Abstract
The electric vehicle relies on electric motors for propulsion. In this report, the case was made for the induction motor as a suitable candidate for this tractive application. Being able to control the electric motor invariably implies efficient control of the vehicle. This work reports the six-step operation of the constant volts/hertz (V/f) control technique for voltage fed induction motor drives. Mathematical equations supporting the principle is discussed and the drive is modelled and implemented in the open loop with results. The line and phase voltage outputs of the laboratory-implemented v/f drive is presented. The modelled results show the behaviour of the v/f drive under varying load conditions. The reference speed is set at 1800rpm while the actual speed only follows the reference speed until torque is applied at 0, 2, 3 and 4 seconds with torque values 0, –11, 11 and 0 Nm respectively. At 2 seconds, the actual speed changes (increases) from the reference speed slightly and reduces slightly when the positive torque is applied, but the speed does not totally match the reference speed until the torque is completely withdrawn. The Direct Torque Control (DTC) scheme is analysed mathematically and the principle of Variable Structure Systems (VSS) theory is applied to the DTC for robustness and tolerance to disturbances. DTC simulation results are presented, the system is run at steady state conditions, at time t = 0.4s, a load-torque disturbance causes it to reduce to one-half of its initial value. The objective of this drive scheme is to keep the load speed constant at its initial value. This causes a sharp increase in speed but it returns to the set reference speed in about 0.4s. This shows the control efficiency and robustness of the DTC scheme as compared with the v/f control method.
Chapter 1
1 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 four- quadrant 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.
Position or speed feedback
Current or voltage feedback
Controller Converter Motor Mechanical system
1 2 3 4
5 Main power supply
Figure 1.1 Drive system elements [1]
ω
| 2 Braking 1 Forward driving Reverse Plugging or driving reverse braking 3 4 |
TL
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.
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
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.
Brake
Electric propulsion subsystem
| Electric motor Mechanical transmission |
Wheel
Accelerator
Vehicle
controller
Electronic power converter
Wheel
Energy management unit
Energy source
Power steering unit
Energy source subsystem
Energy recharging unit
| Auxiliary power supply |
Climate control unit
Auxiliary subsystem
Steering wheel
Figure 1.3 Conceptual illustration of general EV configuration [10]
1.1.1 EV Propulsion Systems
Mechanical link Electric link Control link
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.
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.
| Battery charger Energy storage |
User inputs
θ
Accel.
pedal
Brake
pedal
| Transmission & differential |
Wheel
Electric controller Power converter Electric motor
Software
Classical controls
Modern
controls
| Devices Topology Design IGBT Chopper Finite MOSFET Inverter element GTO PWM CAD MCT Resonant Thermal BJT Force Graphics |
Hardware
μ processor
μ controller
Digital signal processor
Type
DC IM SRM PMSM PMBM
Wheel
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
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:
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; 1����� = 1.34ℎ�.
����� (������) = ������(� − �) × �����(�𝑎�/�) (1.1)
Maximum motor power
Maximum engine torque
IC engine
Electric motor
Constant power region
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.
Power
Torque
0
Constant torque region Constant power region
| Torque Power Base speed Maximum speed |
Speed
Base speed Maximum speed
0
Speed
(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) High- speed 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;
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
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 Self- excited Separately- excited Inductio Synchronous PM Brushless Reluctance PM Hybrid Series Shunt Field Excited PM Excited Wound- rotor Squirrel Cage Wound- PM roto rotor r Reluctance |
n
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].
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.
Torque
Speed of rotation of magnetic field
Angular speed
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 torque- speed 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
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.
DC supply
i*as
λ*r
T*
Field orientation system
i*bs
i*cs
| Inverter ias ibs ics |
Motor
signals M
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.
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 Cost: Rare-earth magnets are very expensive and increases cost implications of the machine considerably. Safety: They are not easy handle due to mechanical friction losses Compactness: BLDC motors have the advantage of being small and light because very large forces that come into play when anything ferromagnetic gets close to them [22], also, in the case of a vehicle wreck rare-earth magnets [6] achieving high flux densities, thus high torque. is still excited by the magnets and high voltages could be present [6]. 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. Low noise: There is no noise associated 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 with the commutation because it is electronic and not mechanical, switching frequencies are high enough so that the harmonics are not audible. torque tends to block the rotor [6]. |
of the recent introduction of high density
and the wheel is spinning freely, the motor
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]:
| � � � � � |
� = � � + � ��� + � (1.2)
��
�� = �� ��� (1.3)
�� = ����� (1.4)
�𝜔 �
�� = �� + �
+ ��� (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
Vt Es
–
(a) Equivalent circuit of BLDC motor
Te Te
ls
Vt
Vt–rated
ωr0 = ωr
Vt/kE
RsIs
ωr0 = ωr
Vt/kE
(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
�� =
(𝑉� − � 𝐸 𝜔 � )� 𝑇
���
(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.
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
Load
Current
sensor
Position
sensor
Control commands
Controller
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
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.
| Index DC motor drives IM drives PM BLDC motor drives Efficiency 2 4 5 Weight 2 4 4.5 Cost 5 4 3 Total 9 12 12.5 |
Table 1.2 Comparison of four types of electric motor drives [11]
SRM drives
4.5
5
4
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.
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.
19
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 |
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.
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.
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.
This material content is developed to serve as a GUIDE for students to conduct academic research
SLIDING MODE DIRECT TORQUE CONTROL OF THREE PHASE INDUCTION MACHINE APPLICABLE IN ELECTRIC VEHICLES
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