08-02-2013, 09:43 AM
Comparison of three-level torque hysteresis controllers for Direct Torque Control
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Abstract
In this paper, a comparison is made between different
implementations of the three-level torque comparator
for a Direct Torque Control (DTC) based induction motor
drive. The DTC scheme controls stator flux and torque by
means of hysteresis comparators, respectively a two-level control
structure for the stator flux and a three-level comparator for
the electromagnetic torque. The standard three-level hysteresis
controller has a DC offset torque error. In this paper, an
additive implementation is investigated in order to remove this
DC offset error. The operational principles of the different
implementations are compared based on continuous and discrete
simulation results, which demonstrate the additional advantages
of the additive structure in a discrete implementation of the
control scheme.
INTRODUCTION
Since its introduction, Direct Torque Control (DTC) has
gained increasing interest from researchers all over the world.
The standard DTC scheme [1], [2] uses hysteresis comparators
for the control of both stator flux magnitude and electromagnetic
torque. The flux control is achieved by a two-level
comparator and a three-level structure is often preferred for the
control of the torque since it enables the use of zero voltage
vectors which allow a smooth torque reduction. Ideally, these
controllers limit both torque and flux to the bounds imposed by
the hysteresis bands. This results in a non-constant switching
frequency. When the DTC scheme is implemented using digital
controllers, the controlled quantities are no longer strictly
limited to these bounds since the discrete scheme operates at
a fixed sampling frequency. However, the discrete scheme will
operate like an analog one if the hysteresis bounds are chosen
sufficiently large [3].
THREE-LEVEL TORQUE HYSTERESIS COMPARATOR
The purpose of the torque hysteresis comparator is to limit
the torque Tel to the bands imposed by the hysteresis controller.
In this section, a detailed description of the hysteresis
controllers is given. The different control structures lead to a
different behavior under continuous and discrete implementation.
Simulations are carried out on a standard 4 kW, 400 V ,
50 Hz 4-pole induction machine. In this section, the ability
of the machine to track a reference torque value of 20Nm is
investigated. The discrete simulations are carried out using a
sampling frequency of 30 kHz, which is in the upper range
of research based implementations but lower than the 40 kHz
used in industrial drives [9]. Furthermore a prediction scheme
presented in [10] is implemented for the discrete simulations
in to order to lower both torque and flux ripple. The prediction
scheme removes the part of the ripples associated with the time
delay caused by the processing of the data.
CONCLUSION
A comparison between different three-level hysteresis controllers
was presented, starting from a description of the
dynamic behavior of the DTC scheme. The additive hysteresis
controller shows significant advantages over the symmetric
and asymmetric structure. Continuous and discrete simulation
results show the possibilities of the additive implementation to
eliminate the DC offset torque error. Furthermore, minimum
torque ripple and detection margin can be set independently,
which offers an additional benefit when the scheme is implemented
on a discrete time base.