Some people worry me!

[ghost123uk]

I don't think OBG means re-polarising (he will be out of bed soon so will clarify ) Re-polarising is for changing dynamos from positive earth to negative (or the other way round should you wish ) I think we are talking about re-energising the weak magnet found in self exciting alternator. But I reckon you know that

Re-polarising dynamos and re-energising magnets definitely requires DC. Mains AC was mentioned above and you cannot re-energise a magnet with AC (but you can destroy a magnet with it ) - please clarify what is happening here for us (me)

[Oldiebut goodie]

I am out of my pit now!
I cannot clarify it any further - all I know is I have resurrected dead generators with the back feeding of 230v ac into it with another generator after researching it on t'internet. It certainly doesn't destroy anything but as to the whys and wherefores as to what actually takes place who knows, I presume that I did read what was actually happening but was not sufficiently interested to retain the information. I am just satisfied that it works!
If you read the link you will see that he does mention that you could reverse the polarity by mistake and end up with not achieving anything.