A motor controller for solar car

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.

Union College,
Upland Road,
St. Lucia QLD 4067.
Ph : (07) 33771500
Fax : (07) 33713826
16 October 1998
The Dean,
Faculty of Engineering,
The University of Queensland,
St. Lucia QLD 4072

Dear Professor Simmons,

In accordance with the requirement of the degree of Bachelor of Engineering in
the division of Electrical and Electronic Engineering, I present the following thesis
entitled :
“A Motor Controller
For the Solar Car
Project”

This work was performed under the supervision of Mr. Geoffrey Walker. I
declare that the work submitted in this thesis is my own, except as acknowledged in the
text and footnotes, and has not been previously submitted for a degree at The University
of Queensland or any other institution.

Yours Sincerely,

Andrew J. Reghenzani.

A Motor Controller For The Solar Car Project

ACKNOWLEDGEDGMENTS

The following people deserve special recognition for their contributions to my
thesis project throughout the year:

My family : who have always supported me throughout University, and have given me
the extra motivation to succeed during difficult times.

My friends : for understanding how important my thesis was and always seeming to ask
the all too familiar question “How’s your thesis going?”.

Members of the Solar Car Team : especially Charles for organizing use of a digital
camera and Anthoney for assistance with writing code. I have thoroughly enjoyed
being in the solar racing team, as it has given me the opportunity to gain valuable work
experience and gain some practical skills which complement my University studies.

My supervisor, Mr. Geoffrey Walker : for all his time, invaluable advice and
encouragement throughout the thesis project.

Keith Aldworth and the electronics workshop personnel : for the manufacture of my
PCB’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 technical
tips regarding PCB design and manufacture.

Keith Lane, Wayne Jenkins and Bill Slack from the electronics workshop : for building
my heatsinks and other hardware from my plans which usually consisted of a page of
dimensions, use of the tools and machines in the workshop at any time and all the
technical advice regarding manufacturing.

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ABSTRACT
The transport needs of our ever growing and evolving society is becoming
increasingly stringent and more demanding. In order to combat this, more efficient
transportation vehicles need to be developed which are faster and cleaner. As the
human race starts to realize the real extent to which the internal combustion engine has
gradually polluted the atmosphere, more research is being concentrated on alternative
forms of propulsion. A number of propulsion systems and energy sources have
undergone feasibility studies to investigate potential commercial and industrial
applications. Some projects have been shown to work successfully, while other
technologies are still well in their infancy stage of development. A handful of examples
of 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 internal
combustion engine, and have been proven to display moderate driving range (up to
300km). An electrically powered vehicle has essentially three major electrical
components. These are an energy source (usually a rechargeable battery bank), an
inverter or motor controller and an electric motor. In the case of a solar car, the energy
source is typically a bank of batteries, which may be recharged by photovoltaic solar
panels. The motor controller is typically a power electronics device which when
supplied 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 to
mechanical energy used to propel the vehicle, usually through a type of transmission.
A motor controller is custom designed for a new hub mounted Brushless DC
Permanent Magnet (BLDC PM) motor, as part of the solar car project. Efficiency and
reliability have been two of the key factors considered when designing the controller.
Due to careful selection of quality components and use of high efficiency control
algorithms, a marketable increase in efficiency over the existing system is expected with
the new controller and motor.

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CONTENTS
ACKNOWLEDGEDGMENTS......................................................................................................I
ABSTRACT.................................................................................................................................. IV
CONTENTS................................................................................................................................... V
LIST OF FIGURES ............................................................................................................ VII
LIST OF TABLES ............................................................................................................. VIII
1. INTRODUCTION................................................................................................................1
1.1 Introduction ............................................................................................................ 1
1.2 Problem Specification ............................................................................................ 2
1.2.1 Thesis Goal .......................................................................................................... 4
1.2.2 Motivation behind the Motor Controller and Motion Control............................. 4
1.3 Organization of the Thesis Document.................................................................... 5
2. THE UNIVERSITY OF QUEENSLAND SOLAR CAR..............................7
2.1 Solar Car Racing and the Races ............................................................................. 7
2.2 A Brief History of the UQ Solar Racing Car ......................................................... 9
2.3 The Nuts and Volts of a Solar Car.......................................................................... 9
2.3.1 Batteries ............................................................................................................. 10
2.3.2 Solar Array ........................................................................................................ 12
2.3.3 Maximum Peak Power Trackers (MPPT’s)....................................................... 13
2.3.4 Motor Controller................................................................................................ 13
2.3.5 Motor ................................................................................................................. 14
2.3.6 Telemetry Functions and Power Supply............................................................ 15
2.4 Necessity for Efficient Systems............................................................................ 15
2.5 The Existing Drive System................................................................................... 16
2.5.1 Controller Type.................................................................................................. 16
2.5.2 Performance Characteristics .............................................................................. 17
2.6 The New Drive System ........................................................................................ 17
2.6.1 Additional Features............................................................................................ 18
2.6.2 Performance Requirements................................................................................ 19
3. MOTOR CONTROL LITERATURE......................................................................20
4. THEORY ...............................................................................................................................25
4.1 The Permanent Magnet Brushless DC Motor ...................................................... 25
4.1.1 Electrical and Mechanical Parameters............................................................... 28
4.2 Controlling a Permanent Magnet Brushless DC Motor........................................ 30
4.2.1 Commutation ..................................................................................................... 30
4.2.2 Current Regulation ............................................................................................ 35
4.2.3 Trapezoidal Current Excitation.......................................................................... 35
4.2.4 Sinusoidal Current Excitation............................................................................ 37
4.3 Power MOSFET Device Characteristics .............................................................. 38
4.4 Heatsink Considerations....................................................................................... 41
5. HARDWARE DESIGN STAGE...............................................................................43
5.1 Design of Power Stage ......................................................................................... 43
5.1.1 Circuit Design.................................................................................................... 44
5.1.2 Sensors............................................................................................................... 45
5.1.2.1 Bus Voltage Measurement .............................................................................. 45
5.1.2.2 MOSFET Heatsink Temperature Measurement ............................................. 46
5.1.2.3 Phase Current Measurement ........................................................................... 46
5.1.3 Manufacture and Construction .......................................................................... 48
5.2 Design of Control Stage ....................................................................................... 50

