Environmental awareness and motor companies

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Abstract

Environmental awareness has resulted in greater emphasis on developing more environmentally friendly and fuel-efficient cars, so the motor companies have turned into electric cars, as they are considered a vial able option towards achieving these goals. Electric cars are now becoming a reality therefore researchers andengineers are looking for new concepts ofEV's in order to improve the overall performance and reliability[1] . At this point transverse flux machines seem to be the best candidate for the electric car drive system.

This project will investigate, model, and simulate a TF machine, for a direct wheel Electric Vehicle. The machine will be modeled using MATLAB/SIMULINK Finite Element Package. The TF machine will be simulated in a real time test environment in order to investigate the different transients. Hopefully the motor performance will be relatively acceptable, in order to be judged fit for implementation in the electric formula car.

The purpose of this literature review is to go throw all relevant publications for TFM and Electric cars. This includes some peripheral literature. To achieve these goals this paper starts with an overview of the previous research.

At this stage there was a general research around permanent magnet machine arrangements using as main sources the Library of the University and the IEEE Portal.

2 Objectives And Specifications

There are several objectives for this project. These can be split into theoretical and practical objectives. (We are only going to deal with the theoretical ones. The practical part will be completed by the School of Mechanical and Systems engineering of Newcastle University.)

The theoretical objectives of this MSc Individual Project involve the following:

  • A research about TFM machines considering their advantages compared to other types of motor arrangements , parameters
  • General investigation (market & puplishments) regarding the impementation of TFMotors in Electric cars.
  • Simuling block model of the machine. The analytical design will be done using the MATALAB element package Developed by The Math Works. With the same program, a simulation will be run to get several results mainly for speed, torque and Power.

For the practical part of the project, the following main objectives will be involved:

  • Using the information collected from MATLAB, The machine will be implemented in the car after submitting the final drawings to the workshop.
  • Testing the motor compatibility with the other chassis components in a lab environment.
  • Testing the car in a track to record times and fix any problems that may occurred

3 Introduction

It has been documented that the increased application of electric vehicles will have positive impact in the environment and the economy. With the increasing interest in electric vehicles, research to determine the suitable machine for a given application is required. Customary candidate drives include Induction Machines, RM and Permanent Magnet Machines. Of particular interest is the Transverse Flux Machine (TFM), which often employs a toroidal, homopolar phase winding, a stator core formed by laminated C-cores and a rotor provided with heterpolar Permanent Magnets [2]. In this literature review, the TFM seems to provides the best power, torque density at low speed, and seems suitable for EV's applications.

4 Back Round - Related Research

A thorough research was done using several sources either from the University's Library or the Internet. From the Internet a lot of portals have been searched, but only in IEEEXplore relevant publications appeared.

Electric Vehicles

In recent years, increased concerns over the environmental impact of gasoline cars due to the green house effect associated with the ever-increasing CO2 emissions. Along with reduced consumer ability to pay for fuel for the gasoline cars, has brought about transformed interest in electric cars, which are perceived to be more environmentally friendly and cheaper to maintain and run, despite the high initial costs.

An electric Vehicle is an unconventional fuel automobile that uses electric motors and motor controllers for propulsion. Electric cars are commonly powered by on-board battery packs (Table.1). Other non-conventional on-board energy storage methods that are anticipated to come into use in the future include ultracapacitors, fuel cells (Table.1), and a spinning flywheel, which stores kinetic energy. Electric vehicles convert stored electric potential into mechanical energy. Electricity can be converted into mechanical energy at very high efficiencies. A quick analysis will show electric vehicles are significantly more efficient compared to hybrid EVs and Fuel EVs [3].Today electric vehicles are mainly used as small vehicles for short distance applications due to energy source boundaries.

The overall EV engineering philosophy essentially is the integration of automobile engineering and electrical engineering. Therefore, system integration and optimization

are primary factors in order to achieve good EV performance at a reasonable cost. Since the characteristics of electric propulsion are fundamentally different from those of conventional engine propulsion, a novel design approach is essential for EV engineering. Moreover, advanced energy sources and intelligent energy management are key factors to enable EVs to enter the competitive car market.

The design approach of modern EVs should include state-of-the-art technologies from automobile engineering, electrical and electronic engineering, and chemical engineering. It should implement unique designs that are particularly suitable for EVs and should develop unique manufacturing technology that is particularly suitable for EVs. Every effort should be made to optimize the energy utilization of EVs. [4].

Comparison Study OfDrive Motor For Electric Vehicle Application

In order to obtain a sufficient comparison for different types of machines is to study the main characteristics of each one by using analytical methods. The TFM will be taken as a reference machine and will be compared with IM, SRM and PMM machines under the same criteria.

