AC and DC reactors for inverters differ significantly in installation location, equipment protection capabilities, and the degree of voltage drop caused on the power grid. Specifically, while the AC Reactor is installed at the input to prevent voltage surges from the power source, the DC Reactor is located in the direct current circuit to optimize the starting inrush current charging the capacitor. Thus, understanding the characteristics of each type will help you design safer and more efficient electrical panels.
Cases where AC reactors should be used include unstable power sources, frequent phase loss, or large voltage surges, while DC reactors are suitable for narrow panel spaces and requirements for reducing harmonics without causing input voltage drops. Therefore, depending on grid conditions and actual motor power, engineers will decide to select the equipment to optimize costs and performance.
You can install both AC and DC reactors simultaneously for the same inverter in extremely harsh industrial environments that require maximum harmonic suppression and absolute inverter protection. However, this combined solution needs to be carefully calculated in terms of investment costs and the cumulative voltage drop level across the entire electrical system.
Below, we will dive into a detailed analysis of each core technical characteristic of these two types of equipment. Let's explore the differences to come up with the most comprehensive and technically accurate protection solution for Variable Frequency Drive (VFD) systems.
How Do AC and DC Reactors for Inverters Differ?
AC reactors excel in input voltage surge protection, DC reactors are optimized for compact size and limiting voltage drop, and both are effective at reducing Total Harmonic Distortion (THD).
More specifically, to clearly understand the differences between these two devices, we need to analyze their technical specifications and operational characteristics. AC reactors provide comprehensive system protection from the outside in, especially blocking interference agents from the national grid before they can penetrate the inverter's Diode bridge. Meanwhile, DC reactors operate silently inside, acting as a flat filter for direct current, helping the capacitor operate durably. The application of which type depends heavily on the circuit design and power quality requirements of each factory.
Below is a comparative analysis table of AC and DC reactors for inverters based on technical criteria. This information table lists the differences in location, function, protection level, and economics.
| Criteria | AC Reactor | DC Reactor |
| Installation Location | In series between the power source and the inverter input | Between the rectifier Diode bridge and the DC Bus capacitor |
| Grid Surge Protection | Very good, eliminates high voltage spikes | Weak, cannot protect against faults ahead of the Diode bridge |
| Harmonic Reduction (THD) | Good (Reduces THD down to 30% - 35%) | Good (Equivalent or slightly better at certain load levels) |
| Voltage Drop Level | Causes input voltage drop (about 2% - 5%) | Causes very little voltage drop, maintains voltage supplied to the motor |
| Size and Price | Larger, heavier, and higher cost | Compact, lightweight, and usually cheaper |
What is the Difference in Installation Location Between AC Reactors and DC Reactors?
The installation location of an AC Reactor is between the three-phase grid power supply and the inverter input, while the location of a DC Reactor is between the rectifier Diode bridge circuit and the DC Bus capacitor.
In detail, this location directly determines the function and operating principle of each type. The AC Reactor (also known as a Line Reactor) acts as the first gateway, receiving alternating current directly from the grid before feeding it into the inverter. It is designed with 3 coils wound on a magnetic core, corresponding to the 3 electrical phases. Conversely, the DC Reactor (also known as a DC Link Choke) is located in the middle compartment of the VFD structure, working in a direct current voltage environment. Because a DC circuit only has positive and negative poles, the DC reactor has a simpler structure, usually just one or two coils integrated directly into the busbar of the inverter.
How Do the Harmonic Reduction (THD) and Inverter Protection Capabilities of These Two Types Differ?
AC Reactors protect the inverter against comprehensive voltage surges but cause grid voltage drops, while DC Reactors limit the initial charging inrush current well and cause less voltage drop, although their THD reduction capability is equivalent.
However, when evaluating performance deeply, each type demonstrates a distinct strength in improving power quality. The AC reactor creates a strong impedance layer to absorb voltage spikes and high-frequency noise pulses from the grid, thereby saving the entire rectifier block of the inverter. The trade-off here is that the voltage supplied to the motor can be reduced by a few percent. Meanwhile, the DC Reactor cannot protect against input phase loss, but it is extremely excellent at smoothing the rough direct current after it passes through the Diode bridge. It restrains the rate of increase of the charging current into the DC capacitor, preventing momentary overcurrent phenomena.
According to research from the IEEE 519 standard on power quality, the accurate installation of one of these two devices, or both, can help improve the system's power factor to 0.95.
In Which Cases Should AC or DC Reactors Be Used for VFDs?
There are 2 main groups of cases where AC or DC reactors should be used, based on the criteria of the factory's grid conditions and the design space requirements of the industrial electrical panel.
