14-04-2014, 03:10 PM
Direct Torque Control of Induction Motors
Torque Control of Induction .pdf (Size: 124.61 KB / Downloads: 28)
Introduction
D.C. motors have been used widely during the last
century in applications where variable-speed
operation was needed, because its flux and torque can
be controlled easily by means of changing the field
and the armature currents respectively. Furthermore,
operation in the four quadrants of the torque-speed
plane including temporary standstill was achieved.
However, DC motors have basically two drawbacks,
which are the existence of commutators and brushes.
These two disadvantages implied not only periodic
maintenance but also difficulty to work in dirty and
explosive environments; difficulty that sometimes
used to become in impossibility.
On the other hand, induction motors are robust, easily
maintained and reliable. Moreover the cost is lower,
as well as the inertia and the weight.
Nowadays, the inverters can act a very fast switching
frequency and the cost of the computers is decreasing.
Due to the advantages and the positive circumstances
explained above, induction motors are replacing DC
motors in the industry applications, even in the
applications where a fast speed and torque response in
four quadrants is required, becoming the new
horsepower of the industry.
Vector Controllers:
In these types of controllers, there are control loops
for controlling both the torque and the flux [3]. The
most spread controllers are the ones that use vector
transform such as either Park or Ku. Its accuracy can
reach values such as 0.5% regarding the speed and
2% regarding the torque, even in stand still.
The main disadvantages are the huge computational
capability required and the compulsory good
identification of the motor parameters.
DTC Control Schematic:
In figure 2 a possible schematic of Direct Torque
Control is shown. As it can be seen there are two
different loops corresponding to the magnitudes of the
stator flux and torque. The reference values for the
flux stator and the torque are compared with the
actual values, and the resulting values are fed into the
two-level and three-level hysteresis comparators
respectively. The outputs of the stator flux (Φ) and
torque (τ) comparators with the position of the stator
flux are used as inputs of the look up table (see table
II). The position of the stator flux is divided into six
different sections. In accordance with the figure 2, the
flux linkage and torque errors are restricted within its
respective hysteresis bands. It can be proved that the
flux hysteresis band affects basically to the stator-
current distortion in terms of low order harmonics and
the torque hysteresis band affects the switching
frequency.
Conclusions
Provided that in Europe the electrical drives business
is worth approximately $1.0 Billion/ Annum and
induction motors are the new horsepower of the
industry, it is obvious that the improvement in the
induction motors drives is a matter of high interest.
Direct Torque Control is considered to be one of the
best methods to drive induction motors, even better
that the well-known Vector Controls. However, DTC
has some disadvantages, being one of the most
important the torque ripple.