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PhD Thesis
PhD candidate: lecturer eng. Alexandru Săvulescu
Scientific coordinator: Prof. PhD. Eng. Stelian Dumitrescu
With a simple and robust construction, the asynchronous motor is used today, more than
ever, in electric drives with adjustable speed. Due to the development of power electronics
in the branch of frequency converters and, especially, in forced commutation, the variable
speed drives with induction motors have now become a wide interest.
Depending on the technological process in which they are used and by the power of the
motor, the systems of automatic electric drive must meet a wide variety of requirements:
- the increased efficiency of energy conversion;
- the continuous control of angular speed and torque;
- the minimizing of the errors and the reducing of the transitory process when the
control variables and the perturbation ones modify;
- the safe operating.
Analysed in terms of settling speed, asynchronous motor is a complex nonlinear,
multivariable system, with input-output interactions . This makes that a properly work both in
dynamic and stationary regimes requires control schemes with multiple control loops.
In this doctoral thesis the author intends to bring some contributions to the development of
the inverter voltage control, making experimental measures on the performance of the existing
converters and designing and simulating two drive systems with induction motors. The
control systems are designed on two different principles and the coefficients of the control
speed are adjusted by an adaptive strategy, named gain scheduling.
In Chapter 1, entitled Models of asynchronous motors used in adjustable speed drives,
the author has made a selective presentation of the induction motor models, developing the
model of equivalent machine with two orthogonal axes, based on the spatial phasor theory,
using relative units due to their main advantages:
- the possibility of comparing behaviors of control systems with the same structure, but
with different nominal power;
- they can be used in numerical simulations, which are realised in chapters 4 and 5, but
also in the real time implementation.
In the second part of the chapter there are presented the principles of vector control of the
induction motor, analyzing the orientation after each of the three fluxes: the rotor flux, the
stator flux and the air gap ones and identifying their advantages and disadvantages.
The main theoretical conclusion is that, if the motor model is oriented after the rotor flux,
the machine is completely decoupled in the two d-q axis and the components of the stator
current phasor are used as variables with control role.
In Chapter 2, entitled Control systems with asynchronous motors fed with variable
frequency, the author has described and analyzed a variety of adjustable speed drives with
asynchronous motors, presenting vector control systems with orientation after each of the
three fluxes of the motor.
In this chapter, there were also described the types of pulse width modulation, natural (of
comparison sinusoid-triangle) or vector modulation, that constitue the base of the working of
the PWM inverters.
The author brings an original contribution by carrying out the principle scheme of a
modernized vector control system with rotor flux estimation and current control whose
performances are highlighted by the simulation performed in Chapter 4.
In Chapter 3, entitled Experimental study of adjustable drive with asynchronous motor
fed by frequency converter, are presented the experimental results obtained from
measurements made on an adjustable drive system with induction motor. For this purpose, the
author has developed a drive system as a test stand, on which has implemented a modern
system of acquisition and Labview processing of data. It was realised the study of the
stationary regimes at different frequencies and the study of the transitory regimes at different
step variations of the frequency.
In Chapter 4, entitled Contributions to the design and simulation of induction motor
drive system with adaptive command and current control, the author has realised the
simulation in Matlab Simulink of an adjustable drive with asynchronous motor, in which the
voltage inverter is controlled by a modernized system with rotor flux orientation, designed
and theoretically motivated in Chapter 2.
The extension of the speed domain of electric drive, which was simulated here, could be
achieved by adding an extra adaptation loop based on gain scheduling strategy. So, the speed
controller parameters were adjusted to their optimal values through functions depending on
the motor speed. Simulations have showed a quality control for speeds slower than the
nominal, with very good transitory responses in speed and torque.
In this way, the advantages of the rotor flux oriented control: the decoupling of the
control-circuit after d-q axes and linearization of the mechanical characteristics, are
successfully combined with the advantage of adaptive command: a quality control on an
extended speed domain.
In Chapter 5, entitled Contributions to the design and simulation of induction motor
drive system with adaptive command and controlled by voltage space vector modulation, the
author has presented a simulation in Matlab Simulink of a different drive from the one
presented in Chapter 4, especially in that the system which commands the voltage inverter
operates on the principle of space vector modulation. This control system does not require the
flux estimation, being an indirect system, and does not need measurement of the stator
currents, but only of the speed.
The last chapter, entitled Original contributions. Conclusions, contains a summary of the
main conclusions to be drawn from this doctoral thesis and its main original contributions.
As a general conclusion we can consider that the simulated control system with voltage
modulation could be implemented in a variety of adjustable electric drives and the simulated
current control system can satisfy, with its superior performances, very demanding uses such
as the position synchronization drives.