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Bi-directional Power Converter for
Flywheel Energy Storage Systems
A Thesis
Submitted for the Degree of
Master of Sciencein the Faculty of Engineering
By
S R Gurumurthy
Department of Electrical EngineeringIndian Institute of Science
Bangalore - 560 012
India
Jan 2006
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i
To
my mother
and
to the memory of
my father
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Acknowledgements
I am grateful to Prof. V. Ramanarayanan and Shri. M. R. Srikanthan, Project Manager
(M), RMP for accepting me as a student. I thank them for guiding me in the project and
providing me all the facilities for experimental work. I express my heartfelt thanks to Prof.
V. Ramanarayanan for giving me an insight into power electronics during my course work
as well as for his continuous support and guidance throughout the project work.
I am extremely grateful to Shri. T. K. Bera, Project Director, RMP, for his guidance and
for giving me an opportunity to undertake the postgraduate studies at Indian Institute of
Science, Bangalore. I also thank Shri. S.Sarkar, Project Manager (Process), RMP, and Shri.
H. A. Balasubramanya for their continuous support throughout my project work.
I thank Prof. V. T. Ranganathan and Prof. G. Naranyanan for their advice and suggestions.
I thank Bhabha Atomic Research Centre, Mysore for all the support, facilities and oppor-
tunities provided to me. I owe my gratitude to IISc administration for providing excellent
hostel and mess facilities during my stay in the Institute campus. I am grateful to Shri.
D. M. Channe Gowda and his team in the Electrical Engineering Department office for the
smooth conduct of administrative activities.
I want to specially thank my colleagues in the power electronics laboratory Venugopal,Kaushik, Kannan, Lakshmi, Debmalya Banerjee, Amit, Mirzaei, Kamalesh, Chandrashekhar,
Milind and Vishal for their help and support during the project work and technical docu-
mentation of the project. My technical discussions with them have helped me learn a lot,
which I am sure, will be useful throughout my life.
I specially thank Kaushik for his help and support during my course work. I owe a lot to
him for his help during the difficult phase of my stay. I will always cherish his friendship. I
cannot forget the helps rendered by Venugopal during experimentation and documentation
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Acknowledgements iii
work of the project. I am very much thankful to him.
Discussions with Dr. R. Anbarasu, A. Nandakumar and J. Nataraj has helped me a lot in
conceptualizing the ideas. I am ever indebted to them. I specially thank Satheesh kumar
who was ever ready for helping me in fabrication, testing of hard ware and documentation
of the work. This work would not have been possible without the help and cooperation of
my wife Seetha and son Viveka. I am very much thankful to them.
I would like to attribute all of my success, my achievements to my father and mother. My
father remain an incessant source of inspiration and support all through my life. Finally
I would like to thank the Almighty for all that I have got in my life and for creating the
opportunities for me to pursue the work of my interest.
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Abstract
In many power processing applications such as traction, elevators, cranes etc, it is common-place to encounter loss of stored energy. The main reason is that, the power converters are
not capable of returning the stored energy during transients. In applications where frequent
transients are involved, this results in substantial loss of energy. Bi-directional converters in
such applications can lead to higher operating efficiency. In a typical traction application,
stored energy while running can be restored during deceleration. This process saves the
energy and improves the efficiency. Such applications need a bi-directional interfacing con-
verter. The bi-directional converter facilitates the energy flow, to and from the device. Basic
requirements of this interface are, simple structure, ease of control and energy efficiency.Such an application is the target of the development work reported in this thesis.
The aim of this work is to develop a bi-directional power converter/controller to facilitate
the energy storage to and from the storage device. The storage device employed in this
application is a flywheel. The bi-directional power converter (BDC) drives the brushless DC
(BLDC) machine coupled to the flywheel. The total system is a Flywheel Energy Storage
(FES) system. The analysis, design, fabrication and evaluation of such a system is covered
in this thesis.
In the FES system considered, there is a flywheel, coupled to the rotor of an electrical ma-chine. This machine uses the power from a dc bus to accelerate the flywheel (in charging
mode) and keep it running. The same machine discharges the flywheel (in deceleration mode)
to provide power back to the dc bus. The machine acts as a motor during charging and as a
generator during discharging. To store the energy in the flywheel, it is run up to the rated
speed of the motor (using input dc power). Under this condition, the flywheel stores the en-
ergy. When the input power fails, the flywheel continues to run due to its inertia, driving the
generator. If an electrical load is connected across the generator terminals, it draws current
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Abstract v
and utilizes the energy stored in the flywheel. In this process, the flywheel discharges its
energy and decelerates. The terminal voltage of the machine exponentially drops during de-
celeration. It is desired that power harvested from the flywheel during the deceleration is at
constant voltage. The machine therefore, is interfaced to the dc bus through a bi-directional
power converter. The bi-directional power converter serves two purposes. It accelerates the
motor while charging and discharges the flywheel while decelerating. Further, the control
in the converter can be exercised to obtain constant dc output voltage for a wider range of
flywheel speed. The amount of power transferred to the load and its duration is a function
of the running speed, overall efficiency and the control strategies adopted.
The primary aim of this thesis is to design and fabricate the bi-directional power con-
verter/controller, identify its operating modes, study the effect of various system parameters
on the performance; and evaluate the system.
To start with, the overall system design is considered. The selection of prime mover and bi-
directional power converter are discussed. Basic design of the BLDC machine is presented.
The design is verified through Finite Element Method (FEM) of analysis and validated
through experimental results. A six-switch voltage source inverter topology is selected as
bi-directional power converter. Operating modes of the power converter are identified as con-
trolled current acceleration (CCA) mode during charging and constant voltage deceleration(CVD) mode during discharging. Transfer function and steady state equations are derived
for both CCA and CVD modes. Controller design for both the modes are proposed. Design
considerations and selection of various circuit elements are explained. Sources of various
losses in the system are discussed in detail. Method of apportioning the losses are given.
The effect of various system parameters on its performance is explained. Design recommen-
dations to move towards lower losses are given.
The controller and Human Machine Interface (HMI) are implemented using a Motorola,
Digital Signal Processor 56F805. All subsystems are fabricated, integrated and tested. Ex-perimental results are presented. Energy transfer, to and from the flywheel is demonstrated.
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Contents
Acknowledgements ii
Abstract iv
List of Tables x
List of Figures xi
Nomenclature xiv
1 Introduction 11.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Two quadrant BLDC drive . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2.1 BLDC machine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2.1.1 Source of Loss and its reduction . . . . . . . . . . . . . . . . 3
1.2.1.2 Machine Type . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2.1.3 Construction . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2.1.4 Sizing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2.2 Bi-directional Power Converter . . . . . . . . . . . . . . . . . . . . . 51.2.2.1 Power Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2.2.2 Sizing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.2.3 Controller . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.3 Overall system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.3.1 Principle of working of the system . . . . . . . . . . . . . . . . . . . . 9
1.4 Scope of the Thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.5 Organization of the thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
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Contents vii
2 Selection, Design and Analysis of Brushless DC Machine 13
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.2 Selection of the type of machine . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.3 Basic design of the BLDC machine . . . . . . . . . . . . . . . . . . . . . . . 14
2.4 Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.5 Equation for the induced voltage . . . . . . . . . . . . . . . . . . . . . . . . 17
2.5.1 Induced voltage equation . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.5.2 Computation of phase and line voltages . . . . . . . . . . . . . . . . . 20
2.6 Armature leakage inductance and coil resistance . . . . . . . . . . . . . . . . 20
2.7 Model of the machine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222.8 FE Method of Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
2.9 Experimental results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.10 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
3 Bi-directional Converter 27
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.2 Power converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.3 Transfer function of BDC in CCA mode . . . . . . . . . . . . . . . . . . . . 30
3.3.1 Simplified Equivalent circuit . . . . . . . . . . . . . . . . . . . . . . . 30
3.3.2 Small signal modeling . . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.4 Transfer function of BDC in CVD mode . . . . . . . . . . . . . . . . . . . . 31
3.4.1 Simplified equivalent circuit . . . . . . . . . . . . . . . . . . . . . . . 34
3.4.2 Small signal modeling . . . . . . . . . . . . . . . . . . . . . . . . . . 34
3.5 Controller . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.6 Switching between controllers . . . . . . . . . . . . . . . . . . . . . . . . . . 36
3.7 Selection of circuit elements . . . . . . . . . . . . . . . . . . . . . . . . . . . 363.7.1 Series Inductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
3.7.2 DC bus capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
3.7.3 Switches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
3.7.4 Switching frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
3.8 Effects of system parameters on the performance . . . . . . . . . . . . . . . . 38
3.8.1 Source resistance of the machine . . . . . . . . . . . . . . . . . . . . . 38
3.8.2 Weight of the flywheel . . . . . . . . . . . . . . . . . . . . . . . . . . 40
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viii Contents
3.8.3 Diameter of the flywheel . . . . . . . . . . . . . . . . . . . . . . . . . 41
3.9 Effect of Backup time on the system performance . . . . . . . . . . . . . . . 41
3.10 Simulation of the system in CVD mode . . . . . . . . . . . . . . . . . . . . . 42
3.11 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
4 Digital Implementation and Performace Evaluation of the System 45
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
4.2 Pulse generation in CCA (motor) mode . . . . . . . . . . . . . . . . . . . . . 45
4.3 Pulse generation in CVD (generator) mode . . . . . . . . . . . . . . . . . . . 48
4.4 Software implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
4.4.1 Human Machine Interface (HMI) . . . . . . . . . . . . . . . . . . . . 48
4.4.1.1 Keypad . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
4.4.1.2 Display . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
4.4.2 Timer interrupt service routine . . . . . . . . . . . . . . . . . . . . . 51
4.4.3 Main program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
4.5 Hardware implementation of the system . . . . . . . . . . . . . . . . . . . . 52
4.5.1 Controller hardware . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
4.5.2 Power converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
4.6 Testing and performance analysis of the system . . . . . . . . . . . . . . . . 55
4.6.1 Apportioning of various losses . . . . . . . . . . . . . . . . . . . . . . 55
4.6.1.1 No load test . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
4.6.1.2 Retardation test with flywheel . . . . . . . . . . . . . . . . . 55
4.6.1.3 Copper losses in the armature winding . . . . . . . . . . . . 57
4.6.1.4 Switching and conduction losses in the converter . . . . . . 58
4.6.2 Power backup time test . . . . . . . . . . . . . . . . . . . . . . . . . . 59
4.6.3 Source resistance effect . . . . . . . . . . . . . . . . . . . . . . . . . . 60
4.6.4 Current dependent eddy current loss in the core . . . . . . . . . . . . 63
4.6.5 Comparison of various loss components . . . . . . . . . . . . . . . . . 64
4.6.6 Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
4.6.7 Harvestable Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
4.6.8 Current waveforms at various speeds . . . . . . . . . . . . . . . . . . 66
4.7 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
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Contents ix
5 Conclusions 69
5.1 The present work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
5.2 Guidelines emerging from the work . . . . . . . . . . . . . . . . . . . . . . . 71
5.3 Spin off technology from the present system . . . . . . . . . . . . . . . . . . 71
5.4 Applications of the system . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
A Specifications of IGBT and Capacitor 72
A.1 IGBT Module . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
A.2 Capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
B Specifications of Digital Signal Processor DSP56F805 73B.1 Digital Signal Processing Core . . . . . . . . . . . . . . . . . . . . . . . . . . 73
B.2 Memory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
B.3 Peripheral Circuits for DSP56F805 . . . . . . . . . . . . . . . . . . . . . . . 74
C Block Diagram of Controller 76
D Specifications of Hall effect position Sensor 77
E Publication 78
F Photographs of the test setup 79
G Further improvements in the system 83
G.1 Method used in apportioning various losses . . . . . . . . . . . . . . . . . . . 83
G.2 Relation between the speed and the loss: . . . . . . . . . . . . . . . . . . . . 83
G.3 Interpretation of the equation : . . . . . . . . . . . . . . . . . . . . . . . . . 85
G.4 Loss reduction techniques: . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
G.5 conclusions: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
References 90
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List of Tables
2.1 Comparison of results at 10000 RPM . . . . . . . . . . . . . . . . . . . . . . 24
4.1 Retardation test data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
4.2 Max voltage gain test data . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
4.3 Comparison of various losses . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
4.4 Test results at various load conditions . . . . . . . . . . . . . . . . . . . . . . 65
G.1 Total loss at various mass as a function of rotor speed . . . . . . . . . . . . . 84
G.2 Various losses with out flywheel . . . . . . . . . . . . . . . . . . . . . . . . . 86
G.3 Various losses in watts with a flywheel of 11 Kg . . . . . . . . . . . . . . . . 86
G.4 Various losses in watts with a flywheel of 15 Kg . . . . . . . . . . . . . . . . 87
G.5 Various losses in watts with a flywheel of 21 Kg . . . . . . . . . . . . . . . . 87
G.6 Loss reduction techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
