New techniques for the detection of faults in closed- loop drives have been recently developed that need to monitor both motor voltages and currents. This paper investigates the diagnosis of rotor asymmetries in field oriented controlled induction machines based on the measured currents and speed. First, an analysis to elucidate how some characteristic features of the control affect the fault propagation is made. In particular, the influence of the outer and the inner control loops, the EMF and cross coupling terms, the estimated angle of orientation and the resolution of the measured rotor angular position are investigated. Then, different scenarios are designed by means of simulations and experiments, and the limitations of using merely currents for the diagnosis are established. Second, an indicator calculated online from the torque producing current and the orientation angle is used to corroborate the conclusions derived from the previous analysis.