Multifunctional Converter Drive for Automotive Electric Power Steering Systems

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Multifunctional Converter

Drive for Automotive Electric

Power Steering Systems

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Multifunctional Converter

Drive for Automotive Electric

Power Steering Systems

Proefschrift

ter verkrijging van de graad van doctor aan de Technische Universiteit Delft;

op gezag van de Rector Magnificus prof.ir. K.C.A.M. Luyben; voorzitter van het College voor Promoties

in het openbaar te verdedigen op maandag 9 september 2013 om 12.30 uur door

Thomas Josef HACKNER

Diplom-Ingenieur (FH), University of Applied Sciences Ingolstadt geboren te Eichstätt, Germany

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Dit proefschrift is goedgekeurd door de promotoren: Prof.dr.eng. J.A. Ferreira

Prof.dr. J. Pforr

Samenstelling promotiecommissie: Rector Magnificus, voorzitter

Prof.dr.eng. J.A. Ferreira, Technische Universiteit Delft, promotor

Prof.dr. J. Pforr, University of Applied Sciences Ingolstadt, Germany, promotor Prof.dr.ir. J. van Mierlo, Vrije Universiteit Brussel, Belgium

Prof.dr. E. Lomonova, Technische Universiteit Eindhoven Prof.dr.ir. J. Hellendoorn, Technische Universiteit Delft Prof.ir. L. van der Sluis, Technische Universiteit Delft Dr.ir. H. Polinder, Technische Universiteit Delft

ISBN: 978-94-6186-193-1

All rights reserved. No part of the material protected by this copyright notice may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording or by any information storage or retrieval system without permission from the author.

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i

Acknowledgements

The research presented in this thesis was performed at the Institute of Applied Research (IAF) at the Technische Hochschule Ingolstadt, University of Applied Sciences, Germany. From 2008 – 2013, I was working towards my Ph.D. in cooperation with the Electrical Power Processing (EPP) group at the TU Delft, The Netherlands.

I would like to thank my supervisor, Professor Johannes Pforr, who gave me the opportunity to research in the interesting field of power electronics and electrical machines. Without the endless discussions with Professor Pforr this thesis would not be possible. Thank you, for all the reviewing and comments to many publications and presentations.

I am very grateful to my promotor, Professor Braham Ferreira, who gave me the opportunity to pursue a Ph.D. in his group and for his very constructive feedback. Further, I would like to thank Dr. Henk Polinder for the all the support and for the fruitful discussions we had in Delft or in Ingolstadt.

I would like to thank the Audi AG for supporting my research project and, in particular, to Josef Winkler my contact person for the great collaboration and the discussions we had about different aspects of automotive power nets.

I would like to express my gratitude to the thesis committee members Professor J. van Mierlo, Professor E. Lomonova, Professor H. Hellendoorn and Professor L. van der Sluis for taking the time to read the draft thesis and giving valuable suggestions.

Special thanks go to Melanie Bailey and Veronika Pisorn for doing the language editing and to Martin van der Geest for translating the propositions and the summary into Dutch.

I am very grateful to my colleagues at the IAF especially to Roland Cziezior, Michael Stadler, Werner Thomas, Sebastian Utz and Christian Augustin for making the time spent doing my Ph.D. enjoyable and rewarding. Further, I would like to thank a few former students Andreas Meilinger, Thomas Schermer, Andreas Gassner, Martin Dehm, Michael Geis, Max Stubenvoll, Benjamin Margraf, Daniel Kremer and Liu Ming that they have contributed to this thesis with their Diploma, Bachelor or Master theses.

I would like also to thank the Ph.D. students at the EPP group. It was always a pleasure to visit the EPP group.

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Acknowledgements

ii

Most of all, I want to thank my family. My parents, for their continuous support and encouragement. My brother and sister for being their when needed. Marlene and Valentina, having you as my daughters is such a God’s gift. You brought so much joy in my live. Katrin, my beloved wife, for your love, for giving me ease when I needed it most and for making my live wonderful.

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iii

List of symbols and abbreviations

Latin Letters

A Current loading [A∙m-1]

b Width [m]

B Magnetic flux density [T]

C Coefficient C Capacitance [F] D Diameter [m] DE Energy density [Wh/m³] E Stored energy [J] f Frequency [Hz]

F Magneto motive force (mmf) [A]

h Height [m]

I Current [A]

J Bessel function k Constant

KRm Resistance correction coefficient

l Length [m]

L Inductance [H]

m Counter for the switching frequency harmonics m Number of layers

M Modulation index

M12 Mutual inductance [H]

n Counter for the baseband harmonics n Number of rows

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List of symbols and abbreviations

x

n Rotational speed [rpm]

N Number

P Power [W]

RS Stator winding resistance [Ω]

R Reluctance [H-1]

t Time [s]

T Torque [Nm]

v Velocity [m/s]

V Volume of the storage element [m³]

V Voltage [V]

x x-coordinate [m]

Greek Letters

αskew Skewing angle [°]

γ Phase angle [°]

η Efficiency [%]

λ Stray conductance value

ϕ Offset phase angle [°]

σ Electrical conductivity [Sm-1] τ Number of periods ϕ Magnetic flux [Wb] Ψ Flux linkage [Wb] ω Angular frequency [Hz]

Latin subscripts

0 Zero-sequence 0 Low-frequency reference 1 Stator phase 1

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xi 2 Stator phase 2

3 Stator phase 3 AC Alternating current

acr Ac phase current reduction Bat Battery

c Storage capacitor c Series connected coils CM Common mode cog Cogging con Conductor DC Direct current DM Differential mode ec Eddy current el Electrical

