Power transfer relay chatter

Discussion in 'General Electronics Chat' started by sws_12, Jul 7, 2011.

  1. sws_12

    Thread Starter New Member

    Jul 7, 2011
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    I added a inverter to my RV to run my 110 appliances from my battery bank. To avoid having to unplug the generator feed and plug in the inverter every time I wish to feed the circuit breaker in the rv I installed a relay. The normally closed position on the relay routes power from the inverter to the circuit breaker. The normally open contacts route power from the generator to the circuit breaker. The relay coil is 120vac and gets power from the generator with a time delay.

    So operation goes like so.
    1. generator is off and inverter is powering the circuit breakers.
    2. I start the generator which begins a 30 second delay to allow the generator to stabilize
    3. after 30 seconds power is applied to the relay coil and the relay closes and now the generator output is feeding the circuit breakers.
    4. I turn off the generator so the coil loses power and it returns to its normally closed position which connects the inverter to the circuit breaker.

    The whole thing works like a charm EXCEPT when I turn off the generator and it winds down the relay chatters and this killed the inverter.

    I tried a cap across the coil to no avail.
    I put a EMI filter before the relay coil and it helps but still too much chatter.

    Is there a way to clamp the coil voltage so when the generator drops to lets say 100vac the relay opens.

    Thanks for any input.
    Best Jason
     
  2. strantor

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    Oct 3, 2010
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    that's wierd. your timer should should reset (open) immediately when it loses power (I assume it is powered by the genny), so unless your genny takes >30sec to wind down, you shouldn't be experiencing any chatter. What is the model# of your timer? can you post a link to the data sheet?
     
  3. praondevou

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    Jul 9, 2011
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    I had a similar situation with a relay. The problem was that when you have a current through a contact ( from the generator in your case) and then you open the contact there will be for a short moment an arc between the two contacts you just opened. When you then use on the other contact (three-poles) a non syncronized source you will actually for that short moment put your inverter in parallel with the generator output. We killed a few fuses like this, until I found out that it takes a few ms for this arc to cease.
    I imagine you are using a the phase on the relay and then the neutral of inverter and generator are connected?

    When you say you turn off the generator , are you cutting the generator output with a switch ? Or does the output voltage/frequency slowly decrease?
     
  4. strantor

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    I was picturing a similar scenario with the contacts on the timer. I suspected that maybe the timer wasn't rated for the voltage; one of the reasons I asked for the model#. But I guess we'll never know
     
  5. Adjuster

    Well-Known Member

    Dec 26, 2010
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    It may be that the relay is dropping out without the timer resetting at all. In this case, the relay may pull back in very quickly if the generator voltage rises as a result of the load being removed. Of course, the generator voltage would fall again as soon as the relay reconnects it, so out goes the relay again... This cycle could repeat many times, with potentially damaging results.
     
  6. praondevou

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    Question is: Is it really chattering (voltage being applied out of specification or intermittently) or just bouncing (as all mechanical relays do to a certain degree)? :confused:
     
  7. sws_12

    Thread Starter New Member

    Jul 7, 2011
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    Here is a link to the timer
    http://www.supco.com/images/pdfproductsheets/TD Series.pdf

    Here is the info on relay
    AMF/Potter & Brumfield
    1HP, 120V; 2HP, 240VAC, 1PH
    25A, 240VAC
    20A, 277VAC
    Made In USA.

    The common terminals on the relay are connected to the circuit breaker panel. The relay switches both hot and common legs, NC are tied to the inverter, NO are tied to the generator.
    The common leg from the generator also goes to one end of the relay coil.
    The hot leg from the generator goes to the timer input.
    The output from the timer goes to the other end of the relay coil.
    The timer is set to 30 seconds.

    When I say I turn off the generator it is not disconnecting the output from the generator it does kill power to the generator coil so the generator winds down. It is during this winding down that the relay chatters like heck. I believe timer never completely looses power during this phase and therefore does not reset. So as the generator tapers off from 120volts to 0 volts over a period of 5 seconds the relay is chattering away.

    My thought at this moment is to put a sidac with a trigger voltage 110 of inline with the timer that will hopefully act clamp the current off as soon as the voltage drops below the trigger voltage.

    Jason

    Here is a link to the data sheet for the sidac
    http://www.littelfuse.com/data/en/Data_Sheets/Littelfuse_Thyristor_Kxxxzy.pdf
     
  8. strantor

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    considering the timer runs on 19-250V, I think it stays on the whole time while it's winding down and what you described is exactly what's happening

    Good Idea about the sideac, but keep in mind, it's continuous current rating is 1A, so only power the relay coil with that
     
  9. praondevou

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    Why do you think that a chattering relay is causing the inverter to burn? Assuming a perfect relay function you would just be connecting and disconnecting the load to the inverter.

