SUMMARY (Induction Motor)

7.7
Field orientation, consisting in the alignment of a revolving reference frame with a space vector of selected flux, allows the induction motor to emulate the separately excited dc machine. In this machine, the magnetic field and developed torque can be controlled independently. In addition, the torque is produced under the optimal condition of orthogonality of the flux and current vectors, resulting in the maximum possible torque-per-ampere ratio.
In ASDs with direct field orientation along the rotor flux vector, \, this vector is determined from direct measurements or estimations of the air-gap flux. The indirect field orientation is based on calculation of the angular position, ©p of A,, as a sum of an integral, O*, of the rotor frequency, a>*, required for the field orientation and the rotor angular displacement, 0O, of the equivalent two-pole motor.
Stator flux orientation has the advantage of employing the easily determinable stator flux vector. Disadvantages include the necessity of a decoupling system making the flux independent of the torque-producing current, as well as the “torque-per-flux-squared” limiting condition on the reference values of torque and stator flux. The field-orientation scheme using the air-gap flux vector is similar to that with the stator flux vector.
Voltage source inverters with closed-loop current control are employed in most field-oriented drives. In ASDs with current source inverters, the required space vector of the stator current is generated by simultaneous adjustment of the dc-link current and inverter state selection.