Advances in motor and generator rotor health

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. In the past decade, digital rotor telemetry technology has come of age, offering new options for rotor condition monitoring on motors and generators. This technology involves placing electronic modules on rotors that perform direct measurements of electrical signals and sensor inputs, digitizing the information, and using wireless technology to pass the data off the rotor. This allows continuous monitoring and long term trending of field voltage and current, insulation resistance to ground, average winding temperatures and hot spot temperatures. On brushless machines, it detects ground faults, issuing an immediate alarm when a ground occurs. This paper discusses application of this new class of rotor condition monitoring options using digital rotor telemetry technology to continuously monitor the health of rotor insulation systems in large motors and generators.