A motor controller for solar car

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A motor controller for solar car

  1. 1. The Department of Computer Science and Electrical Engineering A Motor Controller For the Solar Car Project Andrew James Reghenzani Supervisor : Mr. Geoffrey Walker Submitted for the degree of Bachelor of Engineering (Electrical And Electronic) 16th October 1998.
  2. 2. Union College, Upland Road, St. Lucia QLD 4067. Ph : (07) 33771500 Fax : (07) 33713826 16 October 1998The Dean,Faculty of Engineering,The University of Queensland,St. Lucia QLD 4072Dear Professor Simmons, In accordance with the requirement of the degree of Bachelor of Engineering inthe division of Electrical and Electronic Engineering, I present the following thesisentitled : “A Motor Controller For the Solar Car Project” This work was performed under the supervision of Mr. Geoffrey Walker. Ideclare that the work submitted in this thesis is my own, except as acknowledged in thetext and footnotes, and has not been previously submitted for a degree at The Universityof Queensland or any other institution.Yours Sincerely,Andrew J. Reghenzani.
  3. 3. A Motor Controller For The Solar Car ProjectACKNOWLEDGEDGMENTS The following people deserve special recognition for their contributions to mythesis project throughout the year:My family : who have always supported me throughout University, and have given methe extra motivation to succeed during difficult times.My friends : for understanding how important my thesis was and always seeming to askthe all too familiar question “How’s your thesis going?”.Members of the Solar Car Team : especially Charles for organizing use of a digitalcamera and Anthoney for assistance with writing code. I have thoroughly enjoyedbeing in the solar racing team, as it has given me the opportunity to gain valuable workexperience and gain some practical skills which complement my University studies.My supervisor, Mr. Geoffrey Walker : for all his time, invaluable advice andencouragement throughout the thesis project.Keith Aldworth and the electronics workshop personnel : for the manufacture of myPCB’s and all the labor intensive hand tinning that had to be done for both boards,supply of components, use of the surface mount soldering station and all the technicaltips regarding PCB design and manufacture.Keith Lane, Wayne Jenkins and Bill Slack from the electronics workshop : for buildingmy heatsinks and other hardware from my plans which usually consisted of a page ofdimensions, use of the tools and machines in the workshop at any time and all thetechnical advice regarding manufacturing. - iii -
  4. 4. A Motor Controller For The Solar Car ProjectABSTRACT The transport needs of our ever growing and evolving society is becomingincreasingly stringent and more demanding. In order to combat this, more efficienttransportation vehicles need to be developed which are faster and cleaner. As thehuman race starts to realize the real extent to which the internal combustion engine hasgradually polluted the atmosphere, more research is being concentrated on alternativeforms of propulsion. A number of propulsion systems and energy sources haveundergone feasibility studies to investigate potential commercial and industrialapplications. Some projects have been shown to work successfully, while othertechnologies are still well in their infancy stage of development. A handful of examplesof the technologies under consideration include nuclear energy, fuel cells, steam power,solar power, wind power and tidal power. Electric and hybrid powered cars are emerging as a popular transport alternative.These type of vehicles emit far less pollutants to the atmosphere than the single internalcombustion engine, and have been proven to display moderate driving range (up to300km). An electrically powered vehicle has essentially three major electricalcomponents. These are an energy source (usually a rechargeable battery bank), aninverter or motor controller and an electric motor. In the case of a solar car, the energysource is typically a bank of batteries, which may be recharged by photovoltaic solarpanels. The motor controller is typically a power electronics device which whensupplied with the driver’s input commands, controls the torque in the electric motor.The electric motor converts the electrical energy supplied by the motor controller tomechanical energy used to propel the vehicle, usually through a type of transmission. A motor controller is custom designed for a new hub mounted Brushless DCPermanent Magnet (BLDC PM) motor, as part of the solar car project. Efficiency andreliability have been two of the key factors considered when designing the controller.Due to careful selection of quality components and use of high efficiency controlalgorithms, a marketable increase in efficiency over the existing system is expected withthe new controller and motor. - iv -
  5. 5. A Motor Controller For The Solar Car ProjectCONTENTSACKNOWLEDGEDGMENTS......................................................................................................IABSTRACT.................................................................................................................................. IVCONTENTS................................................................................................................................... VLIST OF FIGURES ............................................................................................................ VIILIST OF TABLES ............................................................................................................. VIII1. INTRODUCTION................................................................................................................11.1 Introduction ............................................................................................................ 11.2 Problem Specification ............................................................................................ 21.2.1 Thesis Goal .......................................................................................................... 41.2.2 Motivation behind the Motor Controller and Motion Control............................. 41.3 Organization of the Thesis Document.................................................................... 52. THE UNIVERSITY OF QUEENSLAND SOLAR CAR..............................72.1 Solar Car Racing and the Races ............................................................................. 72.2 A Brief History of the UQ Solar Racing Car ......................................................... 92.3 The Nuts and Volts of a Solar Car.......................................................................... 92.3.1 Batteries ............................................................................................................. 102.3.2 Solar Array ........................................................................................................ 122.3.3 Maximum Peak Power Trackers (MPPT’s)....................................................... 132.3.4 Motor Controller................................................................................................ 132.3.5 Motor ................................................................................................................. 142.3.6 Telemetry Functions and Power Supply............................................................ 152.4 Necessity for Efficient Systems............................................................................ 152.5 The Existing Drive System................................................................................... 162.5.1 Controller Type.................................................................................................. 162.5.2 Performance Characteristics .............................................................................. 172.6 The New Drive System ........................................................................................ 172.6.1 Additional Features............................................................................................ 182.6.2 Performance Requirements................................................................................ 193. MOTOR CONTROL LITERATURE......................................................................204. THEORY ...............................................................................................................................254.1 The Permanent Magnet Brushless DC Motor ...................................................... 254.1.1 Electrical and Mechanical Parameters............................................................... 284.2 Controlling a Permanent Magnet Brushless DC Motor........................................ 304.2.1 Commutation ..................................................................................................... 304.2.2 Current Regulation ............................................................................................ 354.2.3 Trapezoidal Current Excitation.......................................................................... 354.2.4 Sinusoidal Current Excitation............................................................................ 374.3 Power MOSFET Device Characteristics .............................................................. 384.4 Heatsink Considerations....................................................................................... 415. HARDWARE DESIGN STAGE...............................................................................435.1 Design of Power Stage ......................................................................................... 435.1.1 Circuit Design.................................................................................................... 445.1.2 Sensors............................................................................................................... 455.1.2.1 Bus Voltage Measurement .............................................................................. 455.1.2.2 MOSFET Heatsink Temperature Measurement ............................................. 465.1.2.3 Phase Current Measurement ........................................................................... 465.1.3 Manufacture and Construction .......................................................................... 485.2 Design of Control Stage ....................................................................................... 50 -v-
