When compared to DC drives, AC drives cause very few problems.  However, poorly designed applications do sometimes result in power line voltage distortions.  These voltage distortions can also cause problems for other equipment connected to the same power lines, subsequently resulting in erratic operation of controls, dimming of lights, and overheating motors operating across the line.  The distribution transformers and cables feeding these drives will also experience additional heating, which reduces the power use of those components.

Unlike the way in which a linear load draws current, such as an AC motor operating across the power line, a typical AC drive draws current from a distribution transformer.  This takes place due to the drive taking current from the transformer only during certain times of the cycle to convert the 3-phase AC line voltage to a fixed DC voltage within the drive.  The drive then pulse-width modulates this fixed DC voltage into variable frequency/variable voltage for the motor.

The AC-to-DC conversion is what causes the harmonics.  Current flows only during part of the cycle.  It’s this distorted current that creates the voltage distortion.  This is also why a drive is considered a nonlinear load.

While the number of drives in an automation system may increase, it does not necessarily mean that those drives are the cause of all harmonic problems.  There are other pieces of equipment that rectify AC to DC and create harmonic distortion.  This includes most of the equipment found on the plant floor, as well as phase-to-neutral nonlinear loads in the form of office machines such as computer power supplies and copiers.  Even fluorescent lighting ballasts are known to create harmonic distortion.  That is why all of the electrical loads that could potentially cause problems for a system before you rush out and fit filters to every drive should be analyzed.