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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).