  6. 6. A Motor Controller For The Solar Car Project5.2.1 Circuit Design.................................................................................................... 515.2.1.1 Auxiliary Components and Power .................................................................. 525.2.1.2 Memory Board ................................................................................................ 525.2.1.3 Input/Output Ports........................................................................................... 525.2.2 Manufacture and Construction .......................................................................... 546. SOFTWARE DESIGN STAGE ................................................................................566.1 System Description............................................................................................... 566.2 Main Program....................................................................................................... 586.3 Torque Control ..................................................................................................... 596.3.1 Regeneration...................................................................................................... 596.3.2 Brake.................................................................................................................. 606.4 MOSFET Heatsink Temperature.......................................................................... 616.5 Motor Temperature............................................................................................... 616.6 Speed and Direction ............................................................................................. 616.7 Commutation ........................................................................................................ 616.8 Bus Voltage .......................................................................................................... 617. DISCUSSION .....................................................................................................................637.1 Discussion ............................................................................................................ 638. CONCLUSIONS ................................................................................................................648.1 Thesis Conclusions............................................................................................... 648.2 Possible Future Work ........................................................................................... 648.3 The Future of Solar Car Racing : The Big Picture ............................................... 66APPENDICES ..............................................................................................................................67APPENDIX A: SCHEMATIC AND PCB DESIGNS.................................................68APPENDIX B: MOSFET DATA SHEETS.............................................................69APPENDIX C: CSIRO/UTS MOTOR SPECIFICATIONS........................................70APPENDIX D: MICROCOMPUTER PROGRAM LISTINGS...............................71APPENDIX E: ACCOMPANYING COMPUTER DISK .............................................72MAIN PROGRAM ......................................................................................................................72SCHEMATIC FILES ..................................................................................................................72PCB FILES ...................................................................................................................................72BIBLIOGRAPHY ........................................................................................................................73BOOKS ...................................................................................................................................... 73JOURNAL ARTICLES ............................................................................................................ 73INTERNET RESOURCES ...................................................................................................... 77 - vi -
  7. 7. A Motor Controller For The Solar Car ProjectLIST OF FIGURESFIGURE 1 : BLOCK ELECTRICAL DIAGRAM OF A SOLAR CAR ......................................................................10FIGURE 8 : HALL EFFECT POSITIONING SENSORS........................................................................................28FIGURE 9: NUMBERING PATTERN FOR MOSFET’S IN THE H-BRIDGE ......................................................... 30FIGURE 10 : 120 DEGREES COMMUTATION MODE ...................................................................................... 32FIGURE 11 : 180 DEGREES CONDUCTION MODE ......................................................................................... 34FIGURE 12 : CURRENT FEEDBACK IN A BLDC MOTOR ...............................................................................35FIGURE 13 : TORQUE RIPPLE IN A TRAPEZOIDAL MACHINE ........................................................................ 36FIGURE 14:NON-CONDUCTRING MOSFET[34] ..............................................................................................FIGURE 15:CONDUCTING MOSFET[34] ............................................................................................38FIGURE 16:WAVEFORMS AT TURN-ON[38].....................................................................................................FIGURE 17:WAVEFORMS AT TURN-OFF[38]................................................................................................39FIGURE 20 : THERMISTOR RESPONSE ..........................................................................................................54FIGURE 22 : BLOCK DIAGRAM OF CONTROL ALGORITHM........................................................................... 57FIGURE 23 : A FOUR QUADRANT DRIVE.......................................................................................................58FIGURE 24:ONE SWITCH ACTIVE TOPOLOGY ..................................................................................................FIGURE 25:TWO SWITCH ACTIVE TOPOLOGY .............................................................................................60 - vii -
  8. 8. A Motor Controller For The Solar Car ProjectLIST OF TABLESTABLE 1 : 120 DEGREES COMMUTATION TRUTH TABLE ............................................................................31TABLE 2 : 180 DEGREES COMMUTATION TRUTH TABLE ............................................................................33 - viii -
  9. 9. 1. INTRODUCTION1.1 Introduction The development of the internal combustion engine was certainly considered amilestone for mankind. The focus back in the time of the Industrial Revolution was todesign machines which could fulfill time consuming, labor intensive jobs in a fractionof the time that it took humans alone using conventional methods. Cars were developedas a fast means of transport, and internal combustion engines soon found themselves inmany applications ranging from cane harvesters to outback generator sets. As timeprogressed, most people had realized that although the internal combustion engine hadprovided a much easier lifestyle, there were a number of major drawbacks. Petrol,when combusted, forms a number of gaseous byproducts, consisting mainly of carbondioxide, but also containing traces of other gases such as carbon monoxide andcompounds containing lead. The potency and increasing levels of these gases andcompounds are causing gradual damage to the ozone layer in the Earth’s atmosphere.Such gases are commonly referred to as greenhouse gases. -1-
  10. 10. 1. Introduction Soon people began looking for alternatives to the internal combustion engine.Quite recently, hybrid electric vehicles (EV) have been met with much success, andcommercial versions are being made today. A typical hybrid EV is driven by an electricmotor and usually contains a rechargeable battery bank and a small internal combustionengine. The internal combustion engine still emits greenhouse gases, however only at afraction of the amount. In some of the latest hybrid vehicles, four wheel motors areused (one for each wheel), and four motor controllers are used to control the torque ofeach individual motor for optimal vehicle performance and control. An alternative energy source which is very appealing is solar energy. Solarenergy is a continually advancing technology, and as photovoltaic (PV) solar cells arebeing made more efficient, solar power is finding widespread use in applications suchas outback power supplies and grid connected PV arrays. A large contributor to theincreasing level of pollution is the household car, so solar cars were developed with thevision that an ideal car could be built which could run solely from the sun for thelifetime of the car, and never require fueling up. This indeed is a futuristic dream,however the technology is fast approaching this stage.1.2 Problem Specification Design of a motor controller for the University solar car project has not beenattempted before. The new controller has incorporated a multitude of features which aredesigned to make the drive system highly efficient and safer while providing a moreintuitive driver control. The new motor controller consists of a Hitachi SH1 7032 RISCmicroprocessor operating at a clock speed of 20MHz accompanied by an array ofsensors and a high voltage inverter stage. The work performed in this thesis projectincorporates a number of different fields of work:• Electronic Commutation : the switching of currents to the correct phase windings in order to make the motor rotate and produce torque. This basic operation is common for most types of motors. The brushless DC motor used for the solar car -2-
