If it's a 25Hz motor, I'd hazard a guess that this is very old. In that capacity, a VFD may not be a great choice. They do put extra stresses on the motor windings, both thermally and electrically from the standpoint of voltage spikes. I wouldn't worry too much about the thermal issues if you plan to run it at the design speed of 25Hz. If it is old as I suspect, that motor probably has massive amounts of iron by comparison to modern motors and can likely absorb and dissipate a lot of heat. But that does nothing for you when it comes to the high voltage impulses that can damage the winding insulation. There is a phenomenon called "standing wave generation" in VFD output power circuits that can cause pulses of extra voltage, 1.e. 2.5-3X nominal, and the winding insulation material was not likely designed to deal with that. You can take measures to mitigate this, and you can consider doing it anyway and then having the motor re-wound at a convenient time later rather than scrap it and start over, many people approach it that way. But you need to know what you are getting yourself in to, you don't want any midnight surprises.
Over speeding however comes with a host of other considerations, chief among which are the fact that once you go over base motor speed, the motor power remains constant and since speed is increasing, shaft torque goes down. At the same time if it is a centrifugal load like a pump, the POWER requirement will increase at the cube of the speed change. So at 50Hz, a centrifugal pump will need 8X the kW/HP as it does at 25Hz, yet your motor kW/HP will remain the same. You get in the weeds very fast with that. If your application is not a centrifugal load, then you still need to consider the loss of torque, and the speed rating of the bearings in the motor. it might not be worth it.
"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
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