measuring 200+ amps

Randy

I need to measure the current on a machine that draws about 200 amps at 208 volts. This is to go to a data logger with a bs2pe, an a/d and assorted other components. I am trying to not spend the money on a shunt or clamp on device. I rather poorly remember that you can wrap a wire of a given length around the load wire x number of times and it will produce a measurable voltage. The same idea as a current transformer with out the core. I have searched all over for info and can not find a thing. I would guess that the insulation thickness and having the looped wire be uniform would effect the reading along with some other things but is the principle sound?

Randy

mctrivia

208V so I am guessing it is a 3phase AC machine.

Yes you can just rap a wire(I would use 22 or 30gage) several times arround the hot wires. you will need 1 coil/hot or assume it is balanced and only 1 coil. you can either calulate(would need to drag out text books to find formula) or just try by trial and error.

1) make 10 raps around wire while it is at a known steady load(preferably near max you will want to measure).
2) measure voltage generated with a multimeter. if voltage is equal to 3.3V*80%*(percentage of max load) then stop. if lower add more winding if higher subtract.
3) use electrical tape to secure windings in place. make sure to start at one end and tape with the windings then neatly cut off and tape the other way. cut tape do not tare.

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24 bit LCD Breakout Board now in. $21.99 has backlight driver and touch sensitive decoder.

IRobot2

Randy,

I know you were asking for a specific way of measuring the current on your project but I just had a project that had to measure +200 amps too. The easiest and cheapest thing I found was "ACS754SCB-200-PSF-ND" from Allegro Microsystems. Search for that number at Digikey (or go to http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=ACS754SCB-200-PSF-ND ). This little chip can measure up to 200 amps, works with AC or DC cost about 6 bucks and is about the size of a standard voltage regulator. I ordered a few a couple weeks ago. I blew a couple up just out of curiosity to see how tolerant they were. I was VERY impressed. It is easy to implement (I used a rigged fuse holder for audio equipment). I think I still have a videos of it...

Any way, thought this might help you out. Good luck with your project.

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Alex Burke
"Beware of computer programmers that carry screwdrivers." -Leonard Brandwein

Leon

Current transformers aren't expensive, and are quite convenient.

Leon

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Amateur radio callsign: G1HSM

Randy

Thanks to all. Mctrivia this is what I was looking for. It may be a week or two before I can play with this. Yes this is three phase, a chiller with two 44 ton compressors and four fans. I am wanting to measure and record one leg on each compressor and each fan for load trending plus overall load. Parrallel wires for each phase leg of the compressors and the whole machine, 3 ott each, make for some hefty current transformers. That is why I need something dirt cheap. Besides this is a hobby and learning is what it is all about.

Randy

Prettybird

I have used a torrad for measuring current changes in servos on CNC machines before. Just need a DVM. I had access to a digital ampmeter but had to see changes from distances up to 20"from the control panel. You youl have to do the math for how many amps per MV. Sometimes no loop was necessory.