Line Reactors: Why and When You Should Use One?


Some applications do not require electric motors to run at full speed. You have to install a VFD or Variable Frequency Drive, a motor controller, to drive the motor. VFDs can turn up or down the motor speed according to the application. VFDs are also prone to damage due to input power disruptions. To protect your drives and motors from these current transients, you have to install line reactors.

Why? Let us learn in detail about them.

Table of Contents

variable speed drive inverter

Image: variable speed drive inverter

What are Reactors, and How do They Work?

Reactors, also known as inductors, are mainly electromagnetic devices. In a reactor, electrical grade steel laminations form a steel core. Further, there is copper twisted around each leg of the reactor. Each coil and leg of the reactor signifies one phase of a 3-phase device. The variable frequency drive has a series connection with each leg and ring of the reactor.

Connection of reactor, VFD, and the motor

Image: Connection of reactor, VFD, and the motor

When you connect a line reactor, it gives additional inductance to the circuit. You can calculate line resistance with this inductance. In general, impedance refers to resistance, and however, it is represented in percentage when impedance combines with the system voltage and line current that flows through the reactor. You can rate the line reactor and define its impedance level with this percentage. 

electric motor

Image: electric motor

Reactor Functions

Then, how do reactors protect your system?

VFD Protection and Harmonic Mitigation

Harmonic distortion and line disturbances are common in power systems connected to heavy-duty machinery. In these types of machinery, power is constantly switched on and off. There are fluctuations in current loads, leading to circuit breaks. A reactor helps stabilize the current waveform, mitigate the current surges, and reduce the nuisance tripping of the circuits. 

Along with this, reactors will also minimize harmonic currents taken by the drive, and it also blocks the line voltage harmonics that disrupt the operations of the variable frequency drive. The use of input AC line reactors is a low-cost solution to protect the industry from damage and harmonic mitigation wherever you use the purpose.

Motor protection

Placing the reactor on the load side of a VFD protects the motor and acts as a buffer. The electric motor receives a square wave power supply from a PWM (Pulse Width Modulated) drive. These square waves have sharp edges on the transitions. In the case of modern output transistors, the movement modulates the fast wave to monitor the speed of the motor. However, the fast-switching rate can cause a problem for the engine. This fast-switching speed or DV/DT refers to the rise time and the magnitude of the voltage at the edge of cycle transitions. With these fast waveform pulses, there are chances of voltage distortion and voltage spikes. Such spikes can damage the motor’s insulation system, ultimately affecting the motor’s lifespan. 

Although inverter motor systems can bear these fast pulses, minimal voltage distortion is always sound. On connecting a line reactor, its design inductance will make those sharp edges soft and slow down the voltage distortion that reaches the motor.

pure sine wave and square sine wave

Image: pure sine wave and square sine wave

Reactor Types and When to Use One

The reactors can perform like a line reactor or a load reactor based on the type of protection.

Line Reactors

When you connect the reactor to the power distribution side, it is a line reactor. The reactor acts as an impedance between the VFD and the power source, and this additional impedance will slow down the line voltage distortions that come from spikes or surges. As a result, whenever line voltage deviation arises, it prevents overvoltage faults of the drive. In addition to this, it also contains the drive’s input components. Such reactors protect your VFD and extend its life cycle.

 You can use line reactors when

  • The line side is susceptible to voltage surges, harmonics, and voltage transients.
  • You use VFD devices in the electric circuit
  • If THID (Total Harmonic Current Distortion) exceeds 5%.
  • If you turn on heavy-duty machines that go through heavy loads

Load Reactors

When the motor and the VFD are at a longer distance, you can use a load reactor as a buffer. The load reactor gets its name as it connects to the load side of the VFD. When the drive turns on, it creates high-frequency noises. Due to long cable lengths, the load reactors filter these noises and mitigate the amplified noise.

If the motor and VFD are located over a considerable distance, you can use DV/DT devices to offer a more extended range of protection. These DV/DT devices are better to use in conveyor belts, production lines, and other equipment of long cable lengths.

It would help if you used a load reactor when:

  • When VFD and motor are more than 100 ft apart
  • There is a voltage spike in machines due to constant on and off
  • When using a VFD device, you need a load reactor to protect the motor.

Reactor Impedances: 3% or 5%

So now you know the working and type of reactors. How will you pick one to meet your motor and drive needs?

Each reactor has a marking of impedance, voltage, and current. You can calculate the impedance of the line reactors through the current drawn by the VFD. It is either the percentage impedance of load Z or the percentage of effective resistance of the electrical circuit. The percent impedance of the line reactor is the same as that of the drive at full load. Here the current rating is current that load needed in amps.

Commonly, line reactors have 3% or 5% impedance. It means a line reactor’s impedance applies at the device’s rated current. 

As more current flows through the reactor, a higher impedance is applied. And if the current reduces, the percent impedance also reduces.

Most of the time, 3% impedance is in use as it improves input power factor and reduces harmonic distortions. In extreme harmonic distortions, you have to use 5% impedances. However, you must avoid oversizing the impedance for the reactor of your application.

If the motor is too far from the VFD, it is better to use a 10% impedance. However, the such higher impedance must comply with IEEE 519 standards.


VFDs and motors are costly to install. Reactors are inexpensive devices to protect these expensive devices from power surges. They also prevent the various disturbances created by electrical appliances. If you need any help installing these devices, we will help you. We have decades of experience in handling cable assemblies to support your system. Please feel free to contact us and request a quote.