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5.2.1 Circuit Design.................................................................................................... 51
5.2.1.1 Auxiliary Components and Power .................................................................. 52
5.2.1.2 Memory Board ................................................................................................ 52
5.2.1.3 Input/Output Ports........................................................................................... 52
5.2.2 Manufacture and Construction .......................................................................... 54
6. SOFTWARE DESIGN STAGE ................................................................................56
6.1 System Description............................................................................................... 56
6.2 Main Program....................................................................................................... 58
6.3 Torque Control ..................................................................................................... 59
6.3.1 Regeneration...................................................................................................... 59
6.3.2 Brake.................................................................................................................. 60
6.4 MOSFET Heatsink Temperature.......................................................................... 61
6.5 Motor Temperature............................................................................................... 61
6.6 Speed and Direction ............................................................................................. 61
6.7 Commutation ........................................................................................................ 61
6.8 Bus Voltage .......................................................................................................... 61
7. DISCUSSION .....................................................................................................................63
7.1 Discussion ............................................................................................................ 63
8. CONCLUSIONS ................................................................................................................64
8.1 Thesis Conclusions............................................................................................... 64
8.2 Possible Future Work ........................................................................................... 64
8.3 The Future of Solar Car Racing : The Big Picture ............................................... 66
APPENDICES ..............................................................................................................................67
APPENDIX A: SCHEMATIC AND PCB DESIGNS.................................................68
APPENDIX B: MOSFET DATA SHEETS.............................................................69
APPENDIX C: CSIRO/UTS MOTOR SPECIFICATIONS........................................70
APPENDIX D: MICROCOMPUTER PROGRAM LISTINGS...............................71
APPENDIX E: ACCOMPANYING COMPUTER DISK .............................................72
MAIN PROGRAM ......................................................................................................................72
SCHEMATIC FILES ..................................................................................................................72
PCB FILES ...................................................................................................................................72
BIBLIOGRAPHY ........................................................................................................................73

BOOKS ...................................................................................................................................... 73

JOURNAL ARTICLES ............................................................................................................ 73

INTERNET RESOURCES ...................................................................................................... 77

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LIST OF FIGURES
FIGURE 1 : BLOCK ELECTRICAL DIAGRAM OF A SOLAR CAR ......................................................................10
FIGURE 8 : HALL EFFECT POSITIONING SENSORS........................................................................................28
FIGURE 9: NUMBERING PATTERN FOR MOSFET’S IN THE H-BRIDGE ......................................................... 30
FIGURE 10 : 120 DEGREES COMMUTATION MODE ...................................................................................... 32
FIGURE 11 : 180 DEGREES CONDUCTION MODE ......................................................................................... 34
FIGURE 12 : CURRENT FEEDBACK IN A BLDC MOTOR ...............................................................................35
FIGURE 13 : TORQUE RIPPLE IN A TRAPEZOIDAL MACHINE ........................................................................ 36
FIGURE 14:NON-CONDUCTRING MOSFET[34] ..............................................................................................
FIGURE 15:CONDUCTING MOSFET[34] ............................................................................................38
FIGURE 16:WAVEFORMS AT TURN-ON[38].....................................................................................................
FIGURE 17:WAVEFORMS AT TURN-OFF[38]................................................................................................39
FIGURE 20 : THERMISTOR RESPONSE ..........................................................................................................54
FIGURE 22 : BLOCK DIAGRAM OF CONTROL ALGORITHM........................................................................... 57
FIGURE 23 : A FOUR QUADRANT DRIVE.......................................................................................................58
FIGURE 24:ONE SWITCH ACTIVE TOPOLOGY ..................................................................................................
FIGURE 25:TWO SWITCH ACTIVE TOPOLOGY .............................................................................................60

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LIST OF TABLES

TABLE 1 : 120 DEGREES COMMUTATION TRUTH TABLE ............................................................................31
TABLE 2 : 180 DEGREES COMMUTATION TRUTH TABLE ............................................................................33

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1. INTRODUCTION

1.1 Introduction

The development of the internal combustion engine was certainly considered a
milestone for mankind. The focus back in the time of the Industrial Revolution was to
design machines which could fulfill time consuming, labor intensive jobs in a fraction
of the time that it took humans alone using conventional methods. Cars were developed
as a fast means of transport, and internal combustion engines soon found themselves in
many applications ranging from cane harvesters to outback generator sets. As time
progressed, most people had realized that although the internal combustion engine had
provided a much easier lifestyle, there were a number of major drawbacks. Petrol,
when combusted, forms a number of gaseous byproducts, consisting mainly of carbon
dioxide, but also containing traces of other gases such as carbon monoxide and
compounds containing lead. The potency and increasing levels of these gases and
compounds are causing gradual damage to the ozone layer in the Earth’s atmosphere.
Such gases are commonly referred to as greenhouse gases.

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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, and
commercial versions are being made today. A typical hybrid EV is driven by an electric
motor and usually contains a rechargeable battery bank and a small internal combustion
engine. The internal combustion engine still emits greenhouse gases, however only at a
fraction of the amount. In some of the latest hybrid vehicles, four wheel motors are
used (one for each wheel), and four motor controllers are used to control the torque of
each individual motor for optimal vehicle performance and control.

