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AC Reactors VS DC Chokes for Variable Frequency Drive

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 V&T drive V5-H-4T15G drive with the 3% line reactor and an V&T drive E5-H-4T15G 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.

AC Reactors VS DC Chokes for Variable Frequency Drive
Figure 1: Current draw by drive [1]

Harmonics need to be removed because they can increase the I2R 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 can examine the harmonic content by breaking down a non-sinusoidal waveform into components. The lower harmonics, like the 5th and 7th,have a larger current magnitude and thus have a larger effect on the total harmonic distortion. This is because the current magnitude is inversely proportional to the harmonic number.

What is an AC line reactor?

AC line reactors are added to the input of the drive and placed in series with the incoming line. They help to mitigate harmonics and because they are between the line and the drive, they are able to act as a buffer for surges and other transients. The intended purpose of a line reactor is not to offer high levels of surge protection and, if greater protection is desired, a dedicated protective device such as a metal oxide varister (MOV) or a transient voltage surge suppressor (SPD) is a much better solution. The disadvantage of an AC line reactor is that there is a voltage drop across them. This can cause under-voltage trips in systems where the input voltage is not as stable. Additionally, this voltage drop can lower the DC bus voltage and negatively affect the output of the drive. Finally, AC reactors are normally larger than DC chokes and typically more expensive.

AC Reactors VS DC Chokes for Variable Frequency Drive
Figure 2: AC Choke Attached before the Rectifier of a Drive
What is a DC choke?

DC link chokes are connected between the diodes and the DC bus and can be slightly less or slightly more effective at removing harmonics than AC line reactors depending on the order of harmonic being observed. DC link chokes are typically smaller than AC chokes and they add the necessary impedance for harmonic reduction without a drop in voltage. While the DC choke does not add any extra buffering from voltage surges seen by the rectifier, it will protect against current surges.

AC Reactors VS DC Chokes for Variable Frequency Drive
Figure 3: DC Choke Attached to the DC Bus of a Drive
Grounding with AC and DC Chokes

It is also important to ground fault currents so that they will not flow through the drive and damage it. AC line reactors inherently do this and, while single coil DC chokes do not, newer two-coil DC chokes do.

V5-H-4T15G VS E5-H-4T15G Drive Performance

V&T drive V5-H-4T15G drive offers a standard 3% AC line reactor while the new E5-H-4T15G drive comes standard with a two coil 5% DC choke and MOV surge protection on the line side of the drive. There are two critical aspects to consider when comparing these two solutions: the amount of harmonic reduction and the physical size and layout of the drive.

Harmonic Analysis

Full load tests were run to examine the amount of harmonic content for both a V5-H-4T15G and E5-H-4T15G drive at 380V 15kw. As a point of order, the phase to ground voltage is displayed in the figures below. This is equal to the input voltage divided by root(3). The charts below describe the results.

AC Reactors VS DC Chokes for Variable Frequency DriveAC Reactors VS DC Chokes for Variable Frequency Drive
Figure 4: V5-H-4T15G - Voltage and Current Input (left); Percent Total Harmonic Distortion (right)
AC Reactors VS DC Chokes for Variable Frequency Drive
Figure 5: E5-H-4T15G - Voltage and Current Input
AC Reactors VS DC Chokes for Variable Frequency Drive
Figure 6:Percent Total Harmonic Distortion at Various Harmonics for V5-H-4T15G and E5-H-4T15G

The harmonic analysis indicates that the E5-H-4T15G with the DC choke exhibits lower %THD at the 3rd, 5th, and 7th harmonics which are the most detrimental to meeting IEEE 519. The E5-H-4T15G does allow slightly higher%THD the higher order harmonics which, as was stated before, are more of a concern for EMI sensitive areas. While the E5-H-4T15G does provide better %THD than the V5-H-4T15G, the DC choke is certainly not a cure all for meeting harmonic requirements for a system. Harmonic systems studies are the best option for determining the necessary measures for meeting IEEE 519 as these results may vary with the voltage and power rating of the drives in application. With larger horse power drives, this will typically mean adding a harmonic filter or phase shifting transformer to the drive to truly meet the IEEE 519 requirements.