. It has no effect on what comes out of the drive’s output terminals.
Load reactor: output side
Wired between the drive’s output terminals and the motor. It slows the rate of voltage rise at the motor terminals, which reduces the stress on the motor’s insulation and can lower bearing currents.
It does nothing for the supply. It does not reduce the harmonics the drive draws, and nobody will accept it as part of a harmonic compliance plan.
Where dv/dt filters and sine wave filters fit
These are output-side devices too, and they are the ones that actually solve reflected-wave problems over long cable runs. A load reactor is usually not enough on its own once cable length climbs.
Here is the ladder that matters when a drive sits far from its motor. Treat these as order-of-magnitude guidance and check the drive manual, because published thresholds vary by drive family.
| Cable length (approximate) | Typical mitigation |
|---|---|
| Under 15 to 25 m | None required, assuming an inverter-duty motor |
| 25 to 100 m | Load reactor, or upgrade the motor |
| 100 to 300 m | dv/dt filter |
| 300 to 600 m | dv/dt filter at the upper range, or sine wave filter |
| Over 600 m | Sine wave filter |
The physics behind it: a modern IGBT output stage switches with a voltage rise rate of roughly 1 to 3 kV per microsecond and rise times between 0.05 and 1 microsecond. That is faster than the electrical length of the cable, so the pulse reflects at the motor terminals and travels back. On a 480 V system the reflected wave can peak above 1,600 V, well past what a standard motor’s insulation is rated for. The critical cable length at which this starts can be as short as 8 to 15 m.
This is why Owen’s reactor changed nothing. His problem was at the far end of a 140 m cable, and he had fitted a device at the near end.
If you are already chasing unexplained trips, it is worth separating power-quality symptoms from drive faults before you buy anything. Our article on nuisance trips that are really power-quality problems walks through the diagnostic order.
How to Choose Between 3% and 5% Impedance
Once you have decided on a reactor, the 3% vs 5% line reactor choice is next. Those are the two standard impedance values, and the decision depends on how stiff your supply is.
| 3% impedance | 5% impedance | |
|---|---|---|
| Typical resulting THDi | About 36 to 40 percent | About 30 to 35 percent, and it removes roughly 65 percent of the current distortion |
| Voltage drop at 100 HP / 480 V | 8.3 V line-to-line | 13.9 V line-to-line |
| Power dissipated as heat | 0.9 to 1.5 kW | 1.5 to 2.5 kW |
| Choose it when | Source is reasonably stiff and the goal is protection plus moderate reduction | Available short-circuit current is low, or the drive is a large share of the load |
Go to 5 percent when any of these is true: the available short-circuit current at the drive terminals is below about 10 times the drive’s full-load current, the utility fault level is below roughly 25 kA, or the site does substantial capacitor bank switching.
One caution on the upper end. Going from 3 percent to 5 percent helps. Going to 10 percent does not double the benefit, and it does nearly double the voltage drop and the heat. Past 5 percent you are paying for losses that a different device would avoid.
Rafael learned that the expensive way at a water treatment works in Valencia. A contractor had specified 5 percent reactors on every drive in the plant as a blanket rule. On the 200 kW high-service pump, which ran at close to full load continuously, the reactor was dissipating around 2 kW as heat inside an already warm cabinet.
The enclosure cooling was not designed for it, and the drive began derating in summer. The fix was not a bigger reactor or a bigger fan. The 200 kW drive sat on a stiff 2,000 kVA transformer, where 3 percent was plenty, and the extra 2 percent was pure loss.
Do You Need a Line Reactor at All?
Not always. This is the question most articles skip, and it is often worth several hundred dollars per drive.
The standard rule of thumb compares the supply transformer to the drive. If the transformer’s kVA rating divided by the drive’s kVA rating is less than 20, a line reactor is recommended. On a 500 kVA transformer, that threshold lands at a drive of about 25 kVA. Above that drive size on that transformer, you are in reactor territory.
