Selecting Line Reactors and Output Filters for VFDs

A VFD installation can suffer from two different electrical problems that are often grouped under the word “noise.” On the supply side, the drive’s rectifier draws non-sinusoidal current and can disturb the upstream network. On the motor side, fast PWM voltage edges travel through the cable and can stress insulation, bearings, and nearby signal wiring. A reactor or filter must therefore be selected for the side of the drive and the problem being measured.

Separate the Line Side From the Load Side

An input line reactor is a series inductor installed before the VFD. Its impedance limits the rate at which current changes. It can soften line disturbances, reduce current peaks, and add useful impedance when the supply transformer is stiff relative to the drive. It also introduces voltage drop and heat, so it should not be treated as a universal cure.

An output reactor is installed between the inverter and motor. It acts on the PWM waveform leaving the drive. Its purpose is different from an input reactor: it can reduce peak current, moderate voltage rise time, and help with some motor-cable applications. Whether it is sufficient depends on drive voltage, switching frequency, motor insulation, cable type, cable length, and the VFD manufacturer’s limits.

A harmonic filter belongs on the input side and targets low-order current harmonics. A dV/dt filter or sine-wave filter belongs on the output side. A dV/dt filter reduces the steepness and peak of voltage pulses. A sine filter suppresses high-frequency output components more aggressively and presents the motor with a waveform closer to a sine wave. These devices are not interchangeable.

What an Input Reactor Can and Cannot Do

A six-pulse drive charges its DC bus through a diode rectifier. Current is drawn in pulses near the peaks of the AC voltage waveform. Adding AC line reactance, or using a drive with a DC choke, spreads those current pulses and can lower total harmonic current distortion. ABB’s harmonic-mitigation guidance distinguishes this modest improvement from a tuned passive filter or an active-front-end drive.

The reactor also provides some isolation from line notching and transient events. However, it is not a surge protective device and does not clamp voltage like an SPD. It cannot correct every power-quality problem. Severe voltage imbalance, repeated undervoltage, resonance, or a weak generator supply requires a system-level study.

Before specifying a reactor, record the transformer rating and impedance, available fault current, drive input current, existing capacitors, generator operation, and the measured harmonic spectrum. A percentage-impedance label alone does not prove that the finished installation will meet a harmonic limit.

Choosing an Output-Side Solution

The inverter produces voltage pulses with rapid edges. At the motor terminals, cable impedance and reflected-wave effects can raise peak voltage above the value observed at the drive. The risk generally increases with cable length, voltage, switching behavior, and motor insulation condition. There is no safe universal cable-length threshold.

Use the drive hardware manual as the controlling reference. It should define permitted motor-cable length and when an output reactor, dV/dt filter, common-mode filter, insulated bearing, grounding brush, or sine filter is required. ABB notes that sine filters are low-pass networks made from reactors and capacitors. They suppress high-frequency output voltage components and are used for applications such as extra-long motor cables, step-up transformers, or older motors.

A sine filter can also impose limits on output frequency, switching frequency, cable arrangement, or drive sizing. Its capacitors draw current even when mechanical load is low. Treat the drive, filter, cable, and motor as one engineered system.

A Practical Selection Workflow

1. Define the symptom

Do not start with a component. Identify whether the problem is input current distortion, nuisance overvoltage trips, motor-terminal peak voltage, bearing current, conducted EMI, radiated interference, or audible motor noise. One accessory rarely solves all of them.

2. Measure at the correct points

For line harmonics, capture voltage and current at the point of common coupling under representative load. For output problems, use instruments and probes rated for PWM drive waveforms. A standard multimeter cannot reveal rise time or reflected-wave peaks.

3. Check the manufacturer’s combinations

Confirm that the reactor or filter is approved for the exact drive series, voltage class, continuous current, overload duty, and output frequency. Observe enclosure, ventilation, conductor, grounding, and clearance requirements. Review the motor insulation and bearing arrangement as well.

4. Model operating extremes

Check low and high load, bypass operation, regenerative conditions, generator supply, long leads, multiple motors, and capacitor switching. ABB warns that passive harmonic filters can create leading power factor at light load and can interact with other capacitors through resonance.

5. Commission with evidence

Record line voltage, phase current, DC-bus behavior, motor-terminal waveform where appropriate, filter temperature, drive alarms, and baseline vibration. Repeat the measurements after the machine reaches normal thermal conditions. Keep these records with the drive parameters and single-line diagram.

Installation Details That Decide the Result

Place input components according to the drive and filter drawings. Do not accidentally install an input passive harmonic filter on the output; ABB explicitly identifies this as a field coordination risk. Keep output filter-to-drive conductors short where the manufacturer requires it. Route motor leads away from low-level analog, encoder, and communication cables.

Use continuous protective bonding and terminate shielded motor cable as specified. Filters do not compensate for poor grounding or uncontrolled cable routing. Check reactor and filter surface temperatures, because both add losses inside the enclosure.

Engineering Boundary

Reactors are useful impedance devices, while filters are frequency-selective networks. Their value comes from matching the device to a measured mechanism. For related hardware, review our VFD and AC drive collection and broader drives and motion control range. ABB’s harmonic mitigation technical note and ACQ580 filter guidance provide useful manufacturer context. Final selection must still follow the manuals for the installed drive and motor.