Visualising vector control in cage motors

The principal aim of vector control of cage motors is to obtain excellent dynamic torque response. Ideally, it should offer step changes in torque: in practice, very rapid changes are obtained, comparable with the best obtainable from an armature-controlled DC machine of similar size. This paper aims to assist the understanding of the basic mechanism of vector control, by asking how the stator currents must be controlled to achieve step (or at least extremely rapid) changes in torque. It begins by stating some simple but important underlying facts, recognition of which helps subsequent understanding. This is followed by a discussion of the rotor flux-linkage triangle, which encapsulates all the essential elements necessary for a basic understanding. Finally, scalar and vector control are compared with the help of polar flux-linkage trajectories, which illuminate the mechanism of vector control