This paper studies the performance limits of carrier-signal-injection-based sensorless control methods. When a high-frequency carrier-signal voltage is injected into a salient ac machine, two types of signals are created that can be used to estimate the rotor position: 1) the negative-sequence carrier-signal current and 2) the zero-sequence carrier-signal components. The limitations of ac machine sensorless control using each of these signals are analyzed in this paper. The analysis focuses on three key performance metrics: 1) the accuracy; 2) the bandwidth; and 3) the stability of the estimated position.