26-06-2013, 01:05 PM
V/F Control of Squirrel Cage Induction Motor Drives Without Flux or Torque Measurement Dependency
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Abstract
Based on the popular constant volts per hertz principle, two improvement techniques are
presented: keeping maximum torque constant or keeping magnetic flux constant. An openloop
inverter-three-phase squirrel-cage induction motor drive system that provides constant
maximum torque or increased maximum torque and reduced slip speed at frequencies below
the nominal frequency has been modeled, simulated and tested. Load performance analysis
of the proposed system under different operation conditions was provided. These principles of
operation are extended to the case of operation from variable frequency or variable voltage
control method. Finally, the effects of the non-sinusoidal voltage and/or current wave shapes
are covered.
INTRODUCTION
The three-phase squirrel cage induction motor is the most widely used motor type in the
industry because of its good self-starting capability, simple and rugged structure, low cost and
reliability [1-5]. In spite of this popularity, the induction motor has two basic limitations: (1) The
standard motor is not a true constant-speed machine, its full-load slip varies from less than
1% (in high-horsepower motors) to more than 5% (in fractional-horsepower motors), and (2) It
is not inherently capable of providing variable-speed operation [2,3]. Both of these limitations
require consideration to meet quality and accuracy requirements of induction motor drive
applications.
The limitations of induction motor can be solved through the use of adjustable speed control
based on pulse width modulation techniques [4]. The basic control action involved in
adjustable speed control of induction motors is to apply a variable frequency variable
magnitude AC voltage to the motor to achieve the aims of variable speed operation [5].
Voltage source inverters and current source inverters are used in adjustable speed AC drives.
However, voltage source inverters with constant Volts/Hertz (V/f) are more popular, especially
for applications without position control requirements, or where the need for high accuracy of
speed control is not crucial.
MODELING SYSTEM COMPONENTS
The block diagram of inverter-three-phase squirrel cage induction motor drive system is
presented in figure 1. It consists of IGBT-inverter-based AC to AC converter, three-phase
squirrel cage induction motor and controller. In order to analyze the system performance, all
of these components should be modeled (mathematically described).
Modeling of Three-Phase Squirrel Cage Induction Motor
Many studies of the transient and steady state performance of induction motors have used
two axes (d-q) dynamic machine model for the solution of the motor performance equations
[12], while other studies have used a direct three-phase dynamic model that seemed more
convenient, due to the variables involved in such modeling, in which they are the actual
physical quantities of the motor [13]. Some authors have used dynamic model for small
perturbations and transfer function, or solutions for dynamic behavior in complex symbolic
form [14].
CONCLUSION
Based on the results obtained in this paper, the following conclusions can be made:
1. The derived state-space model of three-phase squirrel cage induction motor can be
used to analyze the performance of induction motor drive systems.
2. The implementation of constant maximum torque and constant flux controllers
improves the performance of inverter-induction motor drive systems.
3. The mechanical characteristics of the drive system with constant flux controller are
harder than that with constant maximum torque controller.
4. It is recommended to use constant maximum torque controller in drive systems
operating with constant torque.
5. It is recommended to use constant flux controller in drive systems operating with
constant power.