Reactors play a crucial role in safeguarding motors, variable frequency drives (VFDs), and other sensitive electrical equipment, enhancing their reliability and longevity. These devices mitigate disturbances from power lines, prevent overvoltage trips, improve overall power factor, and minimize nuisance tripping. Additionally, reactors reduce harmonic distortion by adding impedance to the utility grid, thus improving the quality of the power supply.
I. Understanding Reactors
Reactors, also known as inductors, are electromagnetic devices installed in series between electrical equipment in power systems. They consist of a steel core made from electrical-grade steel laminations and copper-wound coils, which create a magnetic field as current flows through them. This magnetic field limits the rate of current change, thereby reducing harmonics and protecting the drive from power system surges and transients.
Depending on their installation location, reactors can be classified as either line reactors or load reactors. A line reactor, placed between the power system and the VFD, protects the entire VFD from power system surges and transients while also reducing harmonic currents. On the other hand, a load reactor, installed between the VFD and motors, acts as an energy buffer to protect the motors.
II. Advantages of Line Reactors
Line reactors protect VFDs from disturbances in the utility power line, such as surges, spikes, and transients, which can cause unexpected tripping or damage. They also reduce the harmonics generated by VFDs, thereby improving power quality. Line reactors are particularly beneficial in environments where the power line input contains disturbances, where harmonic distortion is a concern (as outlined in IEEE-519 standards), or where the supply line power capacity significantly exceeds the VFD system’s kVA rating.
One of the critical functions of line reactors is minimizing nuisance overvoltage trips caused by capacitor switching transients. Utilities often use capacitor banks for voltage support and power factor correction, which can create momentary short circuits and voltage drops, leading to VFD tripping. Line reactors reduce the DC bus voltage and limit the surge current magnitude, thereby preventing overvoltage trips.
Moreover, line reactors help reduce input line current harmonics, improve VFD power factor, and reduce line losses. VFDs typically exhibit poor power factors due to harmonic content in the current waveform. By applying line reactors, the harmonic currents are reduced, improving the overall power factor and reducing the kVA demand of the drive. This, in turn, leads to better efficiency and lower operational costs.
III. Benefits of Load Reactors
Load reactors, installed after the VFD, protect motors by increasing load inductance and reducing the effects of reflected waves. These reactors are especially useful when long cables are required between the VFD and the motor. The short rise times of the VFD’s output voltage can produce voltage reflections, potentially exceeding the motor’s peak voltage rating and causing insulation breakdown. Load reactors mitigate this risk, ensuring motor longevity and reliability.
In cases where motors have low leakage inductance, applying a load reactor helps balance the inductance, improving drive performance and preventing tripping.
IV. Choosing the Right Reactor: 3% or 5% Impedance
Reactors are categorized by their percent impedance, with 3% and 5% being the most common. A 3% impedance reactor is typically sufficient for most applications in North America, providing adequate protection against line spikes and motor current surges. However, in environments with higher disturbances or where IEEE 519 compliance is required, a 5% impedance reactor may be necessary. Higher impedance reactors are also recommended for reducing noise and extending motor life, though they come at a higher cost and may add weight, require more space, and generate additional heat, reducing overall efficiency.
In summary, while reactors add cost and complexity, their benefits in protecting VFD systems and enhancing equipment reliability far outweigh these drawbacks, making them a critical component in electrical power systems.
