It is unavoidable that some high-frequency measurement noise n will add in the speed sensing, giving a measured speed ωmeas = ωm + n.
Today, many drives are sensorless, i.e., ωm is estimated rather than measured (giving an estimate ˆωm), using variables available in the converter control algorithms. For both IMs and PMSMs, sensorless speed estimation can be made with high bandwidth and accuracy, except perhaps in the low-speed region (typically, |ωm| < 0.1ωnom) [37]. As long as operation outside the low-speed region is of primary interest, it is not necessary to treat sensorless drives separately; it can be assumed that ˆωm = ωm. However, it is not unusual for the measurement noise n to be stronger than that for a comparable sensored drive, because of voltage and current harmonics. For IMs, n also often has a small steady-state bias due to an inaccurate model rotor resistance [38].