EPS Electric power steering ew End-winding fe Iron GEN Generator h magnetizing HF High-frequency hys Hysteresis i i-harmonic in Input j j-harmonic

k Counter for the three motor phases L Motor winding inductance

loss Additional losses m Main

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List of symbols and abbreviations xii M,k Reduction factor max Maximum min Minimum n nominal / rated out Output p Coil p ph Phases r Rotor R Resistor

rms Root mean square S Self S Superimposed s Side-band s Stack s Stator slots sat Saturation slot Slot SP Star-point sw Switching frequency shape Shape u Coil u w Winding factor

Greek subscripts

μ Leakage path σ Leakage or stray

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List of symbols and abbreviations xiii

Abbreviations

AC Alternating Current DC Direct Current DLC Double-Layer Capacitor EMI Electromagnetic Interference EPS Electric Power Steering ESP Electronic Stability Program FEM Finite-Element Method IM Induction Machine

MFCS Multifunctional Converter System mmf Magneto motive force

MOSFET Metal Oxide Semiconductor Field Effect Transistor NEDC New European Driving Cycle

PCB Printed Circuit Board PM Permanent-Magnet PWM Pulse-Width Modulation RMS Root Mean Square SM Synchronous Machine SMD Surface Mount Device

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1

1. Introduction

1.1. Automotive power net

Fuel reduction and improved performance are major driving forces in the automotive industry. Therefore more and more mechanical components are being replaced by mechatronic ones to improve overall efficiency. Many of these systems, especially chassis systems, draw high current pulses with a large di/dt from the automotive power net to provide high-dynamic mechanical power.

This electrification has led to an increase in the power consumption of the automotive power net as shown in Figure 1.1 [Bür 08]. From 1995 to 2006, the average consumed power multiplied by the factor of 1.4. Meanwhile, peak power demand increased from 1800W to more than 4000W by a factor of 2.4 [Bür 08]. Clearly, the requirements for the electrical power net have been rising drastically during the last decade.

Figure 1.1: Evolution of the power consumption in the automotive power net

A typical automotive electrical power net is shown in Figure 1.2. Its components include the starter, generator, battery, electrical loads and a high pulse power load e.g. the electric power steering (EPS) system.

Model year 1995 2006 0 500 1000 2000 1500 2500 3000 4000 3500 845 W 1770 W 4170 W 1200 W average Power [W] peak Power [W]

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1. Introduction

2

Figure 1.2: Typical automotive power net

The generator delivers the power for the electrical loads and charges the lead-acid battery. The lead-acid battery supplies the loads when the combustion engine is off and also stabilizes the automotive power net.

An overview of the different electrical loads in the automotive power net was given in [Fab 06], [Büc 08] and [Rei 11]. In the automotive power net high-power loads are defined as loads that draw a current of more than 30A. Common examples of high-power loads in the automotive power net are heating systems and electrical machines for pumps or fans.

In modern automotive vehicles, high-power heating systems are switched on with active switching devices to reduce the current slope drawn from the power net. The heating systems are low dynamic loads such as the front window heating, rear window heating, the seat heating or the exterior mirror heating. High-power loads that require only a low dynamic are supplied by the Lundell generator.

To prevent the Lundell generator from disrupting the combustion engine’s operation, the rate of change of generator output current must be limited. However, high-dynamic loads powered by a Lundell generator would exceed this limit, making this a poor choice for high-dynamic loads. The maximum current slopes of the generator depend on the rotational speed of the combustion engine. At low combustion-engine speed, the current ramp has to be reduced to a di/dt of about 100A/sec. Hence, every load in the power net that draws a current slope larger than the current slope of the generator cannot be supplied from the generator. The lead acid battery has to deliver the difference between the current from the generator and the required current for the high-dynamic loads.

Different high-dynamic, high-power loads were shown in [Koh 11] and [Klo 11b]. The high-dynamic, high-power loads that are implemented depend on the car manufacturer and the vehicle class and include e.g. electrical machines for the electronic stability program (ESP), electric power steering (EPS), variable rear axle differential, pneumatic shock absorber system and reversible belt pre-tensioners [Klo 11b].

S

G

Starter Generator Battery Electrical loads EPS

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1. Introduction

3 It was shown in [Koh 11] that the electric power steering system is the largest dynamic pulse power load in current automotive power nets. In this thesis the electric power steering system is analyzed because it is a safety-critical load that draws the largest current pulses from the power net. Figure 1.3 shows typical characteristics of the current consumption measured for an electric power steering system and the maximum current slope of the Lundell generator.

Figure 1.3: Typical current waveforms of the electric power steering and generator The maximum current slope (di/dt) of the electric power steering system depends on the dynamic requirements. It was shown in [Koh 11] that the di/dt may be up to 20,000A/s. The maximum current slope of the generator is about 100A/s and hence much smaller than the current slope required from the electric power steering system. Hence, the battery has to deliver the peak power demand, leading to high battery stress and an undesired voltage decrease in the power net. That is especially for currents with a large di/dt. The high current pulses also reduce the lifetime of the lead-acid battery. Therefore a method is required to decouple dynamic pulse power loads from the power net to stabilize the power net voltage.

Other applications to decouple pulse power loads

Besides the automotive power net there are many different applications where a power net with stabilized voltage is required. Airplanes [Ema 00], [Kha 06], busses, trucks, ships [Gut 09], [Ste 07] and [Bur 99] or trains also implement electrical machines for hydraulic pumps or other applications that draw high-dynamic current pulses from the power net.

It was shown in [Kha 06] that in airplanes, weight and volume reduction is a major design driving force. Hence in [Kha 06] an analysis was performed to stabilize the dc-link bus voltage between the generator and an electric flap drive with a storage capacitor that is required to be as small as possible.

