We have a synchronous motor 3250 kW with brushless exciter with rotating diodes. The power resistor ~4 ohmi from rotating part must be replaced. I don't know the manufacturer. Anybody know a manufacturer of such power resistors fitted on the rotating part?
You better approach the motor manufacturer.
Normally, these resistors are custom made in the works of motor manufacturer or in the works of their sub-supplier (complying to the specs of the motor manufacturer).
By power resistor, do you mean the surge arrester in the rotating rectifier assembly? If yes, its resistance is much more than 4 ohms. And the OEM might the only source for a new arrester.
There could be manufacturers that could do it but not have it listed as a product line since it's an OEM part. You could also take the old one (or enough dimensions of it) to some power resistor manufacturers and see if one can make a duplicate.