High current shunt - temperature drifts - Page 1
Forum section: electronics / testgear

author: CosteC
author_rank: Regular Contributor
timestamp: July 21, 2024, 07:41:33 pm
content: Hello everyodyI am in need to measure about 400 A AC (continious) with as good accuracy as possible and fits 6 1/2 digit multimeter (~0.06%). Current is somewhat distorted 50 Hz sine.Easiest way seems shunt. I have found 0.2 class and 0.25 class shunts:https://www.lumel.com.pl/en/catalogue/product/shunts-b2-60mv-3CLASS 0.2 but temperature coefficient seems 100 ppm/K which is rather a lot, but also specified for ambient temperature not shunt temperature.Otherhttps://docs.rs-online.com/18e7/0900766b813bb936.pdfis CLASS 0.25 and has thermal coefficient of 15 ppm/K which is bit questionable, as manganine coefficient is 0ppm for 25'C but -42 ppm for +100'C Obviously there are slightly different alloys under the same name, varying a bit in performance.What is your recomendation for such measurements?

author: Kleinstein
author_rank: Super Contributor
timestamp: July 21, 2024, 08:10:37 pm
content: 400 A is quite high. A good shunt would habe to be phyical large to get good power handling capability. One may want a special low noise amplifier to get away with a low burnden voltage. For AC only the DC drift would be less of an issue. 60 mV burden would still be 24 W and the current path outside the active area can add to this.The shunt would still need to be low TC to handle the heating - no need to care about 100 C, as the shunt should not get that hot at all.Manganin is still a good choice, because of the low resistivity, low thermal EMF (important for DC) and good contacts.The 2 shunts shown are way to tiny - for good accuray consider 10 x or more physical size. This could be shunt for a higher current (e.g. 1000 A) and using less burden with a better amplifier. Here AC is much easier than DC.An alternative to consider would be a good current transformer instead of directly a shunt.

author: Hydron
author_rank: Super Contributor
timestamp: July 21, 2024, 10:56:14 pm
content: This is definitely a job for a CT rather than a shunt - far safer and easier. You should be able to get one that matches the 0.2% shunt accuracy fairly easily (ratio error, so you have to add the meter AC current error too), or I suspect there are other better options if you want to spend more.

author: TimFox
author_rank: Super Contributor
timestamp: July 21, 2024, 11:01:07 pm
content: You can use a CT terminated with an appropriate low-valued resistor to develop maybe 100 mV rms across that resistor and drive an AC voltmeter to avoid any error on an internal current shunt in the DMM.

author: CosteC
author_rank: Regular Contributor
timestamp: July 22, 2024, 06:54:08 am
content: Quote from: Hydron on July 21, 2024, 10:56:14 pmAny proposals for 0.2% class or better CT?Quote from: TimFox on July 21, 2024, 11:01:07 pmWell. Not adding low value resistor in most of cases will ruin CT accuracy... And I studied DMM current measurement accuracy. It is not-so-great for currents > 100 mA.

author: Hawaka
author_rank: Regular Contributor
timestamp: July 22, 2024, 07:12:56 am
content: DaniSense or a LEM Ultrastab should have easily the accuracy you need. But be ready to spend some $$$.

author: CosteC
author_rank: Regular Contributor
timestamp: July 22, 2024, 08:32:22 am
content: Quote from: Hawaka on July 22, 2024, 07:12:56 amIN-xxx series from LEM is indeed almost magically accurate and equally expensiveI made extensive digging in specs and spoke with LUMEL laboratory department.Conclusions:LUMEL does not require de-rating up to +55'C ambient. Murata require de-rating to 2/3 rated current for continuous operation at +25'CLUMEL keeps 0.2% up to +55'C . Over it it is 100ppm/K extra error.MURATA require vertical mounting due to blade shape resistive elements, LUMEL uses round bars which are not as demanding.If Murata accuracy is specified as 0.2% at +25'C then thermal drift makes it 0.295% class at +55'C... unless it is shunt temperature, not ambient. Then it is far worse.So, LUMEL it will be. Maybe with lazy fan to be sure it is away from +55'C limit of ambient temperature.In meantime lets check how much will cost their factory calibrated one with papers.

