TABLE OF CONTENTS
APPROVAL PAGE …………………………………………………………………………………………………………………………… II
CERTIFICATION ……………………………………………………………………………………………………………………………. III
DEDICATION …………………………………………………………………………………………………………………………………IV
ACKNOWLEDGEMENT …………………………………………………………………………………………………………………….V
ABSTRACT …………………………………………………………………………………………………………………………………….VI
CONTENT ……………………………………………………………………………………………………………………………………. VII
LIST OF TABLES ……………………………………………………………………………………………………………………………. IX
LIST OF FIGURES ……………………………………………………………………………………………………………………………. X
CHAPTER 1 ……………………………………………………………………………………………………………………………………. 1
1 INTRODUCTION …………………………………………………………………………………………………………………………….. 1
1.1 Introduction ……………………………………………………………………………………………………………………… 1
1.1.1 EV Propulsion Systems ………………………………………………………………………………………………………………. 4
1.1.2 Electrical Machines and Drives for EVs ……………………………………………………………………………………… 9
1.2 Motivation and Purpose ………………………………………………………………………………………………….. 20
1.3 Objectives of the Study ……………………………………………………………………………………………………. 20
1.4 Scope of the Study……………………………………………………………………………………………………………. 21
1.5 Organization of the Thesis………………………………………………………………………………………………. 21
CHAPTER 2 ………………………………………………………………………………………………………………………………….. 23
2 PRELIMINARY CONCEPTS ……………………………………………………………………………………………………………… 23
2.1 Modelling the EV ……………………………………………………………………………………………………………… 23
2.2 Induction Motor Control …………………………………………………………………………………………………. 29
2.2.1 V/f Control ……………………………………………………………………………………………………………………………….. 30
2.2.2 Encoderless Direct Torque Control ………………………………………………………………………………………….. 33
2.2.3 Variable Structure Systems and Control ………………………………………………………………………………….. 36
CHAPTER 3 ………………………………………………………………………………………………………………………………….. 45
3 THE INDUCTION MOTOR: MATHEMATICAL ANALYSIS ………………………………………………………………………… 45
3.1 Induction Motor Analysis using Space Vectors ………………………………………………………………… 45
3.1.1 Space Vectors ……………………………………………………………………………………………………………………………. 45
3.1.2 Flux Linkages of the Stator and Rotor ………………………………………………………………………………………. 48
3.1.3 Stator and Rotor Voltage Equations in terms of Space Vectors ………………………………………………… 49
3.2 Dynamic Analysis of Induction Motor ……………………………………………………………………………… 50
3.2.1 dq-Winding Representation …………………………………………………………………………………………………….. 50
3.2.2 Mathematical Relationships of the dq Windings (at an arbitrary speed ωd) …………………………… 52
3.2.3 Electromagnetic Torque …………………………………………………………………………………………………………… 57
3.2.4 d- and q-Axis Equivalent Circuits ……………………………………………………………………………………………… 58
3.3 Mathematical Analysis with respect to Sliding Mode Direct Torque Control …………………… 60
3.4 Classification of Induction Motor Control Methods …………………………………………………………. 65
CHAPTER 4 ………………………………………………………………………………………………………………………………….. 67
4 SCALAR BASED CONTROL SCHEMES ………………………………………………………………………………………………… 67
4.1 Scalar Control …………………………………………………………………………………………………………………. 67
4.2 Control of Voltage-Fed Inverters for Induction Machines ………………………………………………… 67
4.2.1 Square-Wave or Six-Step Operation …………………………………………………………………………………………. 68
4.2.2 Pulse Width Modulation …………………………………………………………………………………………………………… 71
4.3 Constant V/f Control ……………………………………………………………………………………………………….. 78
4.4 Design, Modelling and Implementation of V/f Control Scheme ……………………………………….. 80
viii
4.4.1 Proposed V/f Methodology ………………………………………………………………………………………………………. 81
4.4.2 Control Circuit Design and Implementation …………………………………………………………………………….. 82
CHAPTER 5 ………………………………………………………………………………………………………………………………….. 93
5 VARIABLE STRUCTURE DIRECT TORQUE CONTROL OF THE INDUCTION MOTOR ………………………………………. 93
5.1 Sliding Mode Direct Torque Control ………………………………………………………………………………… 93
5.2 Encoderless Direct Torque Control Design………………………………………………………………………. 93
5.2.1 Calculation of 𝝀𝒔, 𝝀𝒓, 𝑻𝒆𝒎, and 𝝎𝒎, in the Estimator Block of DTC …………………………………………. 93
5.3 Sliding Mode Controller Design ………………………………………………………………………………………. 96
5.3.1 Control Law Design by Ljapunov Method …………………………………………………………………………………. 99
CHAPTER 6 ………………………………………………………………………………………………………………………………… 102
6 RESULTS AND DISCUSSION …………………………………………………………………………………………………………..102
6.1 V/f Experimental Results ……………………………………………………………………………………………….102
6.1.1 Six-step Output Voltage ………………………………………………………………………………………………………….. 102
6.1.2 V/f MATLAB® Results …………………………………………………………………………………………………………….. 104
6.2 Direct Torque Control Simulation Results ……………………………………………………………………..104
CHAPTER 7 ………………………………………………………………………………………………………………………………… 108
7 CONCLUSION AND RECOMMENDATIONS…………………………………………………………………………………………..108
7.1 Conclusion ……………………………………………………………………………………………………………………..108
7.2 Recommendations …………………………………………………………………………………………………………109
REFERENCES ………………………………………………………………………………………………………………………………. 110
CHAPTER ONE
Introduction
1.1 Introduction
The block diagram in Figure 1.1 illustrates the elements of typical electric drive system.
