Field Oriented Control (Vector Control) of 3 Phase Squirrel Cage Induction Motors

Abstract

Vector control, also described as field oriented control, is being increasingly used for the speed control of induction motors. This is because, with vector control, it is possible to achieve high dynamic (transient) performance, equalling that of the separately excited DC motor, in variable speed AC drives. Therefore, vector control makes it possible to use, in place of a DC motor, a 3 phase induction motor, which is smaller, mechanically more sturdy, less expensive, and does not have the commutator or other rubbing contacts, and is therefore free of associated maintenance requirements and sparking problems. Vector control, however, requires on-line computation of certain control variables using inputs from sensing elements which continually sense operating motor variables such as motor line currents, motor shaft position or speed. These computed control variables serve as feedback values, which are compared with the appropriate reference values. The errors are continuously corrected by appropriately delaying or advancing the switching instants of the static inverter, from which the motor phases are fed. With modern advances in microprocessors, these-on-line computations can be performed accurately, reliably and with adequate speed. This fact has contributed to the increasing popularity of vector control in high performance variable speed motor drives. However, the underlying concepts of vector control, as applied to the 3 phase cage type induction motor, are often difficult to present with clarity. A reason for this is possibly the difficulty to visualise the physical significance of the mathematical relationships employed, in arriving at the control quantities used in vector control. Often this difficulty is the result of the imperfect definitions of the related variables. Also, most basic text books develop the induction motor theory using the equivalent circuit approach. The commonly used equivalent circuit is convenient only for steady state operating conditions, that is, when the motor is fed from a fixed frequency sinusoidal supply and is spinning at a constant speed. The very purpose of using vector control is to achieve high motor performance under dynamic conditions, when the above conditions do not exist. Therefore an approach, biassed by excessive dependence on the motor equivalent circuit, is not very helpful in presenting the concepts of vector control. This paper is an attempt at the simplified presentation of the basic aspects of vector control, as applied to the 3 phase squirrel cage induction motor. An attempt is made to define in detail and in an exact manner, the variables used in the related mathematical theory. The assumptions used to simplify the theory are clearly stated. The concept of “space vectors” is perhaps the simplest means of presenting the principles of vector control. This concept is, therefore, first explained and used extensively in subsequent analysis. In presenting the practical implementation techniques, the circuits which perform the related on-line computations and control are represented as functional circuit blocks. The hardware design of these circuit blocks is, of course necessary for building the actual control equipment. But this is viewed as an equipment design exercise and is, therefore, left outside the scope of this paper. We limit our treatment to a step by step development of the theory of vector control and to a typical implementation of it, using electronic functional circuit blocks.

Indexing terms:

• Field oriented control
• Squirrel cage induction motor