Comparison of TFM with IM

In comparison between the two machines it is proved that as the number of pole pairs in IM increases, for constant rated speed the power remains constant. Whereas when TFM is operating at a constant rated speed the power increases when n is increased [5].The induced EMF has also the same behavior, by increasing the pole pairs in TFM, induced EMF is increasing whereas in IM machine remains constant.

TFM machine unlike IM is able to generate higher torques and can operate at low speed in that region. This characteristic place TFM as more suitable motor for electric car applications. The efficiency of the induction machine is also lower due to inherent rotor loss. [12

Comparison of TFM with SRM

In order to increase the torque density of the SRM, number of poles must be increased and armature current has to be maintained stable. However in practice, it is impossible to employ large number of poles because of the limited area available for current flow. In addition assembling multiple teeth per pole structure on the same shaft, will cause a winding overhang due to excessive length[6].Therefore a better solution would be the use of the transverse flux machine that demonstrates relatively higher torque density and benefits from the higher number of pole pairs without the downsides of the small current area of SRM . Another downside is that SRM is not yet widely produced as a standard motor, so its cost may be higher than the transverse flux motor.12

Comparison of TFM with PPM

The permanent magnet material usage of the TF machine is larger from an equivalent PM machine, this is and the main reason why TFM exhibits higher torque and power density if compared with a PM machine. The transverse flux structure can manage more magnet volume which provides good performance/cost ratio.TFM due to a large number of pole pairs, is capable to produce high torque with high efficiency at lower speeds [7]. Besides, the back emf can be an issue at high speed: the inverter must be able to withstand the maximum back emf generated by the stator winding due to the existence of a rotor pm field12

TFM Advantages: 1B

  • Each of the motor phases are entirely separated and do not therefore couple electromagnetically with other phases.
  • Motor has a 3-Dimentional flux path.
  • Due to the application of small pole pitches, high force density and high current loading is possiple.
  • Stator cores are double sided active contributing to torque production.
  • Fault tolerance machines.
  • Deliver high torque density compared to other conventional machines.
  • Small Volume and weight for high torque at low speeds.
  • Magnetically independent phases that simplifies the control method.

TFM Disadvantages: 4

  • Low Power factor.
  • Torque Ripples in Shape.
  • Normal Flux Fluctuation.

TF Machine In Electric Vehicles Considerations

Electric machine is one of the key parts in the drive line (Figure 1) of electric vehicles which eventually affects dynamical and fuel consumption. Induction motor, PM synchronous motor, brushless DC motor, and switched reluctant motor are all applied in EV. Traction motors for Electric Vehicles are different from motors applied in industry. Load features are constant torque in low speed and constant power in high speed. Characteristics such as maximum torque and maximum speed, high ratio of maximum speed to base speed is usually larger than 4, high efficiency power are demanded. Moreover, electric machine is usually installed on the chassis on which the environmental features may affect the machines operation. Therefore the characteristics of suitable traction motors for Electric Vehicles should be as follows, high power and torque density, generate high torque at low speed regions, high efficiency, waterproof and shockproof[7].2

The characteristics of the TFM will conclusively affect the vehicles performance so all the above must be taken under consideration. The control strategy for electric propelling will focus on the main characteristics such as extended speed range ability, torque density and power control.

Operating Points Simulation Of The Electric Machine

In order to design the speed and torque range of the electrical machine, especially the base speed, the simulation based on vehicle drive modes and control strategy can be developed on Simulink /ADVISOR, electric vehicle simulation software. Under the drive cycle, the working points of the electrical machine will be simulated. The base speed of the electrical machine is based on the simulation. The electrical machine drive system is designed by emphasizing the common working points. The design process of the car drive line will be similar to the design of the main transverse flux electrical machine.2

Electrical Design and Performance4

According to the reference 4 for each phase of the TFM the operational characteristics of the machine are described by the following equations:

Where:

V is the terminal voltage

i is the phase current

R is the phase resistance

is the coil flux linkage

is the rotor angle

is the angular frequency

p is the number of pole pairsand

T is the torque

Looking at Figure 2 it is obvious that all forms of TFM tend to have a low power factor at full load which show the terminal performance of one motor phase at 80% speed and 64% torque. With proper design of the converter, the low power factor need not be a handicap. Indeed, above 80% speed, the motor would be operated with a trapezoidal drive to allow ‘some reduction of converter rating.

The torque waveform depends on the current wave and upon the degree of saturation in the machine. With a sinusoidal current drive, the single phase torque ripple is close to a sinusoid, cancellation of the torque ripple can be achieved by using large number of motor phases.

5 TfmModeling

With the use of Matlab/Simulink a very powerful real time block model of the machine can be built. The motor, modeled for this simulation will be a TFM three phase dc disc motor fed by a PWM inverter connected to a rechargeable battery.