The elements of 4 and 5 are usually known, 1, 2 and 3 must be chosen. Hence, the
mechanical system must be clearly specified. Also in the preliminary stages of the design,
it is usually discovered that the power supply may, in some way, be inadequate. Therefore,
an understanding of the mechanical system and the demands they make on the power
supply is required.
The mechanical system, from the perspective of the motor, is a torque that must be applied
to a shaft by the motor coupling. The relationship between this load torque and speed must
be defined. For steady state operation, this definition can be described in terms of the fourquadrant
speed-torque diagram (Figure 1.2), where ω is the speed of rotation of the
motor/driven shaft and TL is the load presented at the shaft of the mechanical system.
Controller Converter Motor
Mechanical
system
1 2 3 4
5
Position or speed feedback
Current or voltage feedback
Main power supply
Figure 1.1 Drive system elements [1]
2
ω
TL
Forward
driving
Braking
Plugging or
reverse braking
Reverse
driving
2 1
3 4
Figure 1.2 Four-quadrant speed-torque diagram
The first quadrant in Figure 1.2 above is the normal forward driving, in the second
quadrant, the system demands a negative torque to provide braking. This braking torque
may be produced by: (i) friction braking; a mechanical brake is coupled to the shaft and the
kinetic energy is dissipated as heat due to friction, (ii) eddy-current braking; the kinetic
energy is dissipated largely as eddy-current losses, (iii) dynamic braking; the motor acts
as a generator and the energy generated is dissipated as heat in resistors provided for the
purpose; (iv) Regenerative braking; the motor acts as a generator and transfers power
back to the electric supply system. In the third quadrant, the motor torque and direction of
rotation are reversed, similar to the first quadrant. The fourth quadrant may represent one
of two possible conditions, if the electrical conditions are the same as in the first-quadrant
driving, the mechanical system is driving the motor in a direction opposite to that which
would result from its own developed torque. This is another type of braking referred to as
plugging [1, 2]. Now, if the electrical conditions are changed to that in the third quadrant
(reverse driving), then the types of braking described in the second quadrant are also
obtainable.
3
For the purpose of this research, the mechanical system described here is the electric vehicle. Electric vehicles are highly adaptable and part of everyday society, especially in industrialized economies. Electric cars are found on mountain tops (railway trams, cable cars), at the bottom of the sea (submarines), in space exploration on the moon and even in neighbouring planet mars (Lunar Rover, Spirit and Opportunity Mars Rovers), in tall buildings (elevators), in cities (subways, light rail, buses, delivery vehicles), hauling heavy rail freight or moving rail passengers fast (Pennsylvania Railroad Washington to New York corridor [3], Tohoku Shinkansen Rail Line [4] ) and in sports (Golf Cars, Trolleys) and even for entertainment as found in amusement parks. These are all electric vehicles, they run on rails, shafts, tethers, paved roads and off–road terrains and some run directly on battery power, non-rechargeable and rechargeable alike while some others utilize power directly from the grid.
Electric vehicle technology is now in its third century and still advancing [5] and can be classified in two categories, 1) battery powered electric vehicle (BEV) and, 2) the Hybrid Electric Vehicle (HEV). This study is centred on battery electric vehicles, henceforth referred to as “Electric Vehicle (EV)”. A conceptual modern electric drive train is illustrated in Figure 1.3 [6, 7]. Efforts will be made to describe the details of each sub-system.
The drivetrain comprises of 3 subsystems: electric propulsion subsystem, energy source subsystem and auxiliary subsystem. The energy subsystem, made up of the energy management unit, energy source and energy recharging unit, is directly connected to the other two subsystems. It supplies energy for propulsion and provides the necessary power with different voltage levels for all auxiliaries. The energy management unit cooperates with the vehicle controller to control the regenerative braking, its energy recovery and monitors the energy source. The electric propulsion subsystem is made up of the vehicle controller, electronic power converter, electric motor and the transmission. Signals
4
received by the vehicle controller from the brake and accelerator pedals are processed and
used to generate control signals to the power converter, which functions to regulate power
flow between the electric motor and the energy source. Our interest lies in the electric
propulsion system.
