Bearingless AC Homopolar Machine Design and Control for Distributed Flywheel Energy Storage

210 Pages·2015·6.43 MB·English

Bearingless AC Homopolar Machine Design and Control
for Distributed Flywheel Energy Storage
A DISSERTATION
SUBMITTED TO THE FACULTY OF THE GRADUATE SCHOOL
OF THE UNIVERSITY OF MINNESOTA
BY
Eric Loren Severson
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
Advisor: Ned Mohan,
Co-advisor: Robert Nilssen
June, 2015
(cid:13)c Eric Loren Severson 2015
ALL RIGHTS RESERVED
Acknowledgements
Thisdissertationhasspannednearlysixyears,thesamenumberresearchtripstoNTNU
inTrondheim,Norwaytotalingnearlyoneyear,andcountlessindividualswhohavegone
out of their way to support me. I have been fortunate to be part of a large and active
research group at the University of Minnesota and to have been embraced by an even
larger research department at NTNU.
I am especially thankful to my advisor, Ned Mohan, who has taught me that the
most important thing a person can do is to show respect for others. Professor Mohan’s
rare combination of mischief, humility, passion for mentoring students, and laissez-faire
attitude has molded me as a researcher and encouraged this project to grow wildly
beyond my initial goals. I have received an incredible amount of support from my co-
advisor, Robert Nilssen, in the form of creative ideas and directions for the project,
lessons on winding design, and creating a welcoming research environment at NTNU.
I would also like to acknowledge the support of the graduate students and staff with
whom I have had the pleasure of working with at both the University of Minnesota and
NTNU–especially Tore Undeland, for his help initiating my collaboration with NTNU
and serving as my host advisor on the first research trip; Dominik H¨ager, for all of his
effort, time, and creativity in constructing the protoype machines; NTNU’s Service Lab
fortheirlogistical andtechnical supportofmyexperimentalsetups;SaurabhTewari, for
themanyhourshehasspentreadingmypaperdrafts,providingtechnical guidance, and
sharing an occasional beer; Kristin Bjerke, for her enthusiastic support of my work and
providing my wife and me with an adoptive Norwegian family; Apurva, Ashish, Astrid,
Fritz, Gysler, Kartik, Kaushik, Maggi, Nathalie, Nathan, Rohit, Ruben, Santhosh,
Santiago, Shanker, Srikant, Sverre, Vince, Visweshwar, Zhang, and the numerous other
friends I have made along the way who have made this all worthwhile.
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I would like to acknowledge Karsten Moholt AS, and especially Linn Moholt, for
installing windings on the prototype machines; Infolytica Corp and Chad Ghalamzan
for providing me with finite element analysis tools and support; my father, Verne Sev-
erson, and John Hardy Jr. from Forward Pay Systems, Inc. for assistance with the
power electronic drive fabrication; the Norwegian Centre for International Cooperation
in Education (SIU)for providingfundingto start my collaboration with NTNU; theOf-
fice of Naval Research (ONR) for providing financial support; and the National Science
Foundation, the Department of Defense, and the University of Minnesota for providing
me with six years of fellowship funding in the form of the GRFP, NDSEG Fellowship,
and the Doctoral Dissertation Fellowship. I would also like to thank my dissertation
committee for their careful review of my work.
Finally, I would like to thank Marie Vera for adding urgency to complete my PhD.
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Dedication
To Ann Marie Severson
iii
Abstract
The increasing ownership of electric vehicles, in-home solar and wind generation, and
wider penetration of renewable energies onto the power grid has created a need for
grid-based energy storage to provide energy-neutral services. These services include
frequency regulation, which requires short response-times, high power ramping capabil-
ities, andseveral charge cycles over the courseof oneday; and diurnalload-/generation-
following services to offsettheinherentmismatch between renewablegeneration andthe
powergrid’sload profile,whichrequireslowself-discharge sothatareasonableefficiency
is obtained over a 24 hour storage interval. To realize the maximum benefits of energy
storage, the technology should be modular and have minimum geographic constraints,
so that it is easily scalable according to local demands. Furthermore, the technology
must be economically viable to participate in the energy markets.
There is currently no storage technology that is able to simultaneously meet all
of these needs. This dissertation focuses on developing a new energy storage device
based on flywheel technology to meet these needs. It is shown that the bearingless
ac homopolar machine can be used to overcome key obstacles in flywheel technology,
namely: unacceptable self-discharge and overall system cost and complexity.
Bearingless machines combine the functionality of a magnetic bearing and a mo-
tor/generator into a single electromechanical device. Design of these machines is par-
ticularly challenging due to cross-coupling effects and trade-offs between motor and
magnetic bearing capabilities. The bearingless ac homopolar machine adds to these
design challenges dueto its 3D fluxpaths requiringcomputationally expensive 3D finite
element analysis. At the time this dissertation was started, bearingless ac homopolar
machines were a highly immature technology. This dissertation advances the state-
of-the-art of these machines through research contributions in the areas of magnetic
modeling, winding design, control, and power-electronic drive implementation. While
thesecontributionsareoriented towardsfacilitating moreoptimalflywheeldesigns, they
will also be useful in applying the bearingless ac homopolar machine in other applica-
tions. Example designs are considered through finite element analysis and experimental
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validation is provided from a proof-of-concept prototype that has been designed and
constructed as a part of this dissertation.
