Chattering 240V AC coil relay

Discussion in 'General Electronics Chat' started by Goatman, Oct 22, 2013.

  1. Goatman

    Thread Starter New Member

    Oct 22, 2013
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    A 240V AC coil is connected to a generator output so as to operate when the generator starts to drop out hi load motors. It chatters until the generator output reaches close to 210 volts. Will a resistor in series or a capacitor in parallel ( and what size of each) minimize the chatter?
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The main reasons for an AC contactor or coil to 'chatter' is under voltage, incorrect device (DC coil or no shading ring) or wrong/low freq. It seems in your case it is low voltage?
    It is not quite obvious if you mean the AC is dropping or as it rises?
    Depending on the above, it may be cured by using a bridge and a DC coil with a BEMF diode.
    Max.
     
  3. Goatman

    Thread Starter New Member

    Oct 22, 2013
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    Thanks,
    It is chattering as the voltage rises, then holds solid once the system is close to rated voltage (10% of rated coil voltage or 10% of 240V AC.)
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Then that would be normally expected.
    This will end up shortening the contact life if not corrected.
    Is this some addition to the original equipment?
    There are a few ways to prevent it, the simplest way may be to change the coil out for a DC version, but is this a function you want to only engage the relay within a certain voltage range?
    Max.
     
  5. Goatman

    Thread Starter New Member

    Oct 22, 2013
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    Essentially, I want to insure that two unneeded loads are not able to operate when the generator comes on line at full voltage. In other words, the generator should not "see" the unwanted loads when the transfer switch operates. The transfer switch will not switch to the building load until the generator is up to speed. This is micro-seconds after the chatter stops. The relays are "seeing" generator voltage building up from 0 volts to about 200 volts, and chattering, but then pull in solid. Momentarily after that, the transfer switch, transfers the load to the generator. This is maybe a 7 second process from loss of utility power to the generator being on line carrying the building load. (Less the unneeded relay loads.) I'm sure that the relay contacts are getting scorched during the chattering process.
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
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    So you need a voltage sense level circuit?
    This is very similar to a circuit I built for control of a self excited DC generator, but it would need building a small circuit, or are you looking for an off the shelf solution?
    Max.
     
  7. Goatman

    Thread Starter New Member

    Oct 22, 2013
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    Of course I would like an off the shelf solution.
    But, I could cobble together the items of a circuit, although I don't have a clue as how to design it.
     
  8. #12

    Expert

    Nov 30, 2010
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    Bad design. Move the unnecessary loads to the transfer switch or go with post #4...use a DC approach (because you can't filter AC).

    Even if I'm wrong, you need to re-think the basic design. You just can't band-aid this one.
     
  9. Goatman

    Thread Starter New Member

    Oct 22, 2013
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    What should the voltage range of a DC coil be when applying up to 240V ac to it?
     
  10. Duane P Wetick

    Active Member

    Apr 23, 2009
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    Probably the simplest off-the-shelf solution is a small boost or constant voltage transformer; typ. 190-205 VAC in, will give you a steady 230 VAC out @ VA necessary for your coil to work. You only have to boost the VA a short distance so, the VA requirement will be small. Even a small auto transformer will work; ie use 10% over tap.

    Cheers, DPW [Everything has limitations...and I hate limitations.]
     
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
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    There is a fairly simple method that would not be much to test, I have used it once before in a pinch.
    Place a bridge rectifier across the varying AC, say 300v-400v 10a, from this connect a 200v zener IN5388 positive end to positive of the bridge, connect the other end of the Zener to a small 24vdc relay coil, the other side of the coil to the bridge -ve.
    Across the coil wire a 150μf capacitor.
    The relay can be used to pick up your main contactor or can be replaced by a 24vdc version.
    If the coil picks up slightly early, you can place a smaller value zener in series.
    Max.
     
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