An alternative energy source which is very appealing is solar energy. Solar
energy is a continually advancing technology, and as photovoltaic (PV) solar cells are
being made more efficient, solar power is finding widespread use in applications such
as outback power supplies and grid connected PV arrays. A large contributor to the
increasing level of pollution is the household car, so solar cars were developed with the
vision that an ideal car could be built which could run solely from the sun for the
lifetime 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 been
attempted before. The new controller has incorporated a multitude of features which are
designed to make the drive system highly efficient and safer while providing a more
intuitive driver control. The new motor controller consists of a Hitachi SH1 7032 RISC
microprocessor operating at a clock speed of 20MHz accompanied by an array of
sensors and a high voltage inverter stage. The work performed in this thesis project
incorporates 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

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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

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1. Introduction

™ Temperature monitoring of MOSFET heatsink
™ Fault indicator
™ Wide input voltage range
™ Transient protection
™ Fuse protection
™ Diagnostic capability
™ Cooling fan mounted to heatsink

1.2.1 Thesis Goal

The primary and most important goal of my thesis was:

“To design and construct a Brushless DC motor controller for the University of
Queensland solar car that performs motoring and regeneration at a very high efficiency.
The motor controller should also perform auxiliary functions that make the drive system
more robust, safer and easier to control.”

The controller should operate the motor with the highest possible efficiency
under steady-state operating conditions. Under abnormal conditions, the controller
should respond quickly to resolve the problem and resume normal operation to maintain
a high level of energy efficiency. On completion of the project, the motor controller
will 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 systems
with greater efficiency and more stringent operating specifications. An area in which
efficiency and reliability is an absolute must is motors and their control. Motors are
used in a vast variety of applications ranging from huge crushing mills to pinpoint
accuracy mechanisms in space applications. Some applications require motors to
operate in harsh environmental conditions, e.g. flammable gas leaks, where

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1. Introduction

conventional DC brush motors cannot be used due to the risk of sparks forming between
the brushes and commutator. There are many types of motors available today, however
a 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 the
brushless DC (BLDC) motor. Unlike conventional DC brush motors, the brushless
motor, as it’s name suggests, has no brushes and requires extra electronic circuitry to
perform the job of commutation. The BLDC motor can be constructed in many sizes
and power ratings, and finds widespread application in many motor drives. The primary
motivation behind the thesis was to improve the efficiency and technology of the solar
car. The secondary motivation was related to the popularity of the BLDC motor and it’s
future applications. Factors such as high power to weight ratio and reliability will
definitely see BLDC motor technology improve in years to come. By studying how
such 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 encountered
and how these problems were overcome. Detailed descriptions including theory are
presented to support practical design choices. The following chapters form the body of
the thesis document, and may be summarized as follows:

Chapter 2, The University of Queensland Solar Car, presents first an introduction to
solar racing and how the event was first initiated, followed by a brief history of the UQ
solar racing car. The chapter then presents an electrical system overview in a typical
solar car, and how the main electrical components are interfaced. A short discussion
follows which outlines the importance of efficient systems on a solar car. The chapter
concludes by summarizing the existing drive system, then describing some of the
performance parameters of the new drive system.

Chapter 3, Motor Control Literature, presents a literature review of all relevant work
in the field of BLDC motor control. Useful formulas and control algorithms are

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1. Introduction

extracted from the text and hi-lighted in this chapter. There is a complete list of all
references used in the bibliography section at the very back of the thesis report.

Chapter 4, Theory, provides the background material necessary to understand how a
brushless 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 their
operation down to component level. Design formulas indicate how component values
were obtained. Mechanical factors are presented for construction of the motor
controller and when mounting into the car.

Chapter 6, Software Design Stage, describes the control algorithms implemented in
software which control the motor. There is a full listing of the code completed to date
in Appendix E.

Chapter 7, Results and Discussion, presents a discussion of the motor controller
project and the issues that emerged from such a project.

Chapter 8, Conclusions, concludes the document with a short summary of the findings
throughout the thesis project. Some possible future work is given as suggestions to
improving the motor controller. A final note is then given to the overall picture of solar
racing and where the future of such a technology is headed.

The author hopes the thesis document provides excellent reading and a useful
reference for any future work in motor control.

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2. THE UNIVERSITY OF QUEENSLAND
SOLAR CAR

2.1 Solar Car Racing and the Races

Solar car racing first started out as a novel idea to investigate the limitations of
solar energy as a possible alternative to non-renewable energy sources. From that point
forward, solar car racing has grown in popularity and can be considered a sport, with
annual and biannual racing events being held all around the World. One of the more
prominent races is the World Solar Challenge, which covers some 3100 km from
Darwin to Adelaide along the Stuart Highway. Australian adventurer Hans Tholstrup
organized the first WSC in 1987, and it is now a bi-annual event held in October. The
Sydney City Power SunRace traverses the eastern coast of Australia from Melbourne to
Sydney and is the equivalent of the American SunRace. The American SunRace is the
largest solar event held in the United States. The World Solar Rallye in Akita, Japan is
held every year in July on a purpose-built solar racing track named the Ogata Mura

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2.The University of Queensland Solar Car

Solar Sports Line. Many other countries hold solar related activities to promote solar
energy 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 ability
of solar energy, but have a lot of fun simply making the car perform optimally
regardless of impeding conditions. There is great satisfaction when seeing months of
hard work finally being paid off, as the solar car races through the finish line. The idea
of a solar car race is to reach the finish as fast as possible, obeying the race regulations
at all times to avoid time penalties.

