Inverter Transfer switch for power outage

Discussion in 'The Projects Forum' started by simeonz, Jan 26, 2013.

  1. simeonz

    Thread Starter Member

    Jan 26, 2013
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    Hi all, my quesion is self explanatory, I dont like to pay for people to do things for me because thats expensive, I can wire a few things together.

    I was wondering if this wasd how a transfer switch worked and if this was safe for residential use, the power side will be rated for 15 A use as my breaker, etc... Seems safe to me ??
     
  2. simeonz

    Thread Starter Member

    Jan 26, 2013
    31
    0
    Or perhaps this would be better ?

    I do not know if there high quiescent current from a cheap inverter, the relay is certainly cheaper for low amps.

    Obviously the point of the wiring is that as the power gets cut by power outage, relay switches on power from battery and switches connections from the load.

    If mains power comes back on, it closes power from battery and switches connection from load back to itself...
     
  3. #12

    Expert

    Nov 30, 2010
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    6,751
    The ground is shared. You can hard wire them all together. Where to put the relay depends on the idle current of the inverter compared to the battery capacity but you can get around that by putting a float charger on the battery. Then the battery will stay full no matter where you put the relay.
     
  4. simeonz

    Thread Starter Member

    Jan 26, 2013
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    So I read about transfer switches vague things about ''power feedback'' and voltage surges and time delays. I can make a time delay but I was wondering if this was actually needed ??? One situation I fear is if there is still much inductive energy stored (voltage) and that after the relay switching the inverter still sees high voltage, other than that what else can go wrong ?? Relay gets arc welded ?? More opinions plz...

    I think the voltage surge is when you actually start an petrol generator and you give it time to attain its speed.

    The load will be small, 100 watts, and it already has a timer circuit that I made to turn on 7 minutes every hour...
     
  5. #12

    Expert

    Nov 30, 2010
    16,252
    6,751
    The way I see it is that the mains control a 2 pole, 2 position relay that falls into position to connect the inverter when the mains fail. The battery is always connected to the inverter and a float charger is always connected to the battery. As long as the mains are good, the battery is being kept full. When the mains fail, the inverter is connected to the load.

    After that, your imagination has run wild trying to guess what could go wrong, and most of your imagined faults are imaginary.
    The relay contacts could weld, but you are supposed to choose the right relay to do the job and use fuses to stop any disasters. Add a fuse to the 1A line and the 1B line. That should do it.
     
  6. simeonz

    Thread Starter Member

    Jan 26, 2013
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    This is is a quote from a professionally made transfer switch.

    in case of failure of the main AC power source like the electric utility power, it is desirable to switch
    the critical AC loads to a standby / back up AC power source like a generator or an inverter. The
    switching action should ensure that only one AC power source is connected to the AC loads at any
    one time and that the electric utility power and the generator / inverter output power are never
    connected in parallel but remain isolated
    The electrical loads cannot be connected in parallel with the generator/ inverter and electric utility
    power at the same time. This will cause the following damage and safety hazards:

     
  7. simeonz

    Thread Starter Member

    Jan 26, 2013
    31
    0
    Now that I understand what the point is, I see that my case needs no delay, only a mechanism to assure that contacts dont overlap.
     
  8. BillB3857

    Senior Member

    Feb 28, 2009
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    In my personal opinion, if you need to be asking these questions, you are not qualified to do it safely. Most local codes require licensed installation personnel and inspection after the install for that type equipment.
     
  9. simeonz

    Thread Starter Member

    Jan 26, 2013
    31
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    I connect things very methodically trust me, I dont need anybody to extort money from me for something as easy as putting a schematic together in a metal box. People try to protect their petty little 60$ transfer switches and dont show their schematics, I got a metal box right here that would fit well and some UL relays cheap @ the electronics shop.
     
  10. praondevou

    AAC Fanatic!

    Jul 9, 2011
    2,936
    488
    Yes. Paralleling inverter with mains may kill the inverter.
    Bigger static switches are most often done with SCRs/triacs and a contactor or just SCRs/triacs.
    It doesn't matter if your load is a fan but for many other loads even an interruption if 1 or 2 periods may be too much.

    If the static switch has to transfer the load by detecting a voltage drop at the mains there will always be a delay. The SS I worked with had a guaranteed maximum transfer delay of 4 to 5 ms.

    Your circuit is ok if you consider the mains to be of or on. It can cause problems for your inverter if the mains voltage drops to some intermediate voltage or you have a intermittent contact from your mains wires.
    The relay may flicker.

    More important: Relay contacts to indeed cause a spark between them when opened. Your exact same setup I found in a power panel for a UPS. The load was only some lamps, but it caused sometimes fuses to blow because there was a measurable time where both inputs where shorted via the sparking. Both inputs where not synchronized (same as yours) so with 2x 120VAC the maximum peak voltage from on to the other can be 340V.

    If this can cause problems to your inverter depends on its output protection circuitry.

    The best way is to provide a delay between turning off one source and turning on the other one. This could be done with relay logic just by making use of their contact release time.
     
  11. simeonz

    Thread Starter Member

    Jan 26, 2013
    31
    0
    So basicly my solution is the following.

    Mains outage switches the dpdt on the load, switches on the circuit wich switches on the inverter,inverter gets powered, switches itself thus blocking the path to the coil on the load relay

    Power goes back on, closes circuit to pulse network, inverter gets powered off, switches itself off the load, when cut off permits current to be sent to coil of the load relay, thus switching load back to mains

    This all seems nice, but I wanna know if a relay coil will work as normally with a crappy sinewave inverter !?
     
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