Recently I received a few questions about whether our RIB® current sensors work well with variable frequency drive (VFD) motor controllers. The short answer is yes, but there are a couple key points worth noting.
Determining Sensor Placement
When using a RIB® current sensor to monitor the status of a VFD, the sensor placement is crucial. It should be placed on the line side of (before) the VFD. The ideal frequency of the monitored wire should be at, or around, 60Hz. If it is placed on the load side of (after) the VFD, expect erratic or no operation of the current sensor. This is due to the frequency being altered at the output of the VFD controller.
Using a Model with an Adjustable Threshold
There's a good chance that the VFD consumes some power without the motor turning. That little bit of power is what I call leakage or standby current. It may be enough to make the current sensor contacts close. This could result in a "false-positive" in terms of feedback. By using an adjustable model, the threshold may be fine-tuned to ignore the leakage/standby current, giving you the desired feedback from the current sensor.
Enhancing Building Automation with Current Sensors
Integrating current sensors into a building automation system is a practical way to gain reliable status feedback from VFD-controlled motors, helping facility teams monitor equipment operation efficiently. By using RIB® current sensors correctly, you can track when fans, pumps, and other loads are truly active, adding a valuable layer of visibility within your building automation strategy. This can help identify equipment runtime patterns, verify occupancy-based control sequences, and assist in energy-saving initiatives across your facility.
When planning your building automation projects, using adjustable threshold current sensors can help your system distinguish between true motor activity and low-level standby currents from VFDs. This helps your building automation system avoid false signals while ensuring accurate monitoring and logging of equipment use. With this level of precise feedback, your automation system becomes more intelligent, supporting better maintenance decisions and smoother operation of your building’s mechanical systems.
I hope that this blog helps address any concerns regarding our RIB® current sensors in a Variable Frequency Drive application. As always, feel free to call or email us if there are any other questions or concerns. We'll be happy to help!
About Functional Devices, Inc.
Functional Devices, Inc., located in the United States of America, has been designing and manufacturing quality electronic devices since 1969. Our mission is to enhance lives in buildings and beyond. We do so by designing and manufacturing reliable, high-quality products for the building automation industry. Our suite of product offerings include RIB relays, current sensors, power controls, power supplies, transformers, lighting controls, and more.
We test 100% of our products, which leads to less than 1 out of every 16,000 products experiencing a failure in the field.
Simply put, we provide users of our various products confidence and peace of mind in every box.