AC Shunt Properties

Yes, they are available. What about frequency? Can you tell us that?

Thanks for your interest.

The frequency is 50 Hz.

The device may also be used on 60Hz.

I would be very obliged for an explanation of why you consider frequency a factor.

Is it reactivity?

I ask about the frequency for three reasons:

It tells me a little about your location. I assume you're not in the US since you're interested in 50 Hz (We have, so far as I know, only 25 Hz and 60 Hz). There is a good shunt manufacturer here in Pittsburgh that I could recommend, but you are undoubtedly in another country, so I won't.

Reactance has to be considered for such a high current. It's not terrible at 50 Hz, but it's still something to think about, depending on your desired resolution.

Skin effect has to be considered. Again, it's not really important at 50 Hz, but you need to think about it.

Any shunt manufacturer, and I strongly recommend you go that route, will ask about the frequency.

And, I have to ask: Why not just use an Amprobe? Or a current transducer (there are several types)?

Your title is "AC Shunt Properties". Why would he not inquire about frequency? They are available up to at least 100kHz. Some are cheap and some are expensive. Typically you want 100mV output for high accuracy, so you choose a shunt to give you close to that for the current you want. What are your requirements?

You should be looking for an intrument shunt. They are made from manganin because it has a very low temperature coefficient of resisitance. They are normally rated in mv/amp. As you already know from previous posts, there is sometimes a reactance concern. Another concern is that the output is a floating voltage which means it's not referenced to ground and must be measured with a floating instrument. We use lots of instrument shunts, but hall generator devices and toroidal coils can be useful in certain applications.

Excellent advice here (TVP45 and Welderman). I just happened to see it, but thanks, anyway. Very useful.

Is there no possible problem with the power through the shunt ?

at 25 Amp continous you get 0.625 Watt

at 100 Amp druing 20 sec you get 2.5 Watt

Perhaps connecting serveral shunts in serie and measuring over more shunts at lower current ?

Yes, current shunts are made by a number of companies, here (USA) and abroad, shunts are almost always made with shunt Manganin which is slightly different than 'standard' Manganin. At 50Hz, you are going to have some reactance problems, since the shunt resistance you mentioned using is 4mΩ, reactance of the shunt could exceed that value, a mere 50μH at 50Hz has 15.71mΩ of reactance. I do not know what your actual inductance value may be, the manufacturer should be able to tell you.

Even more problematical though is power dissipation! Your 4mΩ shunt (discounting inductance effects) will dissipate 40 watts at 100 amps and 640 watts at 400 amps. Depending on the thermal characteristics of your shunt, the 40 watts should be treated as a constant power load to effectively handle it and even though your 400 amp load is only for 3 seconds, that will generate even more heat, likely between 128 watts and 160 watts in those three seconds. (Yes, I'm making an estimate based on common shunt builds.)

The shunt is going to have to be able to handle that much power because if the Manganin gets too hot, it will permanently shift in value and TCR (temperature coefficient of resistance) will also change. Manganin is a low temperature copper alloy and does not tolerate even moderate temperatures well. Generally, the top operating temperature is about 55'C-60'C and that includes ambient temperature. Much above that and you will permanently 'damage' the shunt.

The TCR curve of Manganin is parabolic, resistance decreases both above and below 25'C with a relatively small TCR change between 20'C - 30'C, beyond that, the TCR increases rapidly.

I would select one of the listed manufacturers and talk to an applications engineer about this and give him as complete details about your application as is possible.

I thank all for contributing.

I am considering other options, the shunt appealed as it could be easily fitted.

The possible ambient maximum and the high current could easily subject the shunt material to temperatures of over 75C.

The other components maximum allowable temperatures are 75C.

If this temperature makes the shunt unstable it is not suitable for this application.

Wiz gave you a good answer, including the suggestion that you discuss your application with the manufacturer of the shunt. Whether you can use the .004 Ohm shunt you have in mind, will depend on its power rating and your maximum ambient temperature, which you haven't specified. If required, you may be able to increase the capacity of the shunt by cooling. You should also be aware that much higher power shunts are available, but perhaps not at .004 Ohm.