Line Reactors and Electrical Filters
A line reactor is an electromagnetic device that adds impedance to a circuit, protecting VFDs, motors, and sensitive equipment from voltage spikes, harmonic distortion, and electrical noise. At Wistex, we stock TCI’s full line of reactors and filters, plus sizing support to help you choose the right impedance, voltage, and current ratings for your application.
Understanding Line Reactors and Filters
A line reactor is an electromagnetic device, typically a copper coil wound around a steel core, that acts as an inductor in an electrical circuit. By adding impedance to the line, it limits current spikes, reduces harmonic distortion, and protects sensitive equipment such as variable frequency drives (VFDs) from disturbances on the power line.
Line reactors are also known as chokes or inductors, and they’re one of the most common and cost-effective ways to add power quality protection to a drive system. Wistex stocks TCI line reactors and filters across the full voltage and horsepower range, along with sizing and specification support to help match the right impedance and current rating to your application.
What Line Reactors Do
Line reactors serve three main functions in a drive system:
- Absorb voltage transients and line notching coming from the power supply before they reach the drive
- Reduce the harmonic distortion a VFD feeds back into the power system
- Limit current spikes and inrush current that cause nuisance tripping on drives
Each of these functions protects a different part of the system, including the incoming power quality, the facility’s broader electrical distribution, and the drive itself, which is why reactors are frequently specified even on installations that otherwise appear to be running normally.
Input and Output Reactors
Line reactors can be applied on either side of a VFD, and where they’re installed determines what they protect.
Input (Line-Side) Reactors
Input reactors sit between the power supply and the VFD. They absorb voltage surges, transients, and notching coming from the grid, and they stop drive-generated harmonics from feeding back into the power system. Input reactors are especially important in facilities running multiple VFDs, or where a drive is connected to a low-impedance source such as a large utility transformer, conditions that otherwise tend to produce nuisance overvoltage trips.
Output (Load-Side) Reactors
Output reactors sit between the VFD and the motor. They’re generally recommended for cable runs up to 100 feet (beyond 100 ft, a dV/dt filter is a recommended alternative). By smoothing the current waveform, output reactors reduce motor operating temperature and help prevent reflected voltage spikes from damaging the motor’s winding insulation, which is a common failure point on long cable runs where voltage wave reflection allows peak voltages to climb well above the drive’s rated output.
Choosing Impedance: 3% or 5%
Line reactor impedance is one of the most common sizing questions, and it comes down to where the application needs to protect against:
- 3% impedance is the general-purpose choice for voltage spike and transient protection, and is typically sufficient for standard nuisance-tripping and inrush-current concerns
- 5% impedance is specified where harmonic mitigation is a bigger priority, or where the system needs to support IEEE 519 compliance at the point of common coupling
As a starting point, size the reactor’s continuous current rating to be equal to or greater than the drive’s input current rating, then step up to 5% impedance if harmonic performance or a documented IEEE 519 study is part of the specification.
Filters We Supply
Beyond line reactors, Wistex stocks several filter types built to solve more specific power quality problems.
dV/dt Output Filters
On long motor cable runs, mismatched impedance between the VFD and the motor causes voltage waves to reflect back down the cable. As these reflected waves combine with new pulses, peak voltage at the motor terminals can climb well beyond the drive’s rated output, enough to break down standard motor insulation over time. A dV/dt output filter slows the voltage rise time and clamps these peak voltages, protecting motor and cable insulation on longer runs. dV/dt filters are recommended for runs beyond 100ft, up to 3,000 ft.
RFI and EMI Filters
RFI and EMI filters reduce the high-frequency noise VFDs generate in the 150 kHz to 30 MHz range. This noise can interfere with nearby sensitive electronics and communication equipment. These filters support EMC compliance and are current-rated devices, so sizing comes down to matching the filter to the drive's voltage and full-load amps.
Harmonic Filters
For applications where drive-generated distortion must be kept within defined limits. Wistex stocks passive harmonic filters designed to reduce total harmonic distortion at the source. Bringing harmonic current down also improves the system’s power factor, since less of the current drawn by the drive is wasted as harmonic distortion rather than useful power.
Applications
Line reactors and filters are typically specified in:
- Facilities running multiple VFDs on a shared power supply
- Installations with long motor cable runs between the drive and motor
- Sites with sensitive electronics or communication equipment sharing the same power supply
- Facilities subject to power quality or IEEE 519 compliance requirements
Why Choose Wistex
Wistex stocks TCI line reactors and filters across the full voltage, horsepower, and impedance range, backed by sizing and specification support to help match the right device to your drive and application. Orders ship fast, so a reactor or filter added to an existing system doesn’t mean a long wait to get it running.
Frequently Asked Questions
Neither is universally better; it depends on what you’re protecting against. A 3% reactor is the standard choice for voltage spike and transient protection. A 5% reactor is the better choice when harmonic mitigation matters or when the installation needs to support IEEE 519 compliance.
No. A line reactor is a single-winding inductor that adds impedance to limit current and voltage disturbances. A transformer uses separate primary and secondary windings to change voltage levels and provides electrical isolation. Reactors are much smaller and less expensive, and they solve a different problem.
On a 480V system, a line reactor adds impedance ahead of or behind the VFD to absorb voltage transients, reduce harmonic distortion, and limit current spikes, protecting the drive on the input side and the motor on the output side.
Line reactors aren’t required by code for every VFD installation, but they’re widely recommended, and some drive manufacturers require them under specific conditions, including multiple-drive installations and low-impedance power sources, to avoid voiding warranty coverage.
Input reactors protect against power-supply-side disturbances and are commonly added on every VFD, especially in multi-drive installations. Output reactors are recommended up to 100 ft, and a dV/dt filter beyond that, up to 3,000 ft. Many installations use both.