AC and DC contactors

Discussion in 'General Electronics Chat' started by jxm1092, Apr 5, 2013.

  1. jxm1092

    Thread Starter Member

    Jul 16, 2012
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    I've looked around on the net most of the day (and maybe not in the right places) and can't find the answer to my question. I understand the downside of using an AC rated contactor in a DC circuit, but what about using a DC rated contactor in an AC circuit.

    For instance say you used this contactor in a 120 VAC circuit, where the most current you would ever have is 200 AAC. If I'm reading this right you would only get 12 cycles of the contactor before failure? I would think that with the appropriate pre-charge circuit that the life cycle of the contactor could be extended significantly?
     
  2. mcgyvr

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  3. jxm1092

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    Jul 16, 2012
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    You misunderstood my question I think. I'm not asking about using a DC CONTROLLED relay etc with AC. The contacts of the contactor (not the coil) only have DC ratings on the data sheet. What I am curious about is how those CONTACT ratings correlate to AC current across the device.

    I have searched everywhere I can think of for a SPDT contactor/relay/SSR that is rated for 200A @ 120VAC. If you can point me in a better direction I would appreciate it. Ideally I wanted a SSR but, like I said I haven't found anything that would work in the current range I am looking at.
     
    Last edited: Apr 5, 2013
  4. gerty

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    Have you looked at the contactors in an electric forklift. If I'm not mistaken Clark Forklift uses a double pole double throw contactor.
    I know it has an extra pole you don't want, but it's rated for hundreds of amps. Coil voltage is 488vdc best I remember.
     
  5. jxm1092

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    Jul 16, 2012
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    But how does that DC contact rating apply to AC voltage? Besides the TE contactor I found is good up to 500A.
     
    Last edited: Apr 6, 2013
  6. Kermit2

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    DC current WILL be harder on contacts that break the current flow because of the 'arcing' across the contacts. AC current has to pass through zero and therefore the arc is of a shorter duration.

    You average AC household lightswitch can handle 10-15 amps of 120 volt power, but will be quickly destroyed(100 cycles or less) if used to switch 10 amps of 12 volt DC power. (Try it sometime :) )
     
  7. Dodgydave

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    Jun 22, 2012
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    Dc contacts have to put up with arcing due to the disconnection, where as Ac doesn't due to the fact the supply is reversing 50/60 times a second.
     
  8. gerty

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    What is your actual application?
     
  9. BillB3857

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    Feb 28, 2009
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    One feature of DC contactors not previously mentioned.... Blow-Out Coils. These are coils that are in series with the contactor and generate a magnetic field during the arc duration which deflects the arc to a longer path. The longer path causes the arc to extinguish quicker.
     
  10. jxm1092

    Thread Starter Member

    Jul 16, 2012
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    So I'll clarify a little bit more, my apologies. The most that I would ever see is 125A and then I used a 1.5 safety factor which was 187.5A so I rounded up to 200A. I understand the difference between opening the contactor with DC and AC voltage. So seeing how that extrapolates from AC to DC is what no one seems to be able to explain to me. And perhaps its not possible to determine how that affects the contacts?

    I made a rough schematic in paint of what the setup would look like. The AC sources would be separate but synchronous sources for theory sake. So if the load was drawing full power and I wanted to break the contact to stop supplying power at the full load there is no way to counteract that?

    My initial thought was that using a relay with a RC combo to charge the capacitor before make/break of the contactor would mean that the contactor is making/breaking a much smaller current value since the capacitor would be designed to handle the 200A requirment (4.4uF)
     
  11. gerty

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    Aug 30, 2007
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    I don't see the double throw requirement in your drawing. If you don't need that, a motor contactor would work. Or if you prefer to go with solid state a "puck type" triac would work. We used those large triacs to switch the primaries of spot welders 480vac@300amps. Also if your drawing is correct, all of the power must come from the same phase, this would cause an imbalance on a small service.
     
  12. gerty

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    Aug 30, 2007
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    Also, is there a particular reason for disconnecting the parallel power sources. Is it because your load varies? The load will only draw the power it needs.

    A
    gain, what is your actual application
     
  13. jxm1092

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    Jul 16, 2012
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    The application is multiple microgrid networks combined together. So each AC source in the drawing isn't really necessarily a source, but more bi-directional.

    As for the puck type SSR's I have found some but nothing was over 100A which would mean I need to use two per leg and the ones I found at that current rating don't have any type of feedback that could be provided to a controller aside from visual confirmation (LED). I'll keep looking for the triac's your talking about and see if I can find something that works.

    The reason for disconnecting parallel sources would be to control the flow of power. The load doesn't necessarily have to be a load, but could be another "source" that current flows to/from.
     
  14. gerty

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  15. jxm1092

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    Jul 16, 2012
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    I don't think that will work, but I did find this contactor that states what the AC make/break current is and life cycle so I think that is going to work for what I'm trying to do.
     
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