Below, we will analyze practical applications so you can easily make a selection decision. In an industrial environment, no solution is perfect for all systems. Design engineers must consider the nature of the load, the length of the electrical cable from the transformer station to the electrical panel, as well as the project's investment budget. If the needs are identified incorrectly, installation not only causes waste but also reduces motor efficiency due to prolonged voltage drops.
Which Application Groups Must Mandatorily Install an AC Input Reactor?
There are 4 application groups that strictly require the installation of an AC input reactor, including unstable power sources, large power capacities, systems with power compensation devices, and requirements for protecting the rectifier diode bridge.
Specifically, industrial environments with poor power quality always require the equipment of this device.
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The power grid frequently experiences phase loss, short-term voltage drops, or high voltage surges caused by lightning strikes or large switching devices.
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Systems where the power supply transformer has an apparent power capacity (kVA) more than 10 times larger than the inverter's capacity, leading to the risk of a massive short-circuit current.
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Manufacturing workshops using automatic power factor compensation capacitor systems that continuously switch. Each time the compensation capacitor switches, it will create transient waves; the AC Reactor will obstruct these destructive impulse surges.
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Large capacity pumping stations or ventilation fans that need maximum protection for the expensive hardware of the VFD equipment.
Which Inverter Systems Are Suitable for Using a DC Reactor (DC Link Choke)?
There are 3 inverter systems suitable for using DC reactors, including electrical panels with narrow spaces, systems sensitive to voltage drops, and large-capacity inverter lines that already have built-in waiting terminals.
Meanwhile, in less fluctuating environments, the DC Reactor is an optimized cost solution.
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When the space inside the control panel is strictly limited, the DC Link Choke, with its compact design that does not generate much heat radiating to the surrounding environment, is the number one choice.
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For sensitive drive systems that require the voltage supplied to the motor to remain constant, without allowing a 3% to 5% drop at the input like when using an AC Reactor.
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Especially, most medium to large capacity VFD lines today (usually from 22kW and up) are pre-designed by the manufacturer with an open circuit structure in the DC Link section. Engineers simply buy a DC reactor and connect it to the two ready-waiting Domino terminals, making the operation extremely fast and safe.
According to technical statistics, equipping the correct type of reactor helps reduce overcurrent faults on the inverter by up to 60%.
Should Both AC and DC Reactors Be Installed Simultaneously for the Same Inverter?
Yes, you should install both AC and DC reactors simultaneously for the same inverter for 3 reasons: maximum harmonic suppression, absolute inverter protection, and extending equipment lifespan in harsh environments.
Furthermore, the combined use of a Dual Reactor system is becoming a standard trend in projects requiring extremely high reliability. Although the initial cost increases and you must face the level of cumulative voltage loss, the benefit brought is almost absolute safety for the expensive semiconductor elements inside the VFD. The system will be able to withstand any sudden fluctuations from the grid, while suppressing a large amount of harmonics pumped back into the source, protecting other sensitive electronic devices sharing the same network.
How Does the Impact of Not Using a Reactor on the Lifespan of the DC Bus Capacitor Differ Compared to When It Is Installed?
When no reactor is used, the DC bus capacitor's lifespan decreases rapidly due to high inrush currents, whereas installing this device helps the capacitor operate coolly, durably, and maintains stable voltage.
More specifically, to illustrate, this device directly intervenes in the charging and discharging process of the capacitor component. If the system lacks opposed inductance (impedance), the initial starting inrush current when charging into the DC capacitor bank will be extremely large. The intensity of this current creates a high-amplitude voltage ripple phenomenon. This continuous variation causes the capacitor to generate strong localized heat, leading to the drying of the internal solvent layer, causing the capacitor to bulge and severely degrading its lifespan. Conversely, the inductance from the reactor acts to smooth the current, limits the rate of change dI/dt, brings the capacitor back to a safe operating state, and extends its usage time for many years.
How Does the Active Harmonic Filter (AHF) Solution Differ from Using Traditional Reactors?
Active harmonic filters excel in active harmonic suppression capabilities (THD < 5%), traditional reactors are good in terms of low cost, and both are optimal for the goal of improving power quality.
Additionally, modern power technology offers higher-end choices. Both AC and DC reactors belong to the passive filter group. They operate based on the physical structure of the inductor to impede harmonic currents, and at best, they only reduce current THD to around 30% to 40%. Conversely, the Active Harmonic Filter (AHF) is a smart electronic device. It actively measures harmonic components in the system using sensors, then automatically emits a completely out-of-phase current to directly cancel them out. Thanks to this, AHF can force the THD level down below 5%, meeting the strictest standards of the national grid. However, the biggest barrier of AHF is its initial investment cost, which is tens of times higher than traditional reactors.
