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Chapter 7: Field orientation

This chapter begins with a review of conditions of production and control of torque in the dc motor. Principles of the field orientation in the induction motor are introduced, and the direct and indirect field orientation schemes utilizing the rotor flux vector are presented. Stator and air-gap flux orienta-tion systems are described, and we finish with an explanation of the control of stator current in field-oriented motors fed from current source inverters. | 7 FIELD ORIENTATION This chapter begins with a review of conditions of production and control of torque in the de motor. Principles of the field orientation in the induction motor are introduced and the direct and indirect field orientation schemes utilizing the rotor flux vector are presented. Stator and air-gap flux orientation systems are described and we finish with an explanation of the control of stator current in field-oriented motors fed from current source inverters. 7.1 TORQUE PRODUCTION AND CONTROL IN THE DC MOTOR The concept of field orientation proposed by Hasse in 1969 and Blaschke in 1972 constitutes arguably the most important paradigm in the theory and practice of control of induction motors. In essence the objective of field orientation is to make the induction motor emulate the separately excited de machine as a source of adjustable torque. Therefore we will first review fundamentals of torque production and control in the de motor. 119 120 CONTROL OF INDUCTION MOTORS A simplified representation of the de motor is shown in Figure 7.1. The pair of magnetic poles N and s represent the magnetic cữcuit of stator that is the field part of the machine. Therefore the space vector Xf of flux flux linkage generated by the field winding is stationary and aligned with the d axis of the stator. Thanks to the action of commutator not shown and properly positioned brushes the distribution of armature current in the rotor winding is such that the space vector ia of this current is always aligned with the q axis even though the rotor is revolving. Practical de motors are equipped with auxiliary windings designated to neutralize the so-called armature reaction that is weakening of the main magnetic field by the MMF produced by the armature current. Then the developed torque TM is proportional to the vector product of ia and Xf that is to the sine of angle between these vectors. As seen in Figure 7.1 this is always a right angle which ensures the highest .