Electrical Actuators: Applications and Performance

Chapter 6

Observer Gain Determination Based on Parameter Sensitivity Analysis ¹

6.1. Introduction

The major problem with the implementation of vector control in the induction motor involves controlling machine flux because of the problem in measuring this variable.

To avoid measuring the flux, it is possible to estimate it with the help of models using variables that are easier to acquire (stator current and voltage, mechanical speed). This large dependence on a model leads to major sensitivity problems and uncertainties for the control. These uncertainties are caused by the variations of stator and rotor resistances with temperature and skin effect and the variations of inductances with magnetic saturation.

Rotor resistance is the most difficult parameter to identify with precision, especially in the case of squirrel-cage machines, although it plays an important role in vector control. This parameter can vary by 100% with temperature.

Parameter uncertainties lead to errors of amplitude and flux orientation in the machine with the following consequences:

– the system can become unstable when the error of orientation becomes too large;

– an additional stator current is used to develop a given torque, increasing system losses.

In the first section, we suggest an overview of the principle of the two traditional rotor flux observers: a reduced order observer and a full order observer. The major problem linked to the development of a flux observer is the choice of observer gains that ...