For long endurance races such as the WSC, a convoy of cars accompanies the
solar car. One support vehicle usually has onboard computers and radio equipment for
data and voice interchange with the solar cars’ driver and telemetry system. Team
members ride in a scout car and place wooden boards over cattle grids so that the solar
cars’ 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 mechanical
breakdowns. In races such as the World Solar Rallye in Akita, the racing track
consisted of a 30km round circuit, allowing no room for support vehicles. Telemetry
data, which was logged for an entire lap had to be transmitted in a short burst when the
car was in range of the receiving base station antenna. During the normal course of a
race, the drivers must be changed at regular intervals and a number of media stops are
usually anticipated.

There are two aspects that are essential for a highly competitive entry. A major
aspect 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 from
lightweight materials and choosing efficient electrical components. The other aspect,
which is equally important, is to have an effective race strategy. In a race situation, a
race strategy team determines an optimal speed to run the car at, depending on current
weather 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

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resistance). Most often, an unexpected weather pattern emerges or a critical breakdown
occurs. The strategy team must take into account these factors, and make a crucial “on
the 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 was
finally 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 car
was ready to roll. The WSC took Sunshark six days of racing in some of Australia’s
harshest 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 and
achievement from General Motors (GM) Holden.

A decision was made by the newly formed team early next year to participate in
the 1997 World Solar Rallye (WSR) in Akita, Japan. With only minor electrical and
mechanical modifications being made to the car in order to comply with race
regulations, the team and car were ready to compete at the Ogata Mura Solar Sports
Line in Akita. After 5 days of racing in sweltering heat, the car finished in identical
form as the WSC : ranked fifth overall and class winner of the silicon cell/lead-acid
battery category. Major electrical enhancements and some mechanical improvements
are currently underway in preparation for a large testing run near the end of 1998 and
the Sydney CitiPower Sunrace in January. The next WSC has been scheduled for
October 1999 and the team hopes to have a greatly superior car than in previous years
for 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.

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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 Car

The central node of the electrical system is the high voltage (HV) bus. Physically it
may simply consist of a connection point or short strip of copper, however it is at this
point that the flow of current is distributed to all components. The main electrical
components are described in the next section.

2.3.1 Batteries

The primary energy source for the vehicle is the battery bank. The battery bank
usually consists of a number of individual batteries connected in series or parallel. Each
battery in the bank is typically 6 or 12V, and multiple batteries are connected in series
or parallel to obtain the desired system voltage. A single battery is actually made from
multiple “cells” contained within the battery housing. A sealed lead acid type showing

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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 for
lead-acid batteries, fluctuates considerably around the nominal battery voltage, from full
charge to maximum discharge. For this reason, the nominal battery voltage is usually
chosen so that the lowest possible battery voltage is able to sustain a reasonably
competitive speed. An alternative solution to this problem is to implement a boost/buck
converter in the motor controller so that an optimal speed can be obtained for any
battery voltage. There are many types of commercial batteries available today. Some
examples 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 to
obtain 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 full
set of batteries typically weighed in at 96kg. Each type of battery has different
characteristics (e.g. energy density/kg, charge/discharge rate) and uses, however a
comprehensive study of batteries is beyond the scope of this thesis.

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2.3.2 Solar Array

The capacity of batteries set out by race rules and regulations is too small for a
solar car to fully depend on during a race. Energy must be obtained from the sun by a
solar array to supplement the energy taken from the batteries. Under maximum
insolation levels, the solar array can sometimes supply ample energy, and the excess
simply flows back into the batteries. The solar array consists of a configuration of solar
photovoltaic cells, usually encapsulated to protect against the elements and damage.
The encapsulation of cells also increases the overall efficiency of the array. This is
achieved by carefully designing anti-reflective coatings and materials to maximize the
light 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 in
the crystalline lattice, and allowing them to move freely throughout the structure. Solar
cells are constructed from a semiconductor p-n junction, which allows current to flow in
one direction only, similar to the operation of a diode. The SunShark solar car team’s
first array contained 15.5% Sharp cells encapsulated in epoxy, giving a peak power

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output of 1kW. The new array should have higher efficiency cells and enhanced
encapsulation 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 sunlight
intensities, 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 at
it’s maximum power point voltage, while delivering the resulting maximum PV power
to the battery bank. It does this by electronically de-coupling the PV voltage from the
battery voltage by using a high frequency transformer and MOSFET’s. A MPPT
module is shown in Figure 4. The existing array had three MPPT modules
manufactured from the Australian Energy Research Laboratories (AERL).

2.3.4 Motor Controller

The motor controller is designed to convert the electrical energy obtained from
the batteries and solar array to suitable power waveforms to drive the motor. The motor
controller 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

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produced in the motor can be controlled via controls in the cockpit. A more thorough
explanation of the motor controller is given in chapter 4.

2.3.5 Motor

The motors’ function is twofold: to convert the electrical energy to mechanical
energy 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 of
advantages and disadvantages in particular applications ranging from large industrial
roller mills to accurate positioning control. The most popular choice for high efficiency
applications such as solar cars, is the permanent magnet brushless DC motor, or
sometimes known as a synchronous DC motor. The advantages of the BLDC motor
include:

• 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 greater
detail in chapter 4.

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2.3.6 Telemetry Functions and Power Supply

The basic electrical system shown in Fig. 1 can be enhanced with the addition of
telemetry systems and support circuitry. The main aim of the telemetry system is to
calculate an optimal speed and/or power to run the car. One of the tasks performed is to
record data such as bus voltages and motor currents. The existing telemetry in the
SunShark solar car consisted of a signal conditioning board and telemetry board able to
transmit sampled data to the support vehicle via radio modems. It is envisaged the
support vehicle computers will be able to determine an optimal speed of operation, and
even take control of the car, by factoring in all relevant aspects which directly influence
the systems’ performance.