  11. 11. 1. Introduction uses hall effect elements embedded in the motor to provide rotor position feedback information (discussed in chapter 4).• Waveform Shaping : by changing the pulse width modulation (PWM) ratio of the output drive signals, two functions can be implemented simultaneously. Current limiting is the process of regulating the phase currents in the motor to reflect the torque commanded by the driver. Efficiency of the drive may be improved by applying a weighted PWM signal to produce e.g. a sinusoidal output waveform (PWM techniques are discussed in chapter 4).• Sensor Technology : the motor controller has a number of sensors which provide feedback to the software control loops. The sensors used in the motor controller include current transducers for measuring individual phase currents, bus voltage measurement, an integrated circuit temperature sensor for measuring heatsink temperature and a thermistor for measuring temperature of phase windings (sensors are discussed in chapter 5).• Smart Control : the microprocessor is programmed to perform a number of auxiliary functions so that the vehicle performs optimally and safely under all driver input commands and environmental conditions. The following features will be designed into the motor controller, and are discussed in greater detail in chapter 2:™ Regenerative braking capability™ Speed and direction of wheel output™ Cruise control function (performed by telemetry)™ Four quadrant operation™ Reverse at low speed only™ Soft start operation™ Low torque ripple operation™ Sinusoidal PWM phase current excitation™ Temperature monitoring of stator -3-
  12. 12. 1. Introduction™ Temperature monitoring of MOSFET heatsink™ Fault indicator™ Wide input voltage range™ Transient protection™ Fuse protection™ Diagnostic capability™ Cooling fan mounted to heatsink1.2.1 Thesis Goal The primary and most important goal of my thesis was: “To design and construct a Brushless DC motor controller for the University ofQueensland solar car that performs motoring and regeneration at a very high efficiency.The motor controller should also perform auxiliary functions that make the drive systemmore robust, safer and easier to control.” The controller should operate the motor with the highest possible efficiencyunder steady-state operating conditions. Under abnormal conditions, the controllershould respond quickly to resolve the problem and resume normal operation to maintaina high level of energy efficiency. On completion of the project, the motor controllerwill be mounted in the solar car and be interfaced to the other electronic systems.1.2.2 Motivation behind the Motor Controller and Motion Control Many applications in today’s technologically advancing world require systemswith greater efficiency and more stringent operating specifications. An area in whichefficiency and reliability is an absolute must is motors and their control. Motors areused in a vast variety of applications ranging from huge crushing mills to pinpointaccuracy mechanisms in space applications. Some applications require motors tooperate in harsh environmental conditions, e.g. flammable gas leaks, where -4-
  13. 13. 1. Introductionconventional DC brush motors cannot be used due to the risk of sparks forming betweenthe brushes and commutator. There are many types of motors available today, howevera discussion on each type is beyond the scope of this thesis. One type of motor that boasts a very high efficiency and is very reliable is thebrushless DC (BLDC) motor. Unlike conventional DC brush motors, the brushlessmotor, as it’s name suggests, has no brushes and requires extra electronic circuitry toperform the job of commutation. The BLDC motor can be constructed in many sizesand power ratings, and finds widespread application in many motor drives. The primarymotivation behind the thesis was to improve the efficiency and technology of the solarcar. The secondary motivation was related to the popularity of the BLDC motor and it’sfuture applications. Factors such as high power to weight ratio and reliability willdefinitely see BLDC motor technology improve in years to come. By studying howsuch a motor is controlled, the capabilities of this motor are better understood.1.3 Organization of the Thesis Document The remainder of the thesis describes all work completed, problems encounteredand how these problems were overcome. Detailed descriptions including theory arepresented to support practical design choices. The following chapters form the body ofthe thesis document, and may be summarized as follows:Chapter 2, The University of Queensland Solar Car, presents first an introduction tosolar racing and how the event was first initiated, followed by a brief history of the UQsolar racing car. The chapter then presents an electrical system overview in a typicalsolar car, and how the main electrical components are interfaced. A short discussionfollows which outlines the importance of efficient systems on a solar car. The chapterconcludes by summarizing the existing drive system, then describing some of theperformance parameters of the new drive system.Chapter 3, Motor Control Literature, presents a literature review of all relevant workin the field of BLDC motor control. Useful formulas and control algorithms are -5-
  14. 14. 1. Introductionextracted from the text and hi-lighted in this chapter. There is a complete list of allreferences used in the bibliography section at the very back of the thesis report.Chapter 4, Theory, provides the background material necessary to understand how abrushless DC motor operates, and gives an insight of how to control such a motor.Chapter 5, Hardware Design Stage, analyses the circuits designed and describes theiroperation down to component level. Design formulas indicate how component valueswere obtained. Mechanical factors are presented for construction of the motorcontroller and when mounting into the car.Chapter 6, Software Design Stage, describes the control algorithms implemented insoftware which control the motor. There is a full listing of the code completed to datein Appendix E.Chapter 7, Results and Discussion, presents a discussion of the motor controllerproject and the issues that emerged from such a project.Chapter 8, Conclusions, concludes the document with a short summary of the findingsthroughout the thesis project. Some possible future work is given as suggestions toimproving the motor controller. A final note is then given to the overall picture of solarracing and where the future of such a technology is headed. The author hopes the thesis document provides excellent reading and a usefulreference for any future work in motor control. -6-
  15. 15. 2. THE UNIVERSITY OF QUEENSLANDSOLAR CAR2.1 Solar Car Racing and the Races Solar car racing first started out as a novel idea to investigate the limitations ofsolar energy as a possible alternative to non-renewable energy sources. From that pointforward, solar car racing has grown in popularity and can be considered a sport, withannual and biannual racing events being held all around the World. One of the moreprominent races is the World Solar Challenge, which covers some 3100 km fromDarwin to Adelaide along the Stuart Highway. Australian adventurer Hans Tholstruporganized the first WSC in 1987, and it is now a bi-annual event held in October. TheSydney City Power SunRace traverses the eastern coast of Australia from Melbourne toSydney and is the equivalent of the American SunRace. The American SunRace is thelargest solar event held in the United States. The World Solar Rallye in Akita, Japan isheld every year in July on a purpose-built solar racing track named the Ogata Mura -7-
  16. 16. 2.The University of Queensland Solar CarSolar Sports Line. Many other countries hold solar related activities to promote solarenergy as a new energy alternative to existing fossil-based energy. The solar car racing event is the most exciting part of solar car development.Not only do competing teams have the opportunity to showcase to the world the abilityof solar energy, but have a lot of fun simply making the car perform optimallyregardless of impeding conditions. There is great satisfaction when seeing months ofhard work finally being paid off, as the solar car races through the finish line. The ideaof a solar car race is to reach the finish as fast as possible, obeying the race regulationsat all times to avoid time penalties. For long endurance races such as the WSC, a convoy of cars accompanies thesolar car. One support vehicle usually has onboard computers and radio equipment fordata and voice interchange with the solar cars’ driver and telemetry system. Teammembers ride in a scout car and place wooden boards over cattle grids so that the solarcars’ tuned suspension is not put under great mechanical stress. In the 96 WSC,SunShark had an RACQ representative who was able to lend assistance in mechanicalbreakdowns. In races such as the World Solar Rallye in Akita, the racing trackconsisted of a 30km round circuit, allowing no room for support vehicles. Telemetrydata, which was logged for an entire lap had to be transmitted in a short burst when thecar was in range of the receiving base station antenna. During the normal course of arace, the drivers must be changed at regular intervals and a number of media stops areusually anticipated. There are two aspects that are essential for a highly competitive entry. A majoraspect of succeeding in a solar car race is to have a highly efficient and reliable system.This can be accomplished by designing an aerodynamic structure made fromlightweight materials and choosing efficient electrical components. The other aspect,which is equally important, is to have an effective race strategy. In a race situation, arace strategy team determines an optimal speed to run the car at, depending on currentweather conditions (e.g. solar insolation, cloud cover, rain), past weather/race data (e.g.rain patterns, road profiles) and vehicle parameters (e.g. battery state of charge, rolling -8-
