The aim of the paper is to present a new prospective method for on-line monitoring of partial discharge (PD) in large AC motors. The principle of this system consists to measure the weak magnetic high frequency field, due to PD, in the space between the magnetic core and the machine external frame of large generators. Special sensors, trimmed to the machine winding resonances, may perform such field measurements. With several sensors, it will be possible to localise PD activity corresponding to insulation weaknesses.