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jaycee91
January 13, 2019, 09:59 PM
how do you demagnetize a transformer?
phasor
January 15, 2019, 07:51 PM
how do you demagnetize a transformer?

Some winding resistance test sets have a demag function
Behab1
January 17, 2019, 12:42 PM
how do you demagnetize a transformer?

Use a car battery and let the current flow through the winding.
jaycee91
January 23, 2019, 10:29 AM
Use a car battery and let the current flow through the winding.

So DC current is the answer?
Kalbi_Rob
January 23, 2019, 12:47 PM
So DC current is the answer?

AC or DC can be used. AC amps applied close to the excitation knee and slowly lowered can demag, and long tedious alternating of lowering DC currents can demag. Edit: I would highly recommend using AC

Previous TestGuy thread about it:
https://testguy.net/threads/2749-Correct-procedure-for-de-magnetizing-transformer-cores

Article on importance of demag xfmrs with AC explanation:
https://www.tdworld.com/substations/reliable-demagnetization-transformer-cores
SecondGen
January 23, 2019, 01:33 PM
So DC current is the answer?

More like alternating DC, which is effectively AC - just not the 60Hz frequency you might be used to. You can use a car battery to pass current thru a winding in different directions, if you watch a winding resistance test set de-mag function you can see the process in action.

An example would be something like this:
+10A
-10A
+7A
-7A
+5A
-5A
+3A
-3A
+1A
-1A


This isn't exact by any means, I'm just trying to show an example. I'm sure you can also use regular AC voltage but a car battery was mentioned earlier and that's how I was originally taught. Also winding resistance sets usually operate off DC which is why the above method is used. As far as I understand you only need to perform this on one winding to de-mag a polyphase transformer.