  17. 17. 2.The University of Queensland Solar Carresistance). Most often, an unexpected weather pattern emerges or a critical breakdownoccurs. The strategy team must take into account these factors, and make a crucial “onthe spot” decision. Decisions such as these can decide the ultimate outcome of a race.2.2 A Brief History of the UQ Solar Racing Car The University of Queensland Solar Racing Car, commonly known is the“SunShark”, was first conceived by a number of engineering students early in 1995.Being only a concept and a few rough sketches at that early stage, a team decision wasfinally made to build a solar car and enter it in the 1996 World Solar Challenge (WSC).After 10 months of design and 8 months of intense construction work, the $140,000 carwas ready to roll. The WSC took Sunshark six days of racing in some of Australia’sharshest outback conditions. The car finished in fifth place, won the silicon cell/lead-acid battery class, and was presented with the award for technical innovation andachievement from General Motors (GM) Holden. A decision was made by the newly formed team early next year to participate inthe 1997 World Solar Rallye (WSR) in Akita, Japan. With only minor electrical andmechanical modifications being made to the car in order to comply with raceregulations, the team and car were ready to compete at the Ogata Mura Solar SportsLine in Akita. After 5 days of racing in sweltering heat, the car finished in identicalform as the WSC : ranked fifth overall and class winner of the silicon cell/lead-acidbattery category. Major electrical enhancements and some mechanical improvementsare currently underway in preparation for a large testing run near the end of 1998 andthe Sydney CitiPower Sunrace in January. The next WSC has been scheduled forOctober 1999 and the team hopes to have a greatly superior car than in previous yearsfor this major solar event.2.3 The Nuts and Volts of a Solar Car A typical electrical system for a solar car is presented in Fig. 1. -9-
  18. 18. 2.The University of Queensland Solar Car Photovoltaic Solar Maximum Peak Battery Array Power Trackers Bank (MPPT’s) (120V DC) HIGH VOLTAGE BUS Telemetry and Power Motor BLDC Support Circuitary Supply Controller Motor Radio Driver Controls and Modem Driver Display Figure 1 : Block Electrical Diagram of a Solar CarThe central node of the electrical system is the high voltage (HV) bus. Physically itmay simply consist of a connection point or short strip of copper, however it is at thispoint that the flow of current is distributed to all components. The main electricalcomponents are described in the next section.2.3.1 Batteries The primary energy source for the vehicle is the battery bank. The battery bankusually consists of a number of individual batteries connected in series or parallel. Eachbattery in the bank is typically 6 or 12V, and multiple batteries are connected in seriesor parallel to obtain the desired system voltage. A single battery is actually made frommultiple “cells” contained within the battery housing. A sealed lead acid type showing - 10 -
  19. 19. 2.The University of Queensland Solar Car the internal structure is shown in Figure 2. The overall battery voltage is chosen depending on the motor’s EMF constant and the desired nominal cruising speed. For the most efficient operation of the drive system, the battery voltage is chosen so that the motor controller can operate with minimal PWM (i.e. reduced switching losses), at the maximum desirable speed of the car. In practice however, the Figure 2 : A Sealed Lead Acid Battery battery voltage, especially forlead-acid batteries, fluctuates considerably around the nominal battery voltage, from fullcharge to maximum discharge. For this reason, the nominal battery voltage is usuallychosen so that the lowest possible battery voltage is able to sustain a reasonablycompetitive speed. An alternative solution to this problem is to implement a boost/buckconverter in the motor controller so that an optimal speed can be obtained for anybattery voltage. There are many types of commercial batteries available today. Someexamples particularly applicable for solar racing vehicles are sealed (maintenance free)lead-acid, silver-zinc, lithium-iron and zinc-air. The SunShark solar car team chose toobtain sealed lead-acid batteries due to ease of availability and relatively cheap cost.One major drawback however is a relatively large weight/energy density ratio, and a fullset of batteries typically weighed in at 96kg. Each type of battery has differentcharacteristics (e.g. energy density/kg, charge/discharge rate) and uses, however acomprehensive study of batteries is beyond the scope of this thesis. - 11 -
  20. 20. 2.The University of Queensland Solar Car2.3.2 Solar Array The capacity of batteries set out by race rules and regulations is too small for asolar car to fully depend on during a race. Energy must be obtained from the sun by asolar array to supplement the energy taken from the batteries. Under maximuminsolation levels, the solar array can sometimes supply ample energy, and the excesssimply flows back into the batteries. The solar array consists of a configuration of solarphotovoltaic cells, usually encapsulated to protect against the elements and damage.The encapsulation of cells also increases the overall efficiency of the array. This isachieved by carefully designing anti-reflective coatings and materials to maximize thelight energy captured. General categories of solar cells include amorphous, multi-crystalline and mono-crystalline cells. Some types of solar cells include screen printed,buried contact cells (BCC), laser-grooved cells and passive emitter reflective layer(PERL). A screen printed mono-crystalline cell showing the fine metal fingers and busbars which collect the energy from the surface of the cell is shown in Figure 3. The cell shown has a rated efficiency of ~16.5%. Commercially manufactured cells are available with maximum efficiencies in the order of 26%, however cells have been produced with peak efficiencies of 30-35% under laboratory conditions. Solar cells convert sunlight (photons) to electricity (electrons) by the raising Figure 3 : A Screen Printed Solar Cell of the energy level of electrons inthe crystalline lattice, and allowing them to move freely throughout the structure. Solarcells are constructed from a semiconductor p-n junction, which allows current to flow inone direction only, similar to the operation of a diode. The SunShark solar car team’sfirst array contained 15.5% Sharp cells encapsulated in epoxy, giving a peak power - 12 -
  21. 21. 2.The University of Queensland Solar Caroutput of 1kW. The new array should have higher efficiency cells and enhancedencapsulation materials, with an expected power output of 1.5kW.2.3.3 Maximum Peak Power Trackers (MPPT’s) The output voltage of the PV array varies widely with changing sunlightintensities, incident sunlight angles and PV cell temperature. As previously discussed,the battery voltage may also fluctuate, and the PV array may be forced to operate at the voltage depicted by the battery. This can result in a degraded power output from the PV array, because the voltage may not correspond to the maximum power point of the cells. The maximum peak power tracker (MPPT) modules automatically hold the Figure 4 : Maximum Peak Power Tracker Module photovoltaic (PV) panel atit’s maximum power point voltage, while delivering the resulting maximum PV powerto the battery bank. It does this by electronically de-coupling the PV voltage from thebattery voltage by using a high frequency transformer and MOSFET’s. A MPPTmodule is shown in Figure 4. The existing array had three MPPT modulesmanufactured from the Australian Energy Research Laboratories (AERL).2.3.4 Motor Controller The motor controller is designed to convert the electrical energy obtained fromthe batteries and solar array to suitable power waveforms to drive the motor. The motorcontroller used in the solar car is designed to drive a Permanent Magnet Brushless DC(PM-BLDC) motor. The driver becomes part of the speed regulation loop as the torque - 13 -
  22. 22. 2.The University of Queensland Solar Carproduced in the motor can be controlled via controls in the cockpit. A more thoroughexplanation of the motor controller is given in chapter 4.2.3.5 Motor The motors’ function is twofold: to convert the electrical energy to mechanicalenergy when motoring and mechanical energy to electrical energy when regenerating.There are a number of types of motors in use today, ranging from the induction,switched reluctance, brushed DC and stepper motors. Each motor has a number ofadvantages and disadvantages in particular applications ranging from large industrialroller mills to accurate positioning control. The most popular choice for high efficiencyapplications such as solar cars, is the permanent magnet brushless DC motor, orsometimes known as a synchronous DC motor. The advantages of the BLDC motorinclude:• Very high efficiency characteristics over a large power range (98.2% recorded for an optimized Halbach magnet arrangement).• Require minimal maintenance, due to elimination of mechanical commutator and brushes.• Long operating life and higher reliability.• No brushes means no arcing which can be paramount when working in flammable gas locations.• Number of motor geometry’s possible (e.g. interior permanent magnet or surface magnet arrangements).• High power density and torque to inertia ratio give a fast dynamic response.• No brushes eliminates need for a high rotor inertia.• Speed restrictions due to the traditional mechanical commutator are eliminated. The construction and theory of the brushless DC motor is presented in greaterdetail in chapter 4. - 14 -
