VFDs input Filter
Hello,
In this section, you can review EMC filters and input choke coils. In electrical networks where there is a risk of instantaneous high voltage, it becomes essential to use these products. These devices do not regulate or stabilize the voltage; instead, they soften the input current to protect the inverter against transients and spikes. However, it should be noted that they do not absorb 100% of electrical noise or parasites.
It is also highly recommended to review the sections titled “RFI Filters” and “DT Filters.”
As we all know, AC speed control devices (inverters) generate electromagnetic noise that spreads through power conductors and radiates into the air due to their high switching frequencies. This interference can disturb other sensitive electronic equipment located nearby.
So, what should be done to minimize these effects transmitted via cables?
++ Use of RFI Filters:
To suppress high-frequency interference, RFI filters are required, as they are not standard in every inverter. According to EN 61800-3, there are two different noise levels defined:
+ First Environment: For systems powered by low-voltage networks (residential or building installations). For inverters up to 11 kW, the motor cable length should not exceed 100 m, and for above 11 kW, the limit is 30 m.
+ Second Environment: For industrial networks powered by LV grids or dedicated transformers. In this environment, compliance is achieved up to 100 m motor cable length for the entire power range.
+++ Using Input Shock Coils (Input Reactors):
Just as in older inverter models or servo drives, a shock coil at the inverter input provides significant protection. These coils have variable impedance, meaning their impedance value changes depending on load conditions. The purpose of this design is to keep total harmonic distortion (THD) as low as possible.
As the load on the speed control device decreases, the THD ratio of the line current naturally increases. Therefore, the impedance of the coil adjusts itself inversely proportional to the load, effectively reducing harmonics by approximately 24.2% compared to standard inverter systems.
Summary of Key Data:
+ Standard RFI Filter: Level 1 → R1-R2 ≤ 100m, R3-R7 ≤ 30m motor cable.
+ Industrial Level (Level 2): Supports ≤ 100m motor cable for all power ranges.
+ New Technology: “Variable Impedance Input Shock Coil” design.
+ THD Performance: Equal to standard models at full load, but with improved low-load performance.
+ Impedance Behavior: Increases at low load to suppress harmonic distortion.
+ THD Reduction: Approximately 24.2% lower compared to standard inverter inputs.
In conclusion, the use of RFI filters and variable impedance input coils plays a crucial role in ensuring cleaner, more stable inverter operation. These components not only protect the inverter but also help maintain the electromagnetic compatibility (EMC) of the entire system, providing long-term reliability and performance.