G.7 Comparison of various losses . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
G.8 Comparison of various systems . . . . . . . . . . . . . . . . . . . . . . . . . . 89
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List of Figures
1.1 Block schematic of two quadrant BLDC drive . . . . . . . . . . . . . . . . . 2
1.2 Bi-directional Power Converter circuit . . . . . . . . . . . . . . . . . . . . . . 6
1.3 Current vs Time characteristics . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.4 Voltage vs Time characteristics . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.5 Typical flywheel energy storage system . . . . . . . . . . . . . . . . . . . . . 8
1.6 Speed vs Time characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.7 Voltage vs Time characteristics . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.1 Cross sectional view of basic PM BLDC machine . . . . . . . . . . . . . . . 14
2.2 Cross sectional view of PM BLDC machine . . . . . . . . . . . . . . . . . . . 16
2.3 Placement of magnets in the interior of rotor . . . . . . . . . . . . . . . . . . 16
2.4 Magnetic field produced in the air gap . . . . . . . . . . . . . . . . . . . . . 17
2.5 Aig gap flux density vs mechanical angle . . . . . . . . . . . . . . . . . . . . 18
2.6 Stator winding connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.7 Induced voltage as a function of time . . . . . . . . . . . . . . . . . . . . . . 19
2.8 Lumped parameter equivalent circuit for the calculation of leakage inductance. 21
2.9 Magnetic equivalent circuit of the armature leakage inductance. . . . . . . . 21
2.10 Equivalent circuit of the machine . . . . . . . . . . . . . . . . . . . . . . . . 22
2.11 FEM generated air gap flux density plot . . . . . . . . . . . . . . . . . . . . 23
2.12 FEM generated induced voltage waveform plot at 10000 RPM . . . . . . . . 24
2.13 Induced voltage vs speed characteristics . . . . . . . . . . . . . . . . . . . . . 24
2.14 Measured induced voltage waveform of the machine at 10,000 RPM . . . . . 25
3.1 Circuit diagram of the bi-directional power converter . . . . . . . . . . . . . 27
3.2 Converter output voltage waveforms in CCA mode . . . . . . . . . . . . . . 28
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xii List of Figures
3.3 Current path during different intervals of time in the CCA mode . . . . . . . 29
3.4 Equivalent circuit of converter in CCA mode . . . . . . . . . . . . . . . . . . 30
3.5 Simplified equivalent circuit of converter in CCA mode . . . . . . . . . . . . 30
3.6 Induced voltage waveforms of the machine . . . . . . . . . . . . . . . . . . . 32
3.7 Current path during different intervals of time in the CVD mode . . . . . . . 33
3.8 Equivalent circuit of converter in CVD mode . . . . . . . . . . . . . . . . . . 34
3.9 Block diagram of the controller in CCA mode . . . . . . . . . . . . . . . . . 36
3.10 Block diagram of the controller in CVD mode . . . . . . . . . . . . . . . . . 36
3.11 Schematic representation of various losses in the system . . . . . . . . . . . . 39
3.12 Representation of voltage dependent and current dependent losses . . . . . . 393.13 Effect of source resistance on the converter performance . . . . . . . . . . . . 40
3.14 Energy efficiency vs backup time . . . . . . . . . . . . . . . . . . . . . . . . 41
3.15 Energy harvested vs backup time . . . . . . . . . . . . . . . . . . . . . . . . 42
3.16 Plot of speed vs time in CVD mode . . . . . . . . . . . . . . . . . . . . . . . 43
3.17 Plot of output voltage vs time in CVD mode . . . . . . . . . . . . . . . . . . 43
4.1 Placement of the position sensors and their output signals . . . . . . . . . . 46
4.2 Flow chart of the position sensor interrupt . . . . . . . . . . . . . . . . . . . 47
4.3 Sensorwise IGBT gate trigger logic diagram . . . . . . . . . . . . . . . . . . 47
4.4 Power and control schematic in CCA mode . . . . . . . . . . . . . . . . . . . 48
4.5 Power and control schematic in CVD mode . . . . . . . . . . . . . . . . . . . 49
4.6 Flow chart of the key pad interface routine . . . . . . . . . . . . . . . . . . . 50
4.7 Flow chart of the display routine . . . . . . . . . . . . . . . . . . . . . . . . 51
4.8 Flow chart of the timer interrupt service routine . . . . . . . . . . . . . . . . 52
4.9 Flow chart of the main program . . . . . . . . . . . . . . . . . . . . . . . . . 53
4.10 Block diagram of the test setup . . . . . . . . . . . . . . . . . . . . . . . . . 544.11 Various losses and their sources in the system . . . . . . . . . . . . . . . . . 55
4.12 Iron losses in the machine in no load test . . . . . . . . . . . . . . . . . . . . 56
4.13 Speed vs time in the retardation test with flywheel . . . . . . . . . . . . . . 57
4.14 Losses in the machine with and with out flywheel . . . . . . . . . . . . . . . 58
4.15 Equivalent circuit of boost converter for copper loss calculation . . . . . . . . 58
4.16 dc bus voltage control with P o = 818 watts . . . . . . . . . . . . . . . . . . . 60
4.17 dc bus voltage control with P o = 450 watts . . . . . . . . . . . . . . . . . . . 61
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List of Figures xiii
4.18 Voltage gain as a function of duty cycle with α = 0.024 . . . . . . . . . . . . 62
4.19 Source resistance as a function of duty cycle . . . . . . . . . . . . . . . . . . 63
4.20 Overall efficiency as a function of back up time . . . . . . . . . . . . . . . . 65
4.21 Energy harvested as a function of back up time . . . . . . . . . . . . . . . . 66
4.22 Armature current waveform in CCA mode at speed = 2200 RPM . . . . . . 66
4.23 Armature current waveform in CCA mode at speed = 9640 RPM) . . . . . . 67
4.24 Armature current waveform in CVD mode . . . . . . . . . . . . . . . . . . . 67
C.1 Block Schematic of DSP Board . . . . . . . . . . . . . . . . . . . . . . . . . 76
F.1 Bi-directional Power Converter . . . . . . . . . . . . . . . . . . . . . . . . . 80F.2 Brushless DC machine coupled to Flywheel . . . . . . . . . . . . . . . . . . . 81
F.3 Test set up of Flywheel Energy Storage System . . . . . . . . . . . . . . . . 82
G.1 Various losses and their sources in the system . . . . . . . . . . . . . . . . . 84
G.2 Losses in the machine with and with out flywheel . . . . . . . . . . . . . . . 85
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Nomenclature
Symbols : Definitions
α : Ratio Rs
RL
B : Airgap flux density in Tesla
C : Filter capacitor connected across DC bus
D : Duty cycle of switching waveform
Dmax : Steady state maximum duty cycle
Dm : Diameter of the machine
Dmin : Steady state minimum duty cycle
ˆd : Perturbation in duty cycleeb : Instantaneous back EMF induced in armature coils per phase in Volt
E b : Peak value of back EMF induced in steady the state
η : Efficiency
f s : Switching frequency in Hz
H c : Coercivity of the permanent magnet
I a : Armature current in steady the state
ia : Instantaneous armature current
ˆia : Perturbation in armature currentI aRMS : RMS value of armature current
I ∗ : DC current reference
I dc : DC bus current in Ampere
I fb : DC bus current feedback
I Lmax : Peak value of armature current in Amp.
J : Rotational moment of inertia of the flywheel
K i : Current feedback constant
xiv
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Nomenclature xv
K v : Voltage feedback constant
La : Armature leakage inductance per phaseL : Active length of the machine
Lch : External series Inductance per phase
lg : Length of air gap in meters
Lt : Total circuit Inductance (Lt = La + Ls)
N : Number of conductors per slot
ω : Rotor mechanical speed in rad/sec
P ei : Current dependent eddy current loss in the core in watts
P eφ : Flux dependent eddy current loss in the core in wattsP h : Hysteresis loss in the core in watts
Φa : Flux produced by armature current
Φf : Flux produced by field poles
P m : Mechanical losses in watts
p : Number of poles
Ra : Winding resistance in ohms
RL : Load resistor connected across DC bus
Rs : Equivalent source resistance as seen by boost converterR : Stator bore diameter
S : Number of slots per phase per pole
S r : Rotor speed in RPM
T bu : Backup time in seconds
tf : Fall time of the IGBT
T g : Generated torque in the machine
T l : Load torque
T PT : Pole transition timetr : Rise time of the IGBT
T s : Period of switching waveform in sec
V B : Instantaneous voltage applied across B-phase armature winding
V dc : DC bus voltage in Volts
V dc : Perturbation in DC bus Voltage
V ∗ : DC voltage reference
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xvi Nomenclature
V fb : DC bus voltage feed back
V line : RMS value of line voltage in VoltV ph : RMS value of phase voltage in Volt
V R : Instantaneous voltage applied across R-phase armature winding
V Y : Instantaneous voltage applied across Y-phase armature winding
w : Width of magnet in meters
Abbreviations
BDC : Bi-directional Converter
BLDC : Brushless DC
CCA : Controlled Current AccelerationCCM : Continuous Current Mode
CVD : Constant Voltage Deceleration
FES : Flywheel Energy Storage
PMSM : Permanent Magnet Synchronous Machine
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Chapter 1
Introduction
1.1 Introduction
In many drives applications such as traction, elevators, cranes etc, it is commonplace to
encounter loss of stored energy. The main reason is that, the power converters are not capable
of returning the stored energy during transients. In application where frequent transientsare involved, this results in substantial loss of energy. Bi-directional converters in such
applications can lead to higher operating efficiency. In a typical traction application, stored
energy while running can be restored during deceleration. This process saves the energy and
improves the efficiency. Such applications need a bi-directional interfacing converter. The bi-
directional converter facilitates the energy flow, to and from the device. Basic requirements
of this bi-directional converter are, simple structure, ease of control and energy efficiency.
An attempt is made to develop one such interfacing converter.
The topic addressed in this thesis is a bi-directional power converter driving a two quadrantbrushless DC (BLDC) machine. Such a system is simple in structure; easy to control and well
suited for several low cost applications such as electric traction, material handling equipments
etc. The storage device employed in this application is a flywheel. The bi-directional power
converter (BDC) drives the brushless DC (BLDC) machine coupled to the flywheel. The
total system is a Flywheel Energy Storage (FES) system. This thesis covers the analysis,
design, fabrication and evaluation of such a system applied to FES. Key features of the
system are demonstrated and the contributions are outlined.
1
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2 Chapter 1. Introduction
Ifb
BDC
Vfb
Vdc
Controller
Controller
(a)
(b)
ω
T
ω
T
Motor
Generator
BDC
vdc
−
+
−
+
Figure 1.1: Block schematic of two quadrant BLDC drive
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1.2. Two quadrant BLDC drive 3
1.2 Two quadrant BLDC drive
A typical two quadrant drive is shown fig 1.1. The power flow direction in motor mode is
shown in fig 1.1(a). This mode of operation is identified as controlled current acceleration
(CCA) mode. Once acceleration is complete, the motor will be running at constant speed
drawing the loss power from the dc source. The power flow direction in generator mode is
shown in fig 1.1(b). This mode of operation is identified as constant voltage deceleration
(CVD) mode. In this mode, the energy in the flywheel is transfered to the dc bus at constant
voltage.
This system can be divided into following subsystems.
• BLDC machine.
• Bi-directional power converter (BDC).
• Controller.
1.2.1 BLDC machine
In the FES system, the electrical machine accelerates the flywheel during charging and
discharge the flywheel during deceleration. The following are the key features of the machine.
• Capability to be operated at speeds of the order of 10,000 RPM or above.
• Capability to be operated as a motor or a generator during charging or discharging of
the flywheel, respectively.
• High efficiency.
1.2.1.1 Source of Loss and its reduction
In an energy storage/retrieval application, the energy loss is a major concern. The losses in
the machine are listed as,
• Iron losses in the armature magnetic material.
• Copper losses in the armature conductors.
• Friction and windage losses in the rotational system.
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4 Chapter 1. Introduction
The iron loss is a function of property of the core material, its quantity, operating frequency
and flux density. In general, two pole machines are preferred for the high speed motoring
operation since the value of the operating frequency is lower. This in turn, results in lower
iron loss. Iron loss in the machine is related to the specific magnetic loading in the air-gap
(B) [5]. Copper loss in the machine is related to specific electric loading in the armature
periphery (J A/m) [5]. Low electric and magnetic loading will make the losses low; the
machine will however, be bigger.
1.2.1.2 Machine Type
It is preferred to have a commutator-less machine, as it eliminates frequent maintenance
problems, reduce the EMI and increase the efficiency. Permanent Magnet Synchronous
Machines (PMSM) or Brushless DC (BLDC) machines can be used because they can be
operated as generator or motor conveniently. PM machines use magnets to produce air-gap
magnetic flux instead of field coils. This configuration eliminates rotor copper loss as well
as the need for maintenance of the field exciting circuit. This has been made possible by the
easy availability of high performance permanent magnets with high coercivity and residual
magnetism, such as Samarium cobalt and Neodyum-Iron-Boron (NdFeB) magnets. It can
be shown that a permanent magnet excitation circuit does not use any copper and saves 30
percent of the iron used in comparison with an electromagnetic excitation.
1.2.1.3 Construction
The permanent magnet machines consist of a three phase stator windings similar to that of
induction machine and a rotor with permanent magnets. The machine characteristics depend
on the magnets used and the way they are located in the rotor. The permanent magnets
(PM) are either mounted on the surface or buried in the interior of the rotor. Accordingly
they are called as Surface Mounted PM machines or Interior PM machines. PM machines
can be broadly classified into two categories [6].
• Sinusoidal waveform machines: These machines have a uniformly rotating stator field
as in induction machines. The stator winding is sinusoidally distributed or the mag-
nets are shaped to get sinusoidal induced voltage waveforms. Hence sinusoidal stator
currents are needed to produce ripple free torque.
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1.2. Two quadrant BLDC drive 5
• Trapezoidal waveform machines: These are known as brushless DC or electronically
commutated DC machines. Induced voltage is trapezoidal in its shape. The con-
centrated windings on the stator are the reason for the trapezoidal-shaped back emf
waveform. The armature current is switched in discrete steps. The control of such mo-
tors is very simple. Only six discrete rotor positions per electrical revolution are needed
in a three-phase machine to synchronize the phase currents with phase back emfs for
effective torque production. A set of hall effect sensors are mounted on the armature
to provide rotor position information. This eliminates the need for high-resolution
encoder or position sensor required for the PMSM [6]. The back emf waveforms are
fixed with respect to rotor position. Square wave phase currents are supplied such that
they are synchronized with the back emf peak of the respective phase. The controller
achieves this using the rotor position feedback information. The motor basically oper-
ates like a DC motor, with such a controller configuration, from a control point of view;
hence the motor is designated as a brushless DC motor. There are several advantages
of using PM for providing excitation in AC machines. Permanent magnets provide loss
free excitation in a compact way without complications of connections to the external
stationary electric circuits. These types of machines become very attractive option due
to their high torque densities, high power density, excellent performance and with lowrotor losses [6].
