2001 Thesis Project 1 Gareth S Roberts
Design and
Implementation of a
Three-Phase Induction
Motor Control Scheme By
Gareth Stephen Roberts

Department of Information Technology and Electrical
Engineering,
the University of Queensland Submitted for the degree of
Bachelor of Electrical Engineering (Honours).
October 2001
2001 Thesis Project A2 Gareth S Roberts 34 Tolaga Street,
Westlake, QLD, 4074
October 17, 2001


a car with an electric engine has become of particular interest. Dr. Geoff Walker and his
PhD students are working on the creation of the University’s own electric car.

This thesis project is focused on deriving a control scheme to drive an induction machine
that could be applied to the electric car. Induction machines are universally used in
industry because of their robustness, reliability, low price and high efficiency (up to
80%). However, until recent times, it has been hard to control the torque of the induction
motor.

By using the “TMS320F243 DSP controller,” which is embedded in an existing hardware
design and a control scheme called “Field Orientated Control,” we can control the torque
of an induction machine with a high degree of accuracy. Hence, this thesis project
demonstrates how to apply Field Orientated Control with a DSP controller. To do this,
extensive MATLAB analysis was conducted in order to optimize the control system. The
complete physical system is expected to be working on the demonstration day.
2001 Thesis Project A4 Gareth S Roberts
Acknowledgements

I wish to thank the following people:

Dr. Geoff Walker, my supervisor, for taking the time through the whole course of the
year for offering clear and enthusiastic explanations. Dr. Walker was always able to
guide the thesis along the right path.

Mr. David Finn for lending me his motor controller board. David also provided the
required information needed to operate this board.

My family, for offering support through my University years; this year in particular.

My fellow occupants in the Power Electronics labs, for putting up with my company for

2.4. Field-Orientated Control……………………………… 7
2.5. MATLAB analysis……………………………………. 8

Chapter 3 – The Hardware Design………………………………. 9
3.1. The basic control format……………………………... 9
3.2. The existing motor controller………………………... 10
3.3. Current sensing module……………………………… 11
3.4. The speed sensor……………………………………... 12

2001 Thesis Project A6 Gareth S Roberts
Chapter 4 – The Induction Motor………………………………. 14
4.1. The fundamental operating principles for an
Induction Motor……………………………………... 14
4.2. The Electrical principles of an Induction Motor…….. 14
4.3. Torque/Speed generation for an Induction Motor…… 16

Chapter 5 – Field-Orientated Control (FOC)…………………… 19
5.1. An introduction…………………………………….. 19
5.2. Transformation between reference frames…………. 20
5.3. The PI controller……………………………………. 21
5.4. PWM – Pulse-Width Modulation…………………... 22
5.5. The Overall Design…………………………………. 24
5.6. Conclusions drawn from Chapter 5………………… 26

Chapter 6 – The MATLAB design…………………………… 27
6.1. MATLAB – An introduction……………………… 27
6.2. MATLAB simulation design……………………… 27
6.2.1 Field Orientated Control using SIMULINK………... 28
6.2.2 The Current Controller……………………………… 32
6.2.3 The Motor Model…………………………………… 32

Bibliography……………………………………………………… 66
APPENDIX A – The proposed software design
APPENDIX B – PWM test program
2001 Thesis Project A8 Gareth S Roberts
APPENDIX C – Encoder detection test program #1
APPENDIX D – Encoder detection test program #2
APPENDIX E – The schematics for the Motor Controller board
2001 Thesis Project A9 Gareth S Roberts
List of FiguresFigure 1.1 – The Honda Insight
Figure 1.2 – The parallel hybrid car

Figure 3.1 – The basic physical design
Figure 3.2 – The existing motor controller board
Figure 3.3 – The current sensing module

Figure 4.1 – The per phase representation of an Induction motor in steady state
Figure 4.2 – The torque/speed curve
Figure 4.3 – Field weakening

Figure 5.1 – The transformation of the stationary reference frame to the rotating
reference frame
Figure 5.2 – The PI controller
Figure 5.3 – Leg A of the full-bridge inverter
Figure 5.4 – PWM VSI schematic and waveforms
Figure 5.5 – The complete FO controller design in a block representation

Figure 6.1 – The Look-up Table

Figure 7.13 – The encoder detection infrastructure
Figure 7.14 – The flow-chart for Encoder detection

Figure 8.1 – Experimentally measured PWM waveforms on the CRO 2001 Thesis Project 1 Gareth S Roberts
1. Need/Basis for the thesis project

1.1. Project Specification

To design a control scheme for a three-phase induction motor drive. This induction
motor drive is proposed to be incorporated into a “hybrid car” or an “electric car”.

