Abstract:
The induction motor is superior to the dc motor with respect to size, weight, rotor inertia, maximum speed capability efficiency, and cost. However, the case of control of dc motor cannot be equated because the induction motor has a nonlinear control structure, whereas the separately excited dc motor has, a decoupled control structure with independent control of flux and torque. This report describes the technique of field oriented control or vector control which can be used with both induction and synchronous machines and essentially transforms the dynamic structure of the ac machines into that of a separately excited compensated dc motor. As a result, the induction motor drive can achieve four-quadrant operation with a fast torque response and good performance down to zero. speed.
In a dc motor the armature mmf axis is established at 90 degrees electrical to the main field axis. This orthogonal or perpendicular relationship exists between the field flux and armature mmf axis. Hence in a separately excited do motor with a constant flux value, torque is directly proportional to armature current. Direct control of armature current gives direct control of motor torque and fast response; because motor torque. can be altered as rapidly as armature current can be altered.
The principle of field orientation originated in the work of Hasse and Blaschke. A variety of techniques are now developed but these techniques can be broadly classified into two groups: direct control and indirect control. The classification is based on the method used to determine the rotor flux vector. Direct field oriented control, as originally suggested by Blaschke, determines the magnitude and position of the rotor flux vector by direct flux measurement or by computation based on terminal conditions. Indirect flux orientation as suggested by Hasse, requires a high-resolution rotor position sensor to determine the rotor flux position.
