Isolate inverter from shorepower

Discussion in 'OnBoard Electronics & Controls' started by RoyHB, Apr 15, 2010.

  1. RoyHB
    Joined: Apr 2010
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    RoyHB Junior Member

    I'll be installing a 600w or 1kw watt sine wave inverter on our boat. I would like to connect the inverter output to the 240VAC outlets that already exist - they connect to the 240v shore power receptacle. There are two items that are permanently connected to the 240V bus, a water heater and a battery charger.

    I want to accomplish 2 things:
    1) Connect the inverter output to the existing 240VAC bus so that the existing outlets work seamlessly
    2) Remove the water heater and battery charger from the 240VAC bus when the inverter is supplying power

    I don't want to require operator intervention to accomplish this. If shore power is present then I want the inverter output to be isolated from the bus. So manual switching won't work.

    I designed a (primitive) relay switching system that I think will do what I want. It can be viewed here

    At this point, the only problem I can see is that there may be a few milliseconds when the shore power and inverter are both connected - while the shore powered relay is activating.

    Is this a good way to accomplish what I want? Have I missed some basic principle? Is my design safe enough?
     
  2. CDK
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    CDK retired engineer

    You forgot to include the circuit.
    The proper way to wire this is with an AC relay with break before make contacts. Having mains and inverter power connected simultaneously will instantly blow up the inverter.

    The relay coil and NO contact must be powered by shore power, the NC contact is wired to the inverter, the C contact goes to the boat's wiring.
     
  3. RoyHB
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    RoyHB Junior Member

    The schematic

    Thanks - You added the missing ingredient, the break before make contacts.

    Just to restore the worlds balance, here's the schematic I forgot to attach.

    [​IMG]
     
  4. pistnbroke
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    pistnbroke I try

    when the shore power goes off will both live and neutral be disconnected ...many will not realise that in australia they only switch the live at the input not double pole like the rest of the world.......I am concerned that you might have 240v between the boat and the shore when you step aboard.....the first service man to be killed in the fauklands war stepped off a generator onto the ground and was fried .....

    If the shore power has gone off due to the common australian power cut will you not be supplying 240v down the line to the guy trying to mend it and your neigbours on the same phase????

    A 600w inverter will not start a fridge ( compressor type..surge too big)


    you only need the top relay ..the two spare contacts go to the heater/charger
     
  5. Landlubber
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    Landlubber Senior Member

    You also need a reverse polarity switch set up and light and also RCD.
     
  6. Landlubber
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    Landlubber Senior Member

    Roy, Another thing if you have a gen set on board, the neutral has to be connectet to earth at the gen set, yes, a MEN (Multiple Earth Neutral)connection is made at the gen set.
    If you do not know what you are doing , get an qualified AC sparky to do it for you.
     
  7. RoyHB
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    RoyHB Junior Member

    Thanks to all

    Thanks to all for some good advice. I'm quite happy to work with 12 volts but I think I've come to realise that I'd better call Lindsay the sparky for this one.
     
  8. Ike
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    Ike Senior Member

    What you really need is an automatic power transfer switch. You cannot have two or more sources of power connected at the same time. If you do you end up as was said, sending power back down the line, endangering someone else's life. A power transfer switch disconnects the shore power source and connects the inverter automatically. It does the opposite when the shore power is reconnected. It does this seamlessly with no interruption in power.

    There are also manual transfer switches which are less expensive but you have to throw the switch yourself.

    The simplest power transfer switch is what I have on my Motorhome, you unplug the shore power cord form the shore power, and plug it into the generator or inverter. Simple and positive. No danger of feedback.
     
  9. troy2000
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    troy2000 Senior Member

    I suspect my old Georgie Boy motor home was modified somewhere down the line by Artie the Amateur Electrician. To switch from 'shore' power to the generator or vice-versa, I flip one pair of breakers off and another pair on. Your set-up sounds a lot safer.
     
  10. pistnbroke
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    pistnbroke I try

    where roy has gone astray is that he needs two relays with double pole change over contacts ( 4 sets in all) which are energised by the shore supply. When that supply fails one relay disconnects the sockets from the shore suplly and puts them over to the inverter. this gives total isolation.. The other contacts switch in the inverter DC supply..parallel the contacts as you only need to switch the positive . If you wire the heater and charger in before the relays then the shore supply going on and off will run them if present and they dont need switching .
     
  11. RoyHB
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    RoyHB Junior Member

    According to 1st my mother, then later my wife and finally several bosses, I went astray long ago.

    In any event, I'm going to let Lindsay the Marine Sparky deal with this since there's life safety issues involved. Thanks to all for the info and advice.
     
  12. CDK
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    CDK retired engineer

    This is the circuit to power your boat from two sources without conflicts.

    The relay is an industrial AC type with .2" contact travel; they usually have 3 sections, the other 2 could be used to switch the neutral wires as well or exclude items like a boiler or microwave from the inverter power.
     

    Attached Files:

  13. capt littlelegs
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    capt littlelegs New Member

    The simplest way around this is to buy a UPS instead of just an inverter, an uninterrupted power supply that works from battery or power supply without any change over problems and can charge the battery. The water heater and charger are then just connected to the incoming power supply.
     
  14. Ike
    Joined: Apr 2006
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    Ike Senior Member

    That sounds too complex. Mine is safe (it's a 2004 Itasca Sunova made by Winnebago), but you have to remember to make sure everything is turned off and the circuit breaker is off before you pull the plug and move it to the plug on the generator. Otherwise you get arcing when you pull the plug (I've already replaced one plug on my power cord.)

    Ive considered installing an auto transfer switch but what the hell, I'm retired and lazy and pulling the plug is easier than doing all that wiring.
     

  15. Frosty

    Frosty Previous Member

    When shore power comes off you want the invertor to come on by failure of the shore power relay?

    Well I would not want that. My shore power maybe running aircon, 2 fridges battery charger, and anything else that maybe left on.

    Before I manually switch on the Invertor I have to make some adjustments to the power consumption first, like disconnect fridges and certainly no aircon.

    I will then start the invertor and start the fridges one by one.

    I prefer the 3 manual control of a double pole sw for gen, invertor and shore power.

    They are all in a line 123 ,--one day I will make a guard so only one can be on at a time, (for when I sell it).

    Either that or a rotary selector.
     
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