18-07-2013, 04:04 PM
DIRECT TORQUE CONTROL OF INDUCTION (ANALYSIS AND SIMULATION) MOTOR
DIRECT TORQUE CONTROL.pdf (Size: 625.27 KB / Downloads: 45)
Abstract:
Among all control methods for
induction motor drives, Direct Torque
Control (DTC) seems to be particularly
interesting being independent of machine
rotor parameters and requiring no speed or
position sensors. The DTC scheme is
characterized by the absence of PI
regulators, coordinate transformations,
current regulators and PWM signals
generators. In spite of its simplicity, DTC
allows a good torque control in steady state
and transient operating conditions to be
obtained. However, the presence of
hysteresis controllers for flux and torque
could determine torque and current ripple
and variable switching frequency operation
for the voltage source inverter. This paper is
aimed to analyze DTC principles and
describes a generalized model of the three-
phase induction motor and its computer
simulation using MATLAB/SIMULINK.
Constructional details of DTC models for
the induction motor are given and their
implementation in SIMULINK is outlined.
Introduction:
In recent years much
research has been developed in order to find
simpler control schemes of induction motors
that meet the requirements like low torque
ripple, low harmonic distortion and quick
response The IM offers several features,
which make it attractive for use in electric
drive systems. Among various proposals
Direct Torque control (DTC) found wide
acceptance. In the 1980s, Takahashi
proposed a direct torque control for an
induction machine drive Further more DTC
Provides very quick response with simple
control structure and hence, this technique is
gaining popularity in industries In DTC it
is possible to control directly the stator flux
and the torque by selecting the appropriate
inverter state .The main advantages of DTC
are absence of coordinate transformation,
current regulator and separate voltage
modulation block. However common
disadvantages of conventional DTC are high
torque and stator flux ripple, requirement of
torque and flux estimators, implying the
consequent parameters identification and
sluggish speed response during start up and
abrupt change in Torque command .Many
methods have been proposed to reduce the
torque ripple like multi level inverters and
matrix converters.
Method:
Stator flux linkage is estimated by
integrating the stator voltages. Torque is
estimated as a cross product of estimated
stator flux linkage vector and measured
motor current vector. The estimated flux
magnitude and torque are then compared
with their reference values. If either the
estimated flux or torque deviates from the
reference more than allowed tolerance, the
transistors of the variable frequency drive
are turned off and on in such a way that the
flux and torque will return in their tolerance
bands as fast as possible. Thus direct torque
control is one form of the hysteresis or bang-
bang control.