What is a line reactor and when to use it? (2023)

Jun 6, 2019 | Engineers Corner | 7 comments

What is a line reactor and when to use it? (1)

Clean and consistent electrical currents are optimal for the health of your industrial equipment.

The addition of a line reactor, sometimes known as a choke or inductor, helps protect your equipment from input power disruptions that damage the drive.

Line reactors are electro-magnetic devices which consist of a steel core wrapped with copper coils. The coils form a magnetic field which current flows through limiting the rate of rise of current, reducing harmonics and protecting additional electrical devices. There are generally two types of line reactors used with VFDs; AC & DC.

When a reactor is installed between the power system and the VFD, it is known as an AC line reactor. When a DC reactor is inserted into the DC link of a drive, it is known as a DC link reactor.

What is a line reactor and when to use it? (2)

Both AC and DC reactors act as harmonic current limiters but the AC reactor protects more equipment due to being installed between the VFD and power source limiting exposure to power system surges and fluctuations. This limits exposure to power system surges and fluctuations. Reactors can prevent overvoltage trips, increase the reliability and life span of the VFD, improve total power factor, and reduce nuisance tripping.

What is Impedance?

“Resistance in ohms but also commonly referenced in terms of percent when combined with the system voltage and line current flowing through the reactor.

That percentage then becomes the common term used to define the level of impedance for each rating of line reactor. That impedance functions to slow the rate of current changes in the line. The greater the current through the reactor, the greater the percentage of applied impedance will be.

What is a line reactor and when to use it? (3)

If a reactor is said to have an impedance rating of 3% or 5%, that means the reactor will apply that specified percent of impedance when the current flowing through the reactor is at the rated current of the device,TCI, Line and Load Reactor Basics.

Installing a line reactor on the input side of the VFD ensures protection to the drive but line reactors also have the capacity to be installed on the output side of the drive blocking potential incoming background line voltage harmonics.

“In almost all drive applications, the addition of an input AC line reactor is a low cost solution for drive protection and harmonic mitigation,” TCI, Line and Load Reactor Basics.

Want to learn more?

TCI has all sorts of helpful guides for incorporating a line reactor into your application:

  • Application of Line Reactors or DC Link Reactors for Variable-Frequency Drives
  • Connecting a Three-Phase Reactor to a Single-Phase Sour

When to install a line reactor?

  • The input line power is known to have power surges, spikes, transients, etc.
  • The supply line power is very stiff; greater than 10 times the kVA rating of the connected VFD.
  • Where harmonic distortion is a concern. (IEEE-519 Harmonic Control in Electrical Power Systems)
  • You want a buffer between your VFD and your motor to temper the waveform and reduce voltage stress on the motor

The general consensus seems to be the more current regularity the better, for all equipment across the board. If you’re ready to take the next step in harmonic mitigation; feel free to contact our sales staff at: [emailprotected] or get some help figuring the right reactor from our tech support at: [emailprotected].

What is a line reactor and when to use it? (4)

Morgan Spano

Visual thinker in a digital spectrum, or in layman’s terms….I make all the visual content for Marshall Wolf Automation 🙂 With a background in video advertisement and film production, I work with MWA’s marketing department to keep our customers reading our blogs and viewing our products.

  1. What is a line reactor and when to use it? (5)

    Alex Howard on May 16, 2022 at 8:11 am

    Hello .. I have multiple VFD’s and they all were faulting out on Overvoltage, the DC bus voltage was too high. In fact it was reading over 900 Vdc, that is just sitting idle, would not run cause faulted. I did install properly sized Line Reactors and no more fault and DC bus was around 650. I just was looking for an answer as to what caused the VFD to get a high reading, was it other devices causing high frequency or ? I really appreciate a reply.


    • What is a line reactor and when to use it? (6)

      Morgan Spano on May 16, 2022 at 11:51 am

      Hi there Alex,

      Thank you for your question. Unfortunately, this particular issue could have a multitude of different answers that would be difficult for our tech support team to determine. While it sounds like you did exactly what was needed, as a line reactor is designed to absorb power line disturbances, voltage spikes, and harmonics created by the VFD; pinpointing why the VFD was reading so high would require much more direct information about the application and so on. You may have better luck reaching out to the tech team of whichever manufacturer produced your VFDs as they may have a deeper understanding of why they would be faulting out.

      Hopefully you’re able to find your answer, we can definitely sympathize with you wanting to find out WHY the darn thing is doing what it’s doing.


    • What is a line reactor and when to use it? (7)

      Joel on August 19, 2022 at 11:51 am

      Oh that’s super interesting. The line side input generally goes straight into a full-bridge rectifier, the output of which charges the DC bus capacitors. If there’s nothing pulling down those DC bus capacitors, the rectifiers are going to chop the very top off of the input AC waveform and that’ll be your DC bus voltage. So if you have a supply with a ton of line noise with very high peaks, your DC bus capacitor voltage will go crazy.

      Very cool. But it sounds like you have some line voltage problems at your location. Might want to look into that. It’s probably beating the tar out of any switch-mode power supplies you have.


  2. What is a line reactor and when to use it? (8)

    Sam van der Merwe on October 17, 2022 at 4:12 pm

    Can multiple VFDs be connected to one common Input reactor?


    • What is a line reactor and when to use it? (9)

      Morgan Spano on October 18, 2022 at 2:37 pm

      Hi Sam,
      Thanks for reaching out to us.
      Using one line reactor for multiple drives is a tricky application. You would need to size the reactor to the load of all drives in series. By doing that it will protect at full load. The issue is when only 1 or 2 of the series are running, there would lower inductance and may not be enough protection if a single drive is running. Ultimately, it is recommended one reactor per drive. Individual line reactors will provide proper filtering and optimum surge protection for each VFD and reduce crosstalk between each unit.

      TCI is one of our suppliers, their tech number is 414-357-4540
      Best of luck on your application!


  3. What is a line reactor and when to use it? (10)

    John Emerick on October 25, 2022 at 7:32 pm

    What about a line reactor for a VFD with a single phase input that is being used as a phase converter. It seems like all of the line reactors are for 3-phase.


    • What is a line reactor and when to use it? (11)

      Morgan Spano on December 2, 2022 at 9:20 am

      Thank you for your question John,

      The line reactor on single phase VFDs, are normally used on the motor side. They also tend to be used in a higher horse power applications. If you have additional questions in regards to the use of line reactors for single-phase applications, please feel free to contact our tech support directly. They’d be happy to assist you.


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