100A

t

0.03s

C

urr

ent _{current slope of}Maximum

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1. Introduction

4

In maritime vessels, generator systems are implemented to generate electric power. Batteries are used to provide energy to start the diesel engine and during transients that exceed the response time of the generator system [Hun 12]. In [Hun 12] and [Gut 09] the dc bus is stabilized with batteries and an additional dc-dc converter.

In naval ships, large pulse power loads are implemented as shown in [Bur 99]. The required pulse power of these systems is much larger than the energy that could be provided by the generator systems. Therefore large storage batteries are used to supply the pulse power loads [Bur 99].

There are also different industrial applications where pulse power loads require more power than can be provided by the supply grid. In an automotive production line there are hundreds of industrial robots with electrical actuators that are used e.g. to electro weld, to glue or to screw things together. The grid has to be designed in such a way that a stable supply voltage for the industrial robots is assured.

In general, methods to stabilize the power net may be required anywhere electrical machines draw current pulses that lead to an undesired voltage decrease in the power net. The multifunctional converter system, proposed in this thesis to decouple pulse power loads, may be used for different applications to stabilize the power net.

1.2. Electric power steering (EPS)

In the automotive power net, electric power steering (EPS) is an important high-power load. The electrification of the steering system has led to a dramatic improvement of the efficiency in automotive vehicles. Depending on the vehicle class, electric power steering can consume over 90% less energy than the hydraulic servo steering system. Electrification reduces the average power demand because no permanently-running hydraulic pumps are required. The electric power steering system consumes energy only during operation. This leads to a fuel reduction of up to 0.4 liters per 100 kilometers at the New European Driving Cycle (NEDC) and up to 0.8 liters per 100 kilometers in urban traffic [ZF 09].

Further benefits of the electric power steering system are increased safety functions, more comfort and less maintenance (no hydraulic oil system) compared to the conventional hydraulic servo steering system. An electric power steering system allows additional functions to be implemented e.g. the system may assist the driver in critical driving situations. Furthermore, the supporting steering power could be adapted to driving circumstances e.g. increased during parking and decreased on the highway. Even self-parking is now possible with electric power steering systems. In a conventional hydraulic servo steering system the

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1. Introduction

5 hydraulic oil requires routine maintenance. This is not necessary in the case of an electric power steering system which leads to reduced maintenance costs.

An electric power steering system assists the driver by reducing the required human steering force. In current electric power steering systems there is a mechanical connection between the steering wheel and the steering rack. In future steer-by-wire systems there will no longer be a mechanical connection, thereby increasing the reliability of steer-by-wire systems. Although the focus of this thesis is on conventional electric power steering systems, the results presented can be implemented in the design of electrical machines for steer-by-wire systems.

An electric power steering system consists of an electric servo unit and a mechanical gear unit. The mechanical gear unit transforms the rotational speed of the electrical machine into a linear movement of the steering rack. Figure 1.4 shows the principal circuit diagram of the motor drive unit from a state-of-the-art electric power steering system.

Figure 1.4: State-of-the-art voltage source inverter with electrical machine

A pulse-width modulated voltage source inverter draws the energy out of the power net represented by the battery and forces three 120° phase-shifted sinusoidal currents through the motor windings to drive the electrical machine. The cascaded control structure of a state-of-the-art electric power steering system consists of a field-orientated control (FOC) that includes current controllers and an overlaid torque control. The torque at the steering rack is measured with a torque transducer. To achieve convenient steering behavior, the control circuit controls the motor drive unit to reduce the manual steering force to a predefined level. Existing electric power steering systems uses a star connection for the three phases of the electrical machines because circulating currents could occur with the use of delta connections.

1.2.1. Classification

Electric power steering systems are classified based on their mechanical output power. Producers offer different power categories to cover the complete range of different vehicle

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1. Introduction

6

classes. ZF Lenksysteme, for example offers three basic types of electric power steering systems to cover all vehicle classes, from subcompact to mid-size and upper-class cars and vans [ZF 09].

Electric power steering systems can also be classified based on the location of the servo unit and required rack forces. The servo units of ZF Lenksysteme, for example, are assembled either on the steering column, on a second pinion, or parallel to the steering rack [ZF 09]. The servo unit assembled on the steering column can transmit only small rack forces and is therefore only used for subcompact cars. This system has not been taken into consideration in this thesis because of its limited power level.

The power level of the steering system can be increased if the servo unit is placed on the rack. Figure 1.5a shows a system for mid-size cars. The servo unit transmits the forces with a worm gear over a second pinion to the steering rack [ZF 09], [TRW 09].

Figure 1.5: Electric power steering systems

As the transmission capacity of a worm gear is limited, a different gear unit is implemented for high-class vehicles. The electrical machine for high-class vehicles is placed axially parallel to the steering rack as shown in Figure 1.5b. The rotational movement of the electrical machine is transformed with a toothed-belt drive and a recirculating ball gear into a linear

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1. Introduction

7 movement of the steering rack. The recirculating ball gear is a system in which the ball chain is returned through a channel integrated in the ball recirculating nut [ZF 09]. This gear unit is more expensive than a worm gear but has a much higher efficiency.

Another producer of electric power steering systems, Continental, provides a system for upper-class vehicles with a hollow-shaft machine in which the rotor is mounted on a recirculating ball gear unit. This system is more expensive to produce, but doesn’t need a toothed-belt and requires less space [Con 10]. Figure 1.6 shows the electric power steering system with a hollow-shaft machine. The permanent magnet rotor is connected to a recirculating ball gear unit to transform the rotational speed into a linear movement of the steering rack.

Figure 1.6: Electric power steering systems for upper-class vehicles

Figure 1.5a shows a servo unit for mid-size cars where an induction machine is implemented. A permanent-magnet (PM) synchronous machine is used for high-class vehicles as shown in Figure 1.5b and for upper-class vehicles as shown in Figure 1.6. Because induction machines and PM synchronous machines are used for electric power steering systems, both machine types are considered in this thesis.

