[Dick Linn]

So called "universal" motors can be powered from AC or DC. Let's say I have a motor rated for 120 volt (RMS), 60 Hz. AC operation. Would it need 120 volts DC for equal performance? A bit less perhaps?

The application here is running an angle grinder, such as used by welders. Sometimes it is more convenient to power the grinder off the output of a stick welder than to run an extension cord. These machines typically have somewhere around 70 volts maximum. We did note that the grinder ran noticeably slower on the welder AC setting vs. the DC.