Due to their significant energy-saving benefits, Variable Frequency Drives (VFDs) are increasingly used across various industrial applications. A key challenge is how to provide fast protection for VFD loads, such as electric motors.

Since the output current waveform of a VFD is non-sinusoidal and may even contain a DC component, conventional current transmitters cannot measure it accurately. The most direct solution is to use a true RMS current transmitter to measure the distorted current and then process the data through a data acquisition device such as a PLC for protection.

Standard true RMS current transmitters typically provide standard output signals such as DC 0–5V, DC 0–10V, or DC 4–20mA. But how can a fast alarm signal be generated for fault currents without a PLC or similar data acquisition device?

Tests have shown that by combining a true RMS current transmitter with a DC switch output (relay) transmitter, an accurate switch alarm signal can be generated for fault currents. This switch signal can then be used to trip the load circuit or illuminate a fault alarm lamp.

Implementation Example:

HTIC-C21-200P1O2 – True RMS current transmitter

GDUK1-C51-5P1O26 – DC voltage switch transmitter with relay output

When the RMS value of the current (non-sinusoidal or distorted waveform) exceeds 120 A (adjustable), the true RMS transmitter outputs a DC voltage signal. Once this DC voltage exceeds the adjustable threshold of the GDUK1-C51, the relay is activated, providing a dry contact output.

Tests confirm that this method is effective and offers a cost-efficient measurement and control solution.