Vehicle
controller
Electronic
power
converter
Electric
motor
Mechanical
transmission
Wheel
Wheel
Energy
management
unit
Energy
source
Energy
recharging
unit
Auxiliary
power
supply
Power
steering
unit
Climate
control
unit
Steering
wheel
Auxiliary subsystem
Energy source
subsystem
Electric propulsion subsystem
Brake
Accelerator
Mechanical link
Electric link
Control link
Figure 1.3 Conceptual illustration of general EV configuration [10]
1.1.1 EV Propulsion Systems
Electric propulsion systems are the heart of the EV [8, 9, 6, 10].The choice of electric
propulsion system depends on a number of factors such as energy source, motor type and
ratings, vehicle purpose and driver’s expectations such as acceleration, maximum speed,
climbing capability (gradeability), braking and range [6]. Vehicle purpose influences the
weight and volume depending on the vehicle type while energy sources relates to batteries,
fuel cells, ultra-capacitors, etc. Motor type is also critical and relies on vehicle purpose,
available energy and driver’s expectations.
5
The electric motor converts electrical energy into mechanical energy to propel the vehicle
or vice versa, to enable regenerative braking and/or to charge the on-board energy
storage. The power converter is used to supply the motor with appropriate voltage and
current, the controllers command the power converter by sending control signals to it
hence controlling the motor to produce proper torque and speed. The functional block
diagram of an electric propulsion system is illustrated in Figure 1.4.
Electric controller Power converter Electric motor
Wheel
Wheel
Transmission &
differential
Energy storage
Software
Classical
controls
Modern
controls
Hardware
μ processor
μ controller
Digital
signal
processor
Devices
IGBT
MOSFET
GTO
MCT
BJT
Topology
Chopper
Inverter
PWM
Resonant
Design
Finite
element
CAD
Thermal
Force
Graphics
Type
DC
IM
SRM
PMSM
PMBM
User inputs
θ
Accel.
pedal
Brake
pedal
IGBT – Insulated gate bipolar transistor
MOSFET – Metal-oxide semiconductor field-effect
transistor
GTO – Gate turn-off thyristor
MCT – MOS-controlled thyristor
BJT – Bipolar junction transistor
DC – DC motors
IM – Induction motors
SRM – Synchronous reluctance motor
PMSM – Permanent magnet synchronous motor
PMBM – Permanent magnet brushless motor
Battery
charger
Figure 1.4 Functional block diagram of a typical electric propulsion system [7]
There are different types of motors in industrial application that may also be used to propel
EVs. However some performance indexes have to be taken account of when motors are
applied in EVs such as efficiency, weight, cost, dynamic characteristics of EVs [11]. For EV
applications, motor requirements differ and usually require frequent starts and stops, high
rates of acceleration/deceleration; high torque and low-speed hill climbing; low torque
and high-speed cruising, the braking application calls for high torque at high speed, and
holding that torque to low speed [12] and a very wide speed range of operation. It follows
then, that there are characteristics that motors applied in EVs are expected to possess.
Motor ratings compared to the conventional internal combustion (IC) engine power
ratings and motor requirements for EVs are discussed as follows:
6
A. Desired output characteristics of motor drives in EVs:
The size or power output of electric motors and IC engines are typically described in horsepower (hp). The power that an electric motor can continuously deliver without overheating is its rated hp, which is typically a derated figure. For short periods of time, the motor can deliver two to four times the rated hp [3]. At starting, high power is available from an electric motor for acceleration, and the motor torque can be maximum under stall conditions, i.e., at zero speed. Motor type determines whether maximum torque is available at zero speed or not. On the contrary, an IC engine is rated at a specific r/min level for maximum torque and maximum hp. The IC engine maximum torque and hp ratings are typically derived under idealized laboratory conditions. In practical situations, it is impossible to achieve the rated hp; the maximum hp available from an IC engine is always smaller than the rated hp. The torque characteristics of motors are shown in Figure 1.5 [9], superimposed with torque characteristics of IC engines. The characteristics of specific motors and IC engines may differ from these generalized curves. For electric motors, a high torque is available at starting, which is the peak torque of the motor. The peak torque is much higher (typically twice) than that of the rated torque. The peak torque for electric motors in an EV application needs to be sustained for about 60 to 90 s [9].
The torque with which the motor can be expected to deliver continuously without over heating is referred to as rated torque [12] while the peak or rated power is obtained at base speed (when motor characteristics enter the constant power region from the constant torque region, once the voltage limit of the power supply is reached) [9]. The motor rated speed (rated) is at the end of the constant power region. The IC engine peak power and torque occur at the same speed. At this stage, it is helpful to review the power and torque relation, which is as presented in Equation 1.1. Power (watts) can be converted to hp by the relation; 1𝑤𝑎𝑡𝑡=1.34ℎ𝑝.
