We analyze why low-speed sensorless control of the IM is intrinsically
difficult, and what is gained by signal injection. The explanation relies on
the control-theoretic concept of observability applied to a general model of
the saturated IM. We show that the IM is not observable when the stator speed
is zero in the absence of signal injection, but that observability is restored
thanks to signal injection and magnetic saturation. The analysis also reveals
that existing sensorless algorithms based on signal injection may perform
poorly for some IMs under particular operating conditions. The approach is
illustrated by simulations and experimental data