Quick question on mag curves.

[bsmotorsport]

Whenever I have carried out a mag curve I have always been told to drive it into saturation (1A through the secondary), bring it down slowly to 0 then plot various points up to the knee point and carry on all the way up to complete saturation again. The other day I got into a heated discussion with a senior engineer as he stated that you only have to take it to the knee point and that's it. I like to go past the knee point to get a good feel for how matched the CT's are and also to make sure they're no shorted turns. Is what he says right? I can't seem to find any conclusive documents to suggest either way. Any of you got any suugestions to throw in the mix?
Thanks

 

[veritas]

The reason you drive a CT into full saturation (not just to Vkp) is that you may have residual flux in the core which will affect the results of your mag curve test. This is especially true if DC tests such as the flick test (for polarity)is done before the mag curve. Driving it into deep saturation and then bringing it back to zero slowly ensures that as little as possible residual magnetism is left in the core at the commencement of the mag curve test.

Also, you should not need to drive a CT into deep saturation to detect shorted turns.

 

[randyman]

Veritas is exactly correct. And I might add to assure saturation you can take it up to the rated secondary current times its rating factor. That induction level should be high enough to remove any significant residual magnetism that may be in the core. If you want to test that theory then take readings going up and then coming back down at the same points. If they were different then there was residual in the core. I can assue you that CTs tested in the factory are always demagnetized prior to any accuracy or excitation testing. And always after measuring its winding resistance.

Also, in the case of a multi-ratio CT, you can use lower tap connections to do the same thing, which could lower the effective saturation voltage you need to apply.

After you demag the core you do not have to take it up to saturation again unless you want to measure that point, otherwise go only as far as you need to.