IEC overloads

Discussion in 'General Electronics Chat' started by reyamkram, Dec 31, 2013.

  1. reyamkram

    Thread Starter New Member

    Dec 1, 2013
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    I have a situation the other day, I have 2 motors that burnt out from single phasing. Also burnt out the contactor and overload. The overload that was in the circuit was an IEC brand adjustable current. I was told that IEC overloads will not open a power circuit when the contact is welded together. Will only open when normally closed so the contacts in the control circuit. And will not open the power circuit.

    In my experience, every overload that I have ran into during single phasing the power and the control circuit will open and protect the motor. Is it true that IEC overloads that do not open a power circuit and cause the motor to burn out when you have a welded contact and do not open a power circuit during single phasing. What good are they if they do not open a power circuit?

    can you please help or advise me on this? Thank you very much.
     
  2. strantor

    AAC Fanatic!

    Oct 3, 2010
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    ANY overload will continue applying power if the contacts are welded shut. Nothing special about IEC. Nothing is perfect; how many times did the overload work correctly before it failed? If you want to prevent this in the future, use slow blow FUSES.
     
  3. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    There must be something "lost in translation" here.. OBVIOUSLY if a contact has "welded" it won't open properly...

    You MUST determine why the contacts welded in the first place.. excessive overloads? high breaking current? high vibration?
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    For a replacement you may want to look at what is known as a Special Purpose Contactor rating, this is a type used on welders, plasma supplies, HVAC etc, these have extra features that stand up to high cycling rates.
    The majority of contactor O/L's have a N.C. contact to open the coil supply.
    If the contactor has welded shut then the OL is ineffective.
    Max.
     
    Last edited: Dec 31, 2013
  5. #12

    Expert

    Nov 30, 2010
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    Welded contacts are about as useful as a blown fuse. Neither of them will do their job.

    About the, "Special Purpose" contactors, I have seen a contactor so worn that one of the contact buttons was completely gone. Only the flat spring remained, and the contactor was still controlling the compressor correctly. That is amazing performance!
     
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  6. strantor

    AAC Fanatic!

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    Seen that too. Amazing!
     
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  7. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    How would slow blows prevent this?

    I personally have never seen an overload that disconnects power, but rather breaks control, which subsequently drops the main contactor.. Multiple hard starts are a good way to splash contacts, as are dirty armatures, shorted motor circuits, etc..
     
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  8. inwo

    Well-Known Member

    Nov 7, 2013
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    Agree, op may be under the impression that an overload device opens the power circuit.
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
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    It can under some circumstances where the O/L detection will trip the 3ph NFB, (Non Fused Breaker) used as the enclosure disconnect, some have either a coil trip or a N.C. sense contact, or both.
    Max.
     
  10. strantor

    AAC Fanatic!

    Oct 3, 2010
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    Right. It wouldn't prevent the contacts welding, it would prevent hosing the motor and t-leads if/when the contacts do weld. I should have been more clear.
     
  11. strantor

    AAC Fanatic!

    Oct 3, 2010
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    Seen these? I have, but forgot to mention them
    http://forum.allaboutcircuits.com/showthread.php?t=94684
     
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