Why Can’t the Frequency Inverter Be Used with a Leakage Protector?

Introduction

In the world of industrial automation, convertisseurs de fréquence (commonly referred to as variateurs de fréquence ou VFDs) are a key component used to regulate motor speeds and optimize energy efficiency. If you are familiar with programmable logic controllers (PLCs), you probably know that convertisseurs à fréquence variable are widely used in industrial settings. However, one golden rule when using a variateur de vitesse is that it should never be paired with a standard leakage protector. Despite this, some users still attempt to install a leakage protector with their onduleur VFD, only to find that as soon as the inverter starts up, the leakage protector trips and the entire system fails to run. Why does this happen? And what can be done if both are already installed?

Why Does the Leakage Protector Work When the Inverter Is Started, and the System Cannot Run at All?

The primary function of a leakage protector is to detect ground faults by monitoring the system’s neutral and phase currents. Under normal conditions, the leakage protector expects the sum of these currents to be zero (known as zero-sequence current). However, when using a convertisseur de fréquence, this is almost never the case.

Variable frequency drives output power using pulse-width modulation (PWM), which generates high-frequency electrical pulses. These pulses interact with the motor cables, creating a capacitive effect between the cables and the ground. When shielded cables are used, this effect becomes even more pronounced. As the entraînement à fréquence variable operates, the capacitors in the system constantly charge and discharge, resulting in leakage currents flowing into the ground. These currents then return to the inverter through the ground wire, forming a complete current loop.

The leakage protector, detecting this leakage current, interprets it as a fault and immediately shuts down the system to prevent potential hazards. In reality, the system is operating correctly, but the high-frequency leakage current confuses the standard leakage protector. Thus, the system cannot run as long as the leakage protector is in place, and this is why you should avoid pairing a standard leakage protector with a convertisseur à fréquence variable. For safety, it’s enough to ensure that the equipment is properly grounded.

What to Do if a Leakage Protector and Inverter Have Been Installed?

If you already have a convertisseur de fréquence and a leakage protector installed in your system, you may encounter frequent trips, preventing the system from functioning. Fortunately, there are several solutions to this problem:

Adjust the Sensitivity of the Leakage Protector

Most leakage protectors have an adjustable sensitivity setting. You can try increasing the threshold so that it tolerates a higher level of leakage current before tripping. This might prevent unnecessary shutdowns caused by the onduleur VFD.

Use a Specialized Leakage Protector for Inverters

There are leakage protectors specifically designed for use with variateurs de fréquence. These protectors are more resistant to the high-frequency noise and leakage currents generated by VFDs. Replacing the standard leakage protector with one designed for convertisseurs de fréquence will allow the system to function smoothly without constant tripping.

Increase the Load on the Motor

By increasing the motor’s load, the startup current leakage becomes less significant relative to the total current flowing through the system. This can reduce the chances of the leakage protector triggering.

Bypass the Leakage Protector

While this is not generally recommended due to safety concerns, some users may opt to bypass the leakage protector altogether. If safety can be ensured through proper grounding and other measures, this is a potential solution, though it should be approached with caution.

Conclusion

L'utilisation d'un convertisseur de fréquence alongside a standard leakage protector can cause operational issues due to the high-frequency leakage currents inherent in variateurs de vitesse. These currents can trigger the protector unnecessarily, leading to system shutdowns. To resolve these issues, users can adjust the leakage protector’s sensitivity, install an inverter-compatible leakage protector, or consider other solutions like increasing motor load. However, the most reliable solution is ensuring the system is properly grounded and using the appropriate protective devices designed specifically for variateurs de vitesse à inverseur.