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ABSTRACT

This paper presents a modified fuzzy control for speed control of

induction motor (IM). At first, the PI controller is investigated

for speed control of Induction Motor, and then fuzzy logic

controller performance is simulated. Induction Motor

performance is checked through the simulation studies in

MATLAB/SIMULINK environment. Hybridization of fuzzy

logic (FL) and PI controller for the speed control of given motor

is also performed to remove the disadvantages of FL controller

(steady-state error) and PI controller (overshoot and

undershoot). According to the simulation results, hybrid

controller creates better performance in terms of rice time,

overshoot, undershoot and settling time.

INTRODUCTION

Traditionally, dc motors were used for precise wide range speed

control. Nowadays with progress in power electronic industry

and development of inexpensive convertors and many

advantages of ac motors than dc motors, use of ac motors are

usual in electrical drives. Some of these advantages are: lack of

commutator, the reduced maintenance costs, less volume and

weight and consequently lower cost. In addition, induction

motors are robust and have better performance in high speed and

torque.

In recent years, the control and estimation of induction motor

drives is an active research area, and the technology has further

advances in this field. Induction motor drives, especially squirrel

cage rotor-type, have been the workhorses in industry for

variable-speed applications in a wide power range that covers

from fractional horsepower to multi-megawatts.

Generally, the control and estimation of ac drives are

significantly more complex than those of dc drives, and this

complexity increases to a large extent if high performances are

demanded. The need of variable-frequency, harmonically

optimum converter power supplies, the complex dynamics of ac

machines, machine parameter variations, and the difficulties of

processing feedback signals in the presence of harmonics create

this complexity.

Induction motor can be controlled like a separately excited dc

motor, brought a great improvement in the high-performance

control of ac drives especially with the invention of vector

control in the beginning of 1970s. Because of dc machine-like performance, vector control is also known as decoupling,

orthogonal, or transvector control. The vector control and the

corresponding feedback signal processing, particularly for

modern sensorless vector control, are complex and the use of

powerful microcomputer or DSP is necessary. Because of major

advantages of vector control, this method of control will oust

scalar control, and will be accepted as the industry-standard

control for ac drives.

PI controllers are widely used in different industries for control

of different plants and have a reasonable performance. This

performance, however, may not be desirable for some

applications such as ac drive control. Therefore it is essential to

use a more advance controller in these cases.

PI controller can never achieve perfect control, that is, keep the

speed of induction motor continuously at the desired set point

value in the presence of disturbance or set point changes.

Therefore, we need an advance control technique such as fuzzy

logic controller for this goal.

Nowadays, fuzzy systems are applied in wide range of academic

and industrial fields such as modeling and control, signal

possessing, medicine, and etc. An important Fuzzy Logic

application is finding a new solution for control problems that

will be discuses later. The present paper discusses a Fuzzy Logic

Based Intelligent controller. A Fuzzy Logic Controller (FLC)

does not need complex mathematical algorithms and is based on

the IF_THEN linguistic rules (Rajesh Kumar et al, 2008).

In this article we first introduce electrical and mechanical

modeling of an induction motor. Then we will explain the block

diagram of the indirect vector control. In the section 4 we will

discuss the PI, fuzzy logic and hybrid controller, respectively.

Finally we will present the simulation results and a brief

discussion.

2. INDUCTION MOTOR MODELING

The electrical part of an induction motor is represented with a

fourth-order state-space model and the mechanical part with a

second-order system. All electrical parameters and variables are

referred to the stator. This is indicated by the prime symbols in

the machine Equations 1 and 2 for electrical and mechanical

systems. All rotor and stator quantities are in the arbitrary twoaxis

reference frame (d-q frame, see Fig 1).