A controller (500) in an electric drive system is used to determine the operating condition of array of rotating diodes (302B) within a generator (204). During operation, the controller (500) receives a set of generator inputs, a set of machine parameters, and a set of diagnostic values. Next, the controller (500) determines a set of expected operating values corresponding to the set of diagnostic values. The controller (500) then compares the obtained diagnostic values to one or more corresponding expected operating values. If the difference is greater than a threshold, the controller (500) provides a fault indication that one or more of the array of rotating diodes (302B) has malfunctioned.