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E-HELP: Isolating Feedback on VFDs

E-HELP: Isolating Feedback on VFDs

Q: We are a boiler/burner company dealing with a rising amount of computer-based burner controls. We have installed several and have some blowers that are controlled by VFDs (Variable Frequency Drives). We have run into issues with noise on certain systems. We get occasional problems with 4-20mA feedback from the VFD despite going through the proper grounding techniques. We are wondering if installing an opto isolator on the feedback loop might eliminate our issue. We have used the SIX Signal Isolator many times on the signal going to the VFD to offset impedance issues on the VFD. Can we put an opto isolator on both signals?

A: Whether the SIX or another isolator will cure your problem depends on the nature of the noise and how the loop is energized. Sometimes you can install an isolator without additional analysis and see if it works.

If you have data about the noise, compare it to the noise rejection specification of the isolator. For example, the Common Mode Rejection for the ECT-DIN Signal Isolator exceeds 95dB @ 60Hz with a limit of 1500 Vrms.

When selecting the appropriate isolator, take into account which device is providing the loop voltage on the input and output sides. Per the ECT datasheet, you can see that it is available as an output loop powered, input loop powered or line powered device.

Published: October 9, 2015
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