- Control techniques for AC machines for operation in
a wide speeed range.
- Control techniques for multiphase machines.
- Modulation techniques for multilevel converters of dual
two-level topology.
- Modulation techniques for matrix converters.
- Wound rotor synchronous machines for industrial traction
drives
- Diagnostic technique based on rotor modulating signals
signature analysis for doubly fed induction machines in wind
generator systems.
1. Control
techniques for AC machines for operation in a wide speeed
range
When the induction
motors are used for applications at high speed, it is desirable to
retain the maximum torque capability in the field-weakening region.
The torque capability of an induction motor is limited by the
maximum current and the maximum voltage the inverter can apply to
the motor. Several papers were presented in order to achieve the
maximum torque capability of the machine over the whole
field-weakening region. According to these field-weakening
algorithms, the optimal flux value of the motor should be updated
by means of look-up tables or explicit expressions containing the
motor parameters and quantities such as the motor speed, the motor
currents, the dc-link voltage and the requested torque. However,
the performance of these algorithms is strictly related to the
accuracy by which the parameters are known. For these reasons the
stator-flux-oriented drive, more insensitive to parameter
variations than the rotor-flux-oriented one, has received an
increasing attention for field weakening applications. In
particular, a robust method for field weakening operation of DTC
induction motor drives has been developed, where the flux reference
is adjusted on the basis of the torque error behavior. In fact, a
suitable method for robust field weakening is to determine the
optimal flux level using closed-loop schemes that analyze the motor
behavior, rather than look-up tables or explicit expressions
containing the motor parameters.
A further control
method has been analyzed, in which the flux is adjusted on the
basis of the supply voltage requested by the regulators. If this
voltage is greater than the available one, the field-weakening
algorithm reduces the flux. Furthermore, employing a suitable
voltage control strategy allows the motor to exploit the maximum
torque in the whole speed range. In the proposed
rotor-flux-oriented control scheme the main control variables are
the stator flux components instead of the stator current
components. This basic choice simplifies the control scheme,
exhibits a fast torque response and reduces the number of PI
regulators. In addition, the proposed scheme allows the motor to
exploit the maximum torque capability in the whole speed
range.
2. Control
techniques for multiphase machines
The conventional
approach for a variable-speed drive is to supply a three-phase
motor with a three-phase inverter. However, since variable-speed
drives are invariably supplied from power electronic converters,
the number of phases does not have to be equal to three any more
and it can be considered as a design variable. The use of
multiphase inverters together with multiphase ac machines has been
recognized as a viable approach to obtain higher power ratings with
current limited devices, by reducing the stator current per phase
without increasing the voltage per phase. Furthermore, multiphase
motor drives have several advantages over the traditional
three-phase motor drives such as reduction of the amplitude and
increase of the frequency of torque pulsations, and an improvement
of the fault tolerance.
To fully exploit
the potential of M-phase motor drives, a suitable and flexible
modulation strategy for M-phase voltage source inverters (VSIs) has
been defined using the duty-cycle space vector (DCSV)
representation. Basically, this representation describes the state
of the switches by means of complex variables. Using the DCSV
representation, it is possible to combine the multiple space vector
representation, useful in modeling multiphase machines, with
traditional carrier based PWM principle, suitable for the
modulation of multiphase VSIs.
The research
activity has been also focused on the problem of the inverter
output voltage limit, or in other words the capability of a
multiphase inverter to generate simultaneously the required
magnitudes of the multiple space vectors. The determination of the
output voltage limit in multiphase inverters represents a difficult
task, owing to the large number of inherent degrees of freedom
involved in the description of a multiphase system of voltages. An
explicit solution for the voltage limit of a seven-phase inverter
has been determined, leading to a set of inequalities that link the
magnitudes of the multiple voltage space vectors. These
inequalities are valid for any value of the phase angle of the
voltage vectors. The effectiveness of the proposed modulation
strategy and the validity of the voltage limit analysis have been
verified by experimental tests.
3. Modulation
techniques for multilevel converters of dual two-level
topology
The
multilevel inverter technology has been widely recognized as a
viable solution to overcome the voltage limits of power switching
converters in the area of high power medium-voltage drive systems.
The research activity has been focused on the dual two-level
inverter configuration, which has received large attention due to
the simplicity of the power stage. It is based on two standard
three-phase voltage source inverters supplied by two separate dc
sources. The presence of two insulated dc supplies inherently
eliminates common-mode currents and makes it possible to achieve
the maximum output voltage without the need of a common-mode
reactor. Furthermore, the dual inverter topology has a high
reliability because in case of fault in one inverter, its output
terminals can be short-circuited, and the system can operate using
the healthy inverter as a standard three-phase two-level inverter.
