12 vdc starter relay mod to open vs close circuit?

Discussion in 'OnBoard Electronics & Controls' started by sdowney717, Sep 5, 2013.

  1. sdowney717
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    sdowney717 Senior Member

    I have an onan MCCK gen in a boat along with an inverter.
    I would like to be 100% certain the inverter turns off before the gen produces power or they might interconnect the 2 and burnout the inverter. I do have a 4 pole DPDT relay that switches hot and neutral. When inverter turns on, it closes 4PDT relay breaking circuit with gen. However, if load on inverter is too high, the relay will buzz and arc between the two sources and cause a short circuit.

    An easy way to do it would be to open circuit the 12vdc going to inverter when gen runs using a relay. Relay would need to be like a starter relay except they when energized are closed, instead of open circuit.

    Relay will not buzz with gen on if inverter is off And if someone tries to turn inverter on with gen on, inverter can not function.

    So the idea is need something like a starter relay that can open circuit the 12vdc power to inverter when gen starts. There is a delay between gen start command to run and it producing AC power so trigger is better to come from 12 vdc from the gen run circuit and those starter relays are already high current devices.
     
  2. CDK
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    CDK retired engineer

    Most inverters have a remote control socket or a small on/off switch. That you can use to inhibit operation regardless of the presence of 12VDC.
     
  3. sdowney717
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    sdowney717 Senior Member

    Have to remember to turn it off which I don't always remember before turning on gen and vice-versa. So desire an automated circuit.
    The remote inverter switch is a push button momentary on then off then on etc....
    Killing the 12vdc source will cut power to inverter so 4PDT relay will not energize at all and gen powers everything.

    If the inverter load is too high, the points in the 4PDT buzz or bounce due to voltage drop cant hold the relay in and if the gen is also running then you get momentary interconnection between the 2 sources and the inverter will lose the battle.

    I need to think through this, draw out a circuitry. Maybe 2 relays, one big starter type to turn 12vdc power off to inverter controlled by small cube relay getting signal from 12vdc power coming from gen before the gen rotor spins up and the gen's contactor engages sending out AC power. All scenarios need to be thought through about what could happen which I have yet to do.
     
  4. sdowney717
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    sdowney717 Senior Member

    How about this one?
    Will work with existing available parts, I actually have all the parts.

    Power is wasted through starter relay when you desire inverter to be on.

    Bridge switch, one of those white handled ones with LED signal light next to it to tell you if inverter has 12vdc power.

    Key ignition means key has to be on or aux to send power. Either that or full time 12vdc.

    [​IMG]

    Do those starter relays have a high duty cycle?
    What is the coil current draw?
    Think the starter relay will get hot just from being on a while?
     
  5. sdowney717
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    sdowney717 Senior Member

    I dont like that circuit. The starter relay draws 2 amps. And it gets quite hot after a few minutes. So will think of something else.
     
  6. Ike
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    Ike Senior Member

  7. CDK
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    CDK retired engineer

    If you don't want to perform surgery on the inverter and prefer to remove DC from the input, there is an electromechanical solution.
    Find a DC master switch like the ones used in military vehicles (and in boats). It uses a key that is a 90 degrees bent piece of steel with 2 notches that must be inserted against a spring and turned to close the switch.
    Use your starter solenoid, a piece of steel wire and the small relay to yank the key to off position if someone (you) starts the gen while the inverter is switched on. Wire it in such a manner that the starter solenoid is powered from the inverter input terminal, so it cuts off its own power: the mass of the solenoid core is more than enough to complete the operation.

    Of course the power to the inverter must be restored manually.
     
  8. sdowney717
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    sdowney717 Senior Member

    Thanks, I found another way to attack this which is very simple.

    Use a NC cube relay with dc coil in the circuit that supplies power to the 35 amp 4PDT power relay's coil. Just a cube relay 30-40 amps, the kind found in autos.

    Then when gen is set to run, a 12vdc signal opens the relay killing the power to the 4PDT relay coil. The 4PDT relay clicks over to the gen circuit from the inverter circuit. This leaves the inverter idling in run mode but doing nothing. The 4PDT relay points cant buzz or bounce since it is in the unenergized state, so no possibility of interconnection between gen and inverter.

    Tying the small NC relay to the generator DC on signal delays the change over which is good. (takes a while for gen to spin up and make power and the 4PDT relay will be definitely ready for it).
    So I just need a small relay with 12vdc coil to do this and not a whole lot of wire.;)

    This will be the cheapest easiest solution I think. This could easily be done for the incoming grid power to insure gen and grid do not interlock. But I have never noticed a power dip on the grid, like the inverter can do.

    [​IMG]
     
  9. bcervelo
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    bcervelo Junior Member

    Is it wise to mix DC and AC on that relay?
     
  10. sdowney717
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    sdowney717 Senior Member

    The DC coil will be the only thing powered by DC from gen and it is totally insulated from the 120 VAC which will flow through the relay's points only.
    That type cube 12 VDC relay has points designed for DC current.

    AC is easier on points since it goes to zero being a sine wave creating less arcing when points open and close. The current flow will be very small to energize the larger AC relay's coil. The relay would have to physically self destruct I think to be a problem. 40 amps 12 vdc relay is 40 times 12 = 480 watts of power which is way higher than the tiny AC load of the coil to be turned on, so heat will not be an issue.

    You can buy relays with various AC-DC voltage ratings on coils and points.

    I am currently switching on a heat pump water pump with a 12 VDC coil switching a 12 VDC load. The coil is 12 VDC, the relay was designed for 120VAC switching and it has worked flawlessly for years. I had it and decided to use it and its been fine. The load there for that pump is perhaps 12vdc X 3 amps = 36 watts of power. It is an Attwood bait well centrifugal pump and they only use a low amount of amps.
     
  11. Don H
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    Don H Junior Member

    One thing you need to be aware of is that especially with high current relays the Normally closed contact is not rated as high as the normally open contact.
    Your little cube relay wont matter as its only powering a relay coil but

    Your 4 pole relay that is switching the output between inverter and generator will be at risk if you are not careful.(excess heat,fire, welding of contacts) The rating difference can be quite large. eg A 40 AMP RATED relay may have the NO contacts rated to 40 amp but the NC contact can be something like 30A. This is because the solenoid pulls the contacts in tight for a higher current on NO but you are relying on the spring return tension only for the NC contacts. It can be substantially lower and unfortunately many manufacturers do not specify the NC current rating. The better manufacturers do.

    For your application you really need a relay for each power source and some form of interlock. If you use the NO and NC contacts of the small relay to power the final pair it should solve your requirement. You just need the extra power relay. You should also investigate a snubbing circuit on the output to cut down on arcing and possible contact welding.
    You could get very complex here with zero cross switching , just depends on how far you want to go.

    Thanks Don
     
  12. mertkara
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    mertkara New Member

    @sdowney717, hi, I’ve been wanting such a circuit for a long time now but to have a reliable solution for such circuits, one always needs to have a third remote power supply. Now since one of the two supplies will be on at one time, there still is going to be an overlap, a small fraction of a second where both the supplies will be on. So to avoid this, I designed a small micro-controller based circuit which has an option to turn off or turn on relays that connect both power supplies to the output. The relays are getting their power from both power supplies while controller is being powered by a small battery, works like a charm.
     

  13. jonr
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    jonr Senior Member

    For best protection, the circuit would also create several seconds of delay between switching off one source and connecting the new one. And in the case of a generator, it will last longer if it is allowed to warm up for a few minutes before load is applied. Preferably switching would be all solid state (as in SSRs not relays).

    Back-to-back TVS diodes across the relay contacts should increase relay life.
     
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