1.2.2. Automotive requirements for the electric power steering system

In the following the requirements for the design of an electric power steering system for automotive application are listed.

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1. Introduction

8

o Due to the maximum current slope of the generator the current slope drawn from the power net must be limited to 100A/s.

o The torque ripple has to be smaller than 1.5% of the nominal torque to avoid torque ripples on the steering wheel.

o A robust design is necessary as the electric power steering system is a safety-critical system. In case of a failure in the system, the steering system must not fail completely. It must still be possible for the driver to steer.

o The electromagnetic interference specified for automotive systems has to be met. o Safety-critical systems, like the electric power steering system must provide full

functionality when operating with a power net voltage between 10V and 17V. If the power net voltage decreases below 10V, the output power of the systems may be ramped down.

o Some car manufacturers allow only a maximum current of up to 100A [Chr 10] to be drawn out of the power net. This leads to a maximum input power of the steering system of 1kW at a supply voltage of 10V.

o High efficiency of the electric power steering system is required to reduce the complexity and costs of cooling.

o A compact design of the electric power steering system is required to fit in the limited space available in the engine compartment.

o Cost reduction is a general aim in automotive applications.

1.2.3. Comparison of an induction machine with a PM synchronous machine

Figure 1.5 and Figure 1.6 show that induction machines and PM synchronous machines are implemented in motor drive units for the electric power steering system. A discussion of the properties of the two machine types follows.

One important criterion in the automotive industry is the cost of a system. An induction machine has a cheap squirrel-cage rotor with aluminum bars. The PM synchronous machine is more expensive because of the permanent magnets in its rotor.

As has been mentioned, an electric power steering is a safety-critical system and the driver must be able to steer even when a failure occurs. In case of a short circuit, the system with an induction machine can be switched off and the driver can steer without electrical assistance with the mechanical connection. However, in PM synchronous machines, a high braking torque can occur when a short circuit occurs in the winding, e.g. in the case of inverter failure. Therefore additional switching devices, e.g. a star-point relay or MOSFET switches, have to be implemented to open the windings of the PM synchronous machine and avoid a blockage of the steering system.

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1. Introduction

9 The torque speed characteristic of an electric power steering system demands a good field-weakening capability of the electrical machine to achieve high rotational speed. In induction machines, the magnetizing current can easily be reduced, leading to good field weakening with high efficiency. In PM synchronous machines, a field-weakening current is required to reduce the magnetic field of the permanent magnets. This causes high losses, whereby the efficiency of the PM synchronous machine can be reduced below the efficiency of an induction machine at high rotational speed. An additional issue with controlling the PM synchronous machine in the field-weakening region is that at high rotational speed, the internal induced voltage may be higher than the power net voltage. As the field-weakening current reduces the induced voltage this is usually harmless, but in case of a failure, the increased induced voltage may lead to an overvoltage applied to the power net.

In induction machines a magnetizing current is needed, leading to losses in the squirrel-cage rotor and reducing efficiency. The rotor losses are difficult to cool and limit the maximum power. A synchronous machine with a permanent magnet rotor requires no magnetizing current and there are lower losses in the rotor. However, a drawback of the PM synchronous machine is the limited maximum temperature of the magnets.

Another important issue arising in electrical machine design are the power density requirements. Tooth-coil windings may increase the power density of electrical machines. The advantages compared to a distributed winding scheme are the reduced number of stator slots and the small end-windings. This leads to reduced copper losses, higher power density and to simple and low-cost machine production. The drawback of a tooth-coil winding is that the stator mmf generates a lot of harmonics that make no contribution to the torque generation. In induction machines this would lead to increased losses. Therefore, tooth-coil windings are commonly used for PM synchronous machines, but not for induction machines [Hut 05]. Table 1.1 summarizes the benefits and drawbacks of induction and PM synchronous machines.

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1. Introduction

10

Table 1.1

Benefits and drawbacks of induction and PM synchronous machines

Induction machine PM synchronous machine

Low costs + - Safety + - Field weakening + - Cooling - + Efficiency - + Power density - +

The table shows that the machines are comparably attractive for the chosen application of this thesis. The choice of one of the two types of machines should be based on the more appropriate characteristics for the vehicle’s class.

An induction machine is cheaper and more robust and therefore very suitable for the electric power steering systems in mid-class vehicles. Electric power steering systems for upper-class vehicles have been developed in the recent past. These vehicles are heavier and require more steering power. Hence, electrical machines with more mechanical power are required. As the space in the engine compartment and the power drawn from the power net are limited, the power density of the electrical machines needs to be increased. It follows that PM synchronous machines are more appropriate for these vehicle classes because of their high power density.

1.3. Problem description

The above discussion of high-power loads in the automotive power net has shown that a large di/dt drawn from the power net leads to an undesired voltage decrease. Therefore a system topology is required to decouple high-dynamic, high-power loads from the power net with a suitable energy storage element.

In this thesis a system topology incorporating a multifunctional converter is analyzed. A multifunctional converter uses the motor drive system as a dc-dc converter by connecting the star-point of the electrical machine to the power net. When the motor drive system is used as a dc-dc converter, dc currents are superimposed in the motor windings. Hence, a

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1. Introduction

11 multifunctional converter leads to new requirements for the design of a motor drive system, and the best design must be found.

Existing models of motor drive systems have not taken into consideration the fixed star-point connection or the dc phase currents. Therefore new models are required to analyze the influence of a multifunctional converter on high-frequency ripple currents, additional losses and mechanical torque.

