Local weak observability conditions of sensorless AC drives
  Alternating current (AC) electrical drive control without mechanical sensors is an active research topic. This paper studies the observability of both induction machine and synchronous machine sensorless drives. Observer-based sensorless techniques are known for their deteriorated performance in some operating conditions. An observability analysis of the machines helps understanding (and improving) the observer's behavior in the aforementioned conditions. 
Mohamad Koteich, Abdelmalek Maloum, Gilles Duc, Guillaume Sandou
math.CA