7
𝑃𝑜𝑤𝑒𝑟 (𝑤𝑎𝑡𝑡𝑠) = 𝑇𝑜𝑟𝑞𝑢𝑒(𝑁 − 𝑚) × 𝑆𝑝𝑒𝑒𝑑(𝑟𝑎𝑑/𝑠) (1.1)
Maximum motor power
Maximum engine torque
Electric motor
IC engine
Constant power region
Torque
ωb ωrated Speed (r/min)
Figure 1.5 Electric motor and IC engine torque characteristics [9]
Vehicle performance usually includes acceleration performance, evaluated by the time
used to accelerate the vehicle from zero speed to a given speed (starting acceleration,
modelled in Chapter 2), or from a low speed to a given high speed (passing ability) [11];
gradeability, evaluated by the maximum road grade that the vehicle can overcome at a
given speed, and the maximum speed that the vehicle can reach. Since in EVs, it is
dependent only on the traction motor to deliver torque to the driven wheels, the vehicle
performance is completely determined by the torque-speed or power-speed characteristic
of the traction motor. From figure 1.5, it can be observed that the EV motor drive is
expected to be capable of offering a high torque at low speed for starting and acceleration,
and a high power at high speed for cruising. At the same time, the speed range under
constant power is desired as wide as possible. For general electric motor drives in
industrial applications, their output performances are shown in Figure 1.6a [11] and
desired performance in EV application in Figure 1.6b. Under the normal mode of operation,
the electric motor drive can provide constant rated torque up to its base or rated speed. At
this speed, the motor reaches its rated power limit.
8
0
Torque, Power
Torque
Power
Base speed Maximum speed
Speed
Constant torque region Constant power region
Torque, Power
Base speed Maximum speed
Speed
0
Torque
Power
(a) (b)
Figure 1.6 Typical performances of electric motor drives (a) performance in industrial applications,
(b) desired performance in EV applications [11]
The operation beyond the base speed up to the maximum speed is limited to this constant
power region. The range of the constant power operation depends primarily on the
particular motor type and its control strategy.
B. EV motor requirements:
The important characteristics of a motor for an EV include flexible drive control, fault
tolerance, high efficiency, and low acoustic noise. The motor drive must be capable of
handling voltage fluctuations from the source. Another important requirement of the
electric motor is acceptable mass production costs, which is to be achieved through
technological advancement. The requirements of an EV motor, not necessarily in order of
importance, are itemized in the following [9, 11]: 1) Ruggedness; 2) High torque-to-inertia
ratio; large ratios results in “good” acceleration capabilities; 3) Peak torque capability of
about 200 to 300% of continuous torque rating; 4) High power-to-weight ratio; 5) Highspeed
operation, ease of control; 6) Low acoustic noise, low electromagnetic interference
(EMI), low maintenance, and low cost; 7) Wide speed range with constant-power region;
8) Fast torque response; 9) High efficiency over the wide speed range with constant torque
and constant power regions; 10) High efficiency for regenerative braking; 11) Downsizing,
weight reduction, and lower moment of inertia; 12) High reliability and robustness for
various vehicle operating conditions; 13) Fault tolerance;
9
1.1.2 Electrical Machines and Drives for EVs
Every electric vehicle has at least one electric machine, and some have several similar motors working in unison, sharing the load and operating under the same conditions of speed and shaft torque, depending on their drivetrain architecture [1, 13]. The motor responds to the drive control signals fed through the power converter and they both determine the behaviour and characteristics of the propulsion system, the power ratings of the power semiconductors and devices present in the power converter.
As earlier established, the electric machine delivers processed power or torque to the transaxle for propulsion. The machine also processes the power flow in the reverse direction during regeneration, when the vehicle is braking, converting mechanical energy from the wheels into electrical energy. The term “motor” is used for the electric machine when energy is converted from electrical to mechanical, and the term “generator” is used when power flow is in the opposite direction, with the machine converting mechanical energy into electrical energy [9].
Motor drives for EVs can be classified into two main groups, namely the commutator motors and commutatorless motors as illustrated in Figure 1.7 [6]. Commutator motors are traditionally DC machines, DC motor drives have been widely used in applications where dc voltages are available and variable-speed operation, good speed regulation, frequent starting, braking and reversing are required. The DC machines have two sets of windings, one in the rotor and the other in the stator, which establish the two fluxes; hence, the magnetomotive forces (mmfs) that interact with each other produce the torque. The orthogonality of the two mmfs, which is essential for maximum torque production, is maintained by a set of mechanical components called commutators and brushes [9]. They require brushes and commutators to feed current to their armatures making them less reliable and not suitable for maintenance-free operation and high speed [6], they are
10
usually not suitable choices for use in hazardous/explosive environments and are very
susceptible to wear and tear [14]. The associated limitation especially due to their
commutator and brushes make them a less attractive option for the EV. Compared to DC
machines, AC machines have none of these limitations, thus emphasis will be laid on AC
machines in EVs.