The models are needed to improve the design of both induction machines and PM synchronous machines that would include the reduction of high-frequency ripple currents and the superimposed dc phase currents must not influence the average torque or the torque ripple. An analysis is required to determine the influence of the dc phase currents on the mechanical torque to obtain guidelines for the selection and design of electrical machines with winding schemes where the superimposed dc phase currents do not influence the mechanical torque.

An analytical model of induction and PM synchronous machines is needed to determine the influence of the fixed star-point connection on the high-frequency ripple currents and to calculate the high-frequency losses. With the analytical model the machine design and the switching pattern can be improved. It is important that the methods to reduce the high-frequency ripple currents do not influence the mechanical torque.

For the verification of the developed models the induction machines and PM synchronous machines of existing electric power steering systems have to be driven by multifunctional converters. In addition, the design of the existing induction and PM synchronous machines should be improved, prototypes built up and tested to verify the developed design improvements. Two possible machine types may be chosen. Hence, a comparison between the induction machine and the PM synchronous machine is necessary to show which machine type is more suitable to be driven by a multifunctional converter.

An electric power steering system is a safety critical application and hence the fault tolerance is a further design aspect. Therefore, an examination of new remedial strategies that are possible when electrical machines are operated from multifunctional converters is necessary.

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1. Introduction

12

Thesis objective

The main objectives of the thesis are derived from the problem description and include the following:

reduce the current slope of the electric power steering system with a selected storage element and a multifunctional converter and analyze the new requirements for an electrical machine driven by a multifunctional converter to derive design guidelines and to increase the reliability for induction and PM synchronous machines.

To achieve these objectives the following points have to be determined:

i. Finding the most suitable energy storage element and selecting a converter topology to achieve the decoupling of the electric power steering system from the automotive power net

ii. Integration of the front-end dc-dc converter in the inverter and the electrical machine and deriving the new requirements for the design of the electrical machine with a multifunctional converter

iii. Developing models to analyze the influence of a multifunctional converter on high-frequency ripple currents, mechanical torque and losses in induction machines and PM synchronous machines

iv. Designing of induction machines and PM synchronous machines to the new requirements of multifunctional converters in order to avoid any influence on the mechanical torque and to reduce the high-frequency ripple currents to keep the additional losses of the multifunctional converter as small as possible, and comparing the two machine types

v. Increasing the reliability of the electric power steering system with the two-phase operation of the induction machines and PM synchronous machines driven by multifunctional converters

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1. Introduction

13

1.4. Thesis layout

The Figure 1.7 shows the layout of the thesis.

Figure 1.7: Layout of the thesis

In Chapter 2 an overview is given of different methods of decoupling pulse power loads from the power net and a new method of decoupling pulse power loads with a multifunctional converter system is introduced. In a multifunctional converter system, the motor drive system is also used as a dc-dc converter.

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1. Introduction

14

In Chapter 3 the new requirements for the design of electrical machines with multifunctional converter drives are presented. Machine-independent models are developed to analyze the influence of the multifunctional converter on high-frequency ripple currents and losses in the electrical machines. In Chapter 3 an improved switching pattern is developed to reduce the high-frequency ripple currents in electrical machines with a multifunctional converter drive. Further, a remedial strategy is introduced to drive three-phase electrical machines operated from a multifunctional converter with only two active phases.

In Chapter 4, a multifunctional converter system with induction machines is analyzed, continued in Chapter 5 for PM synchronous machines. Models of both machine types are developed to analyze the influence of the multifunctional converter on the machine losses and torque dependent on different winding schemes. These models of the multifunctional converter systems are used to improve the design of the induction machine and of the PM synchronous machine. Further, the influence of the fault-tolerant two-phase operation on the mechanical torque is analyzed for induction machines in Chapter 4 and the PM synchronous machines in Chapter 5.

The results achieved for both machine types are compared in Chapter 6. The conclusions drawn from the research are outlined in Chapter 7 including recommendations for further research.

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2. Methods of decoupling an EPS from the power net

2.1. Introduction

As identified in Chapter 1 the electric power steering system is a dynamic high power load and has to be decoupled from the automotive power net to release the battery. To reduce the current slope of the system an energy storage element is required.

In this chapter, the requirements for an energy storage element are determined. Afterwards different storage elements are compared in order to select the most suitable for the electric power steering system. Several topologies have already been proposed to integrate a storage element into the automotive power net to release the battery. In Section 2.3 an overview of four existing topologies is given. In Section 2.4 a new method of decoupling pulse power loads with a multifunctional converter system is introduced. An overview of existing multifunctional converter topologies is given and the operation principle is shown. The different system topologies are compared in Section 2.5 with the result that the multifunctional converter system is selected for further research.

2.2. Energy storage

Here, the energy required to reduce the current slope of the electric power steering system to the maximum current slope accepted by the generator is calculated. The required energy density is defined together with the available space for the storage element. The automotive requirements for an energy storage element are specified. Different types of the selected storage technology are compared in terms of their energy density to select a storage element.

2.2.1. Requirements for energy storage

As a first step, the energy required to release the battery is determined. For a complete release of the battery the current slope of the electric power steering system has to be limited to the current slope of the generator. This energy calculation is based on the assumption that the power net voltage is constant and the storage element can be represented by a current source. Figure 2.1 shows the power net with the electric power steering system and the storage element.

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2. Methods of decoupling an EPS from the power net

16

Figure 2.1: Automotive power net with energy storage element

To release the battery (Ibat = 0), a storage element is required to cover the difference

between the current drawn by the electric power steering system and the current delivered by the generator. Figure 2.2 shows idealized waveforms of the electric power steering input current, the generator current and the current of the storage element. The current is drawn out of the storage element when the electric power steering system is turned on and supplied back into the storage element when it is turned off.

Figure 2.2: Current waveforms of the EPS, the generator and the storage element The required energy is calculated from the difference between the energy drawn by the electric power steering system and the energy delivered by the generator. The required storage energy E is calculated with (2.1) assuming an ideal input step of the electric power steering system with maximum power for the complete pulse.