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Contents
Acknowledgements i
Dedication iii
Abstract iv
List of Tables xi
List of Figures xiii
1 Introduction 1
1.1 Distribution grid-based energy storage . . . . . . . . . . . . . . . . . . . 1
1.1.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1.2 Requirements of an energy storage device . . . . . . . . . . . . . 2
1.1.3 Overview of energy storage technologies . . . . . . . . . . . . . . 3
1.2 Flywheel systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2.1 Overview of conventional flywheel systems . . . . . . . . . . . . . 4
1.2.2 Flywheel materials . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2.3 Commercial products . . . . . . . . . . . . . . . . . . . . . . . . 7
1.2.4 Problems with current technology . . . . . . . . . . . . . . . . . 9
1.2.5 Relevant flywheel research projects . . . . . . . . . . . . . . . . . 10
1.3 Proposed flywheel system . . . . . . . . . . . . . . . . . . . . . . . . . . 11
1.4 Research contributions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
1.5 Dissertation outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
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2 Model of the Bearingless AC Homopolar Motor 14
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.2 Description of bearingless ac homopolar motor . . . . . . . . . . . . . . 17
2.2.1 Torque production . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.2.2 Suspension force production . . . . . . . . . . . . . . . . . . . . . 17
2.3 Analysis of torque production . . . . . . . . . . . . . . . . . . . . . . . . 19
2.3.1 Sinusoidal airgap profile . . . . . . . . . . . . . . . . . . . . . . . 23
2.3.2 Square airgap profile . . . . . . . . . . . . . . . . . . . . . . . . . 25
2.4 Suspension winding configurations . . . . . . . . . . . . . . . . . . . . . 27
2.4.1 MMF calculations . . . . . . . . . . . . . . . . . . . . . . . . . . 27
2.4.2 Flux linkage calculations . . . . . . . . . . . . . . . . . . . . . . . 28
2.5 Complete machine inductance matrices . . . . . . . . . . . . . . . . . . . 29
2.5.1 Sinusoidal airgap profile . . . . . . . . . . . . . . . . . . . . . . . 31
2.5.2 Square airgap profile . . . . . . . . . . . . . . . . . . . . . . . . . 31
2.6 Analysis of suspension force production . . . . . . . . . . . . . . . . . . 31
2.6.1 Sinusoidal airgap profile . . . . . . . . . . . . . . . . . . . . . . . 32
2.6.2 Square airgap profile . . . . . . . . . . . . . . . . . . . . . . . . . 33
2.7 Analysis of interference torque . . . . . . . . . . . . . . . . . . . . . . . 33
3 Outer-Rotor AC Homopolar Motors for Flywheel Energy Storage 34
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
3.2 Outer-rotor ac homopolar motor . . . . . . . . . . . . . . . . . . . . . . 36
3.3 Comparison to the PM motor . . . . . . . . . . . . . . . . . . . . . . . . 37
3.3.1 Torque density . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
3.3.2 Motor losses. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
3.4 Machine sizing equations for flywheel energy storage . . . . . . . . . . . 39
3.4.1 Flywheel sizing . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.4.2 AC homopolar motor sizing . . . . . . . . . . . . . . . . . . . . . 40
3.5 Example design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
3.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
4 Magnetic Equivalent Circuit Modeling of the AC Homopolar Motor 47
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
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4.2 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
4.3 Overview of the MEC model . . . . . . . . . . . . . . . . . . . . . . . . 50
4.4 Flux tube definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
4.4.1 Stator flux tubes . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
4.4.2 Rotor flux tubes . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
4.4.3 Airgap flux tubes and rotor motion . . . . . . . . . . . . . . . . . 56
4.4.4 Axial flux tubes . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
4.5 Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
4.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
5 Rigid Body Rotor Model Under Variable Excitation 68
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
5.2 Rigid body model. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
5.3 Machine design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
5.4 Finite element analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
5.5 Force ripple and interference . . . . . . . . . . . . . . . . . . . . . . . . 73
5.6 Dependence on field excitation . . . . . . . . . . . . . . . . . . . . . . . 74
5.7 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
5.8 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
6 Dual Purpose No Voltage Winding Design for Bearingless Motors 79
6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
6.2 Dual purpose no voltage windings. . . . . . . . . . . . . . . . . . . . . . 82
6.3 Winding design theory and properties . . . . . . . . . . . . . . . . . . . 86
6.3.1 Phasor star diagram . . . . . . . . . . . . . . . . . . . . . . . . . 86
6.3.2 Key terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
6.3.3 Winding layout techniques . . . . . . . . . . . . . . . . . . . . . 88
6.3.4 Relevant bearingless motor and DPNV winding properties . . . . 88
6.4 Winding design requirements . . . . . . . . . . . . . . . . . . . . . . . . 89
6.4.1 Symmetry requirements . . . . . . . . . . . . . . . . . . . . . . . 89
6.4.2 No voltage requirements . . . . . . . . . . . . . . . . . . . . . . . 90
6.4.3 Non-Zero suspension winding distribution factor . . . . . . . . . 91
6.4.4 Layout requirements . . . . . . . . . . . . . . . . . . . . . . . . . 95
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