machines for aerospace

272 Pages·2016·19.65 MB·English

Modelling of high power density electrical
machines for aerospace
By
David James Powell
A thesis submitted for the degree of PhD
in the department of Electronic and
Electrical Engineering at the University
of Sheffield
May 2003
SUMMARY
This thesis is concerned with the electrical, thermal and mechanical modelling of
electrical machines for the 'more-electric' aircraft. Two specific applications are
considered viz. a permanent magnet brush less DC (BLDC) machine for an electro
hydraulic actuator for a primary flight control surface, and a switched reluctance (SR)
starter/generator for the HP spool of a large civil aero-engine. As a consequence of the
highly variable and often hostile ambient environment and constrained available space
envelope, these electrical machines can rarely be designed in isolation, with thermal and
mechanical constraints often having a significant influence on the design.
In view of these considerations, a transient lumped parameter thermal model has been
developed for the BLDC machine, and validated by experimental measurements on a
prototype machine at various stages of manufacture. Since the rotor cavity of the BLDC
machine is flooded with hydraulic fluid leaking from the pump, fluid friction losses
have been modelled, and validated by tests on a prototype machine. Optimisation of the
BLDC machine airgap has also been investigated using analytical electromagneticlfluid
dynamic modelling.
Detailed investigation of the mechanical stresses in the rotor of the HP spool machine
have led to the development of a novel rotor structure for SR machines which is shown
to have comparable electromagnetic performance with a conventional SR machine. A
specific design of SR machine is analysed in detail in terms of dynamic current
waveforms and the subsequent iron losses, and its thermal performance is modelled in a
representative aero-engine environment.
ACKNOWLEDGEMENTS
I would like to express my sincere thanks to Geraint Jewell, and Dave Howe for their
continuous guidance and encouragement during the course of the research. I am also
grateful to Nigel Schofield for his wealth of experience in the field of electrical
machines.
I would also like to thank the Engineering and Physical Sciences Research Council for
the award of a research studentship, the staff in the Rolls-Royce strategic research centre,
and Goodrich systems for the use of their test-rigs. Special thanks are also due to the
technical staff in the Electrical Machines and Drives group who were particularly helpful
in the installation of the oil cooling test-rig.
I am indebted to my colleagues in the Machines and Drives group for their friendship,
good humour, and support. Finally, thanks to Pippa for all her love, support and
understanding throughout the course of writing this thesis.
CONTENTS
NOMENCLATURE
1 INTRODUCTION
1.1 Background 1
1.2 The more-electric aircraft and more-electric engine 3
1.3 Historical background to the more-electric aircraft 4
1.4 Perceived benefits of the more-electric aircraft 5
1.5 Electrically actuated flight control surfaces 7
1.6 Future aircraft electrical power generation 11
1.7 Design of electrical machines for aerospace application 14
1.8 References 17
2 THERMAL MODELLING OF A BRUSHLESS PERMANENT MAGNET
MACHINE IN AN EHA SYSTEM
2.1 Introduction to thermal modelling of machines 31
2.2 Mechanical construction of the BLDC machine 32
2.3 Sources of loss in the stator 34
2.4 Heat transfer modelling techniques 36
2.5 Stator lamination and frame model 38
2.6 Thermal properties of electrical machine stator slots and coils 39
2.7 Finite element modelling of stator slots and coils 42
2.8 Influence of random conductor distribution within a coil 47
2.9 Summary of conductor bundle modelling 50
2.10 Experimental measurements on conductor bundles 51
2.11 Lumped parameter model of BLDC machine 54
2.12 Experimental measurement to validate stator radial model 56
2.13 Efficiency measurements 59
2.14 References 61
3 FLUID LOSSES IN THE EHA ELECTRICAL MACHINE
3.l Introduction 90
3.2 Slipperless pump leakage characteristics 91
3.3 Fluid physical properties 91
3.4 Calculation of fluid friction losses 92
3.5 Acceleration losses 96
3.6 Heat transfer in the air gap 96
3.7 Drag loss measurements on a dummy rotor 100
3.8 Optimisation of mechanical air gap in the machine 102
3.9 Discussion of results 107
3.l0 Conclusions 108
3.11 References 109
4 HP SPOOL EMBEDDED MACHINE DESIGN
4.l Introduction 124
4.2 Limitations of conventional rotor topologies 129
4.3 Modular rotor topologies 133
4.4 Performance comparison 139
4.5 Conclusions 143
4.6 References 145
5 DYNAMIC MODELLING
5.l Introduction 166
5.2 Selection of the number of turns for the stator winding 166
5.3 Basic dynamic operation of SR machine and converter 167
5.4 Description of the non-linear dynamic model 170
5.5 Winding design 173
5.6 Calculation of iron losses 174
5.7 Procedure for calculating iron loss in the SR machine 176
5.8 Consideration of alternative soft magnetic materials 177
5.9 References 179
6 THERMAL MODELLING OF THE SWITCHED RELUCTANCE
STARTEWGENERATOR
6.1 Introduction 196
6.2 Thermal properties of the HP spool machine 198
6.3 Thermal model 199
6.4 Procedure for determining cooling requirements 202
6.5 References 209
7 CONCLUSIONS 219
APPENDIX A: T -network of thermal resistances 225
APPENDIXB: Brushless DC permanent magnet machine thermal model 228
APPENDIXC: Switched reluctance machine thermal model 244
NOMENCLATURE
Nomenclature is listed in the order in which it appears in this thesis.
