The thesis presents complete solution for control and management of permanent
magnet synchronous machine at elevated operating temperatures with torque
estimation and field weakening mode. The solution is made as an expansion of
basic FOC scheme and does not need any hardware modifications. As a result it
can be considered a cost-effective and relatively simple upgrade to existing control
system.
Introduction of the thesis provides the overview of different areas considered
in this research, from torque estimation, drive protection measures and demagnetization
modelling followed by a problem definition of controlling the electric
drive at elevated temperatures. After the definition of general model equations of
permanent magnet synchronous machine and presentation of field oriented control
in field weakening mode, the thesis focuses on non-salient pole type of machine.
The main reason for this is the simplification of the problem approach. Decreased
performance of machine at elevated temperatures and the corresponding machine
torque estimation are the initial guidelines of the present work, following from
permanent magnet material properties. Demagnetization mechanism, together
with demagnetization modelling represent a great part of the thesis. Central
and longest chapter deals with modeling of demagnetization with finite element
method, through which an appropriate magnetic structure for simplified magnetic
circuit is derived. The results from both the magnetic circuit and finite
element method are compared to justify the accuracy of proposed magnetic circuit.
The stator reverse field estimation, which directly affects the permanent
magnets, is determined using the magnetic circuit approach. The permanent
magnet temperature is determined with simple modified flux observer. Using
both the permanent magnets temperature and magnetic circuit, the permanent
magnets operating point on B(H) curve is obtained. Followed by calculation of
safe operating area, which is defined as the distance between current operating
point of permanent magnets and the knee point of B(H) curve, the stator field
weakening current is limited to prevent their demagnetization.
The results chapter presents the operation of proposed system in both simulations
and in practical laboratory model. Results are obtained in two regimes,
namely at constant field weakening operation and a rising temperature of permanent
magnets and at constant temperature and progressive field weakening. The
efficiency of proposed torque and temperature estimation method along with the
permanent magnet demagnetization protection mechanism are presented. It is
also shown that the accuracy of torque estimation enables the compensation of
the decreased torque generation due to thermal effects. For a clearer illustration
of position of the operating point of permanent magnets according to B(H) characteristics,
3D graphs in both regimes are also shown at the end of the chapter.
The results of both the simulation and laboratory model are gathered together
on the same graph to better present their correspondence. It can be seen how
the operating point stays within reversible demagnetization area in case of active
demagnetization protection and how in the absence of protection the operating
point shifts below the knee of B(H) characteristics into the irreversible demagnetization.
The final chapter briefly summarizes the work and definition of its main features
and indicates the possibility of further work development of the proposed
system. Particularly, the salient pole permanent magnet synchronous machines
are to be considered in the future work in this field.
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