1.2.1.4 Sizing
The machine has to deliver rated power only for a short time - during acceleration while
charging and during break time while discharging. In the motoring mode the machine is
required to provide only the losses under steady state. In other words, the machine is used
for intermittent operation only. A low loss short time rated PM machine is the best choice.
1.2.2 Bi-directional Power Converter
1.2.2.1 Power Circuit
The bi-directional power converter is of the voltage source bridge topology. Such a converter
can transfer power from a constant voltage dc source to an ac load, or from an ac source
to a dc voltage bus. The power converter is made up of solid-state devices. This controls
the flow of bulk power from source to motor terminals or from generator terminals to the
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6 Chapter 1. Introduction
dc power bus. The schematic of the converter is shown in the fig 1.2. In drive mode, the
converter operates as a standard voltage source inverter with six switches. In the generating
mode, the same converter operates as 3-phase boost converter pumping energy from each of
the phases to the dc bus.
ea
eb
ec
La
La
La
LoadC
+dc bus
−dc bus
T1 T3 T5
T2T6T4
Figure 1.2: Bi-directional Power Converter circuit
Current
Time
Load current
Armature current
Figure 1.3: Current vs Time characteristics
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1.2. Two quadrant BLDC drive 7
1.2.2.2 Sizing
In drive mode (CCA), the power converter supplies the required power to the motor, to
accelerate the flywheel and to keep it running. With the closed loop current controller, the
current during the acceleration is controlled. In the generating (CVD) mode, bi-directional
power converter supplies the rated power at constant voltage to the load connected across
the dc bus. The characteristics of the bi-directional converter in CVD mode is shown in the
fig 1.3. It may be noticed that, the machine (armature) current increases (to deliver constant
power) as the flywheel is slowing down. Accordingly the current rating of the converter is
decided based on the output voltage and power requirement, at lowest operating speed of
the flywheel.
1.2.3 Controller
The controller block is shown in the fig 1.1. This is based on Digital Signal Processor. The
controller senses the input commands and feedback signals. These signals are processed to
generate the switching pulses for the power converter semiconductor switches.
The motor is supplied with controlled current at the desired frequency through the bridge
circuit. This ac current decides the accelerating torque of the motor. This is the operating
mode during charging of the flywheel. This may be termed as the current (ac) controlled
acceleration (CCA) mode.
In the discharge duration, the same converter transfers the energy from the flywheel through
the machine to the dc bus. In this mode, the machine voltage varies widely since the
flywheel is decelerating and loosing the energy. However, the converter is capable of operating
over a wide range of machine voltage, pumping power to dc bus at constant voltage. This
mode is called the constant voltage (dc) deceleration (CVD) mode. The current versustime characteristics of the boost converter is shown in the fig 1.3. The voltage versus time
characteristics of the boost converter is shown in fig 1.4. It may be seen from the figures 1.3
and 1.4, that with the boost converter it is possible to draw power at constant voltage right
down to 50 percent of the speed. This corresponds to a harvest of 75 percent of the stored
energy.
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8 Chapter 1. Introduction
Tbu
Energy harvestingtime
Input Voltage
Output voltage
Voltage
Time
Figure 1.4: Voltage vs Time characteristics
Vfb
Bearing (B)Ifb
DSP 56F805
Motorola make
Controller Converter(D)
(C)
(E)
In Out
(A)
SignalsPositionRotor
dc power
Two−Quadrant
drive motor
Flywheel
Bi−directional
Figure 1.5: Typical flywheel energy storage system
1.3 Overall system
FES system has the following subsystems.
• Flywheel that stores the energy (A).
• Bearings that support the rotor and the flywheel (B).
• Two quadrant drive motor (C).
• Bi-directional power converter (D).
• DSP based electronic controller (E).
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1.3. Overall system 9
1.3.1 Principle of working of the system
The flywheel is coupled to the rotor of the electrical machine. This machine is used to accel-
erate the flywheel (charging mode). The same machine discharges the flywheel (deceleration
mode) to provide power to the dc bus. The machine acts as a motor during charging and as
a generator during discharging. The machine is therefore driven by a bi-directional power
converter as shown in the fig 1.5. To store the energy in the flywheel it is run up to the
rated speed of the motor. Under this condition, the flywheel stores the energy. When the
input power fails, the flywheel continues to run due to its inertia driving the generator. If an electrical load is connected across the generator terminals, it draws current and utilizes
this energy. In this process, the flywheel discharges its energy and decelerates. Speed-time
and voltage-time characteristics are shown in the fig 1.6 and fig 1.7.
0
2000
4000
6000
8000
10000
0 20 40 60 80 100
S p e e d i n R P
M
Time in seconds
Figure 1.6: Speed vs Time characteristics
The amount of power transferred to the load and its duration is a function of the running
speed, overall efficiency and the control strategies adopted. From fig 1.7 it is seen that
the terminal voltage of the machine exponentially drops during deceleration. It is desired
that power harvested from the flywheel during the deceleration is at constant voltage. The
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10 Chapter 1. Introduction
0
50
100
150
200
250
300
0 20 40 60 80 100
G e n e r a t o r v o l t a g e i n v o l t s
Time in seconds
Figure 1.7: Voltage vs Time characteristics
bi-directional power converter serves two purposes. It accelerate the motor while charging
and discharges the flywheel during deceleration. Further, the control in the converter can
be exercised to obtain constant dc output voltage for a wider range of flywheel speed. More
about the control methodologies are explained later in the chapter 3.
1.4 Scope of the Thesis
This thesis covers the Analysis, Design, Fabrication and Evaluation of a two quadrant bldc
drive. This system consists of a bidirectional converter/controller, a brushless dc machine
and a flywheel. The scope of the work is partitioned as follows,
• Overall system design
• Basic design and performance analysis of the brushless dc machine.
• Dynamic modeling and development of control strategy for the power converter to
operate as bldc drive in motoring mode (Current controlled acceleration) & as boost
converter in generating (constant voltage deceleration) mode.
• Verification of the design through simulation
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1.5. Organization of the thesis 11
• Development of a suitable control platform (DSP) and the associated software to realize
the control strategy.
• Development of suitable Human Machine Interface (HMI).
• Performance evaluation of the full system.
1.5 Organization of the thesis
The thesis is organized as follows:
Chapter 2: Chapter 2 covers the design and performance of the bldc machine. Basic de-
sign of a permanent magnet bldc machine under idealized condition is presented. The bldc
machine constructed employs standard sizes of rotor/stator frame/stampings etc, that are
readily available. The Finite Element Method of analysis is carried out to verify the basic
design. The analysis results are validated through the speed vs emf characteristics of the
machine running as a generator.
Chapter 3: Chapter 3 presents the bi-directional power converter. The operating modes
are identified. The circuit topologies of the converter and defining equations of the drive in
acceleration and deceleration modes are presented. The control strategies under both modes
of operation are also outlined. Design considerations like device rating, critical values of in-
ductor and dc link capacitor, Switching frequency are high lighted. Effects of non-idealities
in the machine parameters and other circuit components on the performance of the BDC
are also discussed. Simulink modeling of the overall system in CVD mode is also presented
in this chapter.
Chapter 4: The central processing unit of the controller employed is a Digital Signal
Processor (DSP). The hardware realization of the power converter and the DSP controlleris presented in this chapter. The software issues covering flow charts, priority allocation
and scheduling of events etc, are presented. Experimental results covering the charging and
discharging mode of the FES system are presented to validate the design.
Chapter 5: Chapter 5 gives contributions and the conclusions made from this work. Fea-
tures of this system and important findings of this thesis are presented. Design guidelines
emerging from this work, the spin off technology from the present system and other appli-
cations of the system are presented.
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12 Chapter 1. Introduction
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Chapter 2
Selection, Design and Analysis of
Brushless DC Machine
2.1 Introduction
This chapter covers the selection, basic design, construction, analysis and testing of the
brushless dc machine. Equation for induced voltage in the machine is obtained analytically.
Analytical computation of induced voltages are verified by the FEM analysis and validated
through the experimental results.
2.2 Selection of the type of machine
The prime mover for the flywheel energy storage system is an electrical machine. The
machine accelerates the flywheel to charge the same and discharge the flywheel during the
deceleration.
Following are the key features of the machine.
• Capability to be operated at a speed of the order of 10,000 rpm.
• Capability to be operated as a generator or a motor during charging and discharging
mode respectively.
• High efficiency.
• No brushes.
13
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14 Chapter 2. Selection, Design and Analysis of Brushless DC Machine
Permanent Magnets
Radius
L e n g t h
Rotor
Armature conductors
Figure 2.1: Cross sectional view of basic PM BLDC machine
Permanent magnet synchronous machine (PMSM) or Brushless DC (BLDC) machine can be
used for the above features. This is due their capability to be operated as a generator or a
motor conveniently. BLDC machine is selected for this application on account of simplicity
of control.
2.3 Basic design of the BLDC machine
With reference to fig 2.1, basic design of the machine has been done by using idealized
permanent magnet BLDC machine equations [1]. Design specifications of the machine are
as follows:
dc bus voltage (V dc) = 300V.
Out put power (P o) = 1000W.Specific electric loading (J) = 12000A/m.
Specific magnetic loading (B) = 0.2 T.
Efficiency (η) = 0.75.
Rotor speed (S r) = 10,000 RPM.
It may be noted that the flux density taken is 0.2 T. The operating frequency will be 333
Hz at 10,000 RPM. To reduce the iron losses in the magnetic material of the machine, the
air gap flux density is kept substantially low. Idealized design equation relating the machine
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2.4. Construction 15
output power to the mechanical dimensions is given by [1] [2],
P o = π2
BJ Dm
2
LS rη60
(2.1)
Substituting the design parameters in the equation 2.1, we get,
• Dimensions:
Dm2L = 338 e -6 m3; If L and Dm are same in magnitude, then the we have,
L = 70 mm, Dm = 70 mm.
Nearest dimensions of the standard available frame is Dm = 86 mm and L = 75 mm.
• Coil current:
I aRMS =
2
3P o
V dcη = 3.64A. (2.2)
• No of slots and poles:
No of slots and poles are selected as 36 and 4 respectively.
• Turns per coil:
Voltage per turn = Φtotal
T PT , Where T PT is the pole transition time.
e1 =2BL
πDm
4 (4S r)
60 = 1.35V. (2.3)
Total no of turns = V dc
e1 = 222 Turns.
Turns per coil = Total no of turns / (2 * No of slots per phase/2) = 18.5 Turns.
A suitable magnet of dimensions 12 mm x 18 mm is selected for the permanent magnets.
With the above design data, the machine is fabricated. The design validation is done through
back emf test as well as the FEM analysis of the magnetic circuit of the machine.
2.4 Construction
The winding distribution is uniform throughout the periphery of the stator as shown in the
fig 2.2. Permanent magnets are buried in the interior of the rotor as shown in the fig 2.3.
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16 Chapter 2. Selection, Design and Analysis of Brushless DC Machine
α( )t
L
28
29
10
11
20 19
35 1363
B
R
Y
γ
R2
B
Y
Figure 2.2: Cross sectional view of PM BLDC machine
These magnets are used for producing air-gap magnetic flux. The magnetic field produced
in the air gap is shown in the fig 2.4. A set of hall sensors are mounted on the stator,
facing the rotor magnets. These sensors are placed 120o (Electrical) apart to give the rotor
position information. The machines with this type of construction are known as brushless
dc or electronically commutated dc machines.
Rotor shaft
Magnets
0o
90o
o
270o
180
Figure 2.3: Placement of magnets in the interior of rotor
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2.5. Equation for the induced voltage 17
Figure 2.4: Magnetic field produced in the air gap
2.5 Equation for the induced voltage
The arrangement of the magnets in the rotor and their magnetization directions are shown
in the fig 2.4. The flux distribution in the air gap is rectangular in shape. This is shown in
the fig 2.5. Following assumptions are made in order to derive the expression for the induced
voltage in terms of electrical, magnetic and mechanical quantities.
• No saturation in the active magnetic material circuit of the core.
• No eddy current and hysteresis losses.
• Air gap is uniform
• Magnets have infinite resistivity.
• Permeability of the magnet is equal to that of air.
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18 Chapter 2. Selection, Design and Analysis of Brushless DC Machine
Flux density
0 90o 180o 270oθm
B
−B
Figure 2.5: Aig gap flux density vs mechanical angle
Vphase
Vline
R Y B
Figure 2.6: Stator winding connection
2.5.1 Induced voltage equation
The flux linked by each coil as a function of mechanical angle is shown in the fig 2.7(a). The
induced voltage across a conductor moving in the magnetic field is given by,
e = BLv (2.4)
Where ‘B’ is the flux density in Tesla, ‘L’ is the length of the conductors in meters and ‘v’
is the relative velocity of the conductor w.r.t the flux.
Using this relation one can find the total emf induced in one coil (there are N conductors in
one coil). It is given by,
eb = 2BLNR ω (2.5)
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2.5. Equation for the induced voltage 19
φa
(a)
(b)
(c)
(d)
(e) V
3V
e2
e3
V
−V
θ
θm
θm
θm
m
0o 60o40o20o 80o 100o 120o 140o 160o 180o
60o
e1
θm
Flux
−3V
−V
Figure 2.7: Induced voltage as a function of time
The shape of the induced voltage is rectangular as shown in fig 2.7(b). Induced voltages in
the conductors in all other slots are also identical in their shapes and magnitudes. But they
are shifted in phase depending on their position in the stator slot. As all the coils are in
series, the total induced voltage across the coil is the algebraic sum of the induced voltage
of each coil as shown in fig 2.7(e). Peak of the line voltage is given by the equation 2.7.
Total induced emf per phase per pole is of trapezoidal wave shape. The peak value is
given by,
V = 6BLNR ω (2.6)
For a ‘p’ pole machine, this value is multiplied by p
2. The machine under consideration is
a four pole machine; the phase voltage is two times V. The line voltage as referred in the
fig 2.6, is given by equation 2.7.
