Vector control in cage motors-underlying mechanisms

In any electric motor drive, good dynamic performance requires the motor to respond rapidly to demands for changes in the motor torque. In separately-excited d.c. machines, torque is directly proportional to armature current, and all that is required for rapid torque changes is a fast-acting armature current controller. This is not difficult to achieve because the armature inductance is modest. In the induction machine however the rotor currents have to be induced from the stator side and the relationship between torque and stator current is more complex. The aim of this paper is to explain briefly the conditions which have to be satisfied for an induction motor to produce step changes in torque without any of the unwelcome transient phenomena traditionally associated with such machines. The discussion avoids any reference to mathematical transformations