  23. 23. 2.The University of Queensland Solar Car2.3.6 Telemetry Functions and Power Supply The basic electrical system shown in Fig. 1 can be enhanced with the addition oftelemetry systems and support circuitry. The main aim of the telemetry system is tocalculate an optimal speed and/or power to run the car. One of the tasks performed is torecord data such as bus voltages and motor currents. The existing telemetry in theSunShark solar car consisted of a signal conditioning board and telemetry board able totransmit sampled data to the support vehicle via radio modems. It is envisaged thesupport vehicle computers will be able to determine an optimal speed of operation, andeven take control of the car, by factoring in all relevant aspects which directly influencethe systems’ performance. The power supply is responsible for converting the bus voltage down to supplyvoltages for the circuitry. The current system converts 120V to +/-15V, 8V and 5V.The power supply is usually a switch mode type to keep losses to a minimum.2.4 Necessity for Efficient Systems The photovoltaic array for solar cars is very dependent on weather conditions.Although the sun has as much energy as a million hydrogen bombs, a fractional amountof that energy actually reaches the Earth’s surface. Furthermore, the amount of energyreceived from the sun by a photovoltaic solar array depends upon multiple factors suchas cloud cover, angle of incident sunlight, cell temperature and cell efficiency. Due tothe obvious difficulties in obtaining energy from the sun, any wasted energy (i.e. energythat is not contributing to the forward motion of the car) is regarded as a limiting factoron the maximum speed obtainable from the system. For the SunShark solar car,approximately every kilogram of vehicle weight relates to rolling friction power lossincreasing by 1W. Mechanical systems and frames can be made lighter by usingdifferent materials in an attempt to reduce rolling friction power loss. There are anumber of methods in which electrical systems can be made more efficient. Throughcareful circuit design with energy efficient components, substantial power savings canbe made. The heating losses due to current flow in conductors can become substantial - 15 -
  24. 24. 2.The University of Queensland Solar Carin high power parts of the circuitry. It is advisable in this case to use over-rated cablesto help bring the conductor resistance and hence power loss down. Layout ofcomponents is also important to reduce conductor lengths and parasitic inductiveelements. In a solar car race, the maximum velocity of the solar car is limited by theefficiency of the system and race weather conditions. Since the weather conditions on arace are at best highly unpredictable, in some instances a solar car may be fully relianton the batteries for power. At the end of the day, the performance of a solar car isheavily determined by the overall efficiency of the system.2.5 The Existing Drive System The existing motor consisted of a PM-BLDC motor with toroidal flux. Themotor had no iron in either its rotor or stator and consisted of a number of cylindricalmagnets with poles opposing one another, fixed around the circumference of the rotor.The winding were arranged so as to enclose the magnets of the rotor in a “C” or “U”shape. The windings and the coils formed a toriodal shape, thus the name toriodal flux(or T-Flux) motor. The back EMF waveform was of a sinusoidal shape, due to thenature of its construction. The motor required a transmission system consisting of atoothed drive belt. The motor was supplied from Lillington Manufacturing.2.5.1 Controller Type The nominal input voltage to the motor controller was 120VDC. The controllerused trapezoidal phase current excitation waveforms. A PWM chip (NE5568) was usedtogether with a ROM (N82S123AN) programmed with a commutation truth table todecode the hall effect signals from the motor, and provide excitation to the correctphases. All logic circuitry was supplied power using a linear 5V regulator, which hasan efficiency of ~50%. The inverter stage was a common three phase H bridge design,using three paralleled MOSFETs (IRFP260) in one switch, i.e. a total of 18 MOSFET’s.The MOSFET’s had transient suppressing metal oxide varistors (MOV) to clamp thevoltage over each MOSFET switch to a safe level. DC link capacitors (12 X 220uF - 16 -
  25. 25. 2.The University of Queensland Solar Carstandard electrolytic) were used in the DC link. The MOSFET gates were driven by anIR2130 3-phase bridge driver chip. All three lower inverter switches had a 20k ohmresistor connected in parallel, which meant each time the upper switches were activated,0.72 W power dissipation occurred. A simple shunt resistor was used to measure theconstant current in the DC bus, instead of in the DC link. Driver controls consisted oftwo potentiometers: one to adjust speed and the other to adjust the current limit value.A direction switch was also available however care had to be exercised when moving atfast speeds to not bump the switch in the opposite direction, otherwise excess currentswould flow and destroy the controller and possibly the motor.2.5.2 Performance Characteristics The most undesirable aspect of the previous controller was the characteristic ofthe driver control. The controller was speed controlled, which meant the driver had tobasically guess where to position the potentiometer for a certain desired speed. Thiscaused a lot of concentration by the driver as the speedometer had to be constantlymonitored and potentiometer adjusted to obtain the desired speed. Moreover the speedramp was not a linear function of potentiometer position, but had a slow response at lowspeeds and a fast, uneven response at moderate to high speeds. This made fineadjustment of speed a large problem. It was discovered that potentiometers are notalways fully reliable devices, and a number had to be replaced during the course of therace. The controller experienced a number of IC faults during the 96 WSC race,probably due to the high temperature levels. Care had to be taken if the hall effect plugwas to come out, because the controller would set the speed to maximum.2.6 The New Drive System The new motor is made by CSIRO/UTS and is of the permanent magnet type. Themotor features two rotors, has no iron loss and is of an axial field construction. Themotor is specifically designed to fit inside the wheel of a solar car which has a numberof distinct advantages over the original reduction belt system: - 17 -
  26. 26. 2.The University of Queensland Solar Car• All drive transmissions (e.g. indirect shaft coupling, chain, belt) are eliminated. This can result in savings of up to 15%(dependent on drive train configuration) of the total motor output energy in a conventional drive train arrangement, which would have usually been lost as heat and noise.• No need to replace broken belts/chains or dust entering transmission system.• Better aerodynamic performance due to streamlined design.• Motor can be sealed against dust and waterThe technical specifications for the motor can be found in appendix C.2.6.1 Additional Features A number of improved features are to be designed into the new motor controllerto increase overall efficiency, reliability and safety:• Torque Control Input : torque is directly controlled instead of a speed control, which will make the driver control more intuitive. A handgrip will be used which may be rotated in one direction for motoring and rotated in the other direction for regeneration.• Regenerative Braking : allows electrical braking whereby the solar car’s kinetic energy can be reclaimed. Mechanical friction brakes will still be present for fast stopping ability.• Cruise Control Function : a feature which allows the driver constant speed or torque operating modes. (performed by the telemetry unit)• Four Quadrant Operation : meaning the motor can be driven throughout the entire torque-speed plane, i.e. forward and reverse motoring/regeneration.• Reverse Speed Limited : provides a safe reversing speed for better control.• Soft Start : limits starting jerk which will improve handling and reduce tyre wear due to wheel slip.• Low Torque Ripple : advanced PWM modulation algorithms reduce torque ripple to ensure smooth rotation at high and low speeds. - 18 -
  27. 27. 2.The University of Queensland Solar Car• Sinusoidal Phase Current Excitation : improves efficiency when interfacing to a motor containing sinusoidally varying back emf and develops maximum torque production.• Fault Indicators : faults identified immediately by displaying fault codes when: 1. Temperature exceeded in motor stator windings (thermistor used). 2. Temperature exceeded in MOSFET heatsink (IC temp sensor used). 3. Overvoltage detected on HV bus. 4. Overcurrent detected (e.g. shorting power components)• Diagnostic Capability : faulty components can be identified by running a number of tests on different parts of the circuit using a microprocessor.• Wide Input Voltage Range : 0-200V capability for different battery configurations.• Transient Protection and Safety Devices : peripheral device for limiting inrush current when connecting batteries and protection for power devices and microprocessor devices.• Fused Inputs : Protects circuitry from continued current draw.• Cooling Fan : small fan mounted on the MOSFET heat sink to ensure extended operation in extra hot conditions.2.6.2 Performance Requirements The new motor controller is designed for a more intuitive control interface andsafer operation. The controller will contain robust features and be fully self contained.It is envisaged the overall efficiency of the system will be improved, and the averagespeed of the car can be increased. - 19 -
  28. 28. 3. MOTOR CONTROL LITERATURE An extensive literature search was carried out to review work completedpreviously. A list of keywords relating to the topic for searching databases: e.g. motorcontroller, electric drive, motion control was drawn up. A general WWW searchresulted in a number of results however I found many of the web sites were usually acompany trying to sell their product, and offer little or no technical information. TheWWW is a very convenient way of obtaining product data sheets. The main source ofinformation was books from the Physical Sciences and Engineering (PSE) library.There is a reasonable selection of books in the library ranging from Power Electronicsto books specifically on motor drives and their controls. A comprehensive search usingthe networked databases Inspec, Compendex, Engineering & Applied Science, NationalTechnology Information Service (NTIS), Current Contents and Computer ASAP wasalso undertaken. This search resulted in some 32 journal and magazine articles relevantto aspects on motor controllers.Most articles contained an example of a motor and motor controller designed todemonstrate a particular feature. The experimental setup was commonly explained by - 20 -