V line = 2E b = 24B pLNR ω (2.7)
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20 Chapter 2. Selection, Design and Analysis of Brushless DC Machine
The plot of the induced voltage waveforms is given in the fig 2.7.
2.5.2 Computation of phase and line voltages
Machine data:
ω = 1047.2rad/s (Corresponding to a speed of 10,000 rpm)
B = 0.18 T
L = 0.075 m
N = 19
R = 0.043 m
Air gap length = 1.1 mm.
Substituting the above data of the machine in the equations 2.7, we get,
V ph = 138.6 V
V line = 277.2V
The line to line induced voltage can also be expressed as a function of speed in RPM. This is
done by substituting the actual dimensions in the above equation. If S r is the motor speed,
the equation 2.7 becomes,
V line = 2E b = 0.0277S r (2.8)
2.6 Armature leakage inductance and coil resistance
Following assumptions are made to find an expression for leakage inductance.
• There are no slots in the stator and the air gap is uniform throughout the periphery.
• The flux produced by one armature coil is not linked by the other armature coil.
• The relative permeability of magnets used is equal to 1.0.
Inductance of the coil is given by,
La = N 2
R (2.9)
Where ‘N’ is the no of turns and the ‘R’ is the reluctance of the magnetic path. The path
of the flux produced by an armature conductor and the equivalent magnetic circuit is shown
in fig 2.8 and fig 2.9 respectively. It may be noted that, the ‘R’ is the series combination of
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2.6. Armature leakage inductance and coil resistance 21
w
Air gaps length
MagnetPermanant
Armature coil l
l
g
g
Rotor core
Stator core
Figure 2.8: Lumped parameter equivalent circuit for the calculation of leakage inductance.
Rm
R
aNI
aφ
g1
Rg2
Figure 2.9: Magnetic equivalent circuit of the armature leakage inductance.
Rg1 (reluctance of air gap1), Rm (reluctance of magnet) and Rg2 (reluctance of air gap2).
Therefore,
R = Rg + Rm + Rg (2.10)
R =
4πDmLµo
(lg + w + lg) (2.11)
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22 Chapter 2. Selection, Design and Analysis of Brushless DC Machine
b2E
Vline
+
_
(2.096mH) (0.752 Ω)
1.94mH 0.776 Ω
Figure 2.10: Equivalent circuit of the machine
Using this relation we get,
La = 2
3N 2πDm
4 Lµo
(2lg + w)
= 0.97mH (2.12)
Where lg (= 1.1 mm) is the air gap length and w (= 12 mm) is the width of the magnet.
The winding resistance is obtained from the conductors cross section (ac = 0.519mm2), mean
length of a turn (L = 0.30m), number of turns (T = 19) and resistivity (ρ = 0.0177Ωm/mm2)
of the copper as given below:
Ra = 2 ρLT
ac = 0.388Ω. (2.13)
The measured values are as follows:
La = 1.048mH.
Ra = 0.376Ω.
The equivalent circuit of the machine is shown in fig 2.10.
2.7 Model of the machine
The model of the machine is as shown in the fig 2.10. The values shown inside the parenthesisare the measured values of inductance and resistance.
2.8 FE Method of Analysis
The machine is analyzed using the FEM software package ‘Magnet’. Mechanical model of
the machine (shape and actual dimensions) and the properties of the materials used in the
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2.9. Experimental results 23
construction of the machine are fed and the analysis is carried out. Following are the machine
parameters given as input.
Figure 2.11: FEM generated air gap flux density plot
Stamping material : Rote alloy
No of stator slots: 36Core length : 75 mm
Stator inside dia : 86 mm
Turns per coil : 19
Hc in kA/m : 723
Radial air gap length : 1.1 mm
Plots of flux density vs mechanical angle and induced voltage vs time are obtained and given
in the fig 2.11 and fig 2.12.
2.9 Experimental results
A flywheel is coupled to the machine. Machine was run up to a speed of 10,000 rpm and
dc voltage applied was 315 volts. Then the supply has been cut off and the machine was
allowed to decelerate freely. The motor speed and the corresponding induced voltages are
recorded. The induced voltage versus the generator speed is plotted as shown in the fig 2.13.
The induced voltage waveform recorded is shown in fig 2.14.
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24 Chapter 2. Selection, Design and Analysis of Brushless DC Machine
Figure 2.12: FEM generated induced voltage waveform plot at 10000 RPM
0
50
100
150
200
250
300
350
400
0 2000 4000 6000 8000 10000
I n d u c e d v o l t a g e i n v o l t s
Speed in RMP
Figure 2.13: Induced voltage vs speed characteristics
The value of induced emf was computed using analytical and FE methods. The induced
emf is measured by experimental method also. The results are shown in the table 2.1.
Method Induced Voltage Wave shape Speed
Analytical Method 138.6 Trapezoidal 10,000rpm
FE Method 135 Trapezoidal 10,000 rpm
Experimental 120 Trapezoidal 10,000 rpm
Table 2.1: Comparison of results at 10000 RPM
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2.10. Conclusions 25
Figure 2.14: Measured induced voltage waveform of the machine at 10,000 RPM
(Time: 1ms/div, Voltage: 50V/div)
2.10 Conclusions
In this chapter the idealized BLDC machine performance equations were given. An ideal-
ized basic design was carried out to get the key geometrical measures of the machine. The
same was modified to take into account the standardized frame sizes available. The ma-chine was fabricated and the design was validated through back emf measurement and the
equivalent circuit measurement. This equivalent circuit is used in the next chapter to design
the bi-directional power converter to drive the machine. A suitable digital controller for the
converter also presented in the chapter-4.
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26 Chapter 2. Selection, Design and Analysis of Brushless DC Machine
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Chapter 3
Bi-directional Converter
3.1 Introduction
The bi-directional power converter (BDC) facilitates running up the flywheel in the presence
of power supply and recovering the stored energy from flywheel when power fails. This
chapter covers the BDC. The operating modes are identified; control strategies are developed.
The system design and the performance evaluation through simulation is presented in this
chapter.
3.2 Power converter
Power converter circuit diagram and the output waveforms are shown in the fig 3.1 and
fig 3.2 respectively.
VR
Ra
La
VY
VB
Ra Ra
La
La
eb
eb
eb
/2dc
V /2dc 1
4
3
6
5
2
Load
+dc bus
−dc bus
−
+
−
+
Figure 3.1: Circuit diagram of the bi-directional power converter
27
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28 Chapter 3. Bi-directional Converter
VR
VY
VB
36003000240018001200600
−Vdc
2
dcV
2
00 t
t
t
Figure 3.2: Converter output voltage waveforms in CCA mode
The voltage is applied across the windings by turning ‘ON’ the appropriate combination of switches in the converter. Two switches will be ‘ON’ at any time; each device conducts for a
duration of 120o. Commutation will take place every 60o. One can divide the whole cycle into
six intervals of 60o each. The equivalent circuit for the intervals 0−60o, 60o−120o, 180o−240o
and 240o − 300o are shown in the fig 3.3. The path of the current is shown in bold line.
From the figure it can be seen that, the current from dc bus flows through two switches and
two armature coils at any instant. The equivalent circuit is as shown in fig 3.4.
The dynamic equations of the system can be written as,
2La
diadt
= V dcu(t) − 2iaRa − 2eb (3.1)
J dω
dt = T g − Bω − T l (3.2)
Where u(t) = 1 during +ve half cycle and u(t) = -1 during - ve half cycle.
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3.2. Power converter 29
VR
Ra
La
VY
VB
Ra
Ra
La
La
VR
Ra
La
VY VB
Ra
Ra
La
La
VR
Ra
La
VY VB
Ra
Ra
La
La
VR
Ra
La
VY
VB
Ra
Ra
La
La
eb
eb
eb
eb
eb
eb
eb
60000 −
1200
600 −
3000
2400−
2400
1800
Vdc /2
Vdc /2
Vdc /2
Vdc /2
Vdc /2
Vdc /2
Vdc /2
Vdc /2
−
+
eb
Ia
Ia
Ia
Ia
1
4
3
6
5
2
Load
1
4
3
6
5
2
Load
1
4
3
6
5
2
Load
1
4
3
6
5
2
Load
+dc bus
−dc bus
+dc bus
−dc bus
+dc bus
+dc bus
−dc bus
−dc bus
Interval1
Interval2
Interval3
Interval4
−
eb
eb
+e
b
eb
−
+
−
+
−
+
−
+
−
+
−
+
−
+
−
+
Figure 3.3: Current path during different intervals of time in the CCA mode
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30 Chapter 3. Bi-directional Converter
Vdc
2
Vdc
2
VR
Ra
La
VY
Ra
La
RL
eb
eb
+
−
6
31
4
+dc bus
− dc bus
aI
−
+
−
+
Figure 3.4: Equivalent circuit of converter in CCA mode
3.3 Transfer function of BDC in CCA mode
3.3.1 Simplified Equivalent circuit
During the charging of the flywheel, the armature current of motor is controlled. The
armature current of the motor can be controlled by adjusting the duty cycle of the applied
voltage. Fig 3.4 can be re-written as shown in fig 3.5
2e b
2Ra2La
Vdc
OFF
ON
Figure 3.5: Simplified equivalent circuit of converter in CCA mode
The dynamic equations are given by, (For period DT s; Switch is ON)
2La
diadt
= V dc − 2iaRa − 2eb (3.3)
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3.4. Transfer function of BDC in CVD mode 31
(For period (1 − D)T s; Switch is OFF)
2La diadt
= −(2iaRa + 2eb) (3.4)
3.3.2 Small signal modeling
Averaged equation is given by,
2La
diadt
= V dcD − 2iaRa − 2eb (3.5)
Assume that the current controller response time is much smaller compared to the mechanical
time constant. Consider the perturbations ia = I a + ia, d = D + d and eb = E b (Mechanical
time constant is high; Speed change takes place slowly). The small signal model is as follows,
2La
diadt
= V dc d − 2iaRa (3.6)
Current control transfer function of the system is,
ia(s)
d(s)=
V dc2(Ra + sLa)
(3.7)
3.4 Transfer function of BDC in CVD mode
When the flywheel is decelerating, the machine works as a generator. The bottom switches
of BDC are continuously gated with pulses with switching frequency f s; top switches are
kept OFF. The control objective is to keep the dc bus voltage constant. For this purpose,
it is necessary to obtain the dc bus voltage control transfer function. Induced voltages of
machine is as shown in fig 3.6.
The current path in the converter during the positive half cycle of R-phase (30o − 150o) is
shown in fig 3.7.
From the fig 3.7, it may be observed that,
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32 Chapter 3. Bi-directional Converter
o30 o900o 150o 210o 270o 330o
t
t
t
VR
VY
VB
b
−Eb
E+
Figure 3.6: Induced voltage waveforms of the machine
• When active switch is ON,
The generator terminals are short circuited through one active ‘ON’ switch, one diode
and two series inductors (2La). The current path is shown in fig 3.7. The machine
current increases. Part of the energy in the flywheel is now transfered to the machine
inductance.
• When active switch is OFF
The generator terminals are connected to the dc bus through two diodes and two
inductors (armature leakage) in series. The current path is shown in fig 3.7. The
inductor current is now pumped into the dc bus and to the load. Eventually the
inductor current drops down.
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3.4. Transfer function of BDC in CVD mode 33
VR
Ra
VY
VB
Ra
Ra
VR
Ra
VY VB
Ra
Ra
VR
Ra
La
VY VB
Ra
Ra
La
La
VR
Ra
La
VY
VB
Ra
Ra
La
La
Activeswitch
"OFF"
Activeswitch"ON"
switch
"OFF"
Active
Active
"ON"switch
Ia
Ia
eb
eb
eb
eb
eb
eb
eb
eb
eb
eb
eb
+
−
+
−
Ia
Ia
+
−
La
La
La
La L
a L
a
300
Interval 900−150
0
eb
1
4
3
6
5
2
Load
1
4
3
6
5
2
Load
1
4
3
6
5
2
Load
1
4
3
6
5
2
Load
+dc bus
+dc bus
+dc bus
+dc bus
−dc bus
−dc bus
−dc bus
−dc bus
Interval
2C
2C
2C
2C
2C
2C
2C
2C
−+
−900
Ia
Ia
Figure 3.7: Current path during different intervals of time in the CVD mode
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34 Chapter 3. Bi-directional Converter
RL
Vdc
2La
2Ra
CON
OFF
2eb−
+
Figure 3.8: Equivalent circuit of converter in CVD mode
3.4.1 Simplified equivalent circuitDuring the deceleration of the flywheel, the power is transfered to the dc bus at constant
voltage. This is achieved by adjusting the duty cycle of the active switches. The equivalent
circuit is shown in fig 3.8
The dynamic equations are given by,
• Period: DT s (Switch is ON)
− C dV dc
dt =
V dcRL
(3.8)
2La
diadt
= 2eb − 2iaRa (3.9)
• Period: (1 − D)T s (Switch is OFF)
C dV dc
dt = ia −
V dcRL
(3.10)
2La
dia
dt = 2eb−
V dc−
2iaRa (3.11)
3.4.2 Small signal modeling
Averaged equations are given by,
C dV dc
dt = ia(1 − D) −
V dcRL
(3.12)
2La
diadt
= 2eb − V dc(1 − D) − 2iaRa (3.13)
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3.5. Controller 35
We may consider that the speed (and back emf) changes are slow on account of high me-
chanical time constant. With small perturbations ia = I a + ia, d = D + d, eb = E b and
vdc = V dc + V dc to the system, the perturbed equations are,
C d V dc
dt = ia(1 − D) − I a d −
V dcRL
(3.14)
2La
diadt
= V dc d − V dc(1 − D) − 2iaRa (3.15)
Transfer function of the system becomes,
V dc(s)
d(s)
= 2E b
(1 − D)2 ×
(1 −2sLa
RL(1 − D)2)
1 + 2La
RL(1 − D)2s + 2LaC
(1 − D)2s2
(3.16)
The transfer function of the system is of second order. Its corner frequency is dependent
on the duty ratio also. At minimum (0.1) and maximum (0.75) duty ratios, these corner
frequencies (ωn) are at 97 rad/s and 350 rad/s. The system also has a right half plane zero
with a corner frequency 2812 rad/s. The response time of the controller of 0.5 second (which
is 2 percent of the power supply backup time) is considered. With a controller band width
of around 10 rad/s (corresponding to a response time of 0.5 sec), dynamics due to the poles
and zeros of the transfer function can be neglected. Then, it is possible to approximate thetransfer function, simply as a gain.