Besides its steady state characteristics it is essential to determine its transient performance and characteristics. For that purpose, a proper modeling of the motor and the whole drive is necessary. Some of the TFM parameters necessary for the MATLAB/SIMULINK model may be need to be determined experimentally.

As a result of the innovative progress in the field of power electronics Pulse Width inverter can be used to supply the machines with variable frequency and voltage, providing better controllability.[9]

The control Structure?

The main requirements of a direct drive are a constant torque and a constant speed at all operation points. To reach these requirements for the TFM an accurate torque and speed control on a basis of a fast current control is needed. Furthermore speed control can also be added to the block diagram [] a pi (current controller) [1b]

Mathematical model ??

According to reference [9] a basic motor simulink is presented. It should be noted that the mathematical model presented in [9] is similar to that of a TFM.

d- q equations of the rotor reference frame where Eddy current and hysteresis losses are negligible, saturation is not considered.

Where and

In above equations: vd and vq are the d, q axis voltages

id and iq are the d, q axis stator currents

Ld and Lq are the d, q axis inductances

λd and λq are the d, q axis stator flux linkages

while Rs and ωs are the stator phase resistance and inverter output

λm is the flux linkage due to rotor magnets linking the two stators

  • The electromagnetic torque equation.
  • The motor dynamics equation.

Where p is the number of pole pairs, TL is the load torque, B is the dumping coefficient, ωr is the rotor speed, and J is the inertia.

The inverter frequency is related to the rotor speed as follows:

  • state-space formfor non-linear model Where ωr, iq, and id are state variables
  • d and q variables are obtained from a,b,c through Parks transformation are shown below.
  • The a, b, c variables are obtained from the d, q variables trough the inverse of the Park transformation defined as follows:
  • Total input power to the machine.

In terms of the a, b, c variables is

while in d, q variables,

  • These equations are implemented as different blocks of the motor model, in the MATLAB/SIMULINK model shown (in Figure 3)

6 Conclusions

While searching at the library for books on machine simulations luck of findings was noticed. Contrary to library findings, the IEEE portal's search was more successful and a lot of relevant papers appeared from 1979 to nowadays.

In the relatively new technological area of electric cars a variation in approaches, modeling methods and machine selection was noticed. The EV engineering philosophy is the marriage of automotive engineering and electrical engineering, which includes the motor, power electronic converter, controllers, battery and energy management system. [4] In order to accomplish the required drive ability at minimum emission and maximum energy efficiency everything must best perform and co-operate harmoniously. Therefore a background related research is encencial so that more compatible components can be selected.

The key component of an EV is understandably the propulsion motor. By comparing the TFM with other motors it was clear that it has some distinguishing advantages over the other types of machines making it is the perfect candidate for EV applications.( TFM can deliver high torque at low speeds, will maintaining high efficiency).

Is essential that Motor drives potential should be investigated in the developing stage. An introductory basic model of the TFM for a Matlab/Simulink analysis has to be presented. Using Simulink block, model measured values and several of the motors parameters will also be shown in real time analysis.

Summary

Matlab/Simuling is a professional software package and there are some difficulties for an amateur user to get familiar with its use. By practicing it will hopefully be a powerful tool for this project. In order to precisely analyze the machine why have to choose the ideal mathematical model. Results, analysis and conclusions will be based on the Simulink model . Therefore the selection of the model is crucial as it will eventually define the course of the Project.

7 References

[1] Profumo, F. Zheng Zhang Tenconi, A. (5-10 Aug 1996) “ Axial flux machines drives: a new viable solution for electric cars”, Industrial Electronics, Control, and Instrumentation, 1996., Proceedings of the 1996 IEEE IECON 22nd International Conference on

[2]Harris, M.R., Pajooman, G.H., (1995). “Electric motors with heteropolar permanent magnets and homopolar windings: computational study of performance limits,”

Electrical Machines and Drives, Seventh Int. Conf. On Pages: 237-241

[3] http://en.wikipedia.org/wiki/Electric_car

[4]Chan, C.C (April 2007). “The State of the Art of Electric, Hybrid, and Fuel Cell Vehicles” Proceedings of the IEEE On page(s): 704-718

[5] Surong Huang,Jian Luo,Lipo T.A., (1997)

“Analysis and evaluation of the transverse flux circumferential current machine”

Conference Record of the IEEE Pages: 378-384 Vol.1

[6]Bahran Arim, (2000)“High density Torque Motor Structures”

Proc.Conference Record on Electric Machines, ICEM Pages: 1936-1940

[7] Kaiyuan Lu, Ritchie, E., (2001)“ Preliminary comparison study of drive motor for electric vehicle application”

Electrical Machines and Systems, 2001. ICEMS 2001. Proceedings of the Fifth International Conference on,Page(s):995 - 998 vol.2

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