Many netizens ask why frequency converters can't be used with residual current devices (RCDs). Actually, you're already making a mistake when asking yourself this question. The usage guidelines for frequency converters clearly state that RCDs should never be used with them. Why is that? Let's discuss the reasons together today.
Some customers select corresponding residual current devices (RCDs) for their frequency converters . The end result is that as soon as the frequency converter starts, the RCD trips, rendering the system completely unusable.
Why?
The principle of a residual current device (RCD) is that the zero-sequence current is zero. However, when using a frequency converter, the zero-sequence current cannot be zero.
The inverter output uses a PWM wave. There is a capacitive effect between the motor cable and the ground due to the long cable length; this effect is more pronounced when using shielded cables. During inverter operation, the capacitor charges and discharges, causing current to flow through the capacitor to the ground and back to the inverter via the grounding wire on the input side, forming a current loop. If a residual current device (RCD) is used on the input side, it will trip and disconnect the system.
Therefore, you don't need to equip your frequency converter with a residual current device (RCD). To ensure safety, simply ground the equipment properly.
Case Study:
Q: An 18.5KW boiler induced draft fan is equipped with an 18.5KW frequency converter. After the frequency converter is installed and started, the circuit breaker in the power distribution room trips immediately.
It turns out that each circuit in the power distribution room was equipped with a residual current device (200mA trip, 30mA pulse).
The customer refused to remove the leakage protection device. Adjusting the carrier frequency to 1kHz and changing the inverter's startup method also failed to resolve the issue.
Finally, it was suspected that there was a leakage in the motor's power cord, which was about 20 meters long and buried underground. However, it would be difficult to ask the customer to replace it (it was working normally at the original power frequency).
Are there any simple and effective solutions?
A: Because residual current devices (RCDs) typically trip when they detect a three-phase imbalance of 20% or higher, and after installing a frequency converter, the three-phase imbalance will generally exceed 50%.
Therefore, the residual current device (RCD) will definitely trip.
Here are a few methods :
Method 1: There is usually an regulator on the residual current device (RCD). Simply increase the regulator value.
Method 2: Replace the residual current device (RCD) with a RCD specifically designed for frequency converters. RCDs specifically designed for frequency converters are available on the market.
Method 3: Increase the equipment load, that is, the motor load, so that the leakage current of the frequency converter will not be very large when it starts up.
Method 4: Short-circuit the residual current device (RCD).
