AC Reactors Harmonic Reduction: Expected THID and Selection Parameters

AC reactors can reduce input current distortion by approximately 30%–50% under suitable operating conditions. A 5% impedance line reactor may achieve around 35% total harmonic current distortion (THID) at full load, while power factor may rise to 0.75–0.85 in certain installations. Actual results depend on the converter, load profile, supply impedance, and measurement method.

AC Reactor Harmonic Reduction: Practical Performance

An electrical line reactor adds series inductance between the power supply and a variable frequency drive (VFD). This inductance limits current spikes caused by rectifier switching, reducing harmonic injection into the electrical distribution system.

The percentage reduction must be distinguished from the final THID value. A 35% THID reading describes the remaining distortion, not necessarily a 35% reduction from the original measurement.

Matching Impedance to Operating Conditions

• 3% impedance: Suited to general-purpose drives where moderate harmonic reduction and input protection are required.

• 5% impedance: Often selected for heavier harmonic mitigation, provided the resulting voltage drop remains acceptable.

• Higher impedance: Requires careful evaluation of starting torque, DC bus voltage, and drive performance before installation.

AC Reactor Selection for Different Applications

The correct ac reactor for inverter installations depends on the drive rating, input voltage, load variation, and allowable voltage drop. A reactor's impedance percentage alone cannot predict harmonic performance across different systems.

For example, a 5% reactor on a fully loaded six-pulse rectifier may produce different THID readings from the same reactor operating at partial load or on a supply with different short-circuit capacity.

Input Versus Output Reactor Functions

Input ac reactors are installed before the drive to limit supply-side harmonic current and reduce line-current peaks. An ac output reactor is installed between the inverter and motor, primarily to reduce voltage rise, suppress some high-frequency effects, and protect motor insulation in suitable installations.

These devices serve different electrical purposes. An output reactor should not be treated as a substitute for an input reactor when the main objective is reducing supply-side current harmonics.

Verifying Harmonic Performance

Before specifying ac reactors, the following checks establish whether the expected reduction is realistic:

1. Measure baseline THID at the point of common coupling under representative loading.

2. Confirm the drive's rectifier topology, rated current, supply voltage, and reactor impedance.

3. Compare post-installation measurements under similar operating conditions.

4. Evaluate voltage drop, temperature rise, and applicable harmonic limits.

For installations requiring strict harmonic compliance, a line reactor alone may not achieve the target. A passive or active harmonic filter may be necessary when measured distortion remains above the applicable limit.