It is strongly advised 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 an inverter, but this often leads to unexpected issues. The result is that the inverter trips along with the RCD, causing the entire system to shut down completely.
Why does this happen?
An RCD works by detecting any imbalance in the current flowing through the live and neutral wires. Ideally, the zero-sequence current should be zero. However, when using a VFD, this isn't the case.
The output from a VFD is a pulse-width modulated (PWM) signal, which can cause high-frequency currents to flow. Additionally, long motor cables have a capacitive effect relative to ground. If the cable has a shield, this effect becomes even more pronounced. As the VFD operates, the capacitance charges and discharges, creating a leakage current that flows through the earth and returns via the grounding wire. This current is detected by the RCD, triggering it and shutting down the system.
Therefore, it's best to avoid using an RCD on the input side of the VFD. Instead, focus on proper grounding of the equipment to ensure safe and stable operation.
Case Study:
Q: A 18.5 kW boiler induced draft fan was equipped with a 18.5 kW VFD. After installation, the power distribution room kept tripping.
Originally, each circuit had a leakage protector (200mA trip, 30mA pulse). The customer refused to remove the RCD, so we tried adjusting the carrier frequency to 1kHz and changing the inverter startup mode, but the issue remained.
Eventually, we suspected that the motor’s power cable had some leakage. It was about 20 meters long and buried underground, making replacement difficult. The original system without the VFD operated normally.
Is there a simple and effective solution?
A: RCDs typically trip when the three-phase current imbalance exceeds 20%. When a VFD is used, the imbalance can easily exceed 50%, causing the RCD to activate.
Here are a few possible solutions:
Method 1: Many RCDs have an adjustable threshold. You can increase the sensitivity setting to reduce false tripping.
Method 2: Replace the standard RCD with one specifically designed for VFD applications. These models are better suited to handle the unique current characteristics of variable speed drives.
Method 3: Increase the motor load. When the motor is under a heavier load, the leakage current during start-up is generally lower, reducing the likelihood of tripping.
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