The power supply is responsible for converting the bus voltage down to supply
voltages 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 amount
of that energy actually reaches the Earth’s surface. Furthermore, the amount of energy
received from the sun by a photovoltaic solar array depends upon multiple factors such
as cloud cover, angle of incident sunlight, cell temperature and cell efficiency. Due to
the obvious difficulties in obtaining energy from the sun, any wasted energy (i.e. energy
that is not contributing to the forward motion of the car) is regarded as a limiting factor
on the maximum speed obtainable from the system. For the SunShark solar car,
approximately every kilogram of vehicle weight relates to rolling friction power loss
increasing by 1W. Mechanical systems and frames can be made lighter by using
different materials in an attempt to reduce rolling friction power loss. There are a
number of methods in which electrical systems can be made more efficient. Through
careful circuit design with energy efficient components, substantial power savings can
be made. The heating losses due to current flow in conductors can become substantial

- 15 -


in high power parts of the circuitry. It is advisable in this case to use over-rated cables
to help bring the conductor resistance and hence power loss down. Layout of
components is also important to reduce conductor lengths and parasitic inductive
elements. In a solar car race, the maximum velocity of the solar car is limited by the
efficiency of the system and race weather conditions. Since the weather conditions on a
race are at best highly unpredictable, in some instances a solar car may be fully reliant
on the batteries for power. At the end of the day, the performance of a solar car is
heavily 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. The
motor had no iron in either its rotor or stator and consisted of a number of cylindrical
magnets 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 the
nature of its construction. The motor required a transmission system consisting of a
toothed 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 controller
used trapezoidal phase current excitation waveforms. A PWM chip (NE5568) was used
together with a ROM (N82S123AN) programmed with a commutation truth table to
decode the hall effect signals from the motor, and provide excitation to the correct
phases. All logic circuitry was supplied power using a linear 5V regulator, which has
an efficiency of ~50%. The inverter stage was a common three phase H bridge design,
using three paralleled MOSFET's (IRFP260) in one switch, i.e. a total of 18 MOSFET’s.
The MOSFET’s had transient suppressing metal oxide varistors (MOV) to clamp the
voltage over each MOSFET switch to a safe level. DC link capacitors (12 X 220uF

- 16 -


standard electrolytic) were used in the DC link. The MOSFET gates were driven by an
IR2130 3-phase bridge driver chip. All three lower inverter switches had a 20k ohm
resistor 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 the
constant current in the DC bus, instead of in the DC link. Driver controls consisted of
two 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 at
fast speeds to not bump the switch in the opposite direction, otherwise excess currents
would 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 of
the driver control. The controller was speed controlled, which meant the driver had to
basically guess where to position the potentiometer for a certain desired speed. This
caused a lot of concentration by the driver as the speedometer had to be constantly
monitored and potentiometer adjusted to obtain the desired speed. Moreover the speed
ramp was not a linear function of potentiometer position, but had a slow response at low
speeds and a fast, uneven response at moderate to high speeds. This made fine
adjustment of speed a large problem. It was discovered that potentiometers are not
always fully reliable devices, and a number had to be replaced during the course of the
race. 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 plug
was 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. The
motor features two rotors, has no iron loss and is of an axial field construction. The
motor is specifically designed to fit inside the wheel of a solar car which has a number
of distinct advantages over the original reduction belt system:

- 17 -


• 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 water

The 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 controller
to 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 -


• 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 and
safer 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 average
speed of the car can be increased.

- 19 -

3. MOTOR CONTROL LITERATURE

An extensive literature search was carried out to review work completed
previously. A list of keywords relating to the topic for searching databases: e.g. motor
controller, electric drive, motion control was drawn up. A general WWW search
resulted in a number of results however I found many of the web sites were usually a
company trying to sell their product, and offer little or no technical information. The
WWW is a very convenient way of obtaining product data sheets. The main source of
information was books from the Physical Sciences and Engineering (PSE) library.
There is a reasonable selection of books in the library ranging from Power Electronics
to books specifically on motor drives and their controls. A comprehensive search using
the networked databases Inspec, Compendex, Engineering & Applied Science, National
Technology Information Service (NTIS), Current Contents and Computer ASAP was
also undertaken. This search resulted in some 32 journal and magazine articles relevant
to aspects on motor controllers.

Most articles contained an example of a motor and motor controller designed to
demonstrate a particular feature. The experimental setup was commonly explained by

- 20 -

3. Motor Control Literature

the use of diagrams. The circuit in most cases was put under a simulation and results
were compared with the actual measured values. Many of the articles obtained are from
the IEEE and IEE publications.

Reference articles [7] and [9] discuss a controller using MOSFET switches with
in-built current sensing (used IRC644, 14A cont. 250V). These MOSFET’s are referred
TM
to by International Rectifier (IR) as HEXSense devices, as they contain integrated
shunt resistors, which can detect the current passing from the drain to the source. This
results in a more compact design and eliminates the need for external shunt resistors or
hall effect current transducers which results in an immediate weight saving. This type
of MOSFET was researched into, however none were found with the required voltage
rating. The only HEXSense TM devices that were found were of the 3-pin type (TO-220
case 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 digital
converter (ADC), which would quickly clutter the available ADC channels on a
microprocessor. The controller mentioned in articles [7] and [9] can operate the motor
in all four quadrants of the torque-speed plane, i.e. forward and reverse motoring and
forward and reverse regeneration. Trapezoidal phase current excitation with 120 degree
switch conduction intervals are used so that current only flows in two of the phase
currents at any one time.

An important comment in [7] as to the position of the current sensors for current
feedback and regulation is made. The most simple method is a resistive shunt on the
DC bus. Although a simple and relatively cheap method of current detection, it cannot
detect dangerous circulating currents which may be developed in the phase windings
and power switches. This current build up can result in switch failure or
demagnetization of the rotor magnets. The only solution to this problem is to have
current transducers mounted in the phase windings so that the current may be monitored
and evasive action taken.