Motor Drives
Commutator Commutatorless
Induction Synchronous PM
Brushless
Switched
Reluctance
PM
Hybrid
Selfexcited
Separatelyexcited
Series Shunt Field
Excited
PM
Excited
Woundrotor
Squirrel
Cage
Woundrotor
PM rotor Reluctance
Figure 1.7 Classification of electric motor drives for EV applications [6]
1.1.2.1 AC Machines and Drives
In the DC motor, there exists friction between the brushes and commutator which will
cause both to gradually wear down, which is a limitation. Also the heat losses are generated
in the middle of the motor (in the rotor) [5]. AC motors are so designed that the heat is
generated on the outside (stator) enabling easy cooling, brushes and commutators are also
eliminated making it maintenance free and applicable in industrial and volatile
environments.
The induction motor (IM) is widely recognized as commutatorless or brushless motor type
for EVs [6, 7, 11]. They are reliable, rugged and maintenance free with highly mature and
proven technology. Conventional control such as the variable voltage variable frequency
control does not provide desired performance in EV applications, but with the advent of
power electronics and the microprocessor, vector control techniques such as the field
oriented control (FOC) have evolved which have been accepted to overcome the IM control
complexities. However, they still suffer from low efficiencies at light loads and limited
constant power operating range [6].
11
By replacing the field windings of a conventional synchronous motor with permanent magnets (PMs), the PM synchronous motor, also referred to as the PM brushless AC motor or sinusoidal-fed PM brushless motor, is obtained. These eliminates slip rings, brushes and field copper losses [15]. When PMs are mounted on the surface of the rotor, they behave as non-salient synchronous motors because the permeability of the magnets is similar to that of air. When PMs are buried inside the rotor magnetic circuit, the saliency causes an additional reluctance torque which facilitates a wider speed range at constant power operation. On the other hand, by neglecting the field windings or PMs, making use only of the rotor saliency, the synchronous reluctance motor (SRM) is obtained. These motors are simple, inexpensive but with relatively low output power. These motors will be introduced here.
A. Induction Motor
The IM is well recognized as the workhorse of industry [16], and most widely used motor [14, 17, 18]. Unlike the DC motor, the IMs derive their name from the way the rotor magnetic field is created. The IM rotor current is induced by the induction between the stator and rotor windings. These interaction produces torque, which is the useful mechanical output of the machine. The bars forming the conductors along the rotor axis linked at their ends. Sinusoidal stator phase currents fed into the stator coils create a magnetic field rotating at the speed of the stator frequency (ωs). The changing field induces a current in the cage conductors, which results in the creation of a second magnetic field around the rotor conductors. As a consequence of the forces created by the interaction of these two fields, the rotor experiences a torque and starts rotating in the direction of the stator field.
12
Torque
Angular speed
Speed of rotation of
magnetic field
Figure 1.8 Typical torque/speed curve for an induction motor [5]
As the rotor begins to speed up and approach the synchronous speed of the stator magnetic
field, the relative speed between the rotor and the stator flux decreases, decreasing the
induced voltage in the stator and reducing the energy converted to torque. This causes the
torque production to drop off, and the motor will reach a steady state at a point where the
load torque is matched with the motor torque. This point is an equilibrium reached
depending on the instantaneous loading of the motor. Figure 1.8 illustrates the torquespeed
graph for an IM. Control strategies include: 1) Constant volts/hertz control, a scalar
control method in which the flux is kept constant by keeping the ratio between the stator
voltage and frequency constant. This technique is explained later on in this work; 2) Field
Orientation Control (FOC): Depending on the EV design considerations and type the
performance of the volt/hertz scheme may not be satisfactory, this is because it is more
suitably applied to motors that operate with slow speed regulation. However, this
approach shows poor response to frequent and fast speed varying resulting in poor
operating efficiency due to poor power factor [6]. The concept of field orientation was
proposed by Hasse in 1969 and Blaschke in 1972 [17]. Generally speaking, the objective of
FOC is to make the IM emulate the separately excited dc machine to always produce
adjustable or maximum torque [6, 17]. It relies on the dynamic analysis of IMs in terms of
dq-windings (analysed in chapter 3). The general block diagram of a vector control system
for an IM is shown in Figure 1.9. A field orientation system produces reference signals i*as,
i*bs, and i*cs, of the stator currents, based on the input reference values, i*as and T*, of the
13
rotor flux and motor torque respectively and the signals corresponding to selected
variables of the motor. An inverter supplies the motor currents ias, ibs, and ics, such that their
waveforms follow the reference waveform, i*as, i*bs, and i*cs.
