I INTRODUCTION
Vector control is a popular closed loop control scheme
that is applied to ac motor control in order to achieve a
fast dynamic response and accurate speed or torque
control. For a properly tuned vector control drive, the
dynamic performance is comparable to that of a
conventional high performance dc drive. In the vector
control scheme, the three phase currents are transformed,
using Park's transformation equations, into a magnetic
flux producing component (equivalent to the field current
of a dc machine) and a torque-producing component
(equivalent to the armature current of a dc machine) [2].
By keeping the flux current constant and varying only the
torque current, the ac machine can be controlled exactly
like an armature voltage controlled separately excited dc
motor. The variation of the torque current alone will vary
both the magnitude and the orientation angle of the
rotating current vector in the ac machine.
Conventional vector control scheme uses a unit vector
[3] to achieve an accurate transformation so that the flux
current aligns with the magnetic flux. It also uses the
mathematical model of the induction motor in which the
rotor parameters, rotor resistance and rotor inductance,
plays a very important role. However, both parameters
vary with operating conditions and therefore it is difficult
for conventional vector drive to maintain consistent
performance under all operating conditions due to
machine parameter variations.