  29. 29. 3. Motor Control Literaturethe use of diagrams. The circuit in most cases was put under a simulation and resultswere compared with the actual measured values. Many of the articles obtained are fromthe IEEE and IEE publications. Reference articles [7] and [9] discuss a controller using MOSFET switches within-built current sensing (used IRC644, 14A cont. 250V). These MOSFET’s are referred TMto by International Rectifier (IR) as HEXSense devices, as they contain integratedshunt resistors, which can detect the current passing from the drain to the source. Thisresults in a more compact design and eliminates the need for external shunt resistors orhall effect current transducers which results in an immediate weight saving. This typeof MOSFET was researched into, however none were found with the required voltagerating. The only HEXSense TM devices that were found were of the 3-pin type (TO-220case style). If 3 such devices were placed together in parallel to reduce on-state losses,a total of 18 current readings would need to be converted using an analog to digitalconverter (ADC), which would quickly clutter the available ADC channels on amicroprocessor. The controller mentioned in articles [7] and [9] can operate the motorin all four quadrants of the torque-speed plane, i.e. forward and reverse motoring andforward and reverse regeneration. Trapezoidal phase current excitation with 120 degreeswitch conduction intervals are used so that current only flows in two of the phasecurrents at any one time. An important comment in [7] as to the position of the current sensors for currentfeedback and regulation is made. The most simple method is a resistive shunt on theDC bus. Although a simple and relatively cheap method of current detection, it cannotdetect dangerous circulating currents which may be developed in the phase windingsand power switches. This current build up can result in switch failure ordemagnetization of the rotor magnets. The only solution to this problem is to havecurrent transducers mounted in the phase windings so that the current may be monitoredand evasive action taken. The controller in [7] and [9] contains separate commutation logic/control andcurrent regulation blocks. The commutation logic/control block was implemented with - 21 -
  30. 30. 3. Motor Control Literaturethe Motorola MC33034 brushless motor controller chip. The MC33034 has inputs fromthe rotor position sensors and driver control (start/stop and forward/reverse), and hascommutation signal outputs which feed into the Harris GS601 HVIC half-bridge gatedrive chip. Current regulation is achieved by difference summing a current commandsignal (from the driver), and the current feedback signal (from one of the lowerswitches). This difference voltage represents the current error, and the GS601 driverchip minimizes the error by varying the switches’ PWM duty cycle, effectivelyregulating the current to the desired value. The current control algorithm is simple inprinciple. In this case a fixed off time, TL is used. The PWM frequency is determinedby the following formula:  E 1f PWM = 1 −   V T  S  Lwhere f PWM = PWM frequency,E = back emf of the motor,VS = supply voltage of supply,TL = off time of the switches. The accuracy of the relationship described by the formula starts to deteriorate atlow speeds when the motor phase resistive drop approaches the magnitude of the back-EMF. Another current regulation algorithm is briefly mentioned, namely holding thetotal PWM frequency constant, so that the current ripple varies with speed. The one and two switch active regeneration schemes are presented in article [9].The two switch active scheme is preferred over the one switch active scheme at lowspeeds as it is not as sensitive to the back EMF amplitude. Both methods takeadvantage of the energy stored in the motor windings and transfer this energy back tothe supply. A simple speed detector circuit which works on the principle of providing apulse for every transition of the hall effect sensors is described. The frequency of thepulses is proportional to the motor speed according to the following formula: - 22 -
  31. 31. 3. Motor Control Literature pf rotation = nv ( Hz ) 120where f rotation = frequency of the pulses,p = no. rotor polesn = number of commutations per electric cycle (typically = 6),v = motor speed in revolutions per minute. Some articles such as [15], [19] and [32], discussed the developments inbrushless DC motors and described how a particular motor was built for the “DesertRose” solar racing car. The articles discuss an axial flux permanent magnet brushlessDC motor designed for an in wheel drive on a solar car. The axial flux geometry wasfound to have advantages over the common radial flux geometry by reducing volumelimits and having the ability to change the air-gap between stator and rotor. Increasingthe air-gap increases the copper loss as the torque constant decreases, but decreases theiron loss as the flux density reduces. The main author of these articles was DeanPatterson of the Northern Territory University. Dean Patterson also has written ajournal article on the electrical system design for a solar powered vehicle [29], whichmade interesting reading material as the system could be compared with our ownsystem and comparisons made. Article [4] describes some common dc drive failures and how to design a controlsystem which can sense the failure and continue to operate normally. The results arepresented both using a simulation and measured results. Articles [28] and [30] describe how to model electronically commutatedmachines using the P-Spice simulation program. This will be very useful informationwhen experimenting with the inverter stage, and comparing measured results withsimulated results. Article [24] describes the application of soft switching inverters in electricdrives. A soft switching, or sometimes known as resonant converters, eliminateswitching losses by causing the inverter switches to switch at zero voltage instants.There are many different resonant converters available, however they all require extra - 23 -
  32. 32. 3. Motor Control Literatureswitches, inductors and capacitors to be arranged on the DC bus. To design a converterof type is by itself a full thesis, so will not be further investigated. The current designhowever, is flexible enough to allow future people to add a resonant converter ifdesired. Article [31] presents a bi-directional dc/dc converter which can control the DClink voltage and control regenerative braking of an electric vehicle. An explanationfollows that describes how the converter can switch currents in both forward andreverse directions. Motor current ripple is claimed to be reduced by constantlychanging the DC link voltage under different operating conditions. - 24 -
  33. 33. 4. T HEORY4.1 The Permanent Magnet Brushless DC Motor There are a number of configurations for the brushless DC motor, however alloperate on the same principal. There are three main components that make up such amotor: Stator Winding : The stator is usually wound in a three phase wye (or star)connection. Three phase windings are usually sufficient to control most motors,however more than three phase windings are common, and simply require additional H-bridges and commutation circuitry. There is the option with the CSIRO motor to usemore than three phases as each phase is broken up into multiple sections. There is alsothe option to connect the windings in a delta configuration, however this may introduceunwanted circulating currents flowing around the windings. The stator of the CSIROmotor is shown in Figure 5. Each of the three phase windings are distributed in a - 25 -
  34. 34. 4. Theory sinusoidal pattern around the circumference of the stator and are encapsulated in a fiberglass resin. By winding the phases in a sinusoidal pattern, a sinusoidal back emf voltage waveform is produced between two phases when the motor is turned by hand. To obtain maximum efficiency, Figure 5 : Stator Winding of the CSIRO Motor sinusoidal phase currentexcitation must be applied to the motor. Rotor Magnets : In conventional DC motors, electromagnets are used to createa magnetic field. The rotor in a BLDC motor consists of rare earth magnets whichproduce a constant flux (hence the name permanent magnet). One of the rotor magnet rings of the CSIRO motor is shown in Figure 6. The NdFeB magnets (neodymium-iron-boron) are glued to the backing iron, and are arranged in a circle comprising 40 magnet pieces (i.e. 40 pole motor), in an alternating N – S – N configuration. The backing iron forms part of the magnetic circuit. There are Figure 6 : Magnet Ring of the CSIRO Motor two identical magnet rings - 26 -
  35. 35. 4. Theorywhich are placed on either side of the stator and are kept separated by special rims. Thestator will be held stationary and fixed to the trailing arm. Both rotor magnet rings arefixed to the wheel rim, and rotate with the movement of the tyre. Hall Effect Sensors : Hall sensors are a popular choice for rotor positionfeedback in brushless DC drives, reasons being they are cheap and do not requirecomplex processing algorithms. Hall sensors are more suited for use with trapezoidallycontrolled motors, as sinusoidal machines usually require a higher resolution sensorsuch as a shaft encoder or transducer. The actual sensor is usually a N-doped InSbsemiconductor, which in the presence of a magnetic flux, an electromotive force causesfree flowing electrons to move to one side of the semiconductor which causes apotential to form on the output terminals. In most hall elements manufactured, a voltage regulator, amplifier and schmitt trigger are all integrated inside the one device. The hall effect sensors are glued to a PCB which is located inside the motor. The PCB can be adjusted manually to align the stator coil position with the hall effect position. The PCB with the hall effect Figure 7 : Hall Effect PCB of the CSIRO Motor sensors mounted is shown in Figure 7. Three hall effects give output six different states for one full electrical cycle,which is usually sufficient for most motor control applications. There are two possibleways of positioning the hall effect sensors around the axis. The hall elements can eitherplaced at 60 or 120 electrical degree intervals (.i.e. the hall code changes every 60 or120 electrical degrees). The hall effects to be used are configured to change every 60electrical degrees. One electrical cycle is equal to 360 electrical degrees, and is defined - 27 -
