How to Select a VFD Output Filter for Cable Length
When a motor cable is long enough to threaten the motor with reflected-wave overshoot, the fix is a device on the drive output that slows the switching edge or absorbs the reflection. But there are three common devices with very different jobs and costs, and picking the wrong one either wastes money or leaves the motor exposed. This page is a selection guide for the engineer specifying output protection. It compares the load reactor, the dv/dt filter, and the sine-wave filter, explains where each one fits, and shows how run length and motor rating drive the choice.
VFD Output Filter Selection in one line: To select a VFD output filter for a long cable, match the device to the severity of the run: a load reactor slows the edge modestly and suits moderately long runs, a dv/dt filter shapes the edge specifically to control reflected-wave overshoot on longer runs, and a sine-wave filter smooths the output nearly to a clean sinusoid for the longest runs or the most sensitive motors, at the highest cost and size. Base the pick on the measured cable length, the motor's peak-voltage rating, and the carrier frequency, then confirm the chosen device brings the terminal peak within the motor's insulation limit.
Know What Each Device Actually Does
A load reactor, also called an output reactor, adds series inductance between the drive and motor. That inductance slows the rate at which the voltage edge rises, which reduces the overshoot a reflection can build and also limits the high-frequency charging current a long cable draws. It is the simplest and cheapest option and does double duty against both reflected wave and capacitive current, but its edge-slowing is modest, so it handles moderately long runs rather than extreme ones. Its general role is covered in the page on the line reactor.
A dv/dt filter is purpose-built to control the rate of voltage change, the dv/dt, at the motor terminals, precisely the quantity that drives reflected-wave overshoot. It combines inductance and capacitance tuned to shape the edge so the peak terminal voltage stays within bounds over a longer cable than a plain reactor manages. The mechanism it targets is described in the page on dv/dt and reflected wave, and a dv/dt filter is the middle tier between a reactor and a full sine filter.
A sine-wave filter is the heaviest option, filtering the drive output so thoroughly that the motor sees a near-sinusoidal voltage instead of a chopped waveform. That all but eliminates reflected wave, motor heating from harmonics, and audible switching noise, and it is what you reach for on the longest runs or where the motor cannot tolerate any overshoot. It is the largest and most expensive device and can constrain how the drive is set, so it is reserved for the cases that genuinely need it rather than used as a default.
Let Length, Motor Rating, and Carrier Drive the Choice
Start from the measured cable length, established per the guide on checking VFD motor cable length, because length is the primary driver. A run just past the critical length may need only a load reactor; a substantially longer run needs a dv/dt filter to keep the overshoot controlled; and a very long run, or one where the motor is delicate, pushes toward a sine-wave filter. The device tiers roughly track the severity of the length problem.
Weigh the motor's insulation rating as the other half of the decision. An inverter-duty motor built to withstand high peak voltage and fast rise times, as described in the page on motor insulation class, tolerates more overshoot before it needs help, so it may accept a longer run with a lighter device or none. A standard-duty motor has far less margin and pulls the choice toward more filtering at a shorter length. The right device is the cheapest one that keeps the terminal peak inside the specific motor's limit.
Factor in the carrier frequency, because it changes how hard the cable is worked. A higher carrier frequency means more switching edges per second and more capacitive charging current, which can push a marginal run toward needing a reactor or filter it would not need at a lower carrier. If you have freedom to lower the carrier, doing so may reduce the filtering you need, so trade the carrier setting against the filter cost rather than treating them separately.
The Selection Table and How to Confirm the Pick
The following table summarizes where each device fits so the choice can be read against a specific installation. Treat the length descriptions as relative severity rather than fixed figures, because the exact thresholds are drive- and cable-specific and come from the manufacturer's documentation.
| Device | Primary job | Best fit | Relative cost/size |
|---|---|---|---|
| Load reactor | Slow the edge, limit charging current | Moderately long runs | Low |
| dv/dt filter | Control terminal dv/dt and overshoot | Longer runs, standard motors | Medium |
| Sine-wave filter | Near-sinusoidal output | Longest runs, sensitive motors | High |
Confirm the device you chose is compatible with how the drive runs. A sine-wave filter in particular can limit the maximum output frequency and interacts with the carrier setting, so verify the drive supports the filter and that the application's speed range still fits. A reactor or dv/dt filter is more forgiving but still adds a voltage drop that can matter at full speed. Read the drive and filter documentation together rather than treating the filter as a bolt-on that cannot affect the drive.
Verify the outcome, which is the whole point of specifying the filter. The device is correct when the peak voltage at the motor terminals stays within the motor's rated withstand, so on a critical machine the terminal waveform can be captured to prove it. In routine service, the practical confirmation is that premature insulation failures stop, checked through periodic testing per the guide on insulation-resistance testing a motor circuit. A filter chosen on paper but never confirmed on the machine is only a hopeful guess.
Frequently Asked Questions
What is the difference between a load reactor and a dv/dt filter?
A load reactor adds series inductance that modestly slows the switching edge and limits charging current, making it a simple, low-cost fix for moderately long runs. A dv/dt filter combines inductance and capacitance tuned specifically to control the rate of voltage change at the motor terminals, holding overshoot in bounds over a longer cable than a reactor manages. The reactor is the lighter, cheaper first step; the dv/dt filter is the purpose-built middle tier for reflected wave.
When do I need a sine-wave filter instead of a dv/dt filter?
Reach for a sine-wave filter on the longest cable runs or where the motor cannot tolerate any overshoot at all, because it filters the output to a near-sinusoid, eliminating reflected wave, harmonic heating, and switching noise. It is the largest and most expensive device and can limit the drive's output frequency, so it is reserved for cases a dv/dt filter cannot handle. If a dv/dt filter keeps the terminal peak within the motor's rating, you do not need the heavier sine filter.
Does lowering the carrier frequency reduce the filter I need?
It can. A lower carrier frequency means fewer switching edges per second and less capacitive charging current on a long cable, which can ease a marginal run enough to need a lighter device. The per-edge overshoot is unchanged, so lowering the carrier does not by itself cure reflected wave, but combined with a reactor or dv/dt filter it reduces how hard the cable is worked. Trade the carrier setting against the filter cost rather than deciding them in isolation.
Automation services
Need help turning this into a working system?
Merobix integrates SCADA, programs Allen-Bradley and Siemens PLCs, and designs and fabricates industrial control panels.
Meeting requests are reviewed before confirmation.