VTdrive AC Line Reactors vs DC Link Chokes0 pages

AC Line Reactors vs. DC Link Chokes
Introduction
This application note is designed to provide an overview of why impedance on the line side of a drive is
beneficial in adjustable frequency drives and compares the expected performance of line reactors and DC
link chokes. In addition, performance testing is presented to provide a harmonic comparison between the
two filtering devices using an VTdrive V5-H-4T15G drive with the 3% line reactor and an VTdrive E5-H4T15G drive with a 5% DC link choke.

Harmonics
All drives naturally create harmonics on a system given the nature of the AC to DC rectifier of a drive.
These harmonics are created because the drive only draws current when the magnitude of the AC, line to
line voltage is higher than the voltage of the DC bus. In Figure 1 below, the current is blue and the time
where the AC, line to line input voltage exceeds the DC voltage is designated as Id. Current is drawn
during the crossover point of the phase voltages and results in a humped waveform. This double hump
waveform is typical of most drives.

Figure 1: Current draw by drive [1]
2

Harmonics need to be removed because they can increase the I R losses of the system and they can
disrupt the proper functioning of other loads on the system as well as potentially damage them.
Additionally, utilities often require customers to adhere to IEEE 519 standards of total harmonic distortion
(THD). This specifies the amount of harmonics that can be injected back onto the grid. Harmonics that
are generated within in a facility, if not properly filtered, will be seen by the utility.

For very small drives, this sinusoidal current draw is not a problem for the utility because the drive is not a
significant load on the system. However, for larger horsepower drives additional impedance is normally
necessary to remove the impact of these periodic pulses of current. In this regard, the two most common
options to choose from are an AC line reactors and a DC link choke.

What is a choke and how does it affect harmonics?
A choke (or line reactor) is a coil of wire around a magnetic core that creates a magnetic field when
current flows through it. This magnetic field increases the impedance of the line and reduces the total
harmonic content injected from the drive onto the facilities electrical system. To analyze harmonics, we