The controller in [7] and [9] contains separate commutation logic/control and
current regulation blocks. The commutation logic/control block was implemented with

- 21 -


the Motorola MC33034 brushless motor controller chip. The MC33034 has inputs from
the rotor position sensors and driver control (start/stop and forward/reverse), and has
commutation signal outputs which feed into the Harris GS601 HVIC half-bridge gate
drive chip. Current regulation is achieved by difference summing a current command
signal (from the driver), and the current feedback signal (from one of the lower
switches). This difference voltage represents the current error, and the GS601 driver
chip minimizes the error by varying the switches’ PWM duty cycle, effectively
regulating the current to the desired value. The current control algorithm is simple in
principle. In this case a fixed off time, TL is used. The PWM frequency is determined
by the following formula:
 E 1
f PWM = 1 − 
 V T
 S  L

where 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 at
low speeds when the motor phase resistive drop approaches the magnitude of the back-
EMF. Another current regulation algorithm is briefly mentioned, namely holding the
total 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 low
speeds as it is not as sensitive to the back EMF amplitude. Both methods take
advantage of the energy stored in the motor windings and transfer this energy back to
the supply. A simple speed detector circuit which works on the principle of providing a
pulse for every transition of the hall effect sensors is described. The frequency of the
pulses is proportional to the motor speed according to the following formula:

- 22 -


p
f rotation = nv ( Hz )
120
where f rotation = frequency of the pulses,
p = no. rotor poles
n = 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 in
brushless DC motors and described how a particular motor was built for the “Desert
Rose” solar racing car. The articles discuss an axial flux permanent magnet brushless
DC motor designed for an in wheel drive on a solar car. The axial flux geometry was
found to have advantages over the common radial flux geometry by reducing volume
limits and having the ability to change the air-gap between stator and rotor. Increasing
the air-gap increases the copper loss as the torque constant decreases, but decreases the
iron loss as the flux density reduces. The main author of these articles was Dean
Patterson of the Northern Territory University. Dean Patterson also has written a
journal article on the electrical system design for a solar powered vehicle [29], which
made interesting reading material as the system could be compared with our own
system and comparisons made.

Article [4] describes some common dc drive failures and how to design a control
system which can sense the failure and continue to operate normally. The results are
presented both using a simulation and measured results.

Articles [28] and [30] describe how to model electronically commutated
machines using the P-Spice simulation program. This will be very useful information
when experimenting with the inverter stage, and comparing measured results with
simulated results.

Article [24] describes the application of soft switching inverters in electric
drives. A soft switching, or sometimes known as resonant converters, eliminate
switching 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 -


switches, inductors and capacitors to be arranged on the DC bus. To design a converter
of type is by itself a full thesis, so will not be further investigated. The current design
however, is flexible enough to allow future people to add a resonant converter if
desired.

Article [31] presents a bi-directional dc/dc converter which can control the DC
link voltage and control regenerative braking of an electric vehicle. An explanation
follows that describes how the converter can switch currents in both forward and
reverse directions. Motor current ripple is claimed to be reduced by constantly
changing the DC link voltage under different operating conditions.

- 24 -

4. T HEORY

4.1 The Permanent Magnet Brushless DC Motor

There are a number of configurations for the brushless DC motor, however all
operate on the same principal. There are three main components that make up such a
motor:

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 use
more than three phases as each phase is broken up into multiple sections. There is also
the option to connect the windings in a delta configuration, however this may introduce
unwanted circulating currents flowing around the windings. The stator of the CSIRO
motor is shown in Figure 5. Each of the three phase windings are distributed in a

- 25 -

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 current
excitation must be applied to the motor.

Rotor Magnets : In conventional DC motors, electromagnets are used to create
a magnetic field. The rotor in a BLDC motor consists of rare earth magnets which
produce 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 -

4. Theory

which are placed on either side of the stator and are kept separated by special rims. The
stator will be held stationary and fixed to the trailing arm. Both rotor magnet rings are
fixed to the wheel rim, and rotate with the movement of the tyre.

Hall Effect Sensors : Hall sensors are a popular choice for rotor position
feedback in brushless DC drives, reasons being they are cheap and do not require
complex processing algorithms. Hall sensors are more suited for use with trapezoidally
controlled motors, as sinusoidal machines usually require a higher resolution sensor
such as a shaft encoder or transducer. The actual sensor is usually a N-doped InSb
semiconductor, which in the presence of a magnetic flux, an electromotive force causes
free flowing electrons to move to one side of the semiconductor which causes a
potential 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 possible
ways of positioning the hall effect sensors around the axis. The hall elements can either
placed at 60 or 120 electrical degree intervals (.i.e. the hall code changes every 60 or
120 electrical degrees). The hall effects to be used are configured to change every 60
electrical degrees. One electrical cycle is equal to 360 electrical degrees, and is defined

- 27 -

4. Theory

as when the hall sequence starts to repeat. The hall effect sequence can be represented
in Figure 8. Since the motor has 40 poles, for one revolution of the motor, each hall
sensor will experience 20 north's and 20 south's (i.e. 20 high and 20 low level outputs).
The mechanical separation of the magnets and hall effect sensors can be calculated
easily from knowing the number of magnets and poles. One mechanical cycle is equal
to one entire revolution of the motor or 360 mechanical degrees. One electrical cycle
no. electrical degrees in one cycle 360
repeats every = = 18 mechanical degrees.
no. poles/2 20

Figure 8 : Hall Effect Positioning Sensors

4.1.1 Electrical and Mechanical Parameters

The main electrical parameters of the CSIRO motor are presented in Appendix C.

Speed of Motor Calculation
Diameter 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 -

4. Theory

= 101.898 km/hr.

