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Load auto-detect

Discussion in 'Power Electronics' started by flippineck, Feb 6, 2021.

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  1. flippineck


    Sep 8, 2013
    This is in relation to an outbuilding with a small single-phase 240V AC ring main, fed by a diesel generator with a remote auto-start feature.

    I saw a device advertised which you attach to the ring, and to the generator's auto start signal input. From the advertising info, it seems it applies a low DC voltage from a battery to the ring, and then uses disturbance of that DC supply to detect the plugging-in of a load. It then sends a signal to start the generator to supply full 240V AC supply.

    I was wondering though, would all loads designed for operation on 240V AC mains, trigger a detection when plugged into a low voltage battery DC supply?

    I'm thinking, a toaster will work because it's just going to present a straight resistance and V=IR will be seen by whatever's in the box, but what about something with a capacitor dropper type design? Like say an LED lamp in a holder on a flex with a UK 13A plug on the end?

    What about things like switch mode power supplies? Neon lamps?
  2. Bluejets


    Oct 5, 2014
    Old Lister and similar diesel generators used to use dc detect systems but were mainly for lighting i.e. detecting bulbs etc.

    Used to work on these but cannot remember the actual circuit schematics but do remember they incorporated relays etc. and not as simple as you might think.
    Main aspect is to isolate the low voltage dc and generator output ac 220v or near thereto.

    Neon lamp definetely not, do remember a minium loading though.

    Bit of general reading here.......
  3. flippineck


    Sep 8, 2013
    My own main generator is a 10kVA Stephill generator powered by a 3 cylinder Kubota diesel engine. As the link above points out, although this is great for 10 minutes when someone plugs a kettle in whilst making toast and heating beans in the microwave, it would be less efficient to have it running 24/7 because someone has left their mobile phone charger plugged in.

    I also have a small solar / wind power setup which charges a bank of 12V lead acid leisure batteries via a decent MPPT controller, and provides 240V AC via a Chinese pure sine wave inverter. This inverter is labelled as "2000W". It seems to be very capable of powering medium loads (lighting in a static caravan plus TV, radio etc) for long periods however it does struggle when heavier loads present. It can handle a hairdryer for a while or a camping kettle for one or two cups of tea but any more than that and the overcurrent trips and / or the batteries flatten quickly.

    I was thinking of trying to rig up the system such that the inverter is left on permanently to service light / zero loads and act as a source of detection power, such that when a load threshold is exceeded, the diesel generator start sequence is triggered and the ring is automatically transferred over to the more capable supply until the load safely subsides.

    I'm hoping that might work better than trying to detect with low voltage DC from a battery?

    What would be the best way to monitor current and voltage on the ring and look out for a changeover threshold? If ultimately I used a relay to perform the changeover between inverter and generator based on an output from some form of analog or digital logic, how would different classes of appliance likely react to the transient drop out of power supply, and I guess noise and sudden phase shift?

    I imagine, trying to transfer the load seamlessly from the inverter supply to the generator supply and back again whilst preserving phase with no voltage or current dropout and minimal noise, could become quite complicated?

    I surmised there'd probably be a certain amount of hidden complexity that would become apparent once I started looking deeper into how the system would operate, the link in the previous post mentions a couple of things I'd forseen but adds extra stuff to think about that I hadn't.. thanks
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