V dc(s)
d(s)=
2E b(1 − D)2
(3.17)
3.5 Controller
A PI controller is employed for maintaining the current in the motoring (CCA) mode and
voltage in the generating (CVD) mode. The block diagram of the controller in CCA mode
and CVD mode is shown in the fig 3.9 and fig 3.10 respectively. The current controller
will facilitate charging the flywheel while operating in CCA mode. The voltage controller
enable the system to deliver power at constant voltage in CVD mode. The current control
bandwidth (in CCA mode) of 100 rad/s with unity gain and the voltage control bandwidth
(in CVD mode) of 10 rad/s with unity gain are achieved. Protection features like dc over
voltage, dc over current, IGBT pulse inhibition in fault conditions are also provided.
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36 Chapter 3. Bi-directional Converter
Vdc
2(Ra+ sLa)
I1/K
ioCurrent
PI+−
I fb
PWM
^
I*d^ ^
Figure 3.9: Block diagram of the controller in CCA mode
v1/K
b2E V ^dc Voltage
PI+−
V fb
PWM 2(1 − D)
^
d^ V*
Figure 3.10: Block diagram of the controller in CVD mode
3.6 Switching between controllers
The software along with the power condition monitoring circuits, switch the controller from
CCA mode to CVD mode or vice-versa depending on the input power supply condition.
During normal operation the converter is operated as a brushless dc motor drive with current
limit. When the supply fails the converter is operated in boost mode maintaining the dc
bus voltage constant. Switching from CCA mode to CVD mode and vice-versa is done
automatically, depending on the input power supply condition.
3.7 Selection of circuit elements
3.7.1 Series Inductor
• Motoring (CCA) mode
Consider that the armature current reaches the rated current within 10 percent of the
conduction time. The circuit equation for establishing the current is given by,
2Ldiadt
= V dc (3.18)
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3.7. Selection of circuit elements 37
Substituting following data
f = 333Hz at 10,000 rpm; T s = 3ms; Conduction time = 1.0ms.
I a = 3.3A
V dc = 300V
We get, L < 4.55mH.
• Generating (CVD) mode
The condition for the CCM of the boost converter is given by [3],
L > RLDminT s(1 − Dmin)2
2 (3.19)
Substituting following data
T s = 300µs
Dmin = 0.1
RL = 240 Ω Corresponding to minimum load of P o = 375 watts
We get, L > 2.9mH.
The machine armature inductance (La) is 1.0mH/phase; inductance across the two ter-minals of the star connected machine is 2.0 mH. The desired external series inductance
(considering both CCA and CVD mode of operation) is,
0.9mH ≤ Ls ≤ 2.55mH .
An external inductance of 2.5mH/phase has been provided.
3.7.2 DC bus capacitor
The dc bus capacitor is selected based on the voltage ripple considerations [3].
C > I dcDmaxT s
∆V (3.20)
The capacitor ripple voltage component on account of ESR (Equivalent Series Resistance)
is also to be kept within 1.0 percent of V dc. The ESR of the capacitor,
ES R ≤∆V
I = 1Ω (3.21)
Selected capacitor value, C = 1650µF with an ESR of 0.1Ω (at 20oC , 100 Hz).
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38 Chapter 3. Bi-directional Converter
3.7.3 Switches
The current rating of the switches is selected based on the instantaneous maximum value.Maximum current through the switch (which is same as that of inductor) is given by the
relation [3],
I Lmax = 2E b
1
RL(1 − Dmax)2 +
DmaxT sLt
(3.22)
Where, Lt = 2(La + Ls)
Substituting following data
T s = 300µs
Dmax = 0.7
2E b = 180 volts (Lowest input voltage)
RL = 180Ω corresponding to P o = 450 watts
We get I max = 11.0 Amps. Device selected is Semikron make, model: SK30 GD 123 (Data
sheet given in Appendix-A)
3.7.4 Switching frequency
Electrical time constant of the machine is La/Ra. With L = 7.096 mH and R = 2.35 Ω,
this value is 3.02 mSec. The switching period is required to be 10 times less ( ≤ 300µsec).Switching frequency selected is 3.3 kHz (T s = 300µsec).
3.8 Effects of system parameters on the performance
3.8.1 Source resistance of the machine
Energy conversion process can be shown as given in the fig 3.11. Losses in the machine can
be represented as shown in the fig 3.12. The resistance connected in parallel to the source
represents the flux dependent losses. They are, hysterisis loss and flux dependent eddy
current loss of the machine. The series component of the source resistance is a combination
of the armature winding resistance and the current dependent eddy current loss component
of the machine. The series resistance can be calculated as follows [4],
Rs =
P ei
I dc1 − D
2 + 2Ra (3.23)
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3.8. Effects of system parameters on the performance 39
= Vdc Idc Tbackup
(ω2
max− ω2min)= 1/2 J
Available Energy
Flywheel
LossesCopper
LossesIron Power
ConverterLosses
Bearing friction& drag
losses in Flywheel
Bearing friction& drag
losses in Machine
Energy available
at Load
BDCand
Machine
Figure 3.11: Schematic representation of various losses in the system
2Eb Rh Re1
Re2
Rh
Re1
Re2
− Flux dependent eddy current loss component
− Hysteris loss component
BDC
− Current dependent eddy current loss and Copper loss component
Figure 3.12: Representation of voltage dependent and current dependent losses
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40 Chapter 3. Bi-directional Converter
0
1
2
3
4
5
0 0.2 0.4 0.6 0.8 1
V o l t a g e g a i n = 2 E b
/ V d c
Duty ratio
Max voltage gain
M
Figure 3.13: Effect of source resistance on the converter performance
On account of resistance Rs, the voltage conversion ratio during discharging
V dc2E b
gets
degraded. The voltage gain can be found out by the following relation [4]:
V dc =
E b
(1 − D)
1
1 +
Rs
RL
(1 − D)2
(3.24)
The effect of source resistance on the voltage gain is shown in the fig 3.13 [4]. As Rs
increases the maximum voltage gain reduces. Better voltage gain, higher efficiency and
increased backup time are achieved by keeping Rs as low as possible.
3.8.2 Weight of the flywheel
The inertia increases linearly with mass of the flywheel [7]. Stored energy will increase by
the same amount. An increase in the mass of the flywheel however, increases the frictional
loss also and is not desirable.
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3.9. Effect of Backup time on the system performance 41
3.8.3 Diameter of the flywheel
The inertia increases with increasing the diameter of the flywheel in square law [7]. Thereforeincreasing the diameter keeping mass constant will result in increase in the backup time.
3.9 Effect of Backup time on the system performance
• Efficiency :
Overall efficiency of the system can be calculated by the relation,
η = P oT backup
0.5J (ω2
max − ω2
min)
= 1 − P lossT backup
0.5J (ω2
max − ω2
min)
(3.25)
From the above relation it is evident that the overall efficiency reduces as the back
up time increases (for a given ωmax and ωmin). The energy efficiency as a function of
backup time is given in the fig 3.14. It may be noted that energy efficiency is maximum
at lowest backup time. The bearing friction and iron losses are depending on speed
(voltage) and independent of load current. Armature copper loss depends on the load
current. If more power (more current at constant voltage) is drawn in less time, the
energy lost due to copper loss will increase; energy lost due to other losses mentionedabove will remain same. The copper loss is much less in comparison with other losses.
Overall energy lost reduces and the efficiency increases. Therefore efficiency is higher
at high power and low back up time.
0
0.2
0.4
0.6
0.8
1
0 100
E f f i c i e n c y
Backup Time in seconds
2.72A
1.80A1.50A
1.12A
19 3841 53
Figure 3.14: Energy efficiency vs backup time
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42 Chapter 3. Bi-directional Converter
• Energy output : The energy output as a function of back up time is shown in fig 3.15.
In the case of low backup time (higher power), the ratio Rs
RL
increases. This is on
account of increase in current dependent eddy current losses. This limits the maximum
duty cycle (in other words maximum voltage gain) operation. Thereby, limiting the
minimum speed down to which output voltage is maintained constant. This in turn,
reduces the amount of energy that can be extracted from the spinning flywheel. In the
case of high backup time (low output power), the energy lost due to iron and bearing
friction losses increases. This case also reduces the amount of energy that is extracted.
The energy extracted is maximum at particular backup time (in other words load).
This is evident from the fig 3.15. Maximum output energy can be extracted from a
given stored energy in the flywheel, if the system is operated at optimum backup time.
0
5000
10000
15000
20000
25000
0 10 20 30 40 50 60 70 80
E n e r g y i n J o u l e s
Time in seconds
Figure 3.15: Energy harvested vs backup time
3.10 Simulation of the system in CVD mode
This model is simulated in MATLAB/SIMULINK to obtain the dynamic performance of the
overall system. Following system parameters are used to simulate the system,
Moment of Inertia of the flywheel : 0.075 kgm2
Armature leakage inductance : 0.97mH / phase
Frictional torque : 2 Nm
Filter capacitance: 1650 µF and Load resistance: 100 Ω
Voltage constant of the machine : 0.2 V/rad
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3.10. Simulation of the system in CVD mode 43
The results obtained are shown in the fig 3.16 and fig 3.17. The flywheel is accelerated to
a constant speed of 1046 rad/s. The input power is switched off when the time is 5 sec as
shown in fig 3.16. The flywheel starts decelerating towards zero speed as shown in fig 3.16.
The dc bus voltage is maintained constant at 300V by the BDC in boost mode (CVD) up
0 5 10 15 20 25 30 350
2000
4000
6000
8000
10000
12000
Time in seconds
G e n e r a t o r s p e e d i n R P M
Figure 3.16: Plot of speed vs time in CVD mode
to the instant of 27 sec. Therefore, the total back up time is 22 seconds (27 - 5) as shown infig 3.17.
0 5 10 15 20 25 30 350
50
100
150
200
250
300
350
Time in seconds
d c
b u s
v o l t a g e i n
v o l t
Figure 3.17: Plot of output voltage vs time in CVD mode
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44 Chapter 3. Bi-directional Converter
3.11 Conclusions
• Speed (and the back emf) changes are slow on account of mechanical time constant.
Accordingly we got a single pole transfer function in the acceleration (CCA) mode and
simple gain in the decelerating (CVD) mode. PI controllers with a bandwidth of 100
rads/sec and 10 rads/sec are employed for the CCA mode and CVD mode respectively.
• The circuit inductor should be low enough to allow the rise of armature current to
reach its rated current within 10 percent of conduction time in the accelerating (CCA)
mode. This inductor should be high enough to maintain the constant current in decel-
erating (CVD) mode. Accordingly an external inductance is added to satisfy both theconditions.
• Low mass, higher diameter flywheel is preferred to higher mass low diameter flywheel.
This gives better efficiency of the system for a given energy storage.
• An increase in equivalent source resistance increases the losses and also put a limit on
the voltage gain. This also reduces the amount of energy that is extracted. Current de-
pendent eddy current loss contributes to this resistance. It has to be kept as minimum
as possible. This can be done by the usage of thinner lamination for the machine.
• Overall energy efficiency is an inverse function of backup time. Energy delivered has a
maxima as function of power and time. This is identified and demonstrated.
Actual implementation of the system and the performance results obtained are presented in
the chapter-4.
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Chapter 4
Digital Implementation and
Performace Evaluation of the System
4.1 Introduction
This chapter covers the hardware implementation, Human Machine Interface and testing
of the system. A power converter based on three phase full-bridge voltage source topology
is used. A Motorola make, Digital Signal Processor (DSP) 56F805 is used for controlling
the converter. Algorithm for the generation of switching pulses using the rotor position
information is presented; flow chart of the software is given; results of various tests conductedare presented in this chapter.
4.2 Pulse generation in CCA (motor) mode
There are three hall effect sensors for sensing rotor position. Placement of these sensors
is shown the fig 4.1(a). It may be noted that the angular distance between any of the
two adjacent sensors is 120o (electrical). Output signals of these sensors with reference to
induced voltage is shown in the fig 4.1(b). A detailed specifications of these sensors is givenin the Appendix-D. Output signals of these sensors are shaped in the signal conditioning
board to make them compatible to the DSP port. These signals are connected to port-B
of the DSP. Port-B is programmed in edge triggered interrupt mode. Highest priority is
given to this interrupt. The program flow of this interrupt service routine (ISR) is shown
in fig 4.2. This interrupt service routine generates six train of pulses (corresponding to six
IGBT switches) at the output of PWMA module as per the logic given in the fig 4.3. Out
of these six, three pulse trains (G1, G3 and G5) are used directly as the triggering signals to
45
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46 Chapter 4. Digital Implementation and Performace Evaluation of the System
30 60 90 120 15000
180 210 240 270 300 330 3600 0 0 0 0 0 0 0 0 0 0 0ae
be
θe
θe
θe
θe
θe
θe
Ha
Hb
Hc
1200
Hc
aH0120
S e n s o r
P o
s i t i o n
S i g n a l s
I n d u c e d V o l t a g e
Hb
(b)
(a)
R
Y
R
B
Y
B
ec
Figure 4.1: Placement of the position sensors and their output signals
the top side power switches (T1, T3 and T5 ; E = 0). Next three train of pulses (G2, G4 and
G6) are ANDed with high frequency PWM pulses and used for driving bottom side power
switches (T2, T4, T6). This is shown in the fig 4.4. This high frequency PWM pulses are
generated by the current controller. The current controller design is given in chapter-3. Thedc bus current is taken as the feedback for the current controller. The output of the current
controller is used to track the dc current feedback. The output of the current controller is
compared with the high frequency (f s) triangular carrier to generate the PWM at desired
duty cycle pulse train. Note that, once the machine accelerates to full speed the current
drawn by the machine will drop below the set value. The machine, then will be floating on
the dc bus at full speed (approx. 10,000 rpm).
