When using a variable frequency drive (VFD), a residual current device (RCD) must never be used. This is a fundamental principle of VFD usage.
Some equipment uses VFDs and has corresponding RCDs installed. The result: as soon as the VFD starts, the RCD trips, and the system cannot operate at all.
Why?
The principle of an RCD is that the zero-sequence current is zero. However, when using a VFD, the zero-sequence current cannot be zero.
The output of the VFD is a PWM waveform. There is a capacitive effect between the motor cable and the ground, and this effect is even more pronounced when using shielded cables. When the VFD is operating, the capacitor is charging and discharging, and current flows through the capacitor to the ground, and then flows back to the VFD from the grounding wire on the input side, forming a current loop. If an RCD is used on the input side, it will trip, shutting down the system.
Therefore, do not use an RCD with your VFD. To ensure safety, proper grounding of the equipment is sufficient. Case study:
Question: A 18.5kW tin furnace induced draft fan is equipped with a 18.5kW VFD. After the VFD was installed and started, the circuit breaker in the power distribution room tripped. It turned out that each circuit in the power distribution room had an RCD installed (200mA tripping current, 30mA pulse).
The request to remove the RCDs was refused. Later, the carrier frequency was adjusted to 1kHz, and the VFD starting method was changed, but the problem remained. Finally, it was suspected that there was leakage in the motor power cable, as it was about 20 meters long and buried underground, but replacing it was difficult (it worked normally with the original mains frequency).
Are there any simple and effective solutions?
Answer: Because RCDs generally trip when they detect a three-phase imbalance of more than 20%, and when a VFD is installed, the three-phase imbalance generally exceeds 50%, the RCD will definitely trip.
Here are a few methods:
Method 1: The residual current device (RCD) usually has an adjustment knob; simply turn the knob to increase the sensitivity.
Method 2: Replace the residual current device with one specifically designed for variable frequency drives (VFDs). These are available on the market.
Method 3: Increase the equipment load, i.e., the motor load. This will reduce the leakage current during VFD startup.
Method 4: Bypass the residual current device.
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