1.2. Available resources

•
Motor controller board. This was constructed by 1999 thesis student, Mr.
David Finn and was designed to control a brush-less DC motor. However, by
constructing a feedback loop that can detect the outputs of an induction motor, we
can use this motor controller to control an induction machine.

research to date [8].
Figure 1.1. The Honda Insight [14] Figure 1.2. The internal infrastructure of a parallel hybrid car
[14]A hybrid car was released commercially this year. It combines two or more sources of
power; the gasoline-electric hybrid car, for instance, does just this. The electric engine
2001 Thesis Project A3 Gareth S Roberts
boosts acceleration and reduces demand on the petrol engine, saving fuel and improving
performance in the process [14]. While cruising, power comes solely from the petrol
engine. When the vehicle is coasting downhill, or during deceleration and braking, the
electric motor recharges a nickel metal hydride battery pack. During periods when the
vehicle is stationary, the engine automatically shuts down to save fuel, and then starts up
again when the throttle is pressed. The Honda “Insight” for instance consumes less than
half the fuel of a conventional small car and harmful exhaust emissions are lowered by a
significant 90% [14]. This model features a 10kW electric motor that delivers power to
the front wheels via a five speed manual gearbox. 1.4 Why do we use an Induction motor?

For this application, the only external input for the electric motor applied by the user is
the accelerator; which is essentially a variable torque input. There are two existing
options for an electric motor: the “Direct current (DC)” type or the “Induction” type.
Induction motors are universally used in industry because of their high robustness,
reliability, low price and high efficiency (up to 80% [15]). However, the brush-less DC
motor has been, traditionally, the more attractive option for variable torque control. This


2001 Thesis Project A5 Gareth S Roberts
2 The literature review

This section of the thesis report reviews the focal sources of information that were
required to compile this thesis project.

2.1. The Hybrid car concept

Honda and Toyota only released the Hybrid car this year. Knowledge of its operational
principles is not commonly known. The web-site, “How Stuff Works”, is an educational
site that is written by Mr. Marshall Brain. It provides a basic introduction onto how a
hybrid car works. Within this article, the definition of the hybrid car and its potential
advantages are stated. The concept of the parallel hybrid car is presented. This is a car
design that simultaneously utilizes both the combustion engine and the electric engine to
turn the wheels. Mr. Brain also offers an explanation on how commercially released
hybrid cars (the Toyota “Prius” and the Honda “Insight”) work.

2.2 Induction Motor Theory and Practice

Wildi [1] is an excellent source of information on the theory behind the operation of an

Operation of the motor controller board requires knowledge of how the power electronic
aspects work. Mohan Undeland and Robbins [2] provides chapters of information on
power electronics; Dr. Geoff Walker utilizes this text to teach his Power Electronics
subject. In chapter two, all the current power-switching devices are presented. These
are: the Diode (the various types of diodes are presented and compared), the Bipolar
Junction Transistor (BJT), the Metal Oxide Field-Effect Transistor (MOSFET) and the
Insulated Gate Bipolar Transistor (IGBT). In this chapter, the requirements of a
switching device are also stated. The general desired characteristics of a power switching
device is to have a high blocking voltage in the reverse direction, to have minimal
switching losses (this is related to fast switching ability) and the power device is required
handle a sufficient amount of average forward current. It is found that the MOSFET
provides the minimal switching losses and is, hence, well suited for voltage switching
purposes.

2001 Thesis Project A7 Gareth S Roberts
In chapter eight, the basic switching topologies are outlined. An inverter is an electronic
configuration that transforms a DC signal into an AC signal in a controlled manner. This
is very relevant for this thesis project as we have available a DC supply and the induction
motor requires an AC supply that needs to be controlled to a certain degree of accuracy.
It compares how each topology generates harmonics. It discusses utilization of the
supply voltage. All voltage-switching designs require a modulation chip to generate
Pulse Width Modulated (PWM) signals that are applied to the gate of the power
switching devices. There are two types of PWM outlined: sinusoidal and square-wave.
While square-wave switching utilizes the supply voltage better, the harmonic content of
the output waveform is too high to really be considered an effective solution. Therefore,
sinusoidal PWM is the best option based on the literature provided in this text. Later in
the chapter, they talk about “dead-time”, which is a time delay that needs to be
introduced to the square-wave to avoid commutation of the power switching devices.

