Using RIB® Relays with Variable Frequency Drives (VFD)

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.

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!