Power circuit diagram and control schematic of the converter in CCA (motor) mode is given
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4.2. Pulse generation in CCA (motor) mode 47
Is it H c
bIs itH
Is it H aIs it H a
Is it H b
Is it H c T3=OFFT5=ON
T1=OFF
T3=ON
T1=ONT5=OFF
Position
ISR
N
N
N
N
N
N Return
Y
Y
YY
Y
Y
T6=OFFT2=ON
T4=OFF
T6=ON
T2=OFFT4=ON
Edge
Figure 4.2: Flow chart of the position sensor interrupt
H
H
H
H
H
H
T1
T2
T3
T4
T5
T6
T5
T6
T1
T2
T3
T4
a
c
b
a
c
b
Signal ON OFF
Figure 4.3: Sensorwise IGBT gate trigger logic diagram
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48 Chapter 4. Digital Implementation and Performace Evaluation of the System
GateDriveCKT
GateDriveCKT
If
G4
Position
G2G6G4
G1G3G5
_+
ea
eb
ec
DSP Core
G3G1
G6 G2
G5T1
T6
T3 T5
T2
Signals
La
La
La
+15V
Iref
E = 0.
T4
POS ISR
Port B
−
+
Figure 4.4: Power and control schematic in CCA mode
in the fig 4.4.
4.3 Pulse generation in CVD (generator) mode
Power circuit diagram and the control schematic of the converter in CVD (generator) mode
is given in the fig 4.5. The dc bus voltage is taken as the feedback for the voltage controller.
The output of the voltage controller is used to track the dc bus voltage feedback. The voltage
controller design is given in chapter-3. The output of the voltage controller is compared with
the high frequency (f s) triangular carrier to generate drive for bottom switches (T2, T4, T6)
at desired duty cycle as shown in the fig 4.5. The topside switches of the bridge are kept off
permanently in this mode (E = 1).
4.4 Software implementation
4.4.1 Human Machine Interface (HMI)
HMI is a digital interface between the controller and the operator. It takes the input com-
mand and data through the keys, and passes it on to the controller. The processor uses these
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4.4. Software implementation 49
GateDriveCKT
GateDriveCKT
+
_
LR
G4
G2G6G4
G1G3G5
G3G1
G6 G2
G5
C
E = 1.
Vref
T1 T3 T5
T2T6T4
+ dc bus
− dc bus
ea
eb
ec
La
La
La
DSP Core
Figure 4.5: Power and control schematic in CVD mode
data to control the machine as desired by the operator. HMI also takes the messages and
data from the controller and display on the LCD panel. This can be read by the operator
to know various system parameters.
4.4.1.1 Keypad
This program is executed whenever ‘KeyPadFlag’ is set. It checks whether one or more
keys are pressed. If so, the processor checks whether it is a command or data and takes the
appropriate action. The flow chart of the keypad interface program is shown in the fig 4.6.
4.4.1.2 Display
This program is executed whenever the ‘DisplayFlag’ is set. Display interface routine sends
the messages to LCD display unit. This LCD diplay unit consists of 4 rows of 20 characters
each. The message is serially transferred, character by character to the LCD unit. The flow
chart of of this routine is shown in the fig 4.7.
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50 Chapter 4. Digital Implementation and Performace Evaluation of the System
ModeData
Y
Y
NKey Pressed
Stop
Start
Mode
Parameter
Key pad routine
Data = Data −1 Pointer = Pointer −1
Mode = ! Mode
Return
Enter
Y
N
Out put the messageDisable the pulsesSwitch OFF input
Start soft start timer
Switch ON inputEnable the pulses
Y
N
N
N
Y
ParameterData Is MODE
Data = Data + 1 Pointer= Pointer + 1
Data = Temp Buffer
N
Y Y
N
Decreament
Increament
Figure 4.6: Flow chart of the key pad interface routine
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4.4. Software implementation 51
Y
N
put the char inTx buff
Return
sentcharacters are
All N
Y Any
message tobe sent
Display routine
Figure 4.7: Flow chart of the display routine
4.4.2 Timer interrupt service routine
Timer is programmed to interrupt the processor at regular intervals of time. The timer in-
terrupt service routine sets flags like DisplayFlag, KeyPadFlag, SoftStartFlag etc at different
intervals of time. The flow chart of the timer ISR is given in the fig 4.8. These flags are used
by different tasks to start the execution. This scheduling of various tasks is given in the flow
chart of main program (next subsection).
4.4.3 Main program
This is the main program. This program monitors and controls different tasks of the to-
tal system. This program calls various functions like StartADC, ReadADC, PIController,
ScanKeyPad, Display, ActOnAlarm etc at predetermined time intervals allowing them to
carry out their tasks. Processor also takes appropriate actions during interrupts like ‘rotor
position sensor interrupt’, ‘timer interrupt’ etc. The flow chart of the main program is shown
in the fig 4.9.
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52 Chapter 4. Digital Implementation and Performace Evaluation of the System
N
N
Y
TrafCharFlag = 1
ADC Flag = 1
Timer ISR
DisplayFlag = 1
N
Y
N
N
Y
Y
Return
Is it 10ms
UpdateFlag =1
KeypadFlag = 1
Is it 50ms
Is it 100msIs it 600us
Figure 4.8: Flow chart of the timer interrupt service routine
4.5 Hardware implementation of the system
4.5.1 Controller hardware
The entire control algorithm is realized on a digital controller platform. The main proces-
sor is a special purpose DSP controller (Motorola make, 56F805) with a set of peripherals
tailored for motor drive applications. It is a 16-bit, fixed point processor and is operated at
an internal clock of 80MHz. A 12-bit, 8-channel (multiplexed) unipolar Analog to Digital
Converter (ADC) enable sampling of analog signals. Besides, there are digital I/O ports
for handling digital variables. Another on-chip module is two sets of carrier based PWM
switching schemes. This processor also has a serial communication port for communicatingwith the user interface units. Programmable internal timers clock a set of counters, which
can be routed to software interrupts. These can be used for start of sampling of analog
signals, scanning the keys, refreshing the display etc. A signal processing card is also a part
of the platform. This scales and conditions the input analog and digital signals to make it
compatible to DSP ports. This also amplify the output signals to suit the interfacing cir-
cuitry external to the processor (DSP). All control functions and HMI interface are carried
out using this processor.
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4.5. Hardware implementation of the system 53
Mode
CallV PI
Call
I PI
Read ADC
YN
Stop keypressed
Receive key siganl
&take actionKeypad flag
Switch OFF input contactor
Disable pulses
to PWM value register
Load the PI output
display
Refresh the
Update theset value
Alarm flag
N
Y
Display flag
Update flag
N
Y
Y
Y
N
Generator Motor
Start ADC
ADC Flag
If
Switch ON input contactor, Start softstart timer
Make PI output = 0, Enable pulses
Y
NStart key
pressed
START
Refresh display to give start message
InterruptsEnable the
N
N
message to display
Trip and send
Initialize the ports
Y
Figure 4.9: Flow chart of the main program
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54 Chapter 4. Digital Implementation and Performace Evaluation of the System
GateDriveCKT
GateDriveCKT
Vf If
BLDC
Machine
ChargeR
G2
G4
G1G3G5
A D C
P o r t B
G6
D1 D3 D5 G3G1
G6 G2
G5
Position
Signal
P W M ’ A ’ P o r t
T1
T6
T3 T5
T2
+15v
E=0
D4 D6 D2
C RL
Signal processingcircuitry
G4 T4
m o n i t o r i n g s i g n a l
I n p u t p o w e r c o n d i t i o n
Raw ACInput
HMI
−dc bus
+dc bus
Power switching module
DSP core
Figure 4.10: Block diagram of the test setup
4.5.2 Power converter
The power converter is built using the IGBT as the power device. Complete block diagram
of the system is shown in the fig 4.10. The system design is validated on an experimental
setup. The rating of the converter are,
• Input voltage: 230volts, 50Hz, line-to-line.
• Output voltage: 300 V DC
• Output power: 1.0 kW.
• Switching frequency: 3.3kHz.
• Line to line inductance (including machine leakage): 7.096mH.
• DC bus capacitance: 1650 µF , 400 V
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4.6. Testing and performance analysis of the system 55
Windage losses
LoadFlywheel Machine BDC
Hysterisis + Eddy Current+ Copper losses
Conduction and
Switching lossesBearing Friction losses
Figure 4.11: Various losses and their sources in the system
4.6 Testing and performance analysis of the system
4.6.1 Apportioning of various losses
Between the flywheel (which stores the energy) and load (which consumes the energy) there
are different devices like, bearing, electrical machine, bi-directional power converter as shown
in the fig G.1. A portion of the energy which is extracted from the flywheel is dissipated as
loss in these devices. It is necessary to find out these losses. Following tests are conducted
to find and separate out the various losses.
4.6.1.1 No load test
The flywheel is decoupled from the motor shaft. The motor is made to run at different
speeds up to a speed of 10,000 RPM. Machine draws power only to meet the losses. The
weight of the rotor is very small compared to that of flywheel. The mechanical losses in this
condition is assumed to be negligible. All the power drawn is to meet the iron loss of the
machine. Power consumed by the machine at various speeds ( V dcI dc) is recorded and plotted
as shown in the fig 4.12. This test enables us to find the iron loss in the machine which is
predominantly speed dependent.
4.6.1.2 Retardation test with flywheel
In order to evaluate the mechanical losses in the machine, the classical retardation test is
done. The machine along with the flywheel is made to run up to a speed of 10,000 RPM.
Power is cut off and the machine is allowed to decelerate. This data is given in the table 4.1.
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56 Chapter 4. Digital Implementation and Performace Evaluation of the System
0
100
200
300
0 2000 4000 6000 8000 10000 12000
I r o n l o s s i n w a t t s
Speed in RPM
No load test
(without flywheel)
Figure 4.12: Iron losses in the machine in no load test
The same data is plotted as shown in the fig 4.13. The stored energy in the flywheel is
Time 0 30 60 90 120 150 180 210 240 270 300
in secs
Speed 9670 8200 6700 5550 4600 3700 2900 2200 1600 1100 650in RPM
Table 4.1: Retardation test data
consumed as the machine decelerates. The losses in this test condition include the bearing
friction losses, drag and the iron losses in the machine. The power loss at each speed can be
calculated by the following relationship,
P (ω) = J ωdω
dt (4.1)
The power lost at various speeds is computed from the retardation test data. The same
is plotted as a function of speed as shown in the fig G.2. For the sake of comparison, the
iron loss calculated from the no load test are also plotted in fig G.2. The point on curve
‘retardation test with flywheel’ gives the total loss and the point on curve ‘No load test
without flywheel’ gives only iron loss of the machine. The difference between these two gives
the mechanical losses (bearing friction, drag) at that speed.
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4.6. Testing and performance analysis of the system 57
0
2000
4000
6000
8000
10000
0 50 100 150 200 250 300
R o t o r s p e e d i n R P M
Time in seconds
Figure 4.13: Speed vs time in the retardation test with flywheel
4.6.1.3 Copper losses in the armature winding
In CVD mode, the power converter works in boost mode. The converter input is connected
to machine terminals. The machine is connected in star. Therefore, the converter sees two
sets of armature windings and series chokes. The equivalent circuit of converter connected
to the machine is shown in fig 4.15. Copper losses in the winding can be calculated by,
P cu = I 2aRs (4.2)
Where Rs = 2(Ra + Rch). In boost mode I a and I dc are related by,
I a = I dc1 − D
(4.3)
V dc and 2E b are related by,V dc
2E b= 1
1 − D (4.4)
Earlier, in chapter-2 (equation 2.8), the back emf of the machine has been shown to be,
2E b = 0.0277S r (4.5)
Where S r is the rotor speed in RPM.
Combining equations 4.3, 4.4 and 4.5 we get,
I a = I dcV dc
(0.0277S r) (4.6)
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58 Chapter 4. Digital Implementation and Performace Evaluation of the System
0
100
200
300
400
500
0 2000 4000 6000 8000 10000 12000
0
100
200
300
400
500
I r o n a n d m e c h a n i c a l l o s s i n w a t t s
I r o n l o s s i n w a t t s
Speed in RPM
Retardation test with
flywheel(No load)
No load test
without flywheel
Figure 4.14: Losses in the machine with and with out flywheel
Eb
RL Vdc
La Lch2( + ) Ra Rch2( + )
Ia
Idc
2
C
Figure 4.15: Equivalent circuit of boost converter for copper loss calculation
Therefore, the copper loss is given by,
P cu = 2
I dcV dc
(0.0277S r)
2
[(Ra + Rch)] (4.7)
These losses are presented later in table G.7.
4.6.1.4 Switching and conduction losses in the converter
Switching and conduction losses in the power converter are computed as follows:
• Switching losses: If I ON is the on state current and V OFF is off state voltage of the
switch, tr and f s are the rise time and the switching frequency respectively of the
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4.6. Testing and performance analysis of the system 59
switch, then the switching loss in a switch driving an inductive load is given by [3],
P sw = 0.5I ON V OFF trf s (4.8)
While BDC is operating in CVD mode, I ON is I a and V OFF is V dc. Substituting for I a
from equation 4.6, we get for each switch,
P sw = 0.5 I dcV dc
(0.0277S r)V dctrf s + 0.5
I dcV dc(0.0277S r)
V dctf f s (4.9)
Switching loss in the diodes is very small and neglected.