Other control methods such as the sensorless control techniques which decouple the
torque and flux, combined with the use of observers [19, 20] to achieve desired control
objectives, rotor position control by quadrature inversion (QI) technique [21] that
eliminates the decoupling problems and the need for torque controllers, are well
documented in literature.
Field
orientation
system
Inverter
M
ias ibs ics
DC supply
i*cs
i*bs
i*as
λ*r
T*
Motor
signals
Figure 1.9 General block diagram of a vector control system for an induction motor [6].
B. Permanent Magnet Brushless DC Motor (PM BLDC)
It was earlier explained that by using high-energy PMs as field excitation mechanisms, one
can achieve a drive with high power density, high speed and high operating frequency,
these advantage makes it quite attractive for EV applications. Pros and cons associated
with BLDC motors is shown in Table 1.1.
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Table 1.1 Advantages and disadvantages of BLDC motors
Advantages
Disadvantages
High efficiency: BLDC motors are very efficient, the PMs used for excitation consume no power, absence of mechanical brushes and commutators also imply low mechanical friction losses
Cost: Rare-earth magnets are very expensive and increases cost implications of the machine considerably.
Safety: They are not easy handle due to very large forces that come into play when anything ferromagnetic gets close to them [22], also, in the case of a vehicle wreck and the wheel is spinning freely, the motor is still excited by the magnets and high voltages could be present [6].
Compactness: BLDC motors have the advantage of being small and light because of the recent introduction of high density rare-earth magnets [6] achieving high flux densities, thus high torque.
Ease of control: The BLDC motor can be controlled easily as a DC motor, the control variables are constant throughout operation of the motor
Magnet demagnetization: Magnets can be demagnetized by large opposing mmfs and high temperatures.
Ease of cooling: No current circulation in the rotor, hence it does not heat up, the only heat generates in on the stator which is easier to cool.
High-speed capabilities: Surface-mounted PM motors cannot achieve high speeds because of the limited mechanical strength between the rotor yoke and PMs
Low maintenance, great longevity and reliability: The absence of commutators and brushes suppresses the need for maintenance and risk of failure associated with these elements. The longevity is only a function of winding insulation, bearings and magnet life-length.
Inverter failures in BLDC motor drives: Because of the PMs on the rotor, they pose a major risk in the case of short-circuit failures of the inverter. The rotating rotor is always energized, constantly inducing large EMFs in the short-circuited windings. Large current circulates in those windings and according large torque tends to block the rotor [6].
Low noise: There is no noise associated with the commutation because it is electronic and not mechanical, switching frequencies are high enough so that the harmonics are not audible.
Figure 1.10a shows the simplified equivalent circuit for the BLDC motor and their speed-torque curves in steady state with constant and variable voltage supplies, where Vt is the voltage of the power supply, Rs is the resistance of the winding, Ls is the leakage inductance (Ls = Ll – Lm, where Ll is the self-inductance of the winding and Lm the mutual inductance), and Es is the back EMF induced by the winding of the rotating motor. Based on the equivalent circuit of Figure 1.10a, the performance of the BLDC can be described by the following equations [6]:
15
𝑉𝑡 = 𝑅𝑠𝐼𝑠 + 𝐿𝑠
𝑑𝐼𝑠
𝑑𝑡
+ 𝐸𝑠 (1.2)
𝐸𝑠 = 𝑘𝐸𝜔𝑟 (1.3)
𝑇𝑒 = 𝑘𝑇 𝐼𝑠 (1.4)
𝑇𝑒 = 𝑇𝐿 + 𝐽
𝑑𝜔𝑟
𝑑𝑡
+ 𝐵𝜔𝑟 (1.5)
where kE is the back EMF constant associated with the PMs and rotor structure, ωr is the
angular velocity of rotor, kT is the torque constant, TL is the load torque and B is the viscous
resistance coefficient.
Rs Ls
ls
Es Vt
+
–
(a) Equivalent circuit of BLDC motor
ωr0 =
Vt/kE
ωr
Te
ωr0 =
Vt/kE
ωr
Vt-rated
Te
ls
Vt
RsIs
(b) speed-torque performance with
constant voltage
(c) speed-torque performance with
constant voltage
Figure 1.10 BLDC motor circuit and speed-torque curves [6]
For steady state operation, Equations 1.2 to 1.4 can simply be reduced to
𝑇𝑒 =
(𝑉𝑡−𝑘𝐸𝜔𝑟)𝑘𝑇
𝑅𝑠
(1.6)
The speed-torque performance with constant voltage is shown in Figure 1.10b, it is
noticeable that at starting, very high torque is produced resulting in high currents due to
low back EMF (this would damage the stator windings). The speed-torque performance
with variable voltage is shown in Figure 1.10c, here the winding current can be restricted
to its maximum by actively controlling the voltage; thus maximum torque can be produced.
