Page 203 - Electric Drives and Electromechanical Systems

198 Electric Drives and Electromechanical Systems

FIG. 7.9 The principle of vector control. The values of I a or I m can be independently controlled by adjustment of
the magnitude of Is and the angle q.

FIG. 7.10 The outline of a vector controller.
The key element in any vector controller is to achieve this in real time as the motor’s
demanded and actual speed vary under the operational requirements. The requirements
of the drive package are summarised in Fig. 7.10, where I m and I constitute the speed
0
0
a
and torque demands to a vector controller; the output of this controller is the current
waveform demand to a conventional three phase inverter.
7.3.1 Vector control principles
In a vector controller, the magnitude and the phase of the supply currents must be
controlled in real time, in response to changes in both the speed and the torque
demands. In order to reduce this problem to its simplest form, extensive use is made of
conventional two-axis theory; by the selection of the correct reference frame, the
three-phase a.c. rotational problem found in an induction motor can be reduced to a
two-axis, stationary d.c. solution. Within the vector controller, the required motor
currents are computed with reference to the rotor’s frame of reference, while the three
phase motor currents are referenced to the stator’s frame of reference; to achieve this,
a set of transformations must be developed. If the supply to an induction motor is a