How Will Line Reactors Help Your Variable Frequency Drive?

Posted by Yin Tang 23/06/2014 2 Comment(s)

The problem:


While VFD's provide great advantages in energy savings and increased efficiency, they are the #1 cause of electrical power system pollution. Reactors have become the most common means of reducing this pollution (or dirty power) and are in widespread use.

VFD's tend to "gulp" current to recharge their capacitors each time the AC voltage reaches a peak. This results in short bursts of current (Fig. 3) as opposed to the normal sine wave that the utility expects to see (Fig 1). 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 the same effect on the electrical power system, causing voltage drops in the power system and distortion of the voltage waveform. The magnitude of this distortion is referred to as either Total Harmonic Current Distortion (THID) or Total Harmonic Voltage Distortion (THVD). The larger the VFD, the more it will affect the power system. Think of this as noise that will pollute the power system, even traveling to neighboring facilities.

Line Reactor

(Figure 1) Normal Current - a Clean Sine Wave

Line Reactor

(Figure 2) Schematic of a VFD

Line Reactor

(Figure 3) Current Waveform Without Line Reactors

The solution:


An AC line reactor can eliminate about 65% of THID. Although not perfect, this is a vast improvement. High peak currents can also cause fuse degradation and intermittent blowing of fuses or tripping of circuit breakers. Note that the peak currents in Fig. 5 are only half those in Fig. 3. AC line reactors also protect the VFD from power system surges and can prevent overvoltage trips. The line reactors have either 3% or 5% impedance. 5% impedance line reactors will provide lower THID and provide better protection than 3% line reactors.

Line Reactor

(Figure 4) VFD with Line Reactors

Line Reactor

(Figure 5) Current waveform with line reactors

Still not convinced why you should use a line reactor with your VFD? See why our engineers at recommend them.

Leave a Comment

2 Comment(s)

S.VASUDEVAN, Independent Consultant:
03/06/2017, 12:26:25 AM

One of my client has frequent failures of VFDs. Supply 11KV 50Hz Double Star 1903V VFD 3300V motor 2200KW
5 units are in service. Suggest suitable reactor size and cost.

S.Vasudevan,BE.,MBA.,Ex. Site Manager ABB/Sweden projects @ Kuwait

Connor (Admin):
05/06/2017, 03:00:36 PM

Hi S.Vasudevan,
We'd love it if you could send an email to with the following questions answered, and we can easily finish answering your question.

What type of failures are occurring? And how did you find out you needed a reactor?

Enrique Gurdian:
20/01/2017, 08:38:52 AM

I have a residential, multiple unit building where we are replacing RTUs operated thru VFDs.
There is a power quality problem where utility power causes the units to trip during power surges. My question is as follows: what is a better application for an AC Line Reactors:
1) Install individual Line Reactors for ea. RTU.
2) Install a Line Reactor on the incoming feeder to the Switchboard that feeds the RTUs.

Yin (Admin):
23/01/2017, 09:12:07 AM


I assume that the RTUs provide a speed reference to the drives. I’m not completely sure what issues you are having. You are replacing RTUs? Are they being damaged by surges? Is it the VFDs that trip during power surges? In any case, here are our suggestions.

Do not install reactors on the incoming feeder to the switchboard. This may make problems worse.

Each VFD should have line reactors installed immediately upstream of the VFD. Use 3% reactors minimum. 5% reactors are better.
The switchboard should have an SPD (Surge Protective Device) installed. It is critical how you install this device. Wire leads to the SPD should be as short as possible and should be twisted. This limits the surge impedance as helps the SPD function better. Purchase an SPD that also has some noise suppression capability.
Outfit the RTU with its own surge suppressor. For this, use a “series” type surge suppressor. These are very inexpensive and are wired in series with the 120VAC power supply, rather than in parallel as with most suppressors.

Hope this helps!