Electrical Parameter Calculation
The surface motor is described by the following formula:
T=kTI where
T = 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 A
and for nominal torque, I = T/3kT = 16.2/(3x0.39) = 13.85 A

the motor can also be described by:
E = kE ωm where
E = 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 V
and for nominal angular velocity, E = kEωm = 0.39x111 = 43.29 V

Battery Voltage Calculation
The battery voltage has to be chosen so that the motor controller may operate at near to
full PWM when running at nominal speed. The motor has a line-neutral RMS emf at
111 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, where
kmodulation = 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 for
overtaking.

- 29 -

4. Theory

4.2 Controlling a Permanent Magnet Brushless DC Motor

Throughout the thesis document, the following numbering pattern for
MOSFET’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 recovery
time comparable to that of a discrete diode placed in parallel with the MOSFET. The
diodes will be referenced with the same numbering as the MOSFET’s, i.e. SW1 has a
corresponding diode D1, and so on.

4.2.1 Commutation

Commutation is the process of reading the hall effect sensor code, which gives
an indication of the position of the rotor. If the position of the rotor is known, then the
positions 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 the
applied voltage is in synchronism with the rotor position. Depending on the magnitude
of the current command, different magnitude torque can be applied to the motor. There
are two basic schemes of commutating a BLDC motor.

- 30 -

4. Theory

120 Degree Conduction
The 120 degree conduction mode switches MOSFET’s on for a length of 120
electrical degrees and off for 240 degrees. The relation between the MOSFET
switching states and hall effect codes is shown in Table 1. When the MOSFET’s are
turned on, they are not simply switched on and left on, rather they are modulated by a
PWM signal. The PWM signal varies in duty cycle depending on what current
regulation algorithm is being used. When the PWM signal is high, only two
MOSFET’s turn on at any one time, one from the high side and one from the low side of
alternate phases1. When the PWM toggles low, the low switch is turned off and the
corresponding high switch is turned on. This method is called synchronous
rectification, as it allows the current to flow through the paralleled high switch and
freewheeling diode, thus reducing conduction losses. A basic two pole motor is
presented in Figure 10 showing the rotation of the rotor magnets and the corresponding
flow of current in the motor windings and hall effect codes for 120 degree commutation.
Input Output
PWM 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 Table

1
Note : Both high and low MOSFET’s of the same phase are never switched on at the same time.

- 31 -

4. Theory

Figure 10 : 120 Degrees Commutation Mode

- 32 -

4. Theory

180 Degree Conduction
The 180 degree conduction mode switches MOSFET’s on for a length of 180
electrical degrees and off for 180 degrees. The relation between the MOSFET
switching states and hall effect codes is shown in Table 2. Similar to the 120 degree
commutation, a PWM signal varies in duty cycle depending on what current regulation
algorithm is being used. When the PWM signal is high, three MOSFET’s turn on at any
one time, either two from the high side and one from the low side, or two from the low
side and one from the high side.2. When the PWM signal toggles low, the side with
only a single switch active toggles and it’s corresponding switch is turned on. Once
again, synchronous rectification takes place as the current flows through the paralleled
high switch and freewheeling diode, thus reducing conduction losses. A basic two pole
motor is presented in Figure 11 showing the rotation of the rotor magnets and the
corresponding flow of current in the motor windings and hall effect codes for 180
degree commutation.
Input Output
PWM 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 Table

2
Note : Both high and low MOSFET’s of the same phase are never switched on at the same time.

- 33 -

4. Theory

Figure 11 : 180 Degrees Conduction Mode

- 34 -

4. Theory

4.2.2 Current Regulation

Since torque is proportional to the fundamental frequency of the current, by
controlling the current, torque is also controlled. All other frequency components
contribute to losses in the motor, inductors and controller. To form a closed loop
system, 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 the
controller to accurately current limit by trying to keep the current error as close to zero
as possible. The actual current limiting is achieved using a PWM scheme for the
switching MOSFET’s. The audible range for humans is approximately between 6kHz
and 20kHz, so a PWM frequency above ~20kHz is sufficient to avoid an annoying
whine 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 turned
on which produces a phase current of trapezoidal shape (hence the name) as shown in
the lower trace of Figure 13. One major disadvantage of a driving a motor with

- 35 -

4. Theory

trapezoidal current, is that there are many frequency components which make up a
trapezoidal 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 is
characteristic for a trapezoidal motor. The torque ripple can be attributed to two major
sources:

Motor Related Torque Ripple : causes the torque waveform to be rounded during the
commutation intervals. This is caused mainly by magnetic flux leakage paths between
adjacent rotor magnet poles. This torque ripple can be minimized by careful motor
design. See label 1 in Figure 13.

Inverter Related Torque Ripple : The first of this type of ripple is caused by a current
imbalance when current is being switched between active phases. Sharp torque spikes
can be produced and are experienced every 60 electrical degrees. Special PWM
switching techniques can be used to reduce this ripple. See label 2 in Figure 13.

- 36 -

4. Theory

The second inverter related torque ripple is directly proportional to the high
frequency 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 of
the solar car usually filters out the ripple.

At high speeds, phase current and motor torque can decrease abruptly when the
supply voltage equals the combined back emf of the two conducting phases. Continued
high speed operation is possible by gradually extending the conduction time from 120
electrical degrees to 180 electrical degree conduction.

4.2.4 Sinusoidal Current Excitation

Sinusoidal current excitation is an advanced method of driving a sinusoidally
varying back emf producing motor. By driving a motor with sinusoidally weighted
PWM phase current waveforms, less frequency harmonics are present in the phase
current waveform, thus an immediate reduction in losses occurs. As a result, larger
torque is produced for the same RMS current. Sinusoidally driven motors also
experience reduced torque ripple, the principal reason being that sinusoidal machines do
not experience the abrupt phase to phase current commutations that characterize the
trapezoidal machine’s excitation waveforms.