• Conduction loss: The current flowing across the switch is I a. Assuming the switch
drop to be V d, the conduction loss in the switch (P c1) can be computed by,
P c1 = I aV d (4.10)
Substituting for I a from equation 4.6, we get loss in each device.
P c1 = I dcV dc
(0.0277S r)V d (4.11)
There are three devices and each conducting for a duration of 120 o in a cycle. The
total loss of all devices is given by,
P c = 3
1
3
I dcV dc(0.0277S r)
V d
(4.12)
These losses are presented later in table G.7.
4.6.2 Power backup time test
This test is conducted to find out following parameters.
• Time duration for which the system supplies the power to the load (in the absence of
input power) at desired (constant) voltage.
• The minimum speed (or induced voltage) up to which the system maintains the output
dc bus voltage.
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60 Chapter 4. Digital Implementation and Performace Evaluation of the System
The input supply is switched off and the flywheel is allowed to decelerate. The time up to
which the dc bus voltage is maintained constant is recorded. This test is conducted with
a load current of 2.72A, at a dc bus voltage of 300V. The power delevered to the load is
818W. Fig 4.16 shows the plot of variations of output voltage with time. It may be noted
that, even though the generator voltage reduces with time (as speed is reduced), the dc bus
voltage is maintained constant for a duration of 19 sec. It is also observed that the minimum
speed (or the voltage) up to which system maintains the output dc bus voltage is 6894 RPM.
(corresponding induced voltage is 185 volts). It is to be noted that when the power drawn
0
100
200
300
400
0 10 20 30 40 50 60 70
V o l t a g e i n v o l t s
Time in seconds
Controlled
operation
Uncontrolled
operation
185V
19 Sec
Generator Voltage (2Eb)
dc bus Voltage (Vdc)
Figure 4.16: dc bus voltage control with P o = 818 watts
is less, the duration of power delivery at constant voltage is longer. Fig 4.17 shows a similar
result at a load of 450W. At this reduced power delivery, the break time is seen to be 41 sec.
4.6.3 Source resistance effect
The voltage gain of the system can be calculated by the following relationship [3].
V dcE b
=
1
(1 − D)
1
1 +
Rs
RL
(1 − D)2
(4.13)
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4.6. Testing and performance analysis of the system 61
0
100
200
300
400
0 10 20 30 40 50 60 70
V o l t a g e i n v o l t s
Time in seconds
Controlled
operation
Uncontrolled
operation
127V
41 Sec
Generator Voltage (2Eb)
dc bus Voltage (Vdc)
Figure 4.17: dc bus voltage control with P o = 450 watts
It may be noted that, Rs is the source resistance which is the series combination of armature
winding resistance, Ra of the machine and the resistance of the series choke Rch. Ra, Rch
and RL in the equivalent circuit is shown in the fig 4.15. Voltage is applied between the
input terminals of the star connected machine. The equivalent source resistance is (due to
two windings coming in series),
Rs = 2(Ra + Rch) (4.14)
In the experimental setup, the values of Ra = 0.38Ω, Rch = 0.8Ω and RL = 100Ω. The
ratio α = Rs
RL
is 0.024. The plot of voltage gain as a function of duty ratio for the value of
α = 0.024 is shown in fig 4.18. It can be seen from this plot that the voltage gain increases as
the duty ratio is increased. After a certain value of duty cycle, it starts decreasing. Prefered
operating duty ratio of the boost converter is from zero to ‘M’ as shown in the fig 4.18. The
peak of this graph is the maximum voltage gain one can get from the boost converter for agiven α. This peak value is a function of α. The ‘α’, will limit the maximum operating duty
cycle. It may be observed from fig 4.18 that the maximum voltage gain that can be obtained
from the setup (with α = 0.024) is 3.23 and the corresponding Dmax = 0.85. This means that
if dc bus voltage to be maintained is 300 volts, then the input voltage can go down to 93 volts
(and corresponding speed is 3900 rpm). It may be observed from the ‘Power backup time
test’ described in the previous section that the minimum voltage down to which converter
maintains the output dc bus voltage at 300V is only 185V. This means that the maximum
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62 Chapter 4. Digital Implementation and Performace Evaluation of the System
0
0.5
1
1.5
2
2.5
3
3.5
0 0.2 0.4 0.6 0.8 1
V o l t a g e g a i n V d c
/ 2 E b
Duty cycle
M
Figure 4.18: Voltage gain as a function of duty cycle with α = 0.024
voltage gain obtained is only 1.6. This is in contradiction to the estimated maximum gain of
3.23. The experiment was carried out at several other load settings to rule out experimental
error. The maximum voltage gain obtained at these loads are given in the table 4.2. It is
Output power Minimum speed Max Voltage Estimated max
in Watts in RPM gain voltage gain
818 6894 1.6 3.23
450 4691 2.36 4.33
338 4291 2.52 4.97
Table 4.2: Max voltage gain test data
observed that the voltage gain is more at lower armature currents and less at higher armature
currents. It is clear that the equivalent series resistance seen by the circuit is more than the dc
resistance measured. It is also seen that, this equivalent resistance is a function of armature
current. To confirm this, a test was conducted to compute Rs at different duty cycles. In this
test, the converter output voltage (dc bus voltage), input voltage (generator voltage) and
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4.6. Testing and performance analysis of the system 63
duty cycle are recorded. The source resistance Rs is computed from the following relation,
V dcE b
=
1
(1 − D)
1
1 +
Rs
RL
(1 − D)2
(4.15)
The ratio of source resistance to load resistance as a function of duty cycle is plotted as
shown in the fig 4.19. It is clear from the fig 4.19 that the source resistance is more at higher
0
0.05
0.1
0.15
0.2
0.25
0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 S o u r c e R e s i s t a n c e / L o a d R e s i s t a n c e
Duty ratio
RL = 180
RL = 100
Ratio = 0.013
Ratio = 0.024RL = 100
RL = 180
Figure 4.19: Source resistance as a function of duty cycle
armature current (low load resistance) and is less at lower armature current. This increase
in Rs may be due to the loss that is taking place in the core, reflecting as series resistance
(since it is current dependent). It is proposed that, this is due to the the current dependent
losses in the core. This may be on account of the the eddy current losses due to the leakage
flux around the teeth of the core. This flux is produced by the current flowing through the
armature conductors. Therefore these losses are armature current dependent. This loss in
the core is reflected as a series resistance.
4.6.4 Current dependent eddy current loss in the core
Duty cycle and the output (dc bus) voltages are recorded at various induced voltages for a
given load resistance. The source resistance Rs is calculated by using the equation 4.15. If
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64 Chapter 4. Digital Implementation and Performace Evaluation of the System
Rs is the equivalent source resistance as seen by the converter, Ra is the armature winding
resistance and Rch is the resistance of the series choke, then the current dependent eddy
current loss is given by,
P ei = I 2a [Rs − 2(Ra + Rch)] (4.16)
Substituting for I a from equation 4.6 we get,
P ei =
I dcV dc
(0.0277S r)
2
[Rs − 2(Ra + Rch)] (4.17)
This loss also presented later in table G.7.
4.6.5 Comparison of various loss components
Various losses are computed/measured at an output power of 450 Watts. This is shown in
the table G.7. It may be noted that the copper loss P copper shown is the total of loss caused
by the armature dc resistance (P cu) and the current dependent eddy current loss (P ei). It is
Speed P mech P iron P copper P c P sw
in RPM in watts in watts in watts in watts in watts
9835 233 163 48 2.5 1
(6.7 + 41.4)
4691 66 50 115 5.25 2
(30 + 85)
Table 4.3: Comparison of various losses
evident from these tests that the highest contribution to the loss is from the bearing friction
and drag (Mechanical losses). Second highest contribution is from the iron losses in the core
of the machine. Next contribution is from the the copper losses of the machine. This copper
loss is the sum of the current dependent eddy current loss in the core and the power loss
in the dc resistance of copper wire used. The switching and conduction losses in the power
converter are very low and negligible.
4.6.6 Efficiency
The above exercises of apportioning the losses and quantifying the same will help us in
understanding the operating efficiency of the FES. The total energy that can be harvested
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4.6. Testing and performance analysis of the system 65
depends on the losses in the system. Tests are conducted with various loads (different backup
time) and the energy efficiency is calculated at each load. The results are plotted as shown
in fig 4.20. It is found that the energy efficiency is an inverse function of time. The bearing
friction and iron losses are dependent on the operating speed (voltage). Amount of energy
lost will increase if backup time is increased and efficiency decreases. This is explained in
detail in the section 3.9 of chapter-3.
0
0.2
0.4
0.6
0.8
1
0 20 40 60 80 100
E f f i c i e n c y
Time in seconds
Figure 4.20: Overall efficiency as a function of back up time
4.6.7 Harvestable Energy
The aim of this test is to find out the amount of energy that can be harvested at different
loads. Tests were conducted with various loads connected across the dc bus. The results are
given in the table 4.4. It is evident from these tests that the energy harvested has a maxima
Output power Backup time Energy harvested Efficiency
in Watts in secs in Joules
818 19 16359 0.78
450 41 18953 0.62
338 53 17937 0.56
Table 4.4: Test results at various load conditions
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66 Chapter 4. Digital Implementation and Performace Evaluation of the System
0
5000
10000
15000
20000
25000
0 10 20 30 40 50 60 70 80
H a r v e s t a b l e E n e r g y i n J o u l e s
Time in seconds
Maximum
energy
harvested
38 SecEnergy loss = 9973J
Figure 4.21: Energy harvested as a function of back up time
as a function of power and time. This is represented graphically as shown in fig 4.21. In this
set up it has occurred at 542 Watts, 38 sec. The reason for drop in harvested energy both
in lower backup time and higher backup time is explained in the section 3.9 of chapter-3.
4.6.8 Current waveforms at various speeds
Sample current waveforms of the machine are recorded in CCA mode and CVD mode. They
are shown in the fig 4.22 to fig 4.24. It may be observed that the current waveform shown
Figure 4.22: Armature current waveform in CCA mode at speed = 2200 RPM
(CH1(Top): Time:1ms/div, Current:1A/div,CH2(Bottom): Time:1ms/div, Voltage:2V/div)
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4.6. Testing and performance analysis of the system 67
Figure 4.23: Armature current waveform in CCA mode at speed = 9640 RPM)
(Time: 1ms/div, Current:1A/div)
Figure 4.24: Armature current waveform in CVD mode
(Time: 2ms/div, Current: 1A/div)
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68 Chapter 4. Digital Implementation and Performace Evaluation of the System
in fig 4.23 has positive slope as well as negative slope in the middle of the waveform. This
is due to the advancement of the triggering pulses given to the switches.
4.7 Conclusions
Various losses in the machine are separated out. It has been found that the major contribu-
tion to the losses is from mechanical losses, iron loss and the copper losses. The total energy
that can be harvested depends on the losses in the system. Current dependent eddy current
loss contribute for increase of source resistance of the converter. This will put a constraint
on the maximum voltage gain. This in turn, limit the total energy that can be extracted
for a given top speed. Usage of low loss core material like Nickel-iron or cobalt-iron for the
machine and ferrite for chokes will improve the overall efficiency of the system. This will
also help in reducing the source resistance of the converter. From the experiments it is found
that the energy extracted has a maxima as a function of time. For a given system there is
clearly a peak energy output, where the energy extracted will be maximum.
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Chapter 5
Conclusions
In many power drive applications such as traction, elevators, cranes etc, it is commonplace to
encounter loss of stored energy. The main reason is that, the power converters are not capable
of returning the stored energy during transients. In application where frequent transients
are involved, this results in substantial loss of energy. Bi-directional converters in such
applications can lead to higher operating efficiency. In a typical traction application, stored
energy while running can be restored during deceleration. This process saves the energy
and improves the efficiency. Such applications need a bi-directional interfacing converter.
The bi-directional converter facilitates the energy flow, to and from the device. The desired
features of such a system are,
• Good energy efficiency.
• Simple control.
• Reliability.
• Low cost.
• Small size.
The aim of this work is to develop a bi-directional power converter/controller to facilitate
the energy storage, to and from the storage device. The storage system employed in this
application consists of a BLDC machine and a flywheel; together they serve as a flywheel
energy storage system. The analysis, design, fabrication and evaluation of such a system has
been covered in this thesis. The full system has been evaluated and design guidelines are
obtained.
69
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70 Chapter 5. Conclusions
5.1 The present work
Chapter 1 focused on the essential basic specifications, guiding the selection of subsystems.
These systems are, bi-directional power converter, controller, BLDC machine and the fly-
wheel. The bi-directional power converter selected is of six - switch voltage source bridge
topology. The BLDC machine is selected for this application is due to its high power density,
low rotor losses and simplicity of control. The operating modes during charging and dis-
charging of the flywheel are identified. The control platform selected is a DSP of Motorola
make, 56F805. A flywheel of suitable dimensions and capable of storing the energy required
to supply 1.0kW load for a duration of 20 sec has been selected.
Chapter 2 is on the prime-mover required to drive the flywheel. Basic design of BLDC
machine has been carried out. The design is verified using a FE method of analysis using
‘Magnet’ software. The machine is fabricated using standard available frame of nearest
dimensions. The machine is tested up to a speed of 10,000 rpm. Design of the machine is
validated through the experimental results.
Chapter 3 is on the bi-directional power converter and its control. A 6-switch IGBT
bridge of voltage source topology is selected. The operating modes are identified as CCA
and CVD modes. Equivalent circuit and the transfer function in both the modes are obtained.
Suitable controllers for both the modes of operations are designed. The system is numerically
simulated to check the performance.
Chapter 4 presents the digital realization of the system. The controller and HMI are
implemented using a digital signal processor of Motorola make, 56F805. Seamless changeover
from CCA mode to CVD mode and vice versa in the controller is implemented. The power
converter and controller are fabricated; bi-directional converter/controller is integrated with
BLDC machine and the flywheel. The complete system is tested to evaluate the performance.
The results are analyzed to obtain design guidelines for such systems.