  36. 36. 4. Theoryas when the hall sequence starts to repeat. The hall effect sequence can be representedin Figure 8. Since the motor has 40 poles, for one revolution of the motor, each hallsensor will experience 20 norths and 20 souths (i.e. 20 high and 20 low level outputs).The mechanical separation of the magnets and hall effect sensors can be calculatedeasily from knowing the number of magnets and poles. One mechanical cycle is equalto one entire revolution of the motor or 360 mechanical degrees. One electrical cycle no. electrical degrees in one cycle 360repeats every = = 18 mechanical degrees. no. poles/2 20 Figure 8 : Hall Effect Positioning Sensors4.1.1 Electrical and Mechanical ParametersThe main electrical parameters of the CSIRO motor are presented in Appendix C.Speed of Motor CalculationDiameter of rear wheel = 510 mm diameter (approx.)Circumference = (π)x(Diameter of rear wheel) = 1602.21mm.Nominal Speed of Motor = 111 rad/s = 111x60/2π = 1059.97 rpm.At the nominal speed, velocity of solar car is thus:Speed of Car = (Circumference)x(Nominal Speed of Motor)x(60/1000000) - 28 -
  37. 37. 4. Theory = 101.898 km/hr.Electrical Parameter CalculationThe surface motor is described by the following formula:T=kTI whereT = torque developed by motor (max. torque = 50.2 Nm, nom. torque = 16.2 Nm)kT = torque constant per phase (0.39 Nm/A)I = current through DC link (A)i.e. for maximum torque, I = T/3kT = 50.2/(3x0.39) = 42.91 Aand for nominal torque, I = T/3kT = 16.2/(3x0.39) = 13.85 Athe motor can also be described by:E = kE ωm whereE = back emf of motor (V)kE = back emf constant (0.39 Vs/rad)ωm = angular velocity (max. angular vel. = 300 rad/s, nom. torque = 111 rad/s)i.e. for maximum angular velocity, E = kEωm = 0.39x300 = 117 Vand for nominal angular velocity, E = kEωm = 0.39x111 = 43.29 VBattery Voltage CalculationThe battery voltage has to be chosen so that the motor controller may operate at near tofull PWM when running at nominal speed. The motor has a line-neutral RMS emf at111 rad/s. Thus battery voltage required can be given as:Vbattery = (L-N RMS EMF(peak))x(2)/(kmodulation) = (2 x 43 2 ) (1.15) = 105.74V, wherekmodulation = PWM factor (=1.15) due to modulation of the MOSFET switches.Thus a battery bank of 120V should be sufficient and will leave a small amount forovertaking. - 29 -
  38. 38. 4. Theory4.2 Controlling a Permanent Magnet Brushless DC Motor Throughout the thesis document, the following numbering pattern forMOSFET’s in the H-Bridge will be as follows: Figure 9: Numbering pattern for MOSFET’s in the H-Bridge Each of the MOSFET’s contain an intrinsic diode which has a reverse recoverytime comparable to that of a discrete diode placed in parallel with the MOSFET. Thediodes will be referenced with the same numbering as the MOSFET’s, i.e. SW1 has acorresponding diode D1, and so on.4.2.1 Commutation Commutation is the process of reading the hall effect sensor code, which givesan indication of the position of the rotor. If the position of the rotor is known, then thepositions of the magnets are also known. To create a continuous rotation of the motor,the correct phases must be switched on and off in the correct sequence so that theapplied voltage is in synchronism with the rotor position. Depending on the magnitudeof the current command, different magnitude torque can be applied to the motor. Thereare two basic schemes of commutating a BLDC motor. - 30 -
  39. 39. 4. Theory120 Degree Conduction The 120 degree conduction mode switches MOSFET’s on for a length of 120electrical degrees and off for 240 degrees. The relation between the MOSFETswitching states and hall effect codes is shown in Table 1. When the MOSFET’s areturned on, they are not simply switched on and left on, rather they are modulated by aPWM signal. The PWM signal varies in duty cycle depending on what currentregulation algorithm is being used. When the PWM signal is high, only twoMOSFET’s turn on at any one time, one from the high side and one from the low side ofalternate phases1. When the PWM toggles low, the low switch is turned off and thecorresponding high switch is turned on. This method is called synchronousrectification, as it allows the current to flow through the paralleled high switch andfreewheeling diode, thus reducing conduction losses. A basic two pole motor ispresented in Figure 10 showing the rotation of the rotor magnets and the correspondingflow of current in the motor windings and hall effect codes for 120 degree commutation. Input OutputPWM H1 H2 H3 SW1 SW2 SW3 SW4 SW5 SW6 1 0 1 1 1 0 0 0 0 1 1 0 0 1 0 0 1 0 0 1 1 1 0 1 0 1 1 0 0 0 1 1 0 0 0 1 0 0 1 0 1 1 1 0 0 0 0 1 1 0 1 0 1 0 1 0 0 1 0 0 0 0 1 1 1 0 0 0 1 0 0 0 0 1 0 0 1 0 1 0 0 1 0 1 1 0 1 0 0 0 0 1 0 0 1 0 0 0 1 0 0 1 1 0 0 0 1 0 1 0 0 0 1 0 1 0 1 0 0 0 Table 1 : 120 Degrees Commutation Truth Table1 Note : Both high and low MOSFET’s of the same phase are never switched on at the same time. - 31 -
  40. 40. 4. TheoryFigure 10 : 120 Degrees Commutation Mode - 32 -
  41. 41. 4. Theory180 Degree Conduction The 180 degree conduction mode switches MOSFET’s on for a length of 180electrical degrees and off for 180 degrees. The relation between the MOSFETswitching states and hall effect codes is shown in Table 2. Similar to the 120 degreecommutation, a PWM signal varies in duty cycle depending on what current regulationalgorithm is being used. When the PWM signal is high, three MOSFET’s turn on at anyone time, either two from the high side and one from the low side, or two from the lowside and one from the high side.2. When the PWM signal toggles low, the side withonly a single switch active toggles and it’s corresponding switch is turned on. Onceagain, synchronous rectification takes place as the current flows through the paralleledhigh switch and freewheeling diode, thus reducing conduction losses. A basic two polemotor is presented in Figure 11 showing the rotation of the rotor magnets and thecorresponding flow of current in the motor windings and hall effect codes for 180degree commutation. Input OutputPWM H1 H2 H3 SW1 SW2 SW3 SW4 SW5 SW6 1 0 1 0 1 0 0 1 1 0 1 0 1 1 1 0 0 1 0 1 1 0 0 1 1 0 1 0 0 1 1 1 0 1 0 1 1 0 0 1 1 1 0 0 0 1 1 0 1 0 1 1 1 0 0 1 0 1 1 0 0 0 1 0 1 0 1 0 1 0 0 0 1 1 0 1 0 1 0 1 0 0 0 1 1 0 1 0 1 0 0 1 0 1 0 1 0 1 0 1 0 1 0 0 1 0 1 0 1 0 0 1 1 0 0 1 0 1 0 1 Table 2 : 180 Degrees Commutation Truth Table2 Note : Both high and low MOSFET’s of the same phase are never switched on at the same time. - 33 -
  42. 42. 4. TheoryFigure 11 : 180 Degrees Conduction Mode - 34 -
  43. 43. 4. Theory4.2.2 Current Regulation Since torque is proportional to the fundamental frequency of the current, bycontrolling the current, torque is also controlled. All other frequency componentscontribute to losses in the motor, inductors and controller. To form a closed loopsystem, there must be current feedback from the motor as indicated in Figure 12. Current + Error ∑ Controller BLDC Motor - Current Current Feedback Command Figure 12 : Current Feedback in a BLDC Motor This feedback signal is subtracted from the desired current input from the driver,and a current error then propagates to the controller. This provides a mechanism for thecontroller to accurately current limit by trying to keep the current error as close to zeroas possible. The actual current limiting is achieved using a PWM scheme for theswitching MOSFET’s. The audible range for humans is approximately between 6kHzand 20kHz, so a PWM frequency above ~20kHz is sufficient to avoid an annoyingwhine when switching.4.2.3 Trapezoidal Current Excitation Trapezoidal phase current excitation is a basic way to control a BLDC motor.The MOSFET switches are activated and use a constant PWM frequency when turnedon which produces a phase current of trapezoidal shape (hence the name) as shown inthe lower trace of Figure 13. One major disadvantage of a driving a motor with - 35 -
  44. 44. 4. Theorytrapezoidal current, is that there are many frequency components which make up atrapezoidal waveform, and these components only contribute to losses in the motor. Figure 13 : Torque Ripple in a Trapezoidal Machine Figure 13 also indicates a waveform describing torque ripple which ischaracteristic for a trapezoidal motor. The torque ripple can be attributed to two majorsources:Motor Related Torque Ripple : causes the torque waveform to be rounded during thecommutation intervals. This is caused mainly by magnetic flux leakage paths betweenadjacent rotor magnet poles. This torque ripple can be minimized by careful motordesign. See label 1 in Figure 13.Inverter Related Torque Ripple : The first of this type of ripple is caused by a currentimbalance when current is being switched between active phases. Sharp torque spikescan be produced and are experienced every 60 electrical degrees. Special PWMswitching techniques can be used to reduce this ripple. See label 2 in Figure 13. - 36 -
