Vector Control of AC Induction Motors

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Introduction

Scalar control of ac drives produces good steady state performance but poor dynamic response. This manifests itself in the deviation of air gap flux linkages from their set values. This variation occurs in both magnitude and phase.

Vector control (or field oriented control) offers more precise control of ac motors compared to scalar control. They are therefore used in high performance drives where oscillations in air gap flux linkages are intolerable, e.g. robotic actuators, centrifuges, servos, etc.


Torque Control of DC Motors

There is a close parallel between torque control of a dc motor and vector control of an ac motor. It is therefore useful to review torque control of a dc motor before studying vector control of an ac motor.

Torque Control of DC Motors

A dc motor has a stationary field structure (windings or permanent magnets) and a rotating armature winding supplied by a commutator and brushes.

Equivalent Circuit of Induction Motor
Since the rotor leakage inductance has been neglected, the rotor flux = , the air gap flux.