In essence, we are endeavoring to design a controller that can vary the torque induced in
the rotor of the motor. To do this, the induction motor controller will be configured in
the following format:

Figure 3.1. The basic physical design [16]

The user applies an input signal (e.g., the throttle of the car) that will be fed into the
command generator of the DSP controller. The DSP controller will manipulate this
control signal to produce signals that the induction motor can operate off. These signals
will be converted to PWM (Pulse-Width Modulated) signals so that the Full-bridge
MOSFET (Metal-Oxide Silicon Field Effect Transistor) inverter on the motor controller
can amplify these signals to substantial voltage levels. It is anticipated that the amplified
PWM signals will then induce a torque in the rotor of the induction motor that is
proportional to the magnitude of the input signal applied by the user. The design will
utilize an encoder that sends down pulses that can be manipulated to calculate the speed
and position of the rotor. The encoder pulses and the measured currents drawn by the
induction motor form the feedback portion of the design. Hence, the major aim of this
thesis project is to develop a control strategy for the DSP controller that controls the
torque production within the induction machine. However, there is no great emphasis
placed on precise torque control.

2001 Thesis Project A10 Gareth S Roberts

3.2. The existing motor controller

David Finn’s motor controller board can be seen in figure 3.2. There are two main
features that are of particular relevance to this thesis project:

A. TMS320F243 DSP controller: provides several key functions of different nature,

phase sinusoidal signals. These are then converted to 6 PWM signals in the DSP, one
for each MOSFET on the full-bridge inverter. Because the PWM signals are square
waves of varying duty-ratio, we can use the MOSFETs to amplify the PWM
waveform to significant voltage levels that the induction machine can operate off.
PWM generation through the software design is discussed in Chapters 5 and 7.

3.3. Current sensing module

The implemented current-sensing module consists of two current transducers. Because
the stator windings of the motor are a three-phase wye connection, we can use the
following relation to find the other unknown current magnitude:
0 = I
a
+ I
b
+ I
c
(3.1)
Where I
a
, I
b
and I
c
are the three-phase stator currents.

2001 Thesis Project A12 Gareth S Roberts
However, application of an effective current sensing module requires the designer to
consider that the DSP A/D converters operate within a voltage range of 0 to +5V. The
problem with this is that the currents sensed by the transducers are sinusoidally


Every time a pulse is detected, the existing integer number in the free-running counter of
the DSP controller is latched into a register that the software design can read (T2CNT).
Therefore, if the software design compares a “new count”(the current value latched in the
T2CNT register) to an “old count” (the previous value), the following formula can be
applied to calculate the speed:
Speed = Clock rate ÷ (encoder rate × (“new count” – “old count”)) (3.2)
[(revolutions per second) = (counts per second) × (revolutions per pulse) × (pulse per count)]

If the induction machine was rotating constantly at the rated speed, the count difference
between consecutive encoder pulses will be (the clock speed is 20 MHz):
Count difference = 20 MHz (counts per second) ÷ (50 × 10
3
pulses per second)
= 400 counts per pulse.
If we now apply formula (3.2):
Speed = 20 MHz / (2000 × 400) = 25 rps

Also, the detection of a pulse means that the rotor has progressed by:
(360
0
per revolution) ÷ (2000 pulses per revolution) = 0.18
0
per pulse.

Considering the rate that pulses are detected, the resultant resolution of the positional
angle of the rotor seems to be more than adequate for torque control for this project. If
the resolution of the rotor angle was not satisfactory, the software design would have to
interpolate between the pulses. This is discussed in the software section (Chapter 7).



4.2 The Electrical principles of an Induction motor

The induction machine is an electrical device. The electrical properties that are of
particular interest to this thesis project are the stator and rotor’s resistance and
inductance, as well as the magnetizing inductance. During steady state, the induction
motor can be modeled in a per phase representation seen in figure 4.1.
2001 Thesis Project A15 Gareth S Roberts Figure 4.1. The per phase representation of an induction motor in steady state [17]

These parameters are important for the control strategies that will be presented in chapter
5. The induction machine used in this thesis project has the following electrical
parameters:
E = rated phase voltage, 127 V
rms

P = power rating, 500W and 0.67hp
p = Pole pairs, 2
r
s
= stator resistance, 4.495 Ω
r
r
’ = rotor resistance, 5.365 Ω
x
s
= stator inductance, 16 mH
x