  45. 45. 4. Theory The second inverter related torque ripple is directly proportional to the highfrequency PWM ripple in the phase currents and produces the fast torque oscillation(see label 3 in Figure 13.). This ripple is not usually a problem because the inertia ofthe solar car usually filters out the ripple. At high speeds, phase current and motor torque can decrease abruptly when thesupply voltage equals the combined back emf of the two conducting phases. Continuedhigh speed operation is possible by gradually extending the conduction time from 120electrical degrees to 180 electrical degree conduction.4.2.4 Sinusoidal Current Excitation Sinusoidal current excitation is an advanced method of driving a sinusoidallyvarying back emf producing motor. By driving a motor with sinusoidally weightedPWM phase current waveforms, less frequency harmonics are present in the phasecurrent waveform, thus an immediate reduction in losses occurs. As a result, largertorque is produced for the same RMS current. Sinusoidally driven motors alsoexperience reduced torque ripple, the principal reason being that sinusoidal machines donot experience the abrupt phase to phase current commutations that characterize thetrapezoidal machine’s excitation waveforms. When controlling a motor using sinusoidal excitation, the input currentcommand must be split into two different currents:Id or “direct” current is aligned with the permanent magnet flux linkage phasor λm.Iq or “quadrature” current is aligned with the back emf phasor Ef. These currents may be related by the following formula:Ef = (p)x( ωr)x(λm) where p = no. pole pairs. ωr = angular velocity of motor (rad/s). and λm = PM flux linkage amplitude. - 37 -
  46. 46. 4. TheoryThe torque developed in a sinusoidal motor can be expressed as : Te = 3p 2 [ λm .iq + id iq ( Ld − Lq ) ]where Ld, Lq are the stator phase inductance’s. Under normal operation, Id is set to zero and Iq is varied proportionally to inputtorque. In an interior PM motor, flux weakening can be used at high speeds. Fluxweakening is the process of increasing Id to oppose the magnet flux. This results in anextended driving range at high speeds. In a surface magnet motor, flux weakening isnot feasible, as it would not have any effect on weakening the magnetic flux, and wouldonly reduce efficiency and increase current drawn by the motor.4.3 Power MOSFET Device Characteristics A MOSFET (or metal oxide semiconductor field effect transistor) is a voltagecontrolled device as opposed to a transistor which is a current controlled device.Diagrammatically, the MOSFET can be represented in the off and on state as depictedin Figures 14 and 15 respectively.Figure 14:Non-Conductring MOSFET[34] Figure 15:Conducting MOSFET[34] When the device is in the off state, the drain is insulated from the source by thep-type region, however when a potential voltage is applied to the gate terminal, currentis allowed to flow freely between drain and source. A MOSFET’s characteristicwaveforms at turn-on and turn-off is shown in Figures 16 and 17 respectively. - 38 -
  47. 47. 4. TheoryFigure 16:Waveforms at Turn-On[38] Figure 17:Waveforms at Turn-Off[38] The device’s switching speed is largely effected by the size of the gate-to-sourcecapacitance. This capacitor has to be charged and discharged in one switch on andswitch off cycle. A summary of the turn-on sequence is as follows:1) The MOSFET is initially turned off with no gate voltage present. At time 0, thegate voltage reaches the threshold gate voltage, Vth, and drain current starts to rise.2) Between time 1 and time 2 : Gate-Source voltage waveform deviates from it’soriginal trajectory due to : a) Series source inductance develops a voltage due to the rising drain current andcauses the gate-source voltage to decrease, and b) The decreasing drain-source voltage is reflected across the drain-gatecapacitance. A discharge current flows through this capacitor causing an increase in thecapacitive load as seen by the driver. The voltage across the source impedanceincreases thus causes a retardation of gate-source voltage.3) Time 2-Time 3 : Drain current increases further due to the reverse recovery ofthe free-wheeling diode.4) Time 3 : The free-wheeling diode starts to support the drain-source voltage andthe rate of fall of the drain-source voltage is mostly dependent on the Miller effect. It isat this point that the MOSFET has a maximum power loss due to a large current passingthrough the device and a large voltage present across the device’s terminals. Due to thefalling drain-source voltage, the drain current settles out to the current determined bythe load and this causes the gate-source voltage to drop. - 39 -
  48. 48. 4. Theory5) Time 4 : The MOSFET is completely turned on and the gate-source voltage risesrapidly to the “open circuit” value.Similarly, the turn-off sequence can be summarized as follows:1) The MOSFET is initially turned on with a gate-source voltage present at time 0.At time 1, the gate-source voltage reaches a level that just sustains drain current. Thedrain-source voltage increases at a rate governed by the miller effect and the gate-sourcevoltage is kept at a constant level which reflects the drain current.2) The MOSFET experiences a maximum power loss again at approximately time2, where both a large drain current and drain-source voltage are present. When the riseof drain-source voltage is complete, both drain current and gate-source voltage decreaseuntil drain current reaches it’s minimum value at time 3.3) Gate-source voltage decreases past the threshold voltage to zero between time 3and 4. The device is fully turned off at time 4. There are two main power losses to consider when looking at the total powerloss in a power MOSFET. Refer to the MOSFET data sheet in Appendix B:Static On Loss : The power loss due to the resistance between the drain and source.This power loss decreases as more MOSFET’s are paralleled together.Pon = I2xRx(duty cycle) = (13.9)2x0.022x50% = 2.13 W.Dynamic (Switching) Loss : The power dissipated when the MOSFET is changingconduction state. This loss stays ~ constant when paralleling other MOSFETs.Psw = (Vds)x(Id)x(∆t)x(fsw) = 120x13.9x200x10-9x20k = 6.67 W.Gate Drive Requirements : the gate drivers must supply enough charge to theMOSFET gate to enable the gate-source capacitance to charge and the device to turn on.The power requirements increase when more MOSFET’s are paralleled.P = VxI = VxQxf = (10V)x(190nC)x(20k) = 38 mW. - 40 -
  49. 49. 4. Theory4.4 Heatsink Considerations It is extremely important that the electronic systems run as efficiently aspossible. This is particularly relevant for the high power systems, as usually a drop inefficiency of only a couple of percent typically relates to an increased powerconsumption of tens of Watts. A potentially large power sinking device is the MOSFETH-bridge. The MOSFET has losses as described previously, and these losses are inmost cases directly formed into heating the device’s junction. As the MOSFET is madeto switch faster, the switching losses become the most significant form of heatgeneration. There is also heat caused by increased conduction losses at higher outputpowers. As the junction temperature of a MOSFET increases, Rds increases, Idsdecreases, however the overall loss will increase. To this end, to keep the losses to a minimum, a heat-sinking system has to bemade. The package of the MOSFET is designed especially to conduct heat away fromthe junction and to the ambient atmosphere surrounding the device. A simple heatsinkdesign was calculated to obtain a feel for the correct size heatsink required:From the MOSFET data sheet in Appendix B,TJmax = maximum junction temp. = 150 degrees C.TAmax = maximum ambient temp. = 70 degrees C.Rds = drain-source resistance = 0.022 ohms.Imax = maximum switching current = 13.9 A.td(on) = on conduction time = 30 ns.tr = rise time = 12 ns.td(off) = off conduction time = 55 ns.tf = fall time = 12 ns.fsw = switching frequency = 20 kHz.RthJC = thermal resistance from junction-case of MOSFET = 0.26 degrees C/W.RthCS = thermal resistance from case of MOSFET-heatsink = 1.5 degrees C/W.RthJA = thermal resistance from junction of MOSFET-ambient = 60 degrees C/W. - 41 -
  50. 50. 4. TheoryVmax = voltage to be switched at nominal speed = 105 V.Vs = battery voltage = 120 V.Pon = MOSFET on power loss = ( I max ) 2 xRds = (13.9) 2 x0.022 = 4.25W TJ max − TA max 150 − 70RthJA = = = 18.82 degrees C/W. Pon 4.25RthSAmax = RthJAmax – RthJC – RthCS = 18.82 – 0.26 – 1.5 = 17.06 degrees C/W.kmax = maximum duty cycle = 105/120 = 0.875.ton = total on time = td(on) + tr = 60 ns.toff = total off time = td(off) + tf = 67 ns.τ = average on and off time = (ton + toff)/2 = 63.5 ns.Thus Psw = MOSFET switching power loss =Vs .I max 120 x13.9 τ . f sw = 63.5 x10 −9 x 20k = 1.06W . 2 2(for an inductive load)Thus TOTAL Power Loss = Ptotal = Pon + Psw = 5.31 W.The maximum power the MOSFET can dissipate without a heatsink is: (TJ max − TA max ) (150 − 70)Pdmax = = = 1.33W . RthJA 60If we use an aluminum plate with the following characteristics:Thermal Conduction for Aluminium = λ = 2.08, Thickness of Al plate = t = 5mm.Heatsink orientation factor = Cf = 0.43 (for a black anodized vertically mounted plate).Area of both sides of heatsink = A. (TJ max − TA max ) (150 − 70)RthSA = − ( Pd max + RthCS ) = − (1.33 + 1.5) = 12.24 degrees C/W. Ptotal 5.31The following equation can be used to describe the heatsink required:  3.3 0.25   650 RthSA =  Cf  + C f  degrees C/W.  λt   A Solving for A, we obtain A = 18.92 square cm.Thus a 5 mm thick piece of aluminum with dimensions ~3X3 cm is required. - 42 -
  51. 51. 5. HARDWARE DESIGN STAGE This section details the process in designing the hardware of the motorcontroller. A similar device should be able to be constructed based on the informationgiven here. The motor controller consists of two PCB boards: a high voltage board anda control board. Each board will be described separately.5.1 Design of Power Stage The power stage of the motor controller comprises all of the high voltagecomponents. It was decided to place all such components on the one PCB so thatpotentially fatal high voltage kept confined to the one board, and didn’t have to berouted across boards. The major components on this board are the MOSFET H-bridge,driver circuits for the MOSFET’s and high voltage capacitors. There are a number ofauxiliary circuits also placed on the board. Most of them have something to do with thehigh power section, therefore it was convenient to include these circuits on the sameboard as the other components. These include bus voltage measurement, temperature - 43 -

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