How Will Line Reactors Help Your Variable Frequency Drive?

By Yin on 23rd Jun 2014

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 over-voltage 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.