author: tszaboo
author_rank: Super Contributor
timestamp: July 22, 2024, 08:43:12 am
content: You can measure this with shunts, easiest way to improve them is to dump them into a bucket of mineral oil.Or direct fans at them if you are not comfortable with oil.LEM sensors are also very good.

author: CosteC
author_rank: Regular Contributor
timestamp: July 22, 2024, 08:59:06 am
content: Mineral oil require cooling too, especially if measurements take hoursLEM is great, but there is lot of issues except of price. I am really not measuring current, I am measuring ratio between two currents (calibration/accuracy control) so I would need to synchronise LEM with other things. Not willing to do it as far as I canThanks!

author: Phil1977
author_rank: Frequent Contributor
timestamp: July 22, 2024, 09:28:04 am
content: Can you feed the currents you want to compare through the same CT?

author: CosteC
author_rank: Regular Contributor
timestamp: July 22, 2024, 11:54:50 am
content: Quote from: Phil1977 on July 22, 2024, 09:28:04 amNo. I am testing CTs. Or Rogowski Coils.Generally, I need to measure large current and its relation to "small current" or "small voltage". In practice those are two "small voltages" - voltage from shunt(s) or voltage output from integrator (rogowski coil)But I would like to hear more about your idea. Can you share?

author: Phil1977
author_rank: Frequent Contributor
timestamp: July 22, 2024, 12:14:10 pm
content: It´s just a very easy idea: A current clamp or CT physically sums up all the currents flowing through it´s port area.This can be used to enhance the measurement accuracy. If you e.g. want to compare two lines carrying around 100A then you need 1% of measurement accuracy to resolve a difference of 1A. If you direct both lines antiparallel through your current sensor, then you just read the current difference, so that a current difference of 1A with a 1% CT gives you a resolution of 10mA.If the ratio is not one to one but any reasonable rational number, you could also feed several windings through the CT - though of course 400A lines may be too thick for that.

author: Fungus
author_rank: Super Contributor
timestamp: July 22, 2024, 01:16:16 pm
content: Quote from: CosteC on July 21, 2024, 07:41:33 pmBigger heatsinks on the shunt.Edit: I should probably say "better thermal management of..." - too many literal pedants around here.

author: tszaboo
author_rank: Super Contributor
timestamp: July 22, 2024, 01:43:06 pm
content: Quote from: CosteC on July 22, 2024, 11:54:50 amYou should just run the same wire through the DUT then several times, and reduce the current significantly.

author: CosteC
author_rank: Regular Contributor
timestamp: July 22, 2024, 03:14:45 pm
content: Quote from: tszaboo on July 22, 2024, 01:43:06 pmNot really practical. LV devices have small apertures, and current density will increase when putting more turns as insulation takes space for copper.For MV devices putting more turns is simply not possible.Putting more turns causes issues with conductor position location and external fields, particularly important for Rogowski coils.I know for sure some labs use special lock in amplifiers and moderate current levels, but it is outside my budget now.

author: CosteC
author_rank: Regular Contributor
timestamp: July 30, 2024, 11:42:31 am
content: If somebody need accurate shunt for -25'C +55'C ambient temperature, IEC 60051-8:500A, 60 mV 0.2% shunt is 200 PLN, here it is almost next day deliveryhttps://www.tme.eu/pl/details/b2-060500ab01b2m0/boczniki/lumel/b2-060500ab01b2m0/500 A, 100 mV 0.2% shunt with traceable calibration cost around 500 PLN, 6 weeks lead time.If you need silver coating it makes 1100 PLN and 12 weeks lead time.Add VAT to prices.