Page 208 - Electric Drives and Electromechanical Systems

Chapter 7 Induction motors 203

torque-producing component of the stator current. The flux component of the stator
current is normally maintained at a constant value, unless field weakening is required.
The current demands are transformed to the stationary reference frame by the appli-
cation of Eqs. (7.5) and (7.13), and this is followed by two to three phase transformation
to determine the three-phase supply currents. The current demand is used to control a
conventional three-phase power bridge, through the use of a local current loop. Vector
controllers can be used with a number of different power stages, depending on the
application; and there is a case for the use of voltage-sourced transistors or MOSFET
inverter bridges because of their fast-current control. Current-sourced inverters or the
cycloconverters are considered to be suitable for high-power, low-speed applications.
The system described above is a closed-loop system, because of the presence of a rotor
encoder. Rotor encoders are required for vector controllers used in those servo
applications which require fine control of speed or position down to and through
standstill. Recently, a number of manufacturers have developed open-loop vector
controllers as replacements for the more conventional variable-frequency inverters. This
approach is based on an accurate knowledge of the motor and the system being
controlled. The positions of the rotational fields are computed from a knowledge of the
supply waveform; the electrical-axis position, of course, rotates at the synchronous
speed determined by the supply frequency. In order to give these drives additional
flexibility, they are being designed with systems that will measure the motor’s
characteristics prior to the initial powering up as part of the commissioning process. This
is achieved by the use of accurate current measurement and voltage injection
techniques, together with the monitoring of the dynamic performance to determine the
system variables. Even with these additions, the performance of open-loop vector
controllers is unlikely to compete with closed-loop systems in servo applications, but
they will prove attractive as replacements for high-performance inverter drives.

7.3.4 Sensorless vector control

Currently there is considerable interest in the development of sensorless induction
motor controllers. It should be noted that this term is somewhat incorrect e a sensorless
controller removes the need for a position or velocity transducer to be located on the
motor, and these are effectively replaced by processing the outputs current and voltage
sensors to derive the same information. The advantage of this approach are reduced
hardware complexity, lower cost and reduction in the size of the induction motor
package. The elimination of the sensor cable leads to better noise immunity and
increased reliability for example, operating an induction motor at the end of a well string
to pump oil to the surface, is an ideal application for a sensorless vector-controlled
induction motor.
As has been previously discussed, the vector control of an induction motor requires
the estimation of the magnitude and location of the magnetic flux vectors in the
machine. For a sensorless controller, using either open or closed loop estimators,