When controlling a motor using sinusoidal excitation, the input current
command 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 -

4. Theory

The 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 input
torque. In an interior PM motor, flux weakening can be used at high speeds. Flux
weakening is the process of increasing Id to oppose the magnet flux. This results in an
extended driving range at high speeds. In a surface magnet motor, flux weakening is
not feasible, as it would not have any effect on weakening the magnetic flux, and would
only 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 voltage
controlled 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 depicted
in 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 the
p-type region, however when a potential voltage is applied to the gate terminal, current
is allowed to flow freely between drain and source. A MOSFET’s characteristic
waveforms at turn-on and turn-off is shown in Figures 16 and 17 respectively.

- 38 -

4. Theory

Figure 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-source
capacitance. This capacitor has to be charged and discharged in one switch on and
switch 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, the
gate 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’s
original trajectory due to :
a) Series source inductance develops a voltage due to the rising drain current and
causes the gate-source voltage to decrease, and
b) The decreasing drain-source voltage is reflected across the drain-gate
capacitance. A discharge current flows through this capacitor causing an increase in the
capacitive load as seen by the driver. The voltage across the source impedance
increases thus causes a retardation of gate-source voltage.
3) Time 2-Time 3 : Drain current increases further due to the reverse recovery of
the free-wheeling diode.
4) Time 3 : The free-wheeling diode starts to support the drain-source voltage and
the rate of fall of the drain-source voltage is mostly dependent on the Miller effect. It is
at this point that the MOSFET has a maximum power loss due to a large current passing
through the device and a large voltage present across the device’s terminals. Due to the
falling drain-source voltage, the drain current settles out to the current determined by
the load and this causes the gate-source voltage to drop.

- 39 -

4. Theory

5) Time 4 : The MOSFET is completely turned on and the gate-source voltage rises
rapidly 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. The
drain-source voltage increases at a rate governed by the miller effect and the gate-source
voltage is kept at a constant level which reflects the drain current.
2) The MOSFET experiences a maximum power loss again at approximately time
2, where both a large drain current and drain-source voltage are present. When the rise
of drain-source voltage is complete, both drain current and gate-source voltage decrease
until drain current reaches it’s minimum value at time 3.
3) Gate-source voltage decreases past the threshold voltage to zero between time 3
and 4. The device is fully turned off at time 4.

There are two main power losses to consider when looking at the total power
loss 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 changing
conduction 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 the
MOSFET 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 -

4. Theory

4.4 Heatsink Considerations

It is extremely important that the electronic systems run as efficiently as
possible. This is particularly relevant for the high power systems, as usually a drop in
efficiency of only a couple of percent typically relates to an increased power
consumption of tens of Watts. A potentially large power sinking device is the MOSFET
H-bridge. The MOSFET has losses as described previously, and these losses are in
most cases directly formed into heating the device’s junction. As the MOSFET is made
to switch faster, the switching losses become the most significant form of heat
generation. There is also heat caused by increased conduction losses at higher output
powers. As the junction temperature of a MOSFET increases, Rds increases, Ids
decreases, however the overall loss will increase.

To this end, to keep the losses to a minimum, a heat-sinking system has to be
made. The package of the MOSFET is designed especially to conduct heat away from
the junction and to the ambient atmosphere surrounding the device. A simple heatsink
design 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 -

4. Theory

Vmax = 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 − 70
RthJA = = = 18.82 degrees C/W.
Pon 4.25
RthSAmax = 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 60
If 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 5.31
The 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 -

5. HARDWARE DESIGN STAGE

This section details the process in designing the hardware of the motor
controller. A similar device should be able to be constructed based on the information
given here. The motor controller consists of two PCB boards: a high voltage board and
a 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 voltage
components. It was decided to place all such components on the one PCB so that
potentially fatal high voltage kept confined to the one board, and didn’t have to be
routed 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 of
auxiliary circuits also placed on the board. Most of them have something to do with the
high power section, therefore it was convenient to include these circuits on the same
board as the other components. These include bus voltage measurement, temperature

- 43 -

5. Hardware Design Stage

sensing of the MOSFET heatsink and two phase current sensing circuits. These circuits
are described in more detail in section 5.1.2. The PCB contained positive and negative
battery terminals on one end of the PCB and three phase connections on the other end.
The interface to the control board was through a 20 pin IDC connector. MOSFET drive
signals, power supply and sensor readings were sent through this connector, as
described in section 5.2.

5.1.1 Circuit Design

The most important issues to consider when designing the high power part of the
motor controller are, in order of priority, as follows:

1) Reduce Stray Inductance : Any conductor of finite length will possess some
form of inductance. Stray inductance, especially in a circuit switching large currents
(i.e. large di/dt), may slow down turn-off of the MOSFET’s and produce unwanted
oscillations. One method of reducing stray inductance, and in some cases stray
magnetic flux (i.e. EMI), is to minimize the effective loop in which current flows. In
other words, forward and returning current paths have to be as close to one another as
possible. The upper limit to reducing stray inductance is the use of bus bars : copper
strips, with a thin insulator sandwiched between them. The first design of the motor
controller inverter section used copper busbars with a thin insulate sheet between layers.
Power components were to bolt onto the copper, however it was later abandoned due to
difficulties in construction and insulating problems.

2) Reduce Power Loss : An envisaged source of power loss is to use PCB tracks.
A calculation waas carried out to determine the power loss in a PCB track (dimensions
4mm wide, 0.0034 mm thick and 80mm long), carrying 13.9 A of (nominal) current.
R = pL/A = 1.7x10-8x80/(0.0034x4) = 100u ohms.
Where p is the resistivity of copper = 1.7x10-8 ohm x metre.
This indicates that using a PCB with wide tracks should pose no problem with losses.

- 44 -

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