Some published guidance uses a looser 10 to 1 threshold instead. The two rules give different answers on mid-sized installations, so treat the 1:20 figure as the common industry standard and confirm it against your own fault current calculation.
Cases where you can skip an external reactor:
- The supply transformer is large relative to the drive, and the fault current at the terminals is high
- The drive already includes a DC link choke. Many modern drives do, and it performs a similar function
- The drive is small, under about 5 kVA, and no distortion limit applies
- The site has no capacitor banks and no known resonance problem
That last point matters. A reactor on a weak supply adds voltage drop at the motor terminals, and on a supply already sagging under load, you can trade a harmonic problem for a starting-torque problem. Fitting a reactor is not automatically the safe choice.
When a Line Reactor Is Not Enough
A reactor reduces distortion. It does not deliver compliance, and vendors who imply otherwise are overselling a coil. If what you actually need is a harmonic filter for VFD compliance rather than drive protection, this section is where the decision gets made.
What a reactor cannot do
At the point of common coupling, IEEE 519 compliance is assessed against total demand distortion, which depends on the whole plant, not one drive. A 3 percent reactor on a single drive typically leaves input current THD in the high 30s. If the plant has many drives and a weak supply, that is nowhere near enough.
A reactor also cannot help with resonant conditions it did not create, and it cannot adapt when the load changes. It is a fixed impedance.
Passive versus active filters
A passive filter is tuned to the dominant harmonic orders and is cost-effective when the load is stable and the orders are known. It becomes risky when the plant has power factor correction capacitors, because the filter and the capacitors can interact.
An active filter adapts to whatever the load produces. It is the right answer for a plant with mixed loads, changing duty cycles, or a genuine compliance deadline, and it costs accordingly.
Why a reactor belongs upstream of an active filter
This is the part that is genuinely counterintuitive. Fitting a reactor can reduce what you spend on a filter.
Active filter manufacturers generally require a minimum impedance between the filter and the non-linear load, commonly a 3 percent line reactor or an equivalent DC link choke. Without it, the filter struggles to control the load it is trying to cancel. Practical experience on these projects puts the saving on the active filter at roughly 20 percent when adequate upstream impedance is present.
So the cheapest device in the comparison table can make the most expensive one smaller. If you are pricing an active filter installation, price the reactors in at the same time.
| Solution | Cost versus a standard 6-pulse drive | Indicative cost per 100 HP |
|---|---|---|
| Line reactor | 1.05 to 1.15x | Low hundreds |
| Passive harmonic filter | 1.35 to 1.70x | 2,000to2,000to4,000 |
| Active harmonic filter | 1.75 to 2.5x | 8,000to8,000to15,000 |
| Active front-end drive | 2.0 to 3.0x | 12,000to12,000to20,000 |
Mei was specifying a packaging line in Shenzhen when she ran these numbers. The compliance target at the PCC needed an active filter, and the first quotation assumed no upstream impedance. Adding 3 percent reactors to the six largest drives cost a few hundred dollars each and let the filter be specified one frame smaller, which took the filter cost down by about a fifth. The reactors paid for themselves inside the same purchase order.
If you want help working out whether your plant needs a reactor, a filter, or neither, send us your load list and single-line diagram and we will tell you which one applies.
Sizing the Reactor Correctly
Two details trip people up after they have picked the right device, and both are line reactor VFD sizing mistakes rather than product mistakes.
Size on the drive’s input current, not the motor’s nameplate FLA. The reactor carries the current the drive draws, which includes the drive’s own losses. A common guideline is to select a reactor whose current rating is at least 1.15 times the drive’s full-load output current, then round up to the next standard frame. Specifying from the motor nameplate alone leaves the reactor running hot.
Remember that percent impedance applies at rated current only. A 5 percent reactor is 5 percent at its rated current, and the impedance falls proportionally as load drops. This is why oversizing a reactor for the load does not buy extra filtering. It just adds voltage drop at the light-load conditions where you did not need it.
