[en] A method for monitoring a synchronous machine 110 includes injecting a narrowband sinusoidal signal at a first end 116A of a field winding 114 of the synchronous machine and monitoring a voltage at a second end 116B of the field winding with respect to ground. The method then identifies a resonant frequency from the monitored voltage, and generates a winding health indicator from the identified resonant frequency and an expected resonant frequency for a known good machine. The resonant frequency will change if the insulation is degraded. The frequency at which the phase difference between the monitored voltage and its corresponding current can also be measured and compared to the resonant frequency: they should be the same for a healthy winding. A ground fault, represented by equivalent circuit 130, can be located by applying the narrowband signal alternately at both terminals 116A, 116B and measuring at the opposite terminal.