To select an electrical reactor for a variable frequency drive (VFD), first identify whether you need protection at the drive input, protection between the drive and motor, or harmonic mitigation on the power system. Match the reactor’s voltage, current, frequency, impedance, and installation location to the VFD and application—not just to the motor’s nameplate rating. As a starting point, input line reactors are commonly specified at 3% or 5% impedance, but the right value depends on the drive manufacturer’s guidance, available fault current, voltage conditions, and harmonic requirements. I recommend confirming the selection against the VFD manual and the project’s electrical design before ordering.
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Before comparing products, determine what is driving the selection. A VFD may need protection from supply disturbances, a motor may need protection from output waveform effects, or a facility may need lower harmonic distortion at a point of common coupling. These are related issues, but they do not always call for the same reactor.
Record the VFD model and ratings, supply voltage, motor data, cable length and type, operating duty, and any manufacturer recommendations. Also note whether the installation has frequent starts, a weak supply, sensitive equipment on the same network, or a defined harmonic limit. If the project requirement is expressed as a specific total harmonic distortion (THD) target, request an engineering assessment rather than assuming a reactor alone will meet it.
An input line reactor is installed on the AC supply side of the VFD. It adds series impedance that can help reduce the impact of supply-side current transients and limit some harmonic current drawn by the drive. It is often considered when the supply is electrically stiff, when several drives share a source, or when the VFD manufacturer recommends input impedance.
Line reactors are a practical measure, but their harmonic benefit varies with the complete electrical system. They do not remove all harmonics, and they are not a substitute for a harmonic study where compliance or a guaranteed distortion level is required. Check whether the drive already includes a DC link choke or another input filtering feature, since this may affect the additional reactor requirement.
An output reactor is installed between the VFD and motor. It can help reduce the rate of voltage change and provide additional impedance on the motor circuit, which may be useful with longer motor leads or applications where the motor and cable arrangement needs further evaluation. It is not interchangeable with an input line reactor because it addresses the drive-to-motor side rather than the supply side.
Ask the VFD manufacturer to review the permitted cable length, switching frequency, and motor insulation requirements. If the application needs stronger output waveform conditioning, compare an output reactor with a dv/dt filter or sine-wave filter. Those options have different performance, size, cost, and drive-compatibility considerations.
A DC link choke is installed in the VFD’s DC bus and may be supplied as part of the drive or offered as an option. Its function and expected harmonic performance depend on the VFD design. Confirm the drive’s internal configuration before specifying an external reactor so that components are not duplicated or selected without a clear system benefit.
Start with the current rating relevant to the installation point. For an input reactor, use the VFD’s specified input current or the manufacturer’s selection table; for an output reactor, use the drive output current and confirm that the reactor is suitable for VFD output waveforms. Do not select solely from motor horsepower, because input and output current can differ by drive design and operating conditions.
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| Selection factor | What to confirm | Why it matters |
|---|---|---|
| Voltage and frequency | System voltage, phase, and operating frequency | The reactor must suit the supply or drive output where it will be installed. |
| Current | VFD input or output current, duty, and overload conditions | An undersized unit may overheat or fail to meet the application requirement. |
| Impedance | Specified percentage and its reference basis | Impedance affects voltage drop, current limiting, and system response. |
| Environment | Ambient temperature, ventilation, enclosure, and mounting | Operating conditions influence thermal performance and installation suitability. |
For many low-voltage VFD applications, 3% and 5% impedance are common line-reactor options. These are selection points, not universal recommendations: a higher impedance can increase voltage drop and may affect drive performance, while a lower impedance may provide less attenuation. Confirm the specified percentage, current rating, and voltage-drop expectations with the VFD supplier and reactor manufacturer.
Also verify whether the rated current is continuous and whether the product is designed for the applicable harmonic waveform and duty cycle. Check terminal arrangement, conductor size, thermal class, losses, enclosure protection, and available space. If several drives share a reactor or transformer, have the arrangement reviewed for the combined load and protection coordination.
Consider an input line reactor when the objective is to add impedance at the drive input and reduce some supply-side current disturbance. Gather the transformer rating, source impedance if available, the number and size of drives, and the location where harmonic performance is assessed. Where a facility has a defined harmonic limit, use a qualified power-quality analysis to determine whether line reactors are sufficient or whether a broader solution is needed.
Consider an output reactor when the VFD manufacturer’s guidance or the motor-cable arrangement indicates a need for additional output impedance. Share the actual cable length in metres, cable construction, motor data, and drive switching settings with the supplier. There is no single cable-length threshold that applies to every VFD, so use the specific drive documentation rather than a generic rule.
If a project requires a specific THD result, compare line reactors with DC link chokes, passive harmonic filters, or active harmonic filters as appropriate. The best option depends on load variation, the existing network, the required measurement point, and whether the limit applies to current distortion or voltage distortion. A reactor can be part of the solution, but the system-level result should be verified rather than presumed.
For a useful quotation, provide the VFD manufacturer and model, voltage, phase, input or output current, target impedance, installation location, and quantity. Include the ambient temperature, enclosure or mounting requirements, cable details for output applications, and any harmonic performance requirement. Complete information helps a supplier assess product fit and identify missing design details before procurement.
At ANNEZEE, we can review the information provided and help buyers compare suitable electrical reactor configurations for their VFD application. Share the project specification or drive nameplate data, and ask us to confirm the applicable ratings, available options, and technical details for quotation. Final selection should be coordinated with the VFD manufacturer or project engineer where system performance, protection coordination, or harmonic limits are critical.
The right electrical reactor depends first on the problem: use an input line reactor for supply-side impedance and potential reduction of some harmonic current, an output reactor for motor-side considerations, and a DC link choke only where the VFD design calls for it. Then verify voltage, current, impedance, frequency, thermal conditions, and compatibility with the drive. For defined harmonic targets or unusual supply and motor-cable conditions, request an engineering review rather than relying on a generic sizing rule.
Your next step is to collect the VFD and motor data, identify the reactor location, and send the requirements to your supplier for confirmation. ANNEZEE can support the product-selection and quotation process with the technical information available for your application. This approach helps you compare options on a clear basis and avoid specifying a reactor that does not address the actual project need.
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