Brass conductivity

Discussion in 'General Electronics Chat' started by tresguey, Dec 2, 2014.

  1. tresguey

    Thread Starter Member

    Apr 22, 2013
    55
    1
    I have a general question about what type of lug I should use. I will be powering up to 60 amps of electric motors with PWM to MOSFETT's basically. I am connecting the power in and out with a #6 - 32 stud. The brass studs have been breaking once in a while. Should I use a different stub material? The swaged standoff I use to install the studs only go up to #6 - 32. Intitially I wanted to go to #8 - 32, but the swaged standoff was unavailable.
     
  2. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,681
    900
    Is that a question of electrical conductivity or mechanical strength? Current rating is basically a heat rating. Brass (you have not identified the alloy) does not heat harden to any great degree, so I suspect your problem is mechanical. Use a bigger screw, like a #10, #12, or 1/4".

    John
     
  3. #12

    Expert

    Nov 30, 2010
    16,261
    6,770
    I believe a size 6 screw is too small. Go to the hardware store and look inside a 60 amp disconnect box. The screws in there are large enough for 6 AWG wire because that's The Law, according to the National Electrical Code. My point is, if the standard screws are that big, I don't know why you think a 6-32 stands a chance.
     
    GopherT likes this.
  4. Gdrumm

    Distinguished Member

    Aug 29, 2008
    684
    36
    If you can't fit anything larger than a 6-32, then go with a Stainless Steel screw (and SS washer and SS lockwasher).
    It will withstand heat, and should not break.
    It's only my opinion.
     
  5. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,681
    900
    I don't think stainless steel is the answer, if ohmic heating is an issue. It has ≥10X the resistance of most brass alloys. A current of 60A can create a lot of heat.

    Alloys of aluminum can have tensile strengths greater than stainless (e.g., 7075T6) and still conductivity of 45% IACS (i.e., better than brass).

    Resistance of various alloys:
    http://eddy-current.com/conductivity-of-metals-sorted-by-resistivity/

    Strength of aluminum alloys vs. stainless steel:
    http://www.americanmachinetools.com/tensile_strength.htm

    John
     
    Gdrumm likes this.
  6. MrAl

    Well-Known Member

    Jun 17, 2014
    2,425
    490
    Hello,

    There are different alloys but the common brass i have come across has resistivity 4 times higher than copper. That means a chunk of a piece of brass the same size as a chunk of copper will measure four times higher resistance than the copper. That's still better than solder.
     
  7. tresguey

    Thread Starter Member

    Apr 22, 2013
    55
    1
    Thanks for your replies. We are looking more for the strength factor due to a few customers breaking the studs. The DC motors are driven at max amps for only a few seconds very rarely. Most of the time they are run at about 25%. I was inquiring more for heat-sink capabilities and if stainless would conduct well enough.
     
  8. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,769
    969
    typically the fastener has very little (or negligible) effect on the resistance of a joint. There is usually plenty of contact between the lug/terminal and its landing point to offset it. You might get lucky if you can even measure a thermal difference of anymore than a degree or 2 of that.
    I'd just use a steel stud and call it a day.

    A #6 brass fastener with a 60A landed lug connection is just about guaranteed to break/be over torqued constantly in the field..
     
    GopherT likes this.
Loading...