Direct Power and Torque Control of AC/DC/AC Converter-Fed ...

1. Introduction
then nominal speed [15], [63], [121], [140]. IM equations represented in the flux
vector oriented coordinates have a good physical basis because they correspond to
the decoupled torque generation in separately excited DC motor. Nevertheless, from
the theoretical point of view, another types of mathematical transformations can be
chosen to achieve decoupling and linearization of IM equations. That methods are
known as modern nonlinear control [58]. Marino et al. and Krzeminski (see
Kazmierkowski [65]) have proposed a nonlinear transformation of the motor state
variables so that, in the new coordinates, the speed and rotor flux amplitude are
decoupled by feedback; the method is called feedback linearization control – FLC
[56]. Also, method based on the variation theory and energy shaping, called passivity
based control – PBC has been recently investigated [61].
In the mid 1980s, there was a trend toward the standardization of the control
systems on the basis of the FOC methodology. However, Depenbrock (see [18]),
Takahashi and Nogouchi [126] have presented a new strategy, which abandon an
idea of mathematical coordinate transformation and the analogy with DC motor
control. These authors proposed to replace the averaging based decoupling control
with the instantaneous bang-bang control, which very well corresponds to on-off
operation of the VSI semiconductor power devices [129]. These strategies are known
as direct torque control – DTC. Since 1985 the DTC has been continuously
developed and improved by many researchers [5], [6], [18], [34], [35], [74], [75],
[102], [127], [140], [149]. Among the main advantages of DTC scheme are: simple
structure, good dynamic behavior and is inherently a motion-sensorless control
method. However, it has a very important drawbacks i.e. variable switching
frequency [132], [133], high torque pulsation, unreliable start up, and low speed
operation performance [80], [144]. Therefore, to overcome these disadvantages a
space vector modulator – SVM was introduced to DTC structure [18], [38], [101]
giving DTC-SVM control scheme [124]. In this method disadvantages of the
classical DTC are eliminated [152].
However, it should be pointed that no commonly shared terminology exists
regarding DTC and DTC-SVM. From the formal considerations DTC-SVM can also
be called as stator field oriented control – SFOC, (Fig.1. 3). In the thesis DTC, and
DTC-SVM scheme will refer to control schemes operating with closed torque and
flux loops without current controllers [18].
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