An Aid for Teaching Field Oriented Control Applied to Induction Machines

During the past six years the method of Field Oriented or Vector Control (FOC) has become accepted as a means of enabling ac machines, in particular the Squirrel Cage Induction Machine (SCIM), to realize a dynamic performance equal or superior to that of the dc machine. However, whilst the method of FOC and its application has been successful, it is a complex method of control and difficult to understand unless one has a thorough understanding of the internal dynamic behaviour of the SCIM. Such an understanding is necessary in order to understand how the method of FOC decouples the internal dynamic structure of the SCIM to enable a high dynamic performance to be realized. The objective of FOC is to establish and maintain an explicit angular relationship (or ORIENTATION) between the stator current vector and the rotor flux (or FIELD) that is established within the air-gap of the SCIM. This explicit angular relationship may be achieved by regulating the slip of the machine to a particular value which causes the rotor flux vector to become aligned with the d-axis component of the stator current vector. FOC is therefore merely trying to fix the ORIENTATION (angle cx in Fig. 1) between the stator current vector and the rotor flux vector or FIELD of the machine; hence the name FIELD ORIENTED CONTROL. Most, if not all the published material on FOC appears in the form of journal papers or conference proceedings.