Embodiments of the disclosure are directed at determining rotor deterioration in a dynamoelectric machine. Various current and voltage measurements are carried out upon an exciter assembly when the dynamoelectric machine is operated at various speeds and at various occasions over an extended period of time. More particularly, a set of current and voltage measurements that are carried out when the dynamoelectric machine is in a slow-roll mode of operation can be used for determining a resistance of a current path through which the exciter assembly provides electrical power to a rotor assembly. A time-domain-to-frequency-domain conversion can be carried out upon this set of resistance measurements in order to generate a spectral signature. The spectral signature can then be evaluated to detect an altered rotor resistance condition and/or a rotor resistance alteration trend.