Hi all.
The accuracy for e.g. 20A current transformers in the shop is 0.5%
there are class 0.1 current transformers that are, if I’m correct, more accurate. Has anyone used those? Or do the shop versions perform better?
flor example 50ma Current Transformer | Mini Transformers | 50ma Current Coil | 100a 50ma | Ac Coil - Mini - Aliexpress
Welcome, @PGSmink to the OEM forum.
Two points about those c.t’s: First is a ring-core c.t. is inherently more accurate than a split-core, because there’s zero possibility of an air gap. This of course means that you need to disconnect the circuit/load to install it. The second point is the c.t’s we use with the emonTx4 (and the new emonPi, when it appears) have a 0.333 V output, not a current, so the 0.5% accuracy includes the burden. To use the c.t’s you mention, you need to add a burden, which even if it’s a 0.1% resistor, straight away pushes the overall accuracy up to 0.2%.
Thanks for the reply. Does that mean that in practice there are no more accurate c.t’s?
You’ve got to realise that c.t. errors come via many different mechanisms, principally from the way the magnetic flux behaves in the iron (or ferrite), clearly and as I mentioned above, a slight air gap in a split core can have a very significant effect, and then there are the usual manufacturing tolerances and consistency of the materials. The manufacturer can control some of these by making small adjustments to the number of secondary turns.
The point about the 0.333 V output means that any error in the burden value can also be compensated within the overall manufacturing process, so the output voltage tolerance you see includes the burden resistor tolerance.
Would I trust a Chinese £1.16 c.t to really be Class 0.1? I don’t think so.