E =



  1 0 ) ( t dt I I

V_Bat _EPS _Gen = ½ · VBat · IEPS · t1 (2.1)

G M EPS Ibat= 0 I_C I_EPS V_bat= const I_Gen Storage element I_EPS I_C I_Gen Generator current

Current consumption EPS

Current storage element

t

t t

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17 The maximum input power of the electric power steering system is 1kW. For a minimum voltage level of VBat = 10V a current of IEPS = 100A is required. With a current slope of

100A/sec delivered from the generator, it takes t1 = 1sec until the generator has reached the

maximum output current. With (2.1) a storage energy of E = 500J is required.

The electric power steering system is placed below the engine compartment, where the available space is extremely limited. Hence the volume V of the energy storage element is limited to approximately 0.2dm³. By using that maximum volume of the storage element the required energy density DE is calculated with (2.2)

DE = E/V (2.2)

leading to a required energy density of DE = 700Wh/m³.

There are several energy storage technologies commercially available today, e.g. electrochemical energy storage, electric field energy storage, magnetic field energy storage and kinetic energy storage [Hol 03a]. However, only a few have already been qualified and implemented in automotive applications.

For the electric power steering system a storage technology is required which fulfills one of the desired specifications for automotive applications of a generally maintenance-free operation for the complete life time of the vehicle. In the automotive industry the required life time of the components are specified. A storage element has to be selected to achieve this life time. Cost reduction is also a major driving force in the automotive industry and therefore more expensive solutions for energy storage elements fall outside the scope of consideration. As stated in the introduction of the thesis, fuel reduction is increasingly important. Hence, the weight of the storage element has to be as small as possible so as not to be counterproductive to the fuel reduction resulting from the electrification of the steering system.

Electrochemical energy storage elements, e.g. lead-acid batteries, have not been selected because of their comparatively short lifetime. Lithium batteries have a high power and energy density but at low temperature the internal resistance rises to high values [Red 11]. The magnetic field energy storage elements do not have the required energy density [Hol 03a]. Kinetic energy storage elements, e.g. flywheels, require an electrical machine with a complex control circuit leading to high costs. Hence, flywheels may be used as storage elements in high power systems, but are not appropriate for an electric power steering system with relatively small power and energy levels [Hol 03a]. Electric field energy storage elements, so-called capacitors, fulfill automotive requirements of a maintenance-free operation for the complete life time, are light, have high energy density and are cheap compared to e.g. a flywheel system. Therefore, capacitors were selected as energy storage elements for this research.

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2.2.2. Energy storage capacitor technologies

The energy density of different capacitor types is compared. The following capacitors were selected for further examination: aluminum electrolytic capacitors, double-layer capacitor cells and double-layer capacitor modules. In the double-layer capacitor modules, several cells are connected in series to increase the voltage level.

The total energy E stored in a capacitor is calculated with (2.3) and is dependent on the capacitance CC, the maximum voltage level VCmax and the minimum voltage level VCmin.

) ( 2 1 2 min 2 max C C C V V C E    (2.3)

The maximum energy density of the capacitors is obtained from (2.2) and (2.3). The energy density decreases when the voltage variation for charging and discharging is limited. Figure 2.3 shows the maximum energy density of different capacitors based on their rated voltage. It is assumed that the capacitors are charged from zero to their rated voltage limit.

Figure 2.3: Energy density of different capacitors based on voltage level

The data for the aluminum electrolytic capacitors have been received from TDK-EPC and the data for the double-layer capacitors cells and modules from MAXWELL and WIMA GmbH [EPC 12], [Max 12], [WIM 12].

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19 Figure 2.3 shows that for electrolytic capacitors, the energy density increases with the increasing rated voltage, and that the energy density in double-layer capacitors is higher than in electrolytic capacitors. The energy density of double-layer modules is slightly lower than the energy density of the cells because the inter-cell connection and the voltage balancing reduces the fill factor of the module package. The drawbacks of the double-layer capacitors are the limited operating maximum temperature of 65°C and the small cell voltage of 2.7V [Max 12].

The dotted line in Figure 2.3 shows the energy density derived from (2.2). It can be seen that the energy density of electrolytic capacitors is too small for the application. Hence, double-layer capacitors are selected as energy storage elements to release the battery.

In normal operating conditions the capacitor cells are not completely charged and discharged. In most applications a small voltage variation (VCmax - VCmin) is desired. Figure

2.4 shows the required minimum voltage VCmin to which the completely charged capacitor

modules have to be discharged in order to achieve the required energy density. For the figure double-layer capacitor cells from Maxwell with a capacitance of 310F are selected [Max 12]. When four 2.7V double-layer capacitor cells are connected in series the maximum voltage of the module is 10.8V. It can be seen from Figure 2.4 that a voltage variation of 0.8V is required to obtain the required energy density.

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2.3. Overview of existing methods

In the literature different methods were proposed to decouple pulse power loads from the power net and hence to stabilize the power net voltage. The methods are based either on passive integration of storage elements into the power net or active stabilization of the power net voltage with dc-dc converter topologies. In the following section, four existing system topologies for the integration of the double-layer storage capacitors into the power net are shown. The topologies include the following:

1) passive integration of the double-layer capacitor 2) active current source

3) serial voltage source 4) front-end dc-dc converter.

2.3.1. Passive integration of the storage capacitor

In [Mil 04] and [Pol 07a] a topology is proposed to connect the storage capacitor in parallel to the power net to release the battery. Figure 2.5 shows a distributed automotive power net with the parasitic wiring inductances and the electric power steering system stabilized locally with a double-layer capacitor module. Because of the spatial distribution of the high-power loads, a large wiring harness is required leading to parasitic impedances. When a storage capacitor with a small internal impedance is placed next to high power loads the storage capacitor acts as a passive filter for the high current pulses.

