The maintenance of electric drives is essential to their efficient and safe operation in any industry. The lack of an effectively designed and implemented maintenance policy will result in increased downtime, increased capital losses from catastrophic failures, and in some cases, a reduced level of personnel safety. Traditional approaches to maintenance include statistical-replacement policies and preventive-maintenance testing. The relative success or failure of these approaches is dependent on the specific industrial setting. However both have inherent limitations. For example, replacement policies tend to increase costs because motors are often taken out of service while still in good condition. Off-line testing procedures, such as insulation-resistance measurements, have limited value since they can be difficult to interpret and are applicable to the detection of a limited number of failure modes. On-line and continuous monitoring methods offer significant advantages in this regard, since incipient failures can be detected, and appropriate maintenance actions can be scheduled, thereby avoiding the costs and problems associated with unexpected failures. Thus far, research in this area has focused on sensing magnetic flux, stator current, rotor current, shaft flux, ground fault current, vibrations, temperature, and speed fluctuations. The first five methods are based on sensing electrical or magnetic parameters, and the remaining are based on monitoring mechanical quantities. A classical type of current transformer is found very effective in sensing the unbalanced currents in the three-phase power system where highresistance grounding is used, such as in an underground coal mine.