How can I remagnetize a demagnetized rotor?
I heard that this can be done by applying a magnetic field around the rotor which is stronger than the magnet.
If you have a hyper powerful electro-magnetic field generator it may be possible, otherwise you have a nice fridge-magnet to play with; time for a new rotor ( what motor is it, are the windings burnt/ black/ smell like an ash-tray? )...
Not too sure that would work very well. Kind of like re-bending something like an aluminum flat bar after it has been bent for the first time. It will always be weaker because of the initial failure. I would think that the only place that would have the equipment to do so might be a University lab, or a place where magnets are made. Considering what it might take to remagnatize your rotor, that you may just be safer to purchase a new rotor.
I too would just get a new rotor. Even if you did remagnetize the old one, it likely won't be as strong and will reduce the efficiency of the motor by a substantial margin; and every 1% can make a BIG difference in temperatures.
At about 3:15 they show how they make magnets for electric motors and the process of magnetizing the material. Not something you can do in your workshop or garage!