Improvements in direct torque control of induction motors

Author

Arias Pujol, Antoni

Director

Romeral Martínez, José Luis

Date of defense

2001-03-01

ISBN

8469957406

Legal Deposit

B.37830-2001



Department/Institute

Universitat Politècnica de Catalunya. Departament d'Enginyeria Electrònica

Abstract

This thesis is mainly devoted to the investigation of speed control methods for three phase cage induction motors with particular emphasis being given to Direct Torque Control (DTC) improved techniques.<br/>Classical Direct Torque Control has inherent disadvantages such as: problems during starting resulting from the null states, the compulsory requirement of torque and flux estimators, and torque ripple. In the classical DTC induction motor drive a voltage vector is applied for the entire period, and this causes the stator current and electromagnetic torque exceeds its reference value early during the cycle, causing a high torque ripple. Switching cycles then follows this, in which the zero switching vectors are applied in order to reduce the electromagnetic torque to reference value. This thesis suggests a technique based on applying to the inverter the selected active states just enough time to achieve the torque and flux references values. The rest of the switching period a null state is selected which won't almost change both the torque and the flux. Therefore, a duty ratio has to be determined each switching time. By means of varying the duty ratio between its extreme values (0 up to 1) it is possible to apply any voltage to the motor. The optimum duty ratio per sampling period is a non-linear function of the electromagnetic torque error, the stator flux position and the working point, which is determined by the motor speed and the electromagnetic torque. It is obvious that it is extremely difficult to model such an expression since it is a different non-linear function per working point. Therefore, this thesis is focused on performing a fuzzy-logic-based duty-ratio controller, where the optimum duty ratio is determined every switching period. Additionally, this Fuzzy Controller is adaptive and may be applied to any induction motor.<br/>A stator flux reference optimum controller is also designed, which not only helps to achieve a smaller torque ripple, but also reduces the reactive power consumption of the drive taken from the main supply. This is achieved by changing the stator flux reference value with reference being made to the correspondent torque reference value. Therefore, the stator flux reference value chosen is to be just of sufficient value to produce the desired torque <br/>Simulated results are shown in order to compare the classical DTC and the Fuzzy Logic based DTC.<br/>The control algorithms have been implemented on a PC/DSP based board that facilitates the use of parallelism in software design. A 1.5kW, three-phase induction motor drive has been designed and experimental data obtained from it in order to verify the results achieved by simulation.

Keywords

Corrent altern; Rotors; Màquines elèctriques; Mecànica; Motors d'inducció; Torsió; Direct Torque Control; Control automàtic; DTC

Subjects

621 - Mechanical engineering in general. Nuclear technology. Electrical engineering. Machinery

Knowledge Area

3307. Tecnologia electrònica - 3311. Tecnologia de la instrumentació

Documents

01Front_page.PDF

11.73Kb

02Index.PDF

12.50Kb

03Acknowledgements.PDF

7.060Kb

04Symbols.PDF

10.75Kb

05Introduction.PDF

27.40Kb

06Chapter1.PDF

109.7Kb

07Chapter2.PDF

92.31Kb

08Chapter3.PDF

837.8Kb

09Chapter4.PDF

82.18Kb

10Chapter5.PDF

160.7Kb

11Chapter6.PDF

24.28Kb

12References.PDF

19.21Kb

13Appendixes.PDF

562.4Kb

 

Rights

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