Filtering kHz noise

Question

I have a variable frequency drive (VFD) that switches at 3kHz, according to the datasheet. I also have a sensor that has a pulsed 5V signal output.

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:

1. 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.
2. 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.
3. 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?

Answer 1

Figure 1. Extract from the FT330 sensor.

That sensor is capable of sinking 25 mA so there should be no problem with noise.

Your problem is likely with the installation or choice of power supply or connection to the counter. You haven't supplied details of these (despite the request).

Answer 2

The VFD switching frequency specification is a maximum, not necessarily a fixed number. It can be directly proportional to output frequency. It can rise with output frequency over a range, then fall then rise again like a car engine rpm rises and falls whenever the transmission shirts to a higher gear. It can also vary in a more complicated way depending on design. Whatever the case, electrical noise from the VFD is likely the source of the problem.

First look at the routing of all wiring connected to the sensor. If any of the wiring is near any of the VFD wiring, particularly the input and output power wiring, reroute the wiring. If the sensor or it's power supply are in the same enclosure as the VFD or motor, they may need to be enclosed in a separate compartment. You may also need to look at the grounding of the VFD, motor, sensor and whatever receives the signal from the sensor.

It would help to have some kind of diagram showing what all equipment is involved in the system, how it is powered and how it is grounded.