24-04-2012, 04:25 PM
VECTOR CONTROL OF THREE-PHASE INDUCTION MOTOR USING ARTIFICIAL INTELLIGENT TECHNIQUE
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INTRODUCTION
The design of fuzzy logic as the name suggests is unique in itself [1-3]. Depending on the speed and torque performances requirement of the three-phase induction motor for various industrial application fuzzy controller with a unique inference system is designed. The performances of PI and fuzzy controller are compared in [4] but it has considered only 15 rules for designing. It is considered that more the rules, better is the performance of the controller. The same has been projected in [5, 6]. Abad et al., in [5] have taken 49 rules for FLC and then compared the results with that of PI controller. [6] Has incorporated 21 rules and simulated the results. All the results obtained are for the step response of speed. Most of the papers have not included sudden changes in speed or load. The proposed FLC in this paper incorporates all the characteristics viz
VECTOR CONTROL OF INDUCTION MOTOR
Induction motor speed control methods are varied in number of which vector or field oriented control is the most widely accepted method. In vector control, the same performance characteristics are obtainable as is the case with a dc motor. This is achieved by decoupling the three-phase winding into two windings (90° apart) so as to facilitate independent control of torque and flux. A lot of literature is available in this regard. The model of the induction motor is a result of standard mathematical equations described in [7, 8]. The model incorporates the non-linear blocks of induction machine and inverter as used in real time mode. Figure-1 shows the complete model in which FLC has been introduced.
RESULTS AND DISCUSSIONS
Starting
Figures 4a and 4b show the starting characteristic using FLC and PI controller respectively. Since the motor is started on no load, the currents of the stator gradually picks up the values to full magnitude with reduced frequency as soon as the motor starts running, the speed attained by it is the ref. speed and full frequency of the currents are obtained. The electromechanical torque becomes zero, as the load torque is zero. The net flux both at stator and rotor remains constant.
Load application and load removal
Load application
When the load is suddenly applied at t = 0.12 seconds, the Tem i.e. actual torque which was following the reference torque earlier, now jumps to the value of load torque and settles to a magnitude of nearly 0.65. Since, torque and speed are related to each other, so the actual speed ωm too decreases when the load is applied.
CONCLUSIONS
The results obtained from the model using Fuzzy Logic Controller and PI Controller is compared. It can be concluded that use of Fuzzy logic improves and smoothens out the ripples in the motor torque and stator currents. It also facilitates in limiting the magnitude of the torque and current values within the specified range in any kind of disturbance, either provided by the speed removal, sudden application and removal of load torque. The responses obtained using PI controller are achieved by having very large values of gains of kp and ki which in practical applications are not feasible as large gains of the controller have repercussions like increase in cost and size.