tailieunhanh - Chapter 10: Sensorless drives

To begin chapter 10, basic issues in sensorless control of induction motors are outlined. We then present example flux and speed estimators and observers and describe parameter adaptation and self-conmiissioning pro-cedures. Finally, commercial ASDs with induction motors are reviewed. | 10 SENSORLESS DRIVES To begin Chapter 10 basic issues in sensorless control of induction motors are outlined. We then present example flux and speed estimators and observers and describe parameter adaptation and self-commissioning procedures. Finally commercial ASDs with induction motors are reviewed. ISSUES IN SENSORLESS CONTROL OF INDUCTION MOTORS A sensor of angular position of the rotor appears in several block diagrams of induction motor drives described in this book. Knowledge of this position is needed in positioning drives and drives with indirect field orientation. The same sensor can be employed to provide control feedback in speed-controlled drives the speed signal being usually obtained by differentiating the position signal. Direct speed sensors such as tachogenerators are also used in these systems. Some motors in high-performance ASDs are equipped with a built-in encoder hidden inside the motor frame. Motors for drives with direct field orientation may have flux sensors 177 178 CONTROL OF INDUCTION MOTORS embedded in the stator fron by the air-gap allowing precise determination of the air-gap flux vector and consequently that of the rotor flux vector. The position speed and flux sensors enhance operating characteristics of induction motor drives but they also increase the cost of the drive and spoil the inherent ruggedness of the induction motor. Therefore the so-called sensorless drives have been receiving a lot of attention. The adjective sensorless is actually a misnomer because no ASD can accurately be controlled without any sensors. Because flux sensors are rare in practice a more suitable name would be encoderless drives. As it is the sensorless drives are still equipped with voltage and cuưent sensors signals from which are used in control algorithms. These sensors are inexpensive and installed away from the motor usually within the power electronic circuitry. As already illustrated in several diagrams see for instance Figure .