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Need to measure high current DC
Q. Can anyone help me find a source for an ammeter to measure DC Current up to 16,000 amps. I would really like to get a portable device that could clamp-around 8" buss bar. Any help would be greatly appreciated. Thanks!David Sugg
steel - Malvern, Pennsylvania
A. The way a commercial ammeter, such as is used on a rectifier, works is like this--
A short section of the busbar is cut out and a block of slightly resistive metal, called a shunt, is inserted so all the current has to flow through the shunt. The resistance of the shunt is chosen so that there will be a specific voltage drop (usually 50 mV) across the shunt when 16,000 Amps are going through it. So if we read a drop across it of 25 mV, the current is 8000 Amps, so the dial of the ammeter is marked accordingly.
Since the bus bar also has a specific resistance, you could measure the drop across a marked length of busbar with a simple hand-held millivoltmeter and translate that to an amperage reading. Install test points in the bus, maybe 10 feet apart, calculate or measure the mV drop at 16,000 Amps and calibrate. This makes any cheap $10 millivolt meter into a high-current ammeter. I haven't actually done it, but at the moment it sounds good on paper.
Ted Mooney, P.E.
finishing.com - Pine Beach, New Jersey
Striving to live Aloha
If you can turn the current off and on again, what you are looking for is called a Rogowski coil. Search for that name on the web, and look for the company that makes one that has a quick connect break in the loop of flexible insulated coil. A Rogowski coil is a spiral coil wound on an insulator with no magnetic permeability difference from air. The ends of the spiral coil wire are led back along the side of the spiral right next to it, so they do not form an effective loop that would "measure" some of the magnetic field. So, what does measure the field is all those little spiral circles measuring fields all along the path of the coil form (which starts and ends at the same spot when closed). The open circuit voltage of all these equal sized, equally sensitive loops is added together because they are connected in series, and the closed path they follow around the big conductor in the middle makes the voltage proportional to that conductor's current CHANGE. IF the big conductor current has to remain undisturbed, a rogowski coil can't help, since it measures change, not steady current, and you integrate it's output to get current vs. time. If you wanted to measure a steady buss bar current, and had a good, accurate, well zeroed integrator on a rogowski coil, you might be able to zero calibrate it off the conductor, then start integrating while undoing the coil break, looping the ends around the conductor, and connecting the coil break, then freeze the measurement. Then the current change would be the DC current.John Griessen
metal works - Austin Texas
June 21, 2020
Q. Hello everyone,
We have Hard Chrome plating shop. Rectifier capacity is 10 V / 6000 Amp. Can anybody suggest me the right way how to measure actual current coming to the component to be plated, or how to measure the current on Bus Bar at nearest point to the Cathode or component ?
Purpose is verify whether current shows on panel is correct or not.
We are Sewing Machine needle manufacturer German based MNC. The size of bus bar is 140 mm / 20 mm.
- Chandigarh, India
We'd love to give credit for this graphic explaining Ohm's Law (A = V / Ω) but we see it on many websites with no hint of whose work it actually was :-(
A. Hi Parmod. Current is actually a count of the number of electrons flowing through the circuit. So current doesn't get 'consumed' the way voltage does. If 6000 Amps is flowing through any point in the bus bar, and the only current path from anode to cathode is through the plating solution, then 6000 Amps is flowing through the plating solution.
It is possible of course that your meter is not reading accurately, but that can be checked pretty easily --
Please see my earlier explanation of how a "shunt" works, and verify the resistance of your shunt preferably by both checking with the manufacturer and by measuring it with a trusted V-O-M. Once you know the resistance of the shunt, you have the correlation between the voltage drop across the shunt and the amount of current flowing, so you can use the millivolt setting on your trusted V-O-M to easily check if the meter on the rectifier is accurate.
Ted Mooney, P.E. RET
finishing.com - Pine Beach, New Jersey
Aloha -- an idea worth spreading
July 16, 2020
I love the cartoon. It should be in all college freshman physics textbooks.
Jeffrey Holmes, CEF
Spartanburg, South Carolina
July 19, 2020
A. If the panel meter is suspect, you may try to use a DC clamp on meter which, as the name suggests, can just be clamped around the bus bar. But I am not sure if you can find one that meets your specs of 6000A and also big enough to fit the bus bar.
Yes, that cartoon is interesting. But, I am afraid, it is not giving a correct picture of resistance/ohms. It rather explains the working of a field effect transistor pretty well. Resistance is more due to collisions electrons suffer with atoms like a person trying to walk through a crowd. Walking becomes more difficult as the number of people increase in the crowd. That is resistance.
bangaloreplasmatek.com - Bangalore Karnataka India