Until recently, power plant operators have had very limited options for on-line monitoring of the health of motor and generator rotors, particularly those with brushless excitation systems. Degradation of field winding insulation can lead to ground faults or shorted turns, with little or no warning. Moreover, the capability to monitor local hot spot temperatures on rotor windings in service has not been available even though monitoring of stator windings with Resistance Temperature Detectors (RTDs) has always been commonplace.