8.2 Sensorless Control of Induction Motor
Two basic approaches are used for sensorless control. The first includes methods that model the induction motor by its state equations . A sinusoidal magnetic field in the air gap is assumed. The models are either implemented as open-loop structures, like the stator model [1,11], or as closed-loop models, like adaptive observers [5,17]. The adaptive flux observers are now receiving considerable attention and many achieving new solutions because of their high precision and relative robustness against machine parameter deviation [1,10].
Open loop models and even adaptive observers have stability limits at very low stator frequencies. The rotor induced voltage at such operating points is then zero or close to zero, which renders the induction motor an unobservable system.
The basic limitation for sensorless operation is the DC offset components in the stator current and voltage acquisition channels at very low speeds . At lower speeds, voltage distortions caused by the PWM inverter become significant.
The second approach used for low speed sensorless operation is the signal injection technique . Carrier injection methods for sensorless control are sophisticated and the design must match the properties of the motor [1,4,18,41]. This makes the method unfeasible for practical application, so will not be discussed in this book.
Eliminating the speed sensor from the drive system requires estimation of the state variables, for example, ...