Line reactor applications for inverters and motors

Both line reactors and load reactors protect the electrical system because they increase the impedance of the assembly, smoothing out harmonic distortions and preventing power surges. Line reactors protect frequency converters, while load reactors protect motors. However, it is important to know how to choose the most suitable line or load reactor for each application.

Line reactors for frequency converters

A line reactor, also known as an input reactor, is designed to help protect the variable frequency drive (VFD) from disturbances in the input power line that could cause damage to the VFD, and also reduces harmonic currents in the power grid.

They act as an additional impedance and are located between the power supply and the frequency converter, as close as possible to the latter.

Line reactors are sized according to the rated power and voltage of the drive. They are classified according to the voltage drop they generate when the rated current passes through them. This voltage drop is expressed as a percentage, and the most commonly used are 3% and 5%.

Below are some cases in which the use of line reactors is recommended:

• The input line power supply is prone to disturbances that cause power surges, voltage spikes, transients, etc.

• The power of the power supply line is very high compared to that of the frequency converter. Installation of a converter is considered necessary when the power in kVA of the power supply is around 10 times greater than that of the converter.

• When harmonic distortion becomes a problem

3% line reactors with impedance

Line reactors with an impedance of 3% are the most commonly used as they are sufficient for the vast majority of cases. They reduce transient voltages in the power line caused by capacitor switching and are capable of preventing untimely tripping of protective elements. They also improve the actual input power factor and reduce interference between drives.

Line reactors provide protection for frequency converters in short-circuit conditions. If the power of the supply transformer is more than ten times the power of the frequency converter, it is recommended to connect a line reactor to minimise damage in the event of a short circuit in the supply transformer. This line impedance depends directly on the short-circuit current of the drive and the short-circuit capacity of the transformer, and must be greater than or equal to the ratio between the rated value of the transformer and the short-circuit current of the drive.

5% line reactors with impedance

There are also line reactors with an impedance of 5%, which are used when there are greater disturbances in the line and it is necessary to minimise the effects of harmonic distortions that produce noise and increase the operating temperature of the motor.

Multiple line reactors

In cases where there are multiple inverters in parallel, it is advisable to connect a line reactor for each frequency inverter to provide adequate filtering and good protection against overvoltages that may occur in each of these frequency inverters. On the contrary, a single line reactor connected to several frequency converters does not guarantee adequate protection or reduction of harmonic distortions. As in the previous cases, the line reactor is placed in series between the power supply and the frequency converter and as close as possible to the converter.

If you are interested in learning about other applications of line reactors in frequency converters Contact one of our specialised technicians now.

Charging reactors

A load or output reactor is used to protect the motor if the wiring distance between the frequency converter and the motor is considerable. The frequency converter causes a high-frequency three-phase PWM output which, together with the long cables between the converter and the motor, generates high-voltage spikes and noise. This overvoltage can exceed the motor's peak voltage rating when insulation breakdown occurs.

 The load reactor must be installed in series between the frequency converter and the motor and as close as possible to the frequency converter. It is generally recommended for installation when the wiring to the motor exceeds 30 metres, but this will depend on each individual circumstance. There are cases in which, if the motor complies with generator and motor regulations, it is possible to have up to 100 metres of cable without any type of load reactor. Another case would be for distances of 150 to 300 metres, which require the installation of a dV/dt low-pass filter.

Line reactors for multiple motors

For an application that includes more than one motor controlled by a single frequency converter, it is possible to use only one load reactor to protect the set of motors. The load reactor is connected in series between the frequency converter and the motors and should always be placed as close as possible to the converter. The load reactor is sized taking into account the total power of the load, in this case, the motor.