Here are the three data points I've collected:
- VFD is off. I measure ~45Hz, consistent with the sensor operating properly and confirmed by analog measurement.
- VFD is on with 60Hz output. I measure pulse count indicating ~2700Hz (I'm just counting pulses, not measuring freq).
- VFD is on with 10Hz output. I measure pulse count indicating ~300Hz.
These suggest to me that the VFD at 60Hz is putting out noise from the switching speed. I'm not entirely clear why reducing the output frequency would decrease the noise frequency.
I can think of several solutions:
- Put a 300Hz low pass filter on signal line. This is obvious but the VFD noise and the signal range overlap so I could potentially cut off my signal.
- Put a filter on the VFD and shield the cables. I did this, but commercial filters seem designed to primarily target MHz noise, so it didn't work.
- Put a low pass filter on the signal line. Its a 120V/2A line, so that doesn't seem like the safest/most efficient option.
Anyone have a better solution?