Basic Operation of AC Induction Motors (4)

"Field Oriented" Control

In order to obtain even better yet control of AC motor torque, adjustable frequency controls often can make use of a regulation scheme known as "field-oriented" or "vector" control. This technique is intended to control the motor flux, and thereby be able to decompose the AC motor current into "flux producing" and "torque producing" components. These current components can be treated separately (in the control), then recombined to create the actual motor phase currents. This results in a solution to the boost adjustment problem, plus provides much better control of the motor torque - which allows much higher dynamic performance.

One way of looking at field oriented control is that the inverter would like to be able to have the same sort of simple, direct control of both flux and torque that is enjoyed with separately-excited dc motors. With dc motors, the flux level is controlled by simply regulating the field current, while the torque is controlled by regulating the armature current. By using field oriented control, the inverter can treat the ac induction motor as if it had the same sort of independently regulated flux and torque characteristic. When the actual induction motor phase currents are decomposed into flux and torque producing components (in the control, not in the motor), this gives the opportunity to “decouple” these two and achieve better system performance as a result.
In order to accomplish field-oriented control, the controller needs to have an accurate “model” of the motor. Over the last several years a large number of different schemes have been proposed to accomplish the “flux and torque control” desired. Many provide this control without the use of a speed feedback (tachometer) signal. These are typically referred to by the generic term of “sensorless” vector control. Many of today’s techniques also involve some sort of self-tuning at startup in order to obtain information which helps to more accurately model the motor – and thereby produce more optimal control. In addition, there are also techniques by which the models can adaptively adjust to changing conditions, such as the motor temperature going from cold to warm (which impacts the slip).