Variable speed drives (VSD), also called variable frequency drives, are becoming more and more common. They enable efficiency improvements in some processes [1, 2] and enable a soft start of the motor which reduces the stress on the windings caused by inrush currents. In these drives, the 50/60 Hz power frequency voltage is rectified to dc and then power electronics creates an output voltage and current of variable frequency (the fundamental frequency). The speed of a squirrel cage induction motor is directly proportional to the fundamental frequency. There are several types of drives. The most popular type at the present time is the voltage-source, pulse-width-modulation (VS-PWM) drive, since this type tends to have a smaller footprint and is usually less In conventional 50 or 60 Hz motors, it is common to monitor the partial discharge (PD) activity in the stator winding insulation system in critical motors rated 6 kV and above [4-7]. PD are small electrical sparks that occur within the stator winding as a symptom, or as a cause, of several aging processes (see below). Some PD is expected in stators rated 3 kV and above, and this PD can be tolerated due to the presence of mica within the insulation system. However, increasing PD activity over time is an indication that the motor stator winding insulation has weakened and may eventually fail, and thus, maintenance intervention would be prudent. Each partial discharge creates a current pulse that can be detected at the machine terminals [4, 5, 7]. When measuring PD during normal operation of the motor, the PD pulses can often be obscured by similar current pulses from other sources such as power system corona, power tool operation, electrostatic precipitators and/or power system PD sources. An important aspect of on-line PD detection is separating this electrical interference from the stator PD, in order to reduce the risk of false indications of stator insulation problems [4, 5, 7].