I need to know when a 3 phase AC submersible pump is turned on, for the sake of logging it to a online database. Thing is, I do not know how tell if it is ON, or OFF. I've given a thought to using a current transformer, specifically the "STC-013-30" one, but it requires to be wrapped around one of the AC wires (live/neutral) but the motor i have has both the wires insulated together forming a single, huge wire. Plus, its 3 phase, and I do now know if the STC can detect 3 phase currents. Can it be done some way? Can I utilise some other tool to tell if the motor is ON?

  • Well, you're just going to have to separate out those wires then aren't you?
    – Majenko
    Oct 8 '17 at 11:06
  • The surprising sensor that tells me my furnace is running? a microphone. unless you have a silent pump (dream on), a mic taped to the housing should work a treat. if you needed measurements, it's complicated, but telling it's merely "on"? An adjustable cheap mic module w/opamp and pot is fine.
    – dandavis
    Oct 8 '17 at 20:52

It's likely that the power to the motor is originating at a panel with breakers. You should be able to confirm that only one breaker set is being used to power the pump motor. At that location, all the wires are separated.

If you do not have the necessary skill to identify the correct wires, consider to consult an electrician. It is advisable to remove power to the panel before you begin to insert your fingers to wrap the sensor wire around the correct lead.

  • I'm afraid I'm not allowed to split the wires. I'm to alter nothing. It's a shame the current transformer is useless when the live and neutral wires are wrapped around each other. Oct 9 '17 at 14:21
  • It seems likely, based on the other comments, that the electrical method is not suited for your purpose. I suggest that water flowing through a pipe is not silent. Using a sensitive microphone on the pipe nearest the pump would provide an on/off level trigger. Credit to dandavis for his suggestion.
    – fred_dot_u
    Oct 9 '17 at 22:38

The long and short of it is that you need to measure the current. And there's two basic methods of doing that, and both require access to just one of the wires in the cable.

So you will either have to take a knife to your cable and carefully remove a section of the outer insulation to access the wires within it, or create some kind of adaptor cable that the existing cable plugs into (however that is done on your system) to allow access to the individual wires.

Then it's a case of either:

  • Use the magnetic field of one wire to monitor the current (current transformer) or
  • Introduce a shunt resistance into the current path and measure the voltage drop across it.

The first option is by far the safest.

Alternatively there may well be some form of commercial three-phase power monitor with an output that you can monitor to achieve what you want with less danger of electrocuting yourself and others in the process.

  • thanks, but I cannot split the wires. Oct 9 '17 at 14:21
  • Then you will either have to build an adaptor for it to plug into (if there is even a plug - but who knows round here except you...?) or do something else further up the electrical chain.
    – Majenko
    Oct 9 '17 at 14:22
  • Ok, thank you. Any way you think I could accomplish it by checking if the water's flowing through the pump's pipe? Oct 9 '17 at 17:21
  • If you can break into (cut) the pipe and add a suitable flow meter, sure.
    – Majenko
    Oct 9 '17 at 17:22
  • Oops like I said, I cannot alter the existing. I mean, not so much as CUTTING open anything. Anyway, thank you. Oct 9 '17 at 17:25

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.