The proposed dual two-level inverter configuration can be usefully
implemented when using a battery supply system because, in this
case, it is very simple to split the dc supply into two
electrically separated dc sources. Being the converter supplied by
two distinct sources, in several applications, it is necessary to
regulate the power flow from the two sources. This requirement can
be demanded in order to equalize the state of charge of two banks
of batteries
or to
exploit the different characteristics of two sources. A new
modulation technique has been developed, which is able both to
perform multilevel operation and to regulate the load power sharing
between the two dc sources within each switching period.
4. Modulation
techniques for matrix converters
The
research on matrix converters has been carried out either from a
theoretical or experimental point of view, and in cooperation with
research groups of Aalborg (DK) University and Nottingham (UK)
University. The analysis has been focused on SVM control,
improvement of the input current quality and identification of
critical operating conditions for matrix converters.
A
new approach for the analysis of matrix converters has been
proposed, which leads to a general solution for the determination
of the optimum modulation technique. This approach is based on the
use of space vectors for the representation of the matrix converter
duty-cycles.
In
addition, a method to analyse the behaviour of system including a
matrix converter has been developed to study possible unstable
operating conditions.
Two
other topics have been developed, which are interesting mainly from
an applicative point of view:
-
determination of a new commutation strategy for matrix converters,
named "3-step commutation", based on the measurements of the input
voltages and the output currents. It allows to avoid the
uncertainty concerning the commutation instant of the current
between two switch and therefore guarantees a better linearity of
the voltage transfer ratio;
-
comparison between the performance of a matrix converter and a
back-to-back converter, in terms of maximum output power delivered
to the load and maximum output power per switch, taking the thermal
stress of the switches into account due to the switching and
conducting losses.
5. Wound rotor
synchronous machines for industrial traction drives
An
electric drive based on the wound rotor synchronous machine (WRSM)
has been developed and proposed for the use in traction systems of
heavy-duty electric vehicles. While the industry in this field was
massive introducing induction machine IM for the traction system of
industrial vehicles, the scientific community was proposing to use
the most elegant permanent magnet synchronous machines PMSM for its
best performance (efficiency and torque density). The problem was
that all the experimentations with PMSM failed due to the high cost
or to reliability problems of this drive system. The solution based
on the WRSM could match industry needs with a product, which is
simple and economic to be manufactured but having performance
comparable to that of a PMSM. This result was obtained through the
development of a completely new motor control algorithm, which
maximize the performance of this machine in any operating
conditions.
6. Diagnostic
Technique based on Rotor Modulating Signals Signature
Analysis
In a wind
generator turbine, the doubly fed induction machine (DFIM)
operating in variable-speed constant frequency mode is widely used
mainly because of the lower rating of the power converter connected
to the rotor side. The vector control strategy is based on the
regulation of the rotor currents by a back-to-back converter
connected between the mains and the rotor. In power-generation
systems, it is interesting to detect incipient faults as soon as
possible in order to minimize maintenance cost and to prevent
unscheduled downtimes by using advanced online diagnostic
techniques for electrical and mechanical faults. In fact, in
distributed generation, predictive maintenance has become an
unavoidable tool in order to improve the gain in energy market.
Many diagnostic systems have been presented for wind turbines using
DFIM. However, up to now, it is very
difficult to find a fault-detection procedure that is really
suitable for DFIM without adding sensors or digital hardware. With
reference to closed-loop induction-machine drives with a digital
control system, as the control itself affects the behavior of
external variables, new diagnostic procedures must be adopted to
perform the machine monitoring. A new technique has been developed,
which is based on the analysis of the modulating signals generated
by current regulators of the back-to-back converter connected to
the three-phase induction machine rotor side. These modulating
signals give spectra containing all the necessary information
required by an effective diagnostic index. Moreover, simulations
and experimental results have shown that the analysis of rotor
modulating signals yields a higher sensitivity with respect to the
current signature analysis performed directly on three-phase stator
currents. The signals used for diagnostic purpose are generated
inside the control system of the back-to-back converter. Therefore,
the proposed diagnostic system is particularly interesting, since
it can be effectively embedded in the control board of the
back-to-back converter connected to the induction-machine rotor
side.