Static Phase Converter in Tools and Tool Making Posted 21 minutes ago · Edited 21 minutes ago by timgunn That looks old. I would not try running it on a VFD unless a Sine-Wave filter was used in the circuit. What follows is "as I understand things": somewhat oversimplified and perhaps not technically accurate, but close enough for me to get my head around. YMMV and it is worth researching things further if you are intending to go that way. Old motors often used a brittle Shellac-based impregnation material for the windings. I think some of the early synthetic impregnation materials were also rather brittle. Later polymeric resins are much "tougher". My rather limited experience is that post-1980-ish motors are no problem with VFDs. Pre-1960-ish motors are a problem and between 1960 and 1980, things are unclear. On a true sine wave supply (mains), the voltage rise is quite slow. As the current rises, the magnetic fields around each of the coils rise and interact with the other coils in the winding. I think they cause the windings to move towards each other, compressing the insulation, with each peak (positive and negative), causing the characteristic transformer hum tone. VFDs use PWM to switch a DC Voltage on and off very fast (most modern VFDs switch at 4 kHz and up), giving an almost instantaneous Voltage (and current) rise time. The force exerted is dependent on the rate-of-change-of-Current, so the mechanical forces on the insulation are very much higher than when run from "real" mains power. On a motor with brittle insulation, the forces are higher than the insulation can withstand and tiny cracks form. The upshot is that old motors tend to fail very quickly when run from VFDs. There are some other factors that come into play as well, but I can get my head around the one I've described and it has been sufficient to keep me from going down that particular rabbit-hole. A Sine-Wave filter can be fitted between the VFD and the motor to smooth out the high-frequency stuff and leave a sinusoidal waveform very similar to "true" mains (though still with the Variable Frequency that we want for speed control). These will often let you run an old motor on a VFD. Sinusoidal filters typically seem to cost "about" as much as the VFD with which they are used, so they are not cheap. If an old motor has been rewound, it will have been done with the materials used at the time, so a 1935 motor rewound in 1995 can be treated as a 1995 motor (unless it was specifically rewound with 1935 materials as a museum piece, for example).