What is a VFD Line Reactor
Many of the components that make up a variable frequency drive (VFD) are semiconductor components, which are sensitive to power or current surges, voltage spikes, line distortion, and general power anomalies. A line reactor is an optional component that can be added to a drive system to protect the VFD and other devices from power surges and transients.
A line reactor is essentially an inductor—a coil of wire that forms a magnetic field as current flows through it. The magnetic field limits the rate of rise of the current, which reduces harmonics and helps avoid unnecessary tripping of the drive.
A reactor can be referred to as either a line reactor or a load reactor, depending on where it’s installed. A line reactor (also called an input reactor) is installed before the VFD and protects the drive, while a load reactor (also called an output reactor) is installed after the VFD and protects the motor.
When a current wave form is not sinusoidal, it’s said to contain harmonics. Harmonic distortion (often referred to as THID, or total harmonic current distortion) can be as much as 85 percent for a three-phase rectifier using six diodes or thyristors and a filter. A line reactor lowers the current peak and extends the wave further over time, making it more sinusoidal. This lowers the harmonic level to around 30 to 35 percent and improves system performance and reliability.
Basic equation for an inductor
V = L (di/dt)
Where:
V = voltage (volts)
L = inductance value of the reactor (Henries)
di/dt = rate of change of current (amps/s)
This equation shows that an increase in current will cause voltage to be induced. But this induced voltage has the opposite polarity of the applied voltage, so it reduces the rate of rise of current.
The inductance value also influences the inductor’s reactance, whose equation is:
XLÂ = 2Ï€FL
Where:
XLÂ = inductive reactance (ohms)
F = applied frequency of AC source (Hz)
L = inductance value of reactor (Henries)
So the reactor adds impedance to the AC circuit in proportion to both its inductance value and the applied frequency.
What is Impedance?
“Resistance in ohms but also commonly referenced in terms of percent when combined with the system voltage and line current flowing through the reactor.
That percentage then becomes the common term used to define the level of impedance for each rating of line reactor. That impedance functions to slow the rate of current changes in the line. The greater the current through the reactor, the greater the percentage of applied impedance will be.Â
If a reactor is said to have an impedance rating of 3% or 5%, that means the reactor will apply that specified percent of impedance when the current flowing through the reactor is at the rated current of the device.Â
Installing a line reactor on the input side of the VFD ensures protection to the drive but line reactors also have the capacity to be installed on the output side of the drive blocking potential incoming background line voltage harmonics.
“In almost all drive applications, the addition of an input AC line reactor is a low cost solution for drive protection and harmonic mitigation.Â
