A line reactor (also referred to as a "choke") is a variable frequency drive (VFD) accessory. It consists of a coil of wire that forms a magnetic field as current flows through it. This magnetic field limits the rate of rise of the current, thus reducing harmonics and protecting the drive from power system surges and transients.

There are two primary types of line reactors used in variable frequency drives: AC & DC.

When an AC reactor is placed between the power system and the drive, it is referred to as an AC line reactor. When a DC reactor is inserted into the DC link of a variable frequency drive, it is known as a DC link reactor.

Both AC and DC reactors limit harmonic currents. AC reactors have a significant advantage, though: because they sit between the power system and the incoming power terminals at the drive, they protect the entire VFD from power system surges and transients. Reactors can prevent overvoltage trips, increase the reliability and lifespan of your variable frequency drives, improve total power factor, and reduce nuisance tripping.

Typical drive applications for AC line reactors include:

• Oil and Gas pumps
• Irrigation fields
• Water and Wastewater Treatment
• HVAC
• Steel Manufacturing
• Pulp and Paper
• Machining
• Extruders

How will a Line Reactor help your Variable Frequency Drive?

VFDs deliver real advantages in energy savings and manufacturing efficiency, but they are also the leading cause of electrical power system pollution. Reactors have become the most common means of reducing this pollution (or dirty power), which is why they show up so often as a recommended as drive solutions.

VFDs tend to draw current in short bursts to recharge their capacitors each time the AC voltage reaches a peak, rather than pulling current in the smooth sine wave the utility expects. Think of it as flushing a toilet when someone is taking a shower; the water pressure in the shower drops suddenly.

These short bursts of current have a similar effect on your electrical power system, causing voltage drops in the power system and distortion of the voltage waveform. 

The magnitude of this distortion is measured as either Total Harmonic Current Distortion (THID) or Total Harmonic Voltage Distortion (THVD). The larger the VFD, the more it affects the power system. Think of this as noise that will pollute the power system, even traveling to neighboring facilities down the road.

^{Line Reactor Diagram}

An AC line reactor can eliminate roughly 65 percent of THID. Although not a perfect fix, this is a substantial improvement. High peak currents can also cause fuse degradation and intermittent blowing of fuses or tripping of circuit breakers.  AC line reactors also protect your VFD from power system surges and help prevent overvoltage trips.  Line reactors typically carry either 3 percent (Low-Z) or 5 percent (High-Z) impedance. 5 percent impedance line reactor delivers lower THID and better protection than a 3 percent unit.

Why do our engineers recommend Line Reactors?

Here's what a line reactor can do for your operation:

• Easy to add to an existing VFD
• Provides surge protection for the VFD
• Increases reliability and lifespan of the VFD
• Provides some protection against overvoltage trips
• Delivers a cost-effective way to reduce harmonic distortion
• Buffers against voltage spikes
• Brings the facility into compliance with utility standards
• Avoids litigation and damage settlements
• Reduces the amount of high frequency noise injected into the power system.
• Improves true power factor
• Reduces cross-talk between drives
• Helps prevent fuses and circuit breakers from blowing or tripping due to high current spikes
• Provides needed impedance between VFDs and power system capacitors
• Protects capacitors and other power system components from harmonic resonance
• Frees up system capacity by reducing current levels
• Reduces intermittent faults and problems of that are otherwise hard to trace

Our team of experienced drive engineers recommends that every variable frequency drive include a reactor, at minimum, to control harmonics and power system pollution. Line reactors are the most commonly preferred method of harmonic correction due to their effectiveness, reliability, and low cost. AC line reactors, or a combination of AC and DC reactors, provide enhanced surge protection in addition to harmonic mitigation.

Standards of Measurement

IEEE 519 (IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems) is the standard used universally to determine acceptable limits for power system distortion. This standard allows for 5 percent to 20 percent Total Harmonic Current Distortion (THID) depending on the "stiffness" of the power system.

The Point of Common Coupling (PCC), the closest point on the utility side of your service where another utility customer is or could be supplied, serves as the point of measurement. A drive using line reactors may produce around 30 percent THID at that specific point, which is above the allowable limit on its own. In practice, that current is typically diluted by other cleaner loads within the facility, bringing the overall facility into compliance. When that isn't the case, other harmonic reduction techniques are available, though at a significantly higher cost than line reactors.

The Bottom Line on Line Reactors:

AC line reactors applied to the line side of a variable frequency drive improve THID and increase reliability and longevity.  It's an optional accessory that delivers strong protection at a reasonable price.

Premier Automation's team of engineers and service personnel are experts and have worked with, upgraded, designed, and repaired hundreds of drive applications. We specialize in a broad portfolio of automation solutions, including control system design, custom panel building, nationally ranked control system integration, and more.

If you're weighing whether a line reactor makes sense for your system, or you're troubleshooting power quality issues you haven't been able to pin down, our team can walk through your specific setup with you.
Contact Premier Automation for a consultation and find out what the right drive solution looks like for your operation.