    However, assuming a non-perfect relay function as I described in my previous post you can easily burn an inverter output because you'd put a low impedance (your generator output) in parallel with the inverters output. Explaination:

    Don't know if it makes any sense in your case, but we had the very same configuration: 2 sources which where not syncronized nor did they have exactly the same voltage (about 120V), both connected to a relay contact, one to the NO the other to the NC, and the COMMON went to the load. When the relay opened/closed (no chattering) and there was a current flowing through the contacts for a few ms there was an arc, effectively connecting both sources (relay contact closes while there is still current from the contact just opened because of the arc). So the problem was not bouncing nor chattering , it was a wrong relay configuration.

    So even if you stop the chattering you may still have the same problem. ;)
     
  10. sws_12

    Thread Starter New Member

    Jul 7, 2011
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    I do understand what you are saying about the relay arc for a moment in time connecting the two sources that are not in synch. My other thought was the one time I did switch back I was running the AC on the RV so there may have been some back EMF as the generator lost power the AC inductive load becomes a source, and inverters dont like to see input on there output. This whole issue was just bad decision making on my part trying to do the transfer during a heavy load. I am hoping getting rid of the relay chatter will be enough and of course switch the feed when there is little to no load. I did buy the extended warranty on the new inverter :)

    So unless someone has some great incite I will be trying out the sdiac and will post my results probably a week out as were heading to the lake this weekend to do some sailing.

    J
     
  11. strantor

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    I would think that if your relay contacts are rated for 120v then arcing wouldnt be an issue. I could be wrong about that. I would suspect that the rapid loading/unloading is what did yourinvertr in.
     
  12. praondevou

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    just from my experience. :cool: In our case the relay was even totally overrated. I made a test on a workbench. Took two 120 degree shifted phases, passed them through two 30A fuses and then through relay NO and NC, COMMON to the load. The NEUTRAL was connected directly to the other side of the load. Even with only 1 A load, as soon as the relay switched, one of the fuses blew. Did some measuring with an oscilloscope, takes a few ms for the arc to cease. We got rid of the problem by using two timer relays, maybe not the cheapest option, but that's what we had there. Of course you increase the time where there is an interruption to the load, even if it's only ms...
    I'm not saying it IS the problem here, but might be a possibility. ;)

    Happy sailing! Weekend? Today is monday.:D
     
  13. ducat

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    May 29, 2011
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    Using a DC circuit will make it a whole lot easier.
    A DC relay, a 555 timer delay circuit will solve the problem and totally remove clattering as the generator powers down.
     
  14. praondevou

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    Depending on your load and the quality of your inverter there are other failure options to consider. It's also possible that for example you just magnetized a load transformer in one direction (end of one half wave). Then you transfer to your inverter. If this one is 180 degree phase shifted it would magnetize the transformer into the same direction . I know that a low frequency transformer doesn't saturate that easy, but still, maybe the current went too high for your inverter to handle it? .. just a thought...

    Now if we take this scenario and addionally consider the chattering of the relay it'd get even worse... So maybe you are right and not allowing it to chatter already resolves your problem.
     
  15. ducat

    New Member

    May 29, 2011
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    Using a DC circuit will make it a whole lot easier.
    A DC relay, a 555 timer delay circuit will solve the problem and totally remove clattering as the generator powers down.
     
  16. strantor

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    Now after more carefully considering you explanation, I no longer understand what the voltage rating on a relay actually means. When I think of how a relay is normally used, I picture it switching 1 power supply to 2 different loads, not the other way around. Maybe that is how the engineers who designed it picture it being used as well. maybe that rating goes out the window when it's used backwards. Consider the physical construction of the relay; the switch goes back and forth between NO and NC (not a lot of clearance between them) so if you had 2 different loads 120V each out of phase, then there is actually 240V present between NO and NC, which is double the rating and arcing would likely occur. I see your point and I resign; you are right.
     
  17. praondevou

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    Yes the rating may be one point but the arcing actually almost always occurs at these voltage levels, and this between the contacts that you just opened (even if the middle contact did not yet arrive at the other contact), if there was current flowing through them.

    What inverter are you using? Doesn't it have a second input to use as bypass? The correect and professional thing to use is a static switch... but surely to expensive for this app.
     
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