EHA Electro Hydraulic Actuator
EMA Electromechanical Actuator
MTBF Mean Time Between Failures
MEA More Electric Aircraft
BLDC Brushless DC
radiation heat transfer, W
surface emissivity
Er
Stefan-Boltzman constant, 56.7 e-9 W/m2K4
2
area considered for heat transfer, m
surface temperature, K
temperature of ambient air, K
electrical conductivity, S
thickness of lamination steel, m
kexc excess loss coefficient
flux density, T
maximum random offset distance in x direction, m
maximum random offset distance in y direction, m
random number (-O.95<P<O.95)
conductor diameter, m
thermal conductivity, W/mK
unidirectional heat flow, W
temperature differential
slot liner thermal conductivity, W/mK
ksl
conductor bundle thermal conductivity, W/mK
kwdg
slot liner thermal conductivity at high pressure, W/mK
kpl
slot liner thermal conductivity at low pressure, W/mK
kp2
conductor bundle thermal conductivity for process I winding, W/mK
kwdgl
Re Reynolds number
p density, kglm3
(J) angular velocity, radls
r radius of cylinder, m
2
J.1 kinematic viscosity, mrn /s
8 radial air gap length, m
Cf surface friction coefficient
Pdrag fluid friction loss, W
L axial length of rotor, m
T torque, Nm
rl rotor outer radius, m
r2 stator inner bore radius, m
tangential velocity, m/s
Vt
axial velocity, mls
Va
C velocity coefficient
v
Nu Nusselt number
h heat transfer coefficient, W/m2K
Ta Taylor number
rm mean airgap radius, m
m mass flow rate, kg/s
C specific heat at constant pressure, J/kg.K
p
D rotor outer diameter, m
ro
Bg airgap flux density, T
Q electric loading, Afm
number of phases
Nph
Nt number of turns
Nc number of coils
phase current, A
Iph
Dsi' stator inner diameter, m
Br remanent flux density, T
permeability of free space, 4nxlO·7 N/A2
J.lo
Ig electromagnetic radial airgap length, m
1m radial magnet length, m
O's hoop stress in a thick walled cylindrical disk, Pa
v Youngs modulus
Rl cylinder outer radius, m
R2 cylinder inner radius, m
d element edge length, m
xy
K normalised element scaling factor
he coil height, m
8 skin depth, m
f electrical frequency, Hz
J.t permeability, Him
Dh hydraulic diameter, m
U axial air velocity, m/s
f friction factor
f s shape factor
tlP pressure drop, Pa
L length of duct, m
Pf power loss due to frictional pressure drop, W
CHAPTER!
INTRODUCTION
1.1 Background
International air transport is presently the fastest growing sector of transport worldwide.
Technological advances in the aircraft industry have improved aircraft efficiency and
reduced the costs of air transport by such a degree that worldwide air passenger traffic
has grown at an average yearly rate of 9% since 1960 [BOE 96], with freight and mail
traffic also growing by some 11 and 7%, respectively. In 1995 for instance, some 1.3
billion passengers were carried by the world's airlines [BOE 96].
Despite this strong historical growth, the industry remains relatively volatile and prone
to sudden short term declines. However, Button [BUT 99] has postulated that passenger
air traffic will grow at a rate of between 5 and 7% into the foreseeable future, with much
of the growth in the Asia-Pacific region (up to 9% a year), while more recent
predictions by the Airbus Group [AlA 02] have suggested that the growth in the
passenger markets will slow to 4.7% up to 2020, again with more of the market share
moving towards Asia-Pacific (the Chinese domestic market is predicted to grow by over
8%). The vast majority of large civil aircraft with >500 seats operate in Asia and the
demand for such aircraft, as the Asian pacific economies grow, is likely to increase.
Further, these very large aircraft will be able to carry a greater volume of passengers
through the worlds' increasingly congested airports and air traffic control systems.
This projected increase in air travel will occur against a background of ever-increasing
concerns regarding the environmental impact of air traffic. Although this was largely
confined to problems of noise pollution in the past, concern has more recently shifted to
atmospheric pollution around airports and damage caused to the atmosphere by jet
engine exhaust emissions at cruising altitude. This change is a result of increased public
awareness of issues such as greenhouse gas emissions and the potential damage to the
1