18-05-2012, 01:12 PM
Sensorless Speed Estimation of an Induction Motor in a Field Orientated Control System
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Introduction
The Induction motor for many years has been
regarded as the workhorse in industrial applications.
In the last few decades, the induction motor has
evolved from being a constant speed motor to a
variable speed, variable torque machine. Its evolution
was challenged by the easiness of controlling a DC
motor at low power applications. When applications
required large amounts of power and torque, the
induction motor became more efficient to use. With
the invention of variable voltage, variable frequency
drives (VVVF), the use of an induction motor has
increased. The formulation of Field Orientated
Control (FOC) has lead to the induction motor
replacing the DC motor as the main source of energy
conversion in industrial applications.
Field Orientated Control
Field Orientated Control (FOC) enables the induction
motor to be controlled in a method similar to a DC
motor. The conversion of the three-stator currents into
two DC currents enables the speed and torque of the
motor to be calculated in a manner similar to that in a
DC motor.
In FOC there are two transformations. The first
transformation converts the three-stator currents into
two components. The second transformation allows
the stationary reference frame to rotate in synchronism
with the stator flux vector.
Rotor Flux Angle Estimation
The dq – DQ transformation is reliant on the angle of
the rotor relative to the stator reference frame. By
analysing figures 10 and 11, the rotor angle, β, can be
determined by finding the angle between the mmf
vector and the stator direct mmf axis. Using
trigonometric laws, the angle β can be determined
from the following equation.
Field Orientated Control Loop
To accurately test the effectiveness of the estimated
speed and torque algorithms, a closed loop control
system is to be implemented. This control system will
calculate the speed and torque of the motor and
compare them against preset values. A three-phase
inverter will be used to alter the stator currents
to enable the speed to be changed. The system will
use Field Orientated Control principles to adjust
the motors speed and torque. A block Diagram of
the proposed control system is shown on the
following pages.