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?

  • \$\begingroup\$ Datasheets for VSD and sensor, please. A wiring diagram would be good too, showing power supply and sensor load. (Put the information in the question and not in the comments.) \$\endgroup\$
    – Transistor
    Sep 1, 2016 at 19:26
  • \$\begingroup\$ "Its a 250W line ..." What does this mean? 250 V? \$\endgroup\$
    – Transistor
    Sep 1, 2016 at 19:27
  • \$\begingroup\$ Try reducing the pullup resistor value such that the current is more like 20mA. VFDs create a lot of noise and some mains filtering is useful. \$\endgroup\$ Sep 1, 2016 at 21:15
  • \$\begingroup\$ I'm working on a wiring diagram. This is just one part of a much more complex system and I'm trying to sort out which parts are relevant. \$\endgroup\$
    – ericksonla
    Sep 1, 2016 at 21:21
  • \$\begingroup\$ You do have the required pull-up resistor in place, don't you? \$\endgroup\$
    – JimmyB
    Sep 2, 2016 at 21:51

2 Answers 2


enter image description here

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).

  • \$\begingroup\$ To clarify, this suggests that the signal to noise ratio should be too high for the noise to get picked up by the microcontroller? \$\endgroup\$
    – ericksonla
    Sep 2, 2016 at 18:39
  • \$\begingroup\$ Not quite. It suggests that if used properly it would. \$\endgroup\$
    – Transistor
    Sep 2, 2016 at 18:56

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.


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