Page 209 - Electric Drives and Electromechanical Systems

204 Electric Drives and Electromechanical Systems

the complexity of the algorithms determines the performance of the complete motor-
drive system. The elimination of the speed sensor is of particular interest as the
mechanical speed is different from the speed of the rotating flux, as shown in Eq. (7.2).
A number of possible solution have been considered as discussed by Holtz (2002), which
provides an overview of a number of available techniques.
In order to illustrate the principles of sensorless vector control we can consider an
approach based on MRAC (model-reference adaptive control). As discussed in
Section 7.3.2 the current vector has to be determined with reference to a specific
coordinate frame that is moving in space. In a MRAC approach the controller contains a
model of the machine that is capable of estimating the machine parameters from the
motor’s line current and voltages.
Fig. 7.15 shows the principles of a MRAC based controller. The system consists of
three elements, a model of the motor, a controller and a conventional current controlled
inverter. The model relies on the principle that the flux in the machine can be computed
from both the stator and rotor model e in this case the stator model is used as the
reference. The rotor model estimates the rotor flux from the measured current and b u or
tuning signal. The tuning signal is obtained from a comparison of the flux generated
from the stator and rotor models and is used in a closed loop to adjust the rotor model.
The model provides the estimated speed, and hence the speed error, and rotor flux that
are used by the controller to generate the current demand for the inverter. In practice
this approach to sensorless control can satisfactorily control the motor’ speed almost to
standstill.

FIG. 7.15 A sensorless controller for an induction motor based on the use of a MRAS, the speed demand is u d ,
and the controller is based on a modified version of the architecture shown in Fig. 7.14.