It is strongly recommended not to use a residual current device (RCD) when operating a variable frequency drive (VFD). This is a fundamental principle in VFD applications.
Some users attempt to install an RCD for safety when using a VFD, but this often leads to unexpected issues. The VFD and the RCD may both trip, causing the entire system to shut down completely.
Why does this happen?
An RCD works by detecting any imbalance in the three-phase current, specifically by monitoring zero-sequence currents. However, when a VFD is in operation, the output is a PWM (Pulse Width Modulation) signal, which introduces high-frequency components and creates a capacitive effect between the motor cable and the ground.
If the motor cable has a shielded layer, this capacitive effect becomes even more pronounced. As the VFD operates, the capacitance charges and discharges, creating a small leakage current that flows through the earth and returns via the protective grounding wire. This current can trigger the RCD, causing it to trip unnecessarily.
Therefore, it's best to avoid using an RCD on the input side of the VFD. Instead, ensure proper grounding of the equipment to enhance safety without interference.
Case Study:
Q: A 18.5 kW boiler induced draft fan was equipped with a 18.5 kW VFD. After installation, the power distribution panel kept tripping.
Originally, each circuit had a leakage protector rated at 200mA with a 30mA pulse setting. The customer refused to remove the RCD, so we tried adjusting the carrier frequency to 1kHz and changing the inverter’s start-up mode, but the issue remained unresolved.
Eventually, we suspected that the motor power cable might be leaking. It was about 20 meters long and buried underground, making replacement difficult. The original system ran smoothly at power frequency, so the problem was likely related to the VFD.
Is there a simple and effective solution?
A: Standard RCDs typically trip when the three-phase current imbalance exceeds 20%. However, when a VFD is used, the imbalance often exceeds 50%, triggering the RCD every time.
Here are some possible solutions:
Method 1: Adjust the sensitivity of the RCD if it has an adjustable setting. Increasing the trip threshold may prevent unnecessary tripping.
Method 2: Replace the standard RCD with a specialized one designed for VFD applications. These models are less sensitive to the high-frequency leakage currents generated by the inverter.
Method 3: Increase the motor load slightly. When the motor is under heavier load, the leakage current during startup tends to be smaller, reducing the chance of the RCD tripping.
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