Figure 2.5: Power net with passive integration of the storage capacitor

This solution is already available from aftermarket suppliers to provide local bus stiffening for high-power audio system amplifiers. The storage capacitor, which can be a module with

Battery M EPS Energy storage element Electrical loads DC DC

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21 double-layer capacitors, is placed next to the audio system in parallel to the automotive power net [Mil 04].

For the passive integration of the storage capacitor, a charging circuit is required to limit the charging current. To control the charge and discharge of the storage capacitor a dc-dc converter is needed. Additionally, a high power switch is necessary to decouple the storage element from the power net at standstill and in case of a failure. The stabilization capacitor has to be designed for the complete power net voltage variation. Therefore a multitude of double-layer cells with a maximum voltage level of only 2.7V has to be connected in series. Further, layer cells with a small internal resistance are required and therefore double-layer capacitor with a small size may not be implemented [Mil 04] and [Pol 07a].

2.3.2. Active current source

A second topology to release the battery is based on an active current source [Klo 11a], [Sta 09b] and [Bür 08]. Figure 2.6 shows the automotive power net with the active current source consisting of a storage capacitor and a dc-dc converter. The dc-dc converter connects the storage capacitor to a second power net with a variable voltage level.

The dc-dc converter is controlled to keep the voltage at one point of the power net constant. The stabilization unit detects a voltage drop due to a current pulse at the input of the electric power steering system and delivers the required pulse current for the steering system. Hence, the stabilization unit acts as an active current source.

Figure 2.6: Power net stabilized with an active current source

Because the storage capacitor is connected to a separate power net it can be completely charged and discharged, thereby achieving maximum utilization of the storage capacitor. The voltage level of the storage capacitor is independent from the power net and can be designed

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22

to optimize the stabilization unit. The drawback of this topology is that the dc-dc converter has to be designed to supply the peak current pulses for the electric power steering system.

2.3.3. Serial voltage source

The serial voltage source can be either a passive storage element connected in series to the load or an active duty-cycle controlled serial voltage source.

Figure 2.7 shows a passive connection of the storage element in series to the loads. A 12V battery or a double-layer capacitor module can be connected in series to the load to increase the dc-link voltage for the electric power steering system [Bür 08] and [Frö 08]. For this method, only a small dc-dc converter is required to deliver the average power for the energy storage element. By connecting a storage element in series to the load, the voltage level is increased, but only a small release of the battery can be achieved.

Figure 2.7: Serial constant voltage source

A novel approach to stabilizing the automotive power net with a floating converter connected in series to the loads was proposed in [Hac 10b]. This floating converter acts as serial voltage source. Figure 2.8 shows the serial voltage source together with the storage element. The serial voltage source balances the dc-link voltage to release the battery by charging and discharging the storage capacitor.

The serial voltage source has no ground connection and therefore no short circuit to ground can occur. This leads to the increased reliability of the module. The serial voltage source can also be used to stabilize the power net voltage for sensitive loads without the storage capacitor connected at Vdc. In this case, the battery is not released however the loads are supplied with a

constant voltage. A detailed analysis of the floating converter stabilizing the power net can be found in the Appendix C.

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23 Figure 2.8: Serial variable voltage source

2.3.4. Front-end dc-dc converter

A further topology to integrate the storage capacitor is based on a front-end dc-dc converter. Figure 2.9 shows the dc-dc converter connecting the automotive power net to a second power net where the storage element is integrated in parallel to the inverter [Pol 07b] and [Pol 06].

Figure 2.9: Front-end dc-dc converter with storage element

There are different topologies to realize the front-end dc-dc converter. In the past, several topologies were proposed for interconnecting automotive power nets based on different series connections of buck and boost converters [Pol 07b] and converterswith galvanic isolation.

For the proposed application the converter has to be designed for high currents. It is shown in [Hu 03] that for high current application single stage converters achieve best efficiency. Therefore, only single stage converters without galvanic isolation are considered [Hu 03]. The

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single-stage dc-dc converter can be a buck, boost or buck-boost converter. It is shown in [Mur 99] that a boost converter with increased voltage level has benefits for the design of the electric power steering system because the losses in the inverter can be significantly reduced.

For high current applications a synchronous boost converter with several phases connected in parallel has been proposed to reduce the size and costs [Miw 92] and [Ger 05]. These multi-phase converters may be further optimized by implementing coupled inductors [Sta 09a].

Integration of the double-layer capacitors

Regarding the boost converter, there are principally two different integration methods of the storage capacitor when only single-stage dc-dc converters are considered. Figure 2.10 shows the equivalent circuit diagram of the two different integration possibilities of the storage capacitor.

a)

b)

Figure 2.10: Two integration methods of the storage capacitor

The power net represented by the battery is connected to the front-end dc-dc converter and the inverter of the electric power steering system is connected to the output of the dc-dc converter. The storage capacitor can be connected either to the dc-link voltage Vdc shown in

Figure 2.10a or, as shown in Figure 2.10b, from the output to the input of the dc-dc converter. In Figure 2.10b, the storage capacitor voltage is the difference between the dc-link voltage and the power net voltage.

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25 Figure 2.11 shows typical waveforms obtained from the proposed input stages. The waveforms include the current consumption of the electric power steering IEPS, the generator

current IGen, the storage capacitor current IC and the input current of the dc-dc converter Iin.