Features of the system are,
• Speed of operation is limited to 10,000 due to drag and safety issues.
• Control is simple
Important findings of this thesis are,
• Low efficiency on account of high iron losses and bearing losses.
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5.2. Guidelines emerging from the work 71
• Current dependent eddy current loss contribute to the increase of source resistance of
the converter. This puts a constraint on the operating maximum voltage gain. This
in turn, limit the total energy that can be harvested for a given top speed.
5.2 Guidelines emerging from the work
In the present system, the source resistance is the result of the core losses in the machine.
This is required to be reduced to improve the efficiency. The current dependent losses has
to be reduced for harvesting maximum energy stored in the flywheel. These are achieved by
the usage of,
• Vacuum enclosure for rotating parts.
• Two pole machine. For two pole machine, the operating frequency is 167Hz. This, in
turn, results in lower iron loss.
• Low specific loss core materials like, Ni-Iron, Co-Iron etc.
• Low loss, contact-less bearing like magnetic bearing.
Study of these solutions have been done and low cost implementation is given in Appendix-G
5.3 Spin off technology from the present system
This system can be tailored to store and extract energy from super capacitor as the storage
device. The same power circuit can be used as multi-phase chopper in the super capacitor
energy storage application.
5.4 Applications of the system
The FES system which is developed, can be used as an UPS where short support time is
required. This will be useful for the installations which are backed up with diesel genera-
tors. This bi-directional converter along with the BLDC machine can be directly used for
hybrid vehicles and material handling equipments to improve their performance in terms of
efficiency, control and reduction of environmental pollution. With some modifications in the
control circuit and software, the same bi-directional converter can be used for energy storage
applications using the ultra capacitors.
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Appendix A
Specifications of IGBT and Capacitor
A.1 IGBT Module
Parameter : Value
Make : Semikron
Model : Semitop; 6-devices pack
PartNumber : SK 30 GD 123
V CES : 1200 V
V GES : +/- 20 V
V CESat : 3.1 V
I C : 22A at 80o
C / 33A at 25o
C tdon : 65ns
tr : 100ns
tdoff : 430ns
tf : 35ns
A.2 Capacitor
Parameter : Value
V alue : 3300 µF
Surge : 440 V
ES R : 49 mΩ at 20oC at 100Hz
Impedance : 36 mΩ at 20oC at 10kHz
I Ripple : 12.7 A at 85oC at 100Hz
: 16.5 A at 85oC at 10kHz
72
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Appendix B
Specifications of Digital Signal
Processor DSP56F805
B.1 Digital Signal Processing Core
• 16-bit DSP engine with dual Harvard architecture
• 40 Million Instructions Per Second (MIPS) at 80 MHz core frequency
• Single-cycle 16-bit parallel Multiplier-Accumulator (MAC)
• Two 36-bit accumulators, including extension bits
• 16-bit bidirectional barrel shifter
• Hardware DO and REP loops
• Three internal address buses and one external address bus
• Four internal data buses and one external data bus
• Instruction set supports both DSP and controller functions
• Controller style addressing modes and instructions for compact code
• Efficient C compiler and local variable support
• Software subroutine and interrupt stack with depth limited only by memory
• JTAG/OnCE debug programming interface
73
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74 Appendix B. Specifications of Digital Signal Processor DSP56F805
B.2 Memory
• Harvard architecture permits as many as three simultaneous accesses to program and
data memory
• 32K, 16 bit words of Program Flash
• 512, 16-bit words of Program RAM
• 2K, 16-bit words of Data RAM
• 4K, 16-bit words of Data Flash
• 2K, 16-bit words of BootFlash
• Off-chip memory expansion capabilities programmable for 0, 4, 8, or 12 wait states
• 64K, 16 - bits of data memory
• 64K, 16 bits of program memory
B.3 Peripheral Circuits for DSP56F805• Two Pulse Width Modulator modules (PWMA and PWMB) each with six PWM
outputs, three Current Sense inputs, and four Fault inputs, fault tolerant design with
dead-time insertion; supports both center and edge aligned modes
• 12-bit Analog-to-Digital Converters (ADC) which support two simultaneous conver-
sions with two 4-multiplexed inputs
• Two Quadrature Decoders
• Two General Purpose Quad Timers
• CAN 2.0 Module
• Two Serialm Communication Interfaces (SCI0 and SCI1)
• Serial Peripheral Interface (SPI)
• 14 dedicated General Purpose I/O (GPIO) pins, 18 multiplexed GPIO pins
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B.3. Peripheral Circuits for DSP56F805 75
• Computer Operating Properly (COP) watchdog timer
• Two dedicated external interrupt pins
• External reset pin for hardware reset
• JTAG/On-Chip Emulation (OnCE) module for debugging
• Software-programmable, Phase Lock Loop-based frequency synthesizer for the DSP
core clock
• Fabricated in high-density CMOS with 5V tolerant, TTL-compatible digital inputs
• Uses a single 3.3V power supply
• On-chip regulators for digital and analog circuitry to lower cost and reduce noise
• Wait and Stop modes available
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Appendix C
Block Diagram of Controller
SP1
SCI#0
SCI#1
CAN
TIMERGPIO
PWM#1
PWM#2
A/D
3.3V,GNDXTAL
JTAG/OnCEJTAGConnector
Low Freq
Crystal
PrimaryUNI−1
SecondaryUNI−3
Power Supply
+3.3V,+5V
RS232Interface
10Bit D/A4 Channel
D Sub 9−Pin
CAN Interface
Debug LED’s
PWM LED’s
Over V Sense
Over I Sense
ZC Detector
Expantion
Pheripheral
Connector
Memory ExpConnector
16KX16 Bit
Data Memory
D Sub
25−Pin
DSP56F805
Signal
Processecing
Circuitry
& Interface
RESET
MODE
16KX16 BitMemoryProgram
MODE/IRQLogic
RESET Logic
Parallel JTAGInterface
Add,Data
Control
Figure C.1: Block Schematic of DSP Board
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Appendix D
Specifications of Hall effect position
Sensor
Make : Honeywell
Part Number : SS413A
Type : Bi-polar
Supply Voltage : 3.8 to 30 V
Supply Current : 10mA
Output Type : Sink
Output Voltage : 40 VOutput Current : 20mA
tr : 0.05 µs typ
tf : 0.15 µs typ
77
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Appendix E
Publication
1. S.R. Gurumurthy, V. Ramanarayanan, M.R. Srikanthan ‘Design and Evaluation of DSP
controlled BLDC drive for Flywheel energy storage system’ presented in National Power
Electronics Conference, NPEC 2005, Indian Institute of Technology, Kharagpur, India, Dec
22 - 24, 2005.
78
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Appendix F
Photographs of the test setup
79
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80 Appendix F. Photographs of the test setup
Figure F.1: Bi-directional Power Converter
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81
Figure F.2: Brushless DC machine coupled to Flywheel
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82 Appendix F. Photographs of the test setup
Figure F.3: Test set up of Flywheel Energy Storage System
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Appendix G
Further improvements in the system
G.1 Method used in apportioning various losses
Various losses take place in the system are shown schematically in the fig G.1. It is possible to
compute the sources of losses and their contributions to the total loss. With this information
one can adopt different techniques that can be adopted to reduce the losses. Following tests
were conducted to find out the sources of losses.
• No load test (without flywheel)
• Retardation test (with flywheel)
No load test is conducted without flywheel and hence the mechanical losses are negligible.
Therefore, with this no load test data the iron loss of the machine is computed. Retardation
test with flywheels of different mass were carried out. The losses in the system are computed
and tabulated using data obtained from retardation test are given in the table G.1. This
gives the sum total of the mechanical losses along with the iron losses (electrical losses).
These losses are plotted as a function of speed as shown in the fig G.2.
G.2 Relation between the speed and the loss:
The relation between the loss as a function of speed is found out using curve fitting. If P Loss
is the total loss in the system and ”N” is the operating speed in RPM of the machine, then
the equation obtained are given as below:
• Without flywheel
P Loss = 0.000001N 2 + 0.0045N (G.1)
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84 Appendix G. Further improvements in the system
Windage losses
LoadFlywheel Machine BDC
Hysterisis + Eddy Current+ Copper losses
Conduction and
Switching lossesBearing Friction losses
Figure G.1: Various losses and their sources in the system
Speed Power loss (watts) Power loss (watts) Power loss (watts) Power loss (watts)in RPM without flywheel with 11Kg flywheel with 15Kg flywheel with 21Kg flywheel
1880 13 31 46 56
3595 35 88 110 128
5395 60 145 195 250
7200 96 206 320 400
8950 138 294 441 550
9892 165 350 - -
Table G.1: Total loss at various mass as a function of rotor speed
• With flywheel of 11 Kg
P Loss = 0.000002N 2 + 0.0163N (G.2)
• With flywheel of 15 Kg
P Loss = 0.000003N 2 + 0.0191N (G.3)
• With flywheel of 21 Kg
P Loss = 0.000004N 2 + 0.0266N (G.4)
It is found that the right hand side of the equation contains two terms, one is proportional
to the speed and the other is proportional to the square of speed. This is in expected lines.
From the theory, it can be shown the term which is proportional to speed is corresponding
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G.3. Interpretation of the equation : 85
0
100
200
300
400
500
600
0 2000 4000 6000 8000 10000 12000
0
100
200
300
400
500
600
I r o n a n d m e c h a n i c a l l o s s i n w a t t s
I r o n l o s s i n w a t t s
Speed in RPM
21 Kg
15 Kg
11 Kg
No load test
without flywheel
Figure G.2: Losses in the machine with and with out flywheel
to loss due to the bearing friction and hysteresis in the core of the machine; the term which
is proportional to the square of the speed is corresponding to loss due to the air drag and
eddy current in the core of the machine.
G.3 Interpretation of the equation :
• Without flywheel (no load test):
When flywheel is not coupled to the rotor shaft, the loss due to the bearing friction as
well as the drag can be neglected. This is on account of low mass and surface area of
the rotor. Therefore, the loss computed from this test can be entirely due to the iron
loss of the machine.
• With flywheel:
When flywheel is coupled to the rotor shaft, loss computed is the sum total of all the
losses in the machine. They are, bearing friction, drag, hysteresis and eddy current
loss. Subtracting the square component of ”no load test” from square component of
loss obtained from this test gives the drag loss in the system. Similarly, subtracting the
linear component of ”no load test” from the linear component of loss obtained from
this test gives the bearing friction loss of the machine.
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88 Appendix G. Further improvements in the system
G.4 Loss reduction techniques:
Drag loss can be reduced by providing the vacuum enclosure for the rotaing parts of the
system. Both eddy current and hysteresis loss can be reduced by the using a two pole machine
(half the supply frequency) as well as by using low specific loss core material. Bearing friction
loss can be reduced by using a active magnetic bearings. This is a complicated technology
and the system becomes expensive. Using a two pole machine will be a cheaper option
compared to costly low specific loss core material. Usage of two pole machine will reduce
the flux dependent as well as armature current dependent eddy current loss to the extent
of 75 percent (compared to 4 - pole machine). Vacuum enclosure will be a cheaper optioncompared to magnetic bearings. Keeping the rotating parts in a enclosure with vacuum
of 0.1 mb, the loss due to drag will get reduce to the extent of 75 percent (compared to
atmosphere). Therefore, implementing the system with vacuum enclosure and two pole
machine will reduce the overall loss by 50 percent. Extra-polation of the results obtained
Speed P d P bf P h P e P cu P conv P Total
in RPM in watts in watts in watts in watts in watts in watts in watts
9835 96 116 44 97 48 3.5 4054691 22 55 21 22 115 7.25 243
Table G.7: Comparison of various losses
from the experiments conducted will validate this point. From the results shown in the
chapter-4 it is found that the energy havested is maximum at a backup time of 41 sec.
Hence various losses are computed at a back up time of 41 seconds. The maximum and
minimum speed considered are 9835 RPM and 4691 RPM. The experimental set up consistsof the flywheel of J = 0.075Kgm2 running in air (atmospheric pressure) coupled to a 4-pole
BLDC machine. Break up of various losses for the existing set up are given in the table G.7.
If a 2-pole machine is used, then the flux dependent as well as armature current dependent
eddy current loss will become 1
4th and the hysteresis loss will become (
1
2)half. The losses are
computed accordingly. If the rotating parts are kept in a vacuum enclosure (with a pressure
of 0.1mb) the drag losses becomes 1
4th. With the reduction in the system losses, the overall
efficiency of the system will improve. Overall efficiency of the system is computed using the
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G.5. conclusions: 89
Power loss Power loss Average Overall System
at 9835 rpm at 4691 rpm power loss efficiency conditions405 243 325 0.60 4-pole machine
Running in air
278 151 215 0.72 2-pole machine
Running in air
206 134 170 0.78 2-pole machine
Running in vacuum of 0.1mb
Table G.8: Comparison of various systems
relation,
η =
P oT backup
0.5J (ω2
max − ω2
min)
= 1 −
P lossT backup
0.5J (ω2
max − ω2
min)
(G.5)
Summary of these loss reduction and the effect on the overall efficiency is given in the
table G.8.
G.5 conclusions:
Low cost and easy solutions for improving the efficiency of the system are vacuum enclosure
for rotaitng parts and usage of two-pole machines. With these techniques the efficiency will
be as high as 80 percent.
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1950; Chapman and Hall Ltd (1950).
[3] Power Electronics Circuits - Issa Batarseh, University of Central Florida; John Wiley
and Sons, Inc (2004).
[4] Fundamentals of Power Electronics - Robert Errickson, Second Edition; Kluwer
Acadamic Publishers.
[5] Electric Drive suitable for Flywheel Energy Storage System - R.Anbarasu; Ph.D Thesis,
IIT, Delhi (1987).
[6] Electric and Hybrid Vehicles, Design Fundamentals - Iqbal Husain; CRC Press (2003).
[7] The key factors in the design and construction of advanced flywheel system and their
application to improve telecommunication power back up - Roger E Horner Interna-