AC Contactors for DC Application

Discussion in 'General Electronics Chat' started by sibi.2009, Dec 26, 2013.

  1. sibi.2009

    Thread Starter New Member

    Dec 26, 2013
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    I need a contactor to control a 48V DC Motor . I am planning to use GE Contactor rated 32A , 415V AC with 48V DC Coil . Can this contactor be used for controlling 48V Dc current.
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    If a 3ph contactor I would use the outer two contacts, normally the contacts are de rated for DC if they are DC capable.
    What is the current of the motor, the other issue is if the contactor is the main source of control, or is there a rpm or other controller between contactor and motor?
    If so it best to try and ensure the motor rpm is at zero when switching the contactor, if at all possible.
    You have the advantage that it is relatively low voltage DC you are switching.
    Is the motor under load at start up?
    Max.
     
  3. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Typically, a relay contact rated for 10Aac is rated for only 1 to 2Adc. It is a fact of life that when switching AC, the AC power zero-center crossings take care of extinguishing the arc; not so when switching off an inductive load like a motor on DC. I would guess that your contactor will struggle with switching 5Adc. Check for welded contacts.

    Previous thread
     
    Last edited: Dec 26, 2013
  4. tcmtech

    Well-Known Member

    Nov 4, 2013
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    It's pretty common to see higher current rated AC contactors used in lower voltage high current DC circuits.

    Miller and Lincoln welder Co both have smaller MIG models that use common three phase 60 amp 600 VAC rated contactors as their primary DC side control.

    For the most part they were fairly reliable switching the highly inductive DC loads unless ran at their top end output capacity in high cyclic applications which to be honest was not what the models they used them in were rated for anyway.
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
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    If you were buying an AC contactor for switching the DC, It would have been an advantage to buy a Special Purpose type, they have high duty cycle contacts, arc chutes etc.
    Welders, Plasma supplies and HVAC systems use this type.
    Max.
     
  6. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    If the contacts are not rated for DC or the level of DC you intend to be switching then they shouldn't be used.. plain and simple..
     
  7. alfacliff

    Well-Known Member

    Dec 13, 2013
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    you cant bet ont he zero crossing extinguishing the arc in contactors. I work with industrial stuff all the time, and the arc must be extinguished with other means. yhou cant bet on the contactor opening on zero crossing, if it opens on peak, there is more current available which takes a while to extinguish,. contact spacing and speed of opening are what are normally used.
    cliff
     
  8. MaxHeadRoom

    Expert

    Jul 18, 2013
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    So far the OP has not stated the current, it is not a definite conclusion across the board.
    In all the cases I have used AC relays for DC there was only one where I had to resort to magnetic arc blowout version.
    This was 240vDC and very high energy.
    With 48vdc this will not be so much of an issue.
    Max.
     
  9. strantor

    AAC Fanatic!

    Oct 3, 2010
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    It depends, on lots of technical things, but it boils down to a concept that is less than technical; reliability. Are you an OEM? Are you going to sell 10,000 units of this DC motor being improperly controlled by an AC contactor? If so, I think you need more oversight. If this is for your own one-off automatic cattle gate, then sure, go for it! The worst that can happen is that the contactor welds shut and the motor runs away, burning up when your gate crashes, and explodes your batteries.

    Since you used the word "contactor" im going to assume its a high current motor. High current dc contactors are NOT cheap, and this is one of the reasons why industry is shifting to AC for variable speed applications instead of old DC standard. They have a different name for these high current dc contactors designed for motor applications; "DC loop contactors" if youre using anything else, youre not doing it right, officially. That doesn't mean it won't work, it just means you should not get upset when it fails catastophically,.
     
  10. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The P&B PRD-1DH0 double pole series with DC magnetic Arc Blow-out goes up to around 50amps and runs around $40.00.
    Max.
     
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