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C. SRM Drives
The SRM drive is considered a competitor for EV applications, due to its low cost rugged
structure, reliable converter topology, higher efficiency over wide speed ranges and
control ease, it is an attractive candidate for variable speed motor drives. These drives are
suitable for EVs, HEV traction applications, aircraft starter/generator systems, door
actuators, etc. [23, 24]. The SRM has a simple rugged, low cost structure with no PM or
windings on the rotor. Unlike the IM and PM machines, the SRM is capable of high –speed
operation without the concern of mechanical failures that result from high-level
centrifugal force [6].
A conventional SRM drive, illustrated in Figure 1.11 consists of the SRM, power inverter,
sensors (voltage, current, position, etc.) and control circuitry. Through proper control, high
performance can be achieved in the SRM drive system.
Electric
energy
input
Power
converter
SRM
Current
sensor
Position
sensor
Controller
Control
commands
Load
SRM Drive
Figure 1.11 SRM drive system [6].
Control of SRM Drives: Excitation of the SRM phases needs to be properly synchronized
with the rotor position for effective control of speed, torque and torque pulsation. Shaft
position sensors are usually used to provide rotor position but this adds to the complexity
and cost of the system and tends to reduce the reliability of the drive system. Sensorless
control strategies can be employed to solve this problem and are reported in literature.
Some of them are the phase flux linkage-based method [25], phase inductance-based
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method [26], modulated signal injection methods [23, 27, 28], observer-based methods [29] among others. Most of these techniques are based on the fact that the magnetic status of the SRM is a function of the angular position of the rotor. As the rotor moves from the unaligned position towards the aligned position, the phase inductance increases from the minimum value to the maximum value [6]. Some sensorless techniques do not use the magnetic characteristics or voltage equations, rather they are based on observer theory or synchronous operation method similar to that applied to conventional AC machines.
1.1.2.2 A Case for the Induction Motor
There is no general consensus as to the type of electric machine best suited for vehicles, according to [13]. Table 1.2 compares four types of electric motor drives listing weight factors in efficiency, weight, and cost of four types of motor drives, where 5 marks represent the highest efficiency, lowest weight, and lowest cost, respectively.
Table 1.2 Comparison of four types of electric motor drives [11]
Index
DC motor drives
IM drives
PM BLDC motor drives
SRM drives
Efficiency
2
4
5
4.5
Weight
2
4
4.5
5
Cost
5
4
3
4
Total
9
12
12.5
13.5
The SRM seems to have the edge in EV applications, they have the simplest design but these machines are generally extremely noisy during operation, have higher torque pulsations and the design has not been advanced to the same extent as the PM or induction motor [30]. However, they are applied in heavy-duty vehicles and research is in progress to apply them in light-weight vehicles [13]. The induction and permanent magnet machines are the two types currently used in EVs and are expected to dominate the market. Table 2.1 highlighted the advantages and disadvantages of the PM. The major disadvantage lies in the soaring price and supply disruptions of magnets due to geopolitical issues [13, 30], hence they are not readily available and expensive.
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Table 1.3 easily helps in the choice of motor, it shows information for electric cars produced (for commercial purposes) and for prototype or experimental versions. The induction has been successfully applied in production vehicles as seen in Table 1.3. One cannot help but notice the dominance of the IM in prototype vehicles (Table 1.4) implying its low cost, mature technology and ability to meet vehicle propulsion requirements.
The motor is a part of the propulsion system required in EV, as discussed in earlier, it provides traction power to the wheels required for moving the vehicle. Taking advantage of electric motors in vehicles could help design more compact, lightweight efficient drivetrains.
Several types of electric machine technologies have been investigated for automotive propulsion. Most of the commercially available electric vehicles use either induction or PM machines for propulsion and are expected to dominate the market [13, 30]. In this research work, the induction motor (IM) was selected as choice of propulsion motor. As discussed above, it meets the propulsion requirements for electric vehicles. At present, IM drives are among the more mature technology in commutatorless motor drives. Compared with DC motor drives, the induction motor drive has additional advantages such as lightweight, small volume, low cost, high efficiency [6], capable of substantially higher speeds than DC motors [8]. Induction machines are a better option over PM machines when you consider the cost of magnetic components in the motor and supply disruptions of magnets due to some recent geopolitical issues [30]. Conventional induction motors use aluminium rotors, however, electrical conductivity of copper is 60% more than aluminium, using copper can also reduce the motor operating temperature by 5-320C, suggesting increased life-time [13, 30].
At this juncture, it should be noted that the term “induction machine” and “induction motor” are used interchangeably in this report.
