With the advent of reliable and inexpensive AC drives, the drive isolation transformer began to loose ground to the line reactor as the input device of choice. In applications where isolation, voltage step-up or down or where shielding is not necessary, reactors provide similar results and are smaller and less expensive. Today, many specifications for drives still call for drive isolation transformers to be installed as protection devices, and manufacturers have been known to claim harmonic mitigation as one of the primary benefits of their products. Many consulting engineers who have traditionally used the devices in specifications have trusted those claims, or have been slow to challenge them. In fact, the only harmonic reduction that a drive isolation transformer offers is due to reactive impedance which slows the rate of change of current flowing to the input rectifier.
Similar results are available with line reactors or DC bus chokes.
Reactors for use with AC and DC drives Line reactors were originally developed as a cost effective replacement to drive isolation transformers sold with DC drives when the other functions of drive isolation transformers were not needed. Source impedance provided by a reactor brings similar reduction in harmonics to source impedance provided by a transformer. They are cost-prohibitive for fractional horsepower drives, however, these small drives are usually insignificant contributors to harmonics, and therefore, manufactures and users are often unconcerned. DC Chokes used in the DC bus of an AC drive DC chokes are used to tame ripple current in the DC bus of an AC drive. They do not, however, protect against incoming line transients.