Purchasing a modern frequency converter (VFD) is an excellent investment in production modernization. However, many professionals, seeking to save money, cut "additional" equipment from the budget. The line choke (input reactor) is most often cut. And that's a shame.

In this article, we'll discuss why skimping on a line choke can result in the failure of an expensive inverter, how this device works, and what parameters to use to choose the right one.
What is a line choke and where is it installed?

A three-phase line choke (AC line reactor) is an inductor with a ferromagnetic core that is installed in the power supply circuit between the industrial power grid and the VFD.

(Important note: do not confuse a line choke with a motor choke. A line choke is installed at the VFD input, while a motor choke is installed at the output, between the VFD and the electric motor.)

The main function of an input choke is to create inductive reactance, which smooths out sudden changes in current and voltage, acting as a kind of "shock absorber" for electronics.
4 Reasons to Install a Line Choke

A frequency converter operates on the principle of double conversion: first, it rectifies alternating current to direct current (via a diode bridge), and then inverts it back to alternating current of the required frequency. This process places significant stress on both the frequency converter itself and the power grid.

A line choke solves several problems at once:

Protecting the frequency converter from power surges. During sudden voltage surges (for example, when powerful nearby loads are turned on or during lightning strikes), the choke limits the rate of current rise. This prevents breakdown of the rectifier diodes and DC link capacitors.

Protecting the power grid from harmonics. The frequency converter draws current in pulses, not as a smooth sine wave. This generates higher harmonics that flow back into the grid, overheating transformers and cables and causing malfunctions in sensitive electronics (PLCs, sensors). A choke significantly reduces the total harmonic distortion (THDi).

Phase imbalance protection. If there is a slight voltage imbalance between phases in the grid, it can cause a significant current imbalance at the inverter input. A choke balances these currents, preventing overheating and inverter shutdown.

Reduction of false protection trips. On unstable grids, inverters often experience an "Overvoltage" fault. The choke's inductance smooths out these fluctuations, ensuring continuous operation of the equipment.

When is installing a line choke absolutely necessary?

Variable frequency drive manufacturers (including Veichi, Siemens, ABB, and others) strongly recommend installing a line reactor in the following cases:

The power transformer's wattage is more than 10 times greater than the inverter's wattage. (In such networks, the impedance is very low, and any surge can severely impact the frequency converter.)

Several frequency converters or thyristor drives are connected to a single power line.

The network frequently experiences voltage dips or has reactive power compensation (RPC) units installed.

You use a frequency converter with a power rating of 22-30 kW or higher (at higher power ratings, harmonic distortion becomes critical).

How to choose the right line reactor? Key Parameters

Choosing a reactor is not difficult if you know the key characteristics of your frequency converter.
1. Rated Voltage

The reactor must match the voltage rating of your network. For a standard industrial network, choose models rated at 380 V - 400 V (or 480 V for some Western standards).
2. Rated Operating Current (A)

This is a crucial parameter. The rated current of the choke should be equal to or slightly greater than the maximum input current of the frequency converter.

Example: If you are using a 22 kW VFD (e.g., Veichi AC10-T3-022G-B), its input current is approximately 45 A. You will need a choke with a rated current of 45 A or 50 A. Choking a choke with a significant margin (e.g., 100 A for a 45 A VFD) is not recommended, as its inductive reactance will be too low to provide effective protection.

3. Voltage Drop (Impedance: 3% or 5%?)

Line chokes are typically classified by the voltage drop (impedance) they create at rated current. The most popular standards are 3%, 5%, or 4%.

2-3% chokes: The most common choice. They provide an excellent balance between inverter protection, harmonic mitigation, and cost. They are ideal for most standard applications in European factories.

4-5% chokes: Used in harsh environments: with extremely unstable grids, very stringent harmonic distortion standards (IEEE 519), or if the inverter is installed close to the main distribution transformer. They are more expensive and run slightly hotter, but provide maximum protection.

Conclusion

A three-phase line choke costs, on average, only 3-5% of the cost of a medium- and high-power frequency converter.