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Demagnetizing Transformer Cores

Demagnetizing Transformer Cores

Demagnetizing Transformer Cores

(OP)
I am currently using a winding resistance meter with a demag function to demagnetize 3Ø power transformers (25MVA ONAN Dy and 50MVA ONAN autos are the largest). It is capable of providing 15A output. Despite showing that the demagnetization is complete on multiple attempts, excitation tests are showing irregular patterns between the three legs. Based on the short period of time it takes to run the demag process with only 15A, it is not surprising that it's not actually demagnetizing the core.

Doble recommends using a battery for successively lower time periods while swapping polarity on the leads at the transformer terminals. We have some safety concerns due to the inrush current and possible arcing due to the inductive load.

Is this something we should avoid? Doble seems to be a trustworthy source and we've seen contractors demag with a battery, but if there are safety concerns, we would prefer to get a device purpose-built for this task.

Thanks

RE: Demagnetizing Transformer Cores

You may be able to use a diode and let the current decay through the diode. You would have to disconnect, wait for the current to decay, swap the polarity and re-energize. This may be best automated.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Demagnetizing Transformer Cores

It must be demagnetizing. The Dc current required to demagnetize the core is 1.414 times the RMS value of rated excitation current which will be less than 0.5 % of rated full load current.

RE: Demagnetizing Transformer Cores

(OP)
Doesn't the current magnitude determine how long it needs to be applied? I was under the assumption that lower currents would require long periods of time to demagnetize.

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