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Table 1.3 Production electric cars [8]
Manufacturer
Citroen
Daihatsu
Ford
GM
GM
Honda
Nissan
Nissan
Nissan
Peugeot
Renault
Model name
AX/Saxo Electrique
Hijet EV
Th!nk City
EV1
EV1
EV Plus
Hypermini
Altra EV
106 Electric
Clio Electric
RAV4
Drive type
Seperately excited DC
PM synch
IM
IM
IM
PM synch
PM synch
PM synch
Separately excited DC
PM synch
PM synch
Max power O/P (kW)
20
27
102
102
49
24
62
20
22
50
Top speed (km/h)
91
100
90
129
129
129
100
120
90
95
125
Claimed max range (km)
80
100
85
95
130
190
115
190
150
80
200
Table 1.4 Prototype and experimental electric cars [8]
Manufacturer
BMW
Daimler Chrysler
Daimler Chrysler
Fiat
Ford
Ford
GM
GM
Lada
Mazda
Mazda
Peugeot
Toyota
Model name
BMW Electric
Zytek Smart EV
A-Class Electric
Seicento
Th!nk Neighbor
e-Ka
Impulse3
Impulse3
Rapan
Roadster-EV
Demio-EV
Ion
E-com
Drive type
PM Synch
BLDC
IM
IM
DC
IM
2×3 Ø IM
2×3 Ø IM
Separately Excited DC
AC
PM Synch
Separately excited DC
PM Synch
Max power O/P (kW)
45
30
50
30
5
65
45
42
30
20
19
Top speed (km/h)
130
97
130
100
40
130
120
120
90
130
100
Claimed max range (km)
155
160
200
90
48
150
80
150
100
180
150
100
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1.2 Motivation and Purpose
The vehicle is a very dynamic environment subjected to different conditions such as, varying road profiles/conditions, vibrations, high temperatures, exposure to the elements, among other factors, impressing stress on the materials and components that make up the system. The components in the electric vehicle will be subjected to these conditions and are expected to perform as desired. As mentioned earlier, the propulsion system is the heart of the electric vehicle, hence, adequate choice and high performance control of the electric motor (in this case the induction motor) invariably results in high performance and efficiency of the electric vehicle. Efforts are to find suitable control strategy that will not only optimize available energy but provide smooth, precise and efficient control of the IM in all four quadrants. Actually, the idea is to ensure the driver does not have to “learn” to drive the vehicle, it responds to normal driving actions such as accelerating, reversing and braking. It is for the designer to implement control methods for smooth operation of the IM.
The Variable Structure System (VSS) Sliding Mode(SM) technique will be employed in the Direct Torque Control (DTC) of encoder-less IM drive. This reduces the cost of the drive system and provides high performance control of the IM considering the continually varying load conditions in the system, interferences and disturbances. Objectives of this research is discussed next.
1.3 Objectives of the Study
Two control methodologies for the IM will be investigated: the constant volts/hertz (v/f) and the direct torque control (DTC) methods. The objective is to design a SM – based DTC, first, the design and implementation of v/f will be discussed and its model results presented and then compared with that of the DTC. The performances of these schemes will be evaluated, compared and conclusions will be drawn from the results.
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Thus, the main objective of this study, which is the contribution of this thesis, is to develop encoder-less operation of a Sliding Mode Direct Torque Controlled IM drive suitable for application in electric vehicles. In order to achieve this, the main objective is broken down to the following sub-objectives:
1) Analysis and modelling of the electric vehicle.
2) Mathematical analysis of the Induction Motor.
3) Design and implementation of the v/f control strategy for the Induction Motor.
4) Development of sliding mode direct torque control (SM-DTC) strategy and algorithm for the Induction Motor.
5) Performance comparisons and inference between the v/f and DTC.
1.4 Scope of the Study
The study of electric vehicles is vast, embodying different disciplines. This work centres on control strategies for propulsion of an electric vehicle. The performances of the v/f and DTC schemes will be evaluated and compared, but the core of the research is to establish the SM-DTC strategy for the motor providing tractive power to the wheels. Other components such as vehicle body shape, chassis, gearing, battery, charging units, tire specifications, nature of terrain will not be given preference or priority. They will be mentioned where necessary for completion.
1.5 Organization of the Thesis
This thesis is divided into 7 chapters, brief overview of these chapters are presented below:
Chapter 2: Preliminary concepts are explained. Basic concepts of electric vehicle, propulsion systems and vehicle modelling are presented, including various electric machines deployed in EV drives and a case is made for the induction motor. Also, control of induction machines, v/f, VSS and DTC is introduced.
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Chapter 3: The mathematical analysis of the induction motor in different coordinates and using space vectors is presented. These equations are a requirement for the development of the control techniques and observers for the SM-DTC scheme discussed in later chapters.
Chapter 4: A review of scalar control methods and schemes of IM is discussed. The v/f control strategy, operating principles, equations and system design are presented. Behaviour of the IM under this technique is also presented and compared with VSS control.
Chapter 5: DTC methodology is discussed and application of VSS theory is applied for improved performance. Mathematical equations are also developed.
Chapter 6: The simulation results of the encoder-less DTC scheme is discussed and compared to the experimental and simulated v/f control.
Chapter 7: A summary is presented, conclusions from the research work are drawn and scope of future work is outlined
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