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Thread: Contactor burning

  1. #16
    Electrical Contractor/Instructor jwelectric's Avatar
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    If I understand this correctly we have an overhead crane that is powered by 480 volts.
    This crane has two 12 volt direct current electric motors that operate an air compressor and a hydraulic pump.
    The operation of the crane and both the compressor and hydraulic pumps are controlled by a pendant that hangs from the crane which the above drawing is supposed to be of.

    Well I am going to leave this one alone as I just don’t see the crane lifting and lowering with only two wires to the motor controler but will ask why these tanks that are designed to be buried underground must be handled so delicately? Are they not reinforced by some sort of rebar?

  2. #17
    DIY scratch-pad engineer leejosepho's Avatar
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    Quote Originally Posted by sbrn33 View Post
    9 out of 10 times this is a burnt coil not any real problem with the actual contacts or mechanism. Low voltage to the coil for a while will burn em up.
    Ah, so I was wrong ... but is it the low voltage that actually does the damage or is it the fact that the coil is straining *because of* the low voltage? I presently have a small 12VDC fan (out of a computer power supply) being supplied with 7.5VDC to cool my modem. I did that because I do not want the fan running at full speed. Is that fan now likely to fail early?

  3. #18
    DIY scratch-pad engineer leejosepho's Avatar
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    Quote Originally Posted by jwelectric View Post
    If I understand this correctly we have an overhead crane that is powered by 480 volts.
    Yes, and that bridge crane has its own wireless controls for travel, lifting and lowering and speed.

    Quote Originally Posted by jwelectric View Post
    This crane has two 12 volt direct current electric motors that operate an air compressor and a hydraulic pump.
    No. What we have here is an I-beam hanging on the crane's hook, and everything I have mentioned is on that beam. Here is a picture from one of my early tests during fabrication:

    Name:  testpic4-.jpg
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    Only the hydraulic pump and leveling system are in this picture, and a 480V turn motor and the compressor and control pendant were added as my work continued.

    Ultimately, I installed a cord reel to bring 480V down to the hook and power everything on the beam from there.
    Last edited by leejosepho; 06-16-2010 at 10:03 AM.

  4. #19
    DIY scratch-pad engineer leejosepho's Avatar
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    Quote Originally Posted by jwelectric View Post
    Well I am going to leave this one alone ...
    Cool!

    Quote Originally Posted by jwelectric View Post
    ... but will ask why these tanks that are designed to be buried underground must be handled so delicately? Are they not reinforced by some sort of rebar?
    Certainly, but the concrete is only a little over 12 hours old when the halves are pulled from their forms.

  5. #20
    Electrical Contractor/Instructor jwelectric's Avatar
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    Cool Dude and thank you for the explanation. I was having a hard time with the pendant and thinking it controlled the crane as well as the beam.

    I can also understand the pulling of green concrete in only 12 hours of pour. Having done some work at a similar plant I suppose I was thinking of your plant being like the one I did service work at. There they had break away molds that were opened in the yard.

  6. #21
    Retired Defense Industry Engineer jadnashua's Avatar
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    Contactor coils do NOT like less than their rated input voltage (which may be a range). The contact ratings of the contactor power leads are a maximum, and using lower voltages (and current) on them than specified won't be a problem. Some motor designs will work and just run slower if you drop the voltage, some will burn up. If you are using DC contactor coils, you want to install a diode across the coil with it reversed, so during normal operation, there is no current flow (i.e., it is blocking the flow and not acting like a short across the relay coil - IOW, put the cathode on the plus side of the coil and the anode on the minus side). Then, when it is de-energized, the coil's induced voltage will be drained off, and not cause the thing to chatter as the spike decays. If the coil is big enough, there can be enough current generated when control power is removed to cause it to momentarily re-energize itself. the diode prevents that and lenghtens the life of the relay contacts for minimial costs.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  7. #22
    DIY scratch-pad engineer leejosepho's Avatar
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    Quote Originally Posted by jwelectric View Post
    Cool Dude and thank you for the explanation. I was having a hard time with the pendant and thinking it controlled the crane as well as the beam.
    Yes, and I should have been more clear in the beginning.

    Quote Originally Posted by jwelectric View Post
    I can also understand the pulling of green concrete in only 12 hours of pour. Having done some work at a similar plant I suppose I was thinking of your plant being like the one I did service work at. There they had break away molds that were opened in the yard.
    Where I worked, I always got corrected whenever I used the word "molds"! For whatever reason, "forms" are what they have there. But either way, most castings such as manholes, drywells, catch basins and just about everything else had forms/molds that opened up on the outside and were easily removed from the inside before the finished part ever got touched. With septic tanks, however, that would have been nearly impossible and completely impractical there since that would have meant flipping the one-piece (inner and outer) form/mold and tank half together in order to be able to remove the inside "plug" (my term), then flipping the form/mold back into place for the next pour. I once made a set of knock-down panels for forming square-and-vertical walls inside some custom 1000-gallon containment vessels, but that kind of thing would be far too labor-intensive in a production setting where the finished parts are going to be buried in the ground and filled with the stuff that typically goes into septic tanks.

  8. #23
    DIY scratch-pad engineer leejosepho's Avatar
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    Quote Originally Posted by jadnashua View Post
    Some motor designs will work and just run slower if you drop the voltage, some will burn up.
    Well then, I must be picking the ones that do not! But yes, I have definitely seen low voltage produce heat and ruin motors.

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