Sorry for the, ahem, late reply.

Arcs always give of quite a lot of RF radiation, but there is a difference between welding and an arc-gap oscillator, as was common during the early years of the 19th century and was kown as the Poulsen arc converter:

https://en.wikipedia.org/wiki/Arc_converter

Quite interesting to study, they use the negative resistance area of operation of an arc gap in order to sustain a continuous oscillation, which were later replaced by vacuum tube oscillators.

Negative resistance oscillators are still applied to day, for instance using the so-called "lambda diode":

https://en.wikipedia.org/wiki/Lambda_diode

This article was about the power supply, though, and not so much about the motor itself, although I do suspect RF oscillations of the motor coils may very well have been used. I don't know, that's just a guess.