How to Handle Electromagnetic Interference on Ultrasonic Level Meters

At a customer's site, a two-wire integrated ultrasonic level meter was connected directly to a variable frequency drive (VFD), and the intention was to use the level measurement data from the ultrasonic level meter to control the motor speed via the VFD. However, the measurement data deviated significantly from the actual values. When the ultrasonic level meter was connected to a switching power supply on site, the measurements were normal, but when connected directly to the VFD, the echo pattern shown here appeared, indicating that the interference was caused by the VFD.

Step 1: Ground the ultrasonic level meter.

The ultrasonic level meter has three terminals, one of which is specifically for grounding. After grounding on site, the echo pattern became as shown below.

As can be seen from the figure, the interference has been significantly reduced, indicating that grounding is a common method to reduce interference in environments with electromagnetic interference. In general, after grounding, correct measurements can be obtained. However, in this case, grounding alone did not resolve the issue. Therefore, an isolation method had to be used. A signal isolator was added between the VFD and the ultrasonic level meter, which blocked the interference from the VFD. At this point, the echo menu pattern is as follows:

At this point, the echo menu is very clear, with no traces of electromagnetic interference. Compared with the actual liquid level on site, the level height measured by the ultrasonic level meter is correct. Through this example, we know that when encountering strong electromagnetic interference sources such as VFDs, motors, and high-voltage wires on site, the following measures should be taken:

First, enter the echo menu (press and hold the Up arrow key, then press the SET key and hold for three seconds; the echo menu will appear.)

1. Connect the grounding terminal of the ultrasonic level meter to a true earth ground independently, and do not share a ground with other equipment.

2. If grounding is ineffective, add a signal isolator between the ultrasonic level meter and equipment such as PLCs and VFDs.

After resolving the electromagnetic interference, exit the echo menu (press and hold the Down arrow key, then press the SET key and hold for three seconds; the echo menu will exit and return to the previous menu.)

If you need more detailed information, please call our hotline: 0591-28082552. The technical staff of Fuzhou Dayu Electronics will be happy to assist you at any time!