One more check before you buy. If the site has power factor correction capacitors, a reactor and a capacitor bank can form a resonant circuit at a harmonic order you did not plan for. The interaction is between the reactor’s inductance and the capacitors’ capacitance, and it can amplify the very harmonic you were trying to reduce. If your site has PFCC, raise it with whoever is sizing the capacitors rather than assuming the two devices are independent.
For the surrounding installation questions, our VFD installation best practices covers cable routing, grounding, and enclosure thermal planning, and how to size a VFD for a motor covers the drive selection that comes before any of this.
Frequently Asked Questions
Do I need a line reactor on my VFD?
Not always. Check whether your supply transformer’s kVA rating is less than 20 times the drive’s kVA rating. If it is, a reactor is worth fitting. If the supply is stiff and your drive already has a DC link choke, you may not need an external one at all.
What is the difference between a line reactor and a harmonic filter?
A line reactor is an inductor that adds impedance and reduces harmonic current by roughly 30 to 50 percent while protecting the rectifier. A harmonic filter is a tuned LC network or an active device that cancels specific harmonic orders to bring distortion down to a compliance limit. The reactor is a protective component. The filter is a compliance solution.
Does a line reactor reduce harmonics?
Yes, meaningfully, but within limits. Adding a reactor typically cuts input current distortion by 30 to 50 percent, because its impedance rises with frequency and opposes the 5th and 7th harmonic currents far more than the fundamental. What it cannot do is remove the distortion entirely, because it changes how much harmonic current flows rather than stopping the drive from generating it.
Is a line reactor the same as a load reactor?
No, and this is the confusion that wastes the most money. A line reactor goes on the input side and reduces supply-side harmonics. A load reactor goes on the output side and slows voltage rise at the motor terminals. They are different devices in different locations doing different jobs, and fitting one will not solve a problem caused by the other.
Will a line reactor alone meet IEEE 519?
Usually not. A 3 percent reactor typically leaves input current THD in the high 30s, and IEEE 519 compliance is assessed against total demand distortion at the point of common coupling across the whole plant. A reactor is a step toward compliance, not a substitute for it. If you have a firm limit to meet, you need to model the site, not just add reactors.
Can a line reactor cause problems?
Yes, in three situations. On a weak supply it adds voltage drop that can affect motor starting. With power factor correction capacitors it can form a resonant circuit. And if it is oversized, it wastes energy as heat without improving filtering, because percent impedance only applies at rated current.
Do I need a reactor if my drive has a DC choke?
Usually not an external one. A DC link choke performs a similar function to a line reactor on the DC bus rather than the AC input. Many modern drives include one as standard. Check the drive datasheet before buying an external reactor, because you may be paying twice for the same impedance.
What cable length needs a dv/dt filter?
As a rough guide, runs under 15 to 25 m normally need nothing, 25 to 100 m calls for a load reactor, and 100 to 300 m calls for a dv/dt filter. Beyond about 600 m you are into sine wave filter territory. These thresholds vary by drive family, so confirm against the drive manual before specifying.
Conclusion
The choice between a line reactor and a harmonic filter gets easier once you separate two questions that usually arrive tangled together.
The first is which side of the drive you are working on. A reactor on the input side addresses harmonics and rectifier protection. A load reactor or dv/dt filter on the output side addresses motor insulation stress and long cable runs. Get this wrong and you buy a correct part that does nothing.
The second is whether you need protection or compliance. A reactor at 3 percent impedance handles protection and takes the edge off distortion for 1.05 to 1.15 times the drive cost. When a distortion limit has to be met at the point of common coupling, a filter is the instrument, at 1.35 to 2.5 times the cost.
Most installations that ask this question need a 3 percent reactor, or nothing at all. The rest need a filter, and should be planned as a system rather than bought as a part.
Our drives from 0.1 kW to 53,000 kW ship ready to accept the reactor or filter your site requires, and our engineers will size it with you rather than leave you to guess from a catalogue. Talk to our engineers or browse our VFD range to get started.
