Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

DC Amp Gauge Vs. AC amp Gauge

  1. Dec 8, 2014 #1
    I was walking around at work today to find an AC amp Gauge on a DC breaker (1000KW, 250 VDC), Which made me scratch my head if there is any difference between the two gauges for this application?

    Any idea?

  2. jcsd
  3. Dec 8, 2014 #2

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    In general, AC and DC ammeters are different creatures.
    What is the application?

    I once ran across a three phase half wave rectified motor controller fed by two synchronous generators connected in parallel.
    The load current drawn by that system was of course DC and indicated by DC ammeters.
    It was necessary to add AC ammeters so that we could match excitation of the parallel machines by minimizing AC current.

    So, there could be a reason. What kind of meters are they? Could it be a simple mislabelling? (see my signature....)
  4. Dec 9, 2014 #3
    Thank Him for the Replay

    I attached pictures reflecting the above situation.
    what's the reasoning behind that gauges difference in terms of their fabrication. More explicitly why DC gauge won't read the AC or at least the RMS value

    Thanks again

    Attached Files:

  5. Dec 9, 2014 #4

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    Judging from your photos i strongly suspect you are in a switchgear room or control room of a substation that's handling AC. Reason i say that is i've simply never heard of a DC substation, and those breakers are labelled "substation tie". I'd expect AC in a substation, in the thousands of amps .those meters report.

    It is vitally important that switchgear have present DC control voltage to operate it. In my plant we used a simple white light, and our DC control voltage was 130 volts. But i see that 250 is also a standard..
    I think the DC meter indicates not only the presence of control voltage like our light did, but its health..

    Now to whhat's the difference in AC and DC meters:

    Those look like industry standard KX241 style meters which come with internals appropriate to their use.
    The basic DC meter is the D'Arsonval galvanometer, described here. Basically it's a coil encircling a magnet such that current through the coil makes the coil rotate, and the needle is affixed to the coil. Deflection is in proportion to the average current throuhg the coil. It only reads DC because the average value of AC is zero....

    To read AC there are a couple things one can do..
    1. Insert a rectifier circuit to change the AC into DC for the D'arsonval movement....
    2. Use a different movement such as "iron vane" or sometimes known as "Moving Iron", , which is basically an AC electromagnet that tugs the needle around. It'll read both AC and DC...

    There's a very basic explanation here:
    and here :

    Search on images for the Daesonval and Iron Vane movements returns tons of hits.

    Here's a catalog from the folks who make the meters in my plant: Caution it's 4 meg, so if you're on dialup better plan ahead...
    as you see meters are really interesting. Pf'er dlgoff has a collection of nice old ones.... see "Beauty of old measuring devices" thread....
    I hope this gets you started.

    old jim
  6. Dec 10, 2014 #5
    Thanks Jim for the detailed replay
    you were right, that is a switchgear room.
    and for the breakers addressed earlier they're fed from different rectifiers (250VDC, 1000KW) that are directly fed different 11.5KV.
    The 250 VDC mainly feeds some crane and some big DC motors (old old company)
  7. Dec 10, 2014 #6

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    Aha !
    Wow what a lot of DC ! Are you an aluminum smelter or something?
    Is that breaker really on the DC not AC side of the rectifier?

    Were I designing that panel I'd measure current going into the rectifier on AC side because a simple current transformer and AC meter will do the job while providing safety isolation from the high voltage. Further, one could add a switch so he can look at currents on all three phases.
    DC meters use a shunt and it takes extra measures to isolate.
    There do exist DC current "transformers" but they are electronic devices that add complexity and unreliability.
    Again that's a guess based on what I'm accustomed to seeing.
    Surely the electricians in your plant have been into that panel and would gladly share their knowledge with a curious engineer?

    old jim
  8. Dec 11, 2014 #7
    That's what I said the went to that Substation Room,
    it is a iron steal plant. and the gauges are after the DC side of the rectifier.
    I believe it was just inexperienced electrician mistake!

    thanks Jim for all the info
  9. Dec 11, 2014 #8

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    well then they probably are ordinary DC meters that use a shunt . I'd guess they just came in with wrong lettering on the face. Be careful the wires on the back are probably at 250 VDC.
    We had a condenser vacuum gauge show up with a scale of 0 to 300 inches of mercury. Some waggish operator put a sticky note on it "Wow - this really sucks".

    I'd guess you use that kiloamp DC for big motors in rolling mills, or arc furnaces ?
    Interesting to glimpse into other worlds....

    thanks, old jim
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook