22-06-2012, 04:24 PM
A HYBRID SYSTEM BASED APPROACH TO DIRECT TORQUE CONTROL (DTC) OFINDUCTION MOTORS
ABSTRACT
In this paper, we study the Direct Torque Control (DTC) of an Induction Motor coupled to an Inverter (Inv- IM). DTC permits to control directly the stator flux and the torque by selecting the appropriate inverter state. DTC has been introduced because it presents several advantages in comparison to other techniques such as voltage/frequency control, vector control and field control. In this paper, we first model the DTC of Inv-IM as a hybrid system (HS). Then, we abstract the continuous dynamics of the HS in terms of discrete events. We thus obtain a discrete event model of the HS. And finally, we use Supervisory Control Theory of DES to drive Inv-IM to a desired working point.
The technique of Direct Torque Control (DTC), permits to control directly the stator flux and the torque by using an appropriate voltage vector selected in a look-up table. The main advantages of DTC are a minimal torque response time and the absence of: coordinate-transform, voltage modulator block, controllers such as PID for flux and torque. For these advantages, DTC is the control method adopted in this paper. The induction motor is a continuous system because its behavior is modeled by algebraic and differential equations on two continuous variables: the stator flux and the electromagnetic torque.
For conciseness, by flux we mean stator flux, and by torque we mean electromagnetic torque. With DTC, the voltage vector generated by the inverter is applied to the IM to control the flux and the torque. We propose two approaches to solve the no determinism of the 6-zone division. The first approach is based on the observation that the no determinism occurs when is in a zone while one of the control vector is applied. we consider uniquely the 6- state automaton because it reduces the state space explosion which is inherent to the use of automata.
An important study in SCT is to synthesize a supervisor Sup when the plant and the specification are given and defined by two FSA P and S, respectively. Sup observes the evolution of P (i.e., the events executed by the plant) and permits only the event sequences accepted by S. We have applied the synthesis procedure of the software tool TTCT. With TTCT, forcible events can be forced to occur before an event tick that models the passing of one
Time unit. To be able to use TTCT, we have adapted P and S by preceding every UN forcible event by the event tick. The solution synthesized provides several possible scenarios of control. The objective of the control is to drive Inv-IM into the set of regions and then to force it to remain in this set. Here, we have performed three major tasks.. The HS is thus modeled as a DES. The advantage of this abstraction is that all the rigorous analysis and design methods for DES can be applied for studying DTC and finally the use of Supervisory Control Theory (SCT) of DES to drive Inv-IM to a desired working point.