Figure 2.11: Current waveforms of the two proposed dc-dc converter topologies

To release the battery, the difference between the current of the electric power steering system and the generator current is delivered from the storage capacitor as the capacitor is charged or discharged. The input current of the boost converter in Figure 2.11a is equal to the generator current. However, in Figure 2.11b the capacitor current has to be considered and the input current of the dc-dc converter is the difference between the generator current and the capacitor current. This leads to an increase of the maximum input current in the dc-dc converter when the electric power steering is switched off and the storage capacitor is charged. The input current increases because the front-end dc-dc converter operates as an inverting converter, charging the storage capacitor. The amplitude of the capacitor current depends on the voltage ratio of the converter.

Due to the increased input current of the dc-dc converter in Figure 2.10b, the size of the components for the dc-dc converter increases. However, the voltage level of the storage capacitor is reduced because the capacitor voltage is the difference between the dc-link voltage and the power net voltage. Therefore fewer double-layer capacitor cells have to be connected in series. For the front-end stage, the double-layer capacitors present the main contribution to the overall volume, weight and costs.

I_EPS

I_C I_Gen

Generator current

Current consumption EPS

Current storage element t t t I_in Inductor current t a) I_EPS I_C I_Gen Generator current

Current storage element t t t Inductor current t I_in b)

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To limit the charging currents of the double-layer capacitors, a separate charging circuit is required for the first topology. The converter in Figure 2.10b operates as a buck-boost converter to charge the double-layer capacitors. Hence, no additional charging circuit is required. A further benefit is that in case of failure, e.g. short circuit in the dc-dc converter, no high voltage is applied to the power net. This leads to improved safety of the dc-dc converter shown in Figure 2.10b.

For the design of an electric power steering system with a front-end dc-dc converter the topology in Figure 2.10b has the advantage that no charging circuit is required, fewer double-layer capacitor cells have to be connected in series and there is an increased safety factor.

The front-end dc-dc converter increases the dc-link voltage. In common electric power steering systems MOSFETS with a break down voltage of 40V are implemented. Parasitic effects during the switching transient of MOSFETS produce a high overvoltage spike. To avoid an avalanche break down of the MOSFETS, the overvoltage spike has to be kept smaller than the break down voltage. This limits the maximum dc-link voltage. With an optimized design, driver unit and a snubber circuit the overvoltage can be reduced [Utz 10]. The maximum dc-link voltage has therefore been limited to 2/3 of the breakdown voltage [Möß 12]. Hence for MOSFETS with a breakdown voltage of 40V the maximum dc-link voltage is therefore 27V. As mentioned before the power net voltage varies between 10V and 17V. Hence, the voltage of the storage capacitor VC is set to 10V.

Double-layer capacitors are typically available with a maximum voltage of 2.7V. To achieve the given minimum capacitor voltage of 10V, four capacitors are connected in series. The maximum ΔV for the energy storage in the capacitors is therefore limited to 0.8V.

The slope of the input current is calculated with (2.1) and (2.3). When an ideal input power step of 1kW of the electric power steering system is assumed and 310F cells are implemented, a slope of the input current of less than 80A/sec is achieved.

With the front-end dc-dc converter the battery can be completely released and the power net voltage stabilized. A further benefit of the front-end dc-dc converter is that the dc-link voltage level for the electric power steering system can be adjusted. This allows an optimization of the design of the electric power steering system to increase overall efficiency. However a serve drawback of the topology is that an additional dc-dc converter is required that have to be designed for large peak currents.

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2.4. Multifunctional converter to decouple pulse power loads

2.4.1. Multifunctional use of components

In the previous section existing system topologies were shown to decouple the power consumption of the electric power steering system and the power drawn from the power net. The current slope drawn from the power net by the electric power steering system was limited to release the battery. This could be realized with a storage capacitor and an additional front-end dc-dc converter. However, the additional front-front-end dc-dc converter increases the volume, weight and costs of the electric power steering system. The power electronic components of the dc-dc converter can be optimized for small volume, weight and costs. A further optimization of the component count can be achieved by integrating the functionality of several components or sub-systems into one.

In the automotive industry, an example of implementing multiple functions in a single component to reduce the component count is the diode bridge rectifier of the Lundell generator. Overvoltage protection is integrated into the diodes by using zener-diodes [Rob 10]. Some examples of ways to reduce the component count of dc-dc converters as given in the literature are the electromagnetic integration of passive components and the integrated heat sink structure [Smi 93], [Pop 05] and [Ger 05]. Electromagnetic integration combines the functionality of several discrete, passive components into a single component, e.g. realizing an L-C filter within a single inductor core [Waf 02]. The integrated heat sink structure uses the housing of passive components as a heat path [Ger 05].

In this section the possibility of integrating the additional front-end dc-dc converter in the motor drive system for a further reduction of volume, weight and cost will be analyzed. The power electronic components of the motor drive system can be given several functions. The windings of the electrical machine can be used as a filter inductor for the dc-dc converter and the half-bridges of the inverter can be used as active switching devices for the dc-dc converter.

The multifunctional use of the motor drive system leads to the name “multifunctional converter system”. Figure 2.12 shows the basic idea of the integration of multiple functions into a single system. The multifunctional converter system carries out both of the functions of the dc-dc converter and the motor drive system.

Multifunctional Converter Drive for Automotive Electric Power Steering Systems

Multifunctional Converter

Drive for Automotive Electric

Power Steering Systems

Multifunctional Converter

Drive for Automotive Electric

Power Steering Systems

Proefschrift

Acknowledgements

Contents

List of symbols and abbreviations

Latin Letters

Greek Letters

Latin subscripts

Greek subscripts

Abbreviations

1. Introduction

1.1. Automotive power net

S

G

1.2. Electric power steering (EPS)

1.3. Problem description

1.4. Thesis layout

2. Methods of decoupling an EPS from the power net

2.1. Introduction

2.2. Energy storage



2.3. Overview of existing methods

2.4. Multifunctional converter to decouple pulse power loads