Missing phase detector

Discussion in 'The Projects Forum' started by geoffers, Aug 17, 2015.

  1. geoffers

    Thread Starter Active Member

    Oct 25, 2010
    Hi all,
    I'm after suggestions for a quick project. I have a dirty water pump that runs on a 3 phase. I've had problems with rats nibbling the wires, and various other things interfering with the supply. There's earth leakage sensing etc on the vfd that drives it but it won't pick up if the feed to one phase is broken. This leads to to much current going through the other two phases and burning one out.

    The simplest way of checking it I can think of is to wrap a few turns of copper wire around each wire going to the motor then using a diode and capacitor as a peak detector. This could be then be checked by the adc on a pic, if a wire gets broken there would be no current through it, therefore to voltage across my mini coil. I can the get the pic to stop the vfd and avoid damaging my pump motor again?

    Is my idea practical/workable, does anyone else have a better one?

    Cheers Geoff
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    Sounds workable and has the merit of being non-invasive.
  3. AnalogKid

    Distinguished Member

    Aug 1, 2013
    Agree. The idea is a variation of a missing pulse detector. If you don't want to wind coils, check your favorite distributors for current sense transformers. The innergoogle has lotsa circuits.

  4. DickCappels


    Aug 21, 2008
    Alec_t likes this.
  5. Ramussons

    Active Member

    May 3, 2013
    Why not create a virtual Neutral using 3 resistors in a star setup and measuring this virtual node voltage against the actual Neutral?
  6. geoffers

    Thread Starter Active Member

    Oct 25, 2010
    Thanks for all the replies,
    After a quick experiment with the kettle some copper wore round the live and a multimeter I ordered some little transformers from rs, I'll update when I have something working!
    Thanks again geoff
  7. geoffers

    Thread Starter Active Member

    Oct 25, 2010
    Hi all,

    I said I'd update....
    Its all wired up and working, thanks for everyones input. There should be some pictures and a schematic attached, I used bi-colour led's and the darlington is a tip121 not 120! When it picks up a missing phase it shuts the pump down by interupting the supply to the float switch using a relay. It then flashes the red led to show which phase is 'missing'. Hopefully my new shiney pump should be safe now! (won't be shiney for long!
    Ps the transformers are rs number 537-4491
    Cheers Geoff