Design, Modeling and Simulation of An PDF [PDF]

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SAE TECHNICAL PAPER SERIES

1999-01-1149

Design, Modeling and Simulation of an Electric Vehicle System Iqbal Husain and Mohammad S. Islam The University of Akron

Reprinted From: Advances in Electric Vehicle Technology (SP-1417)

International Congress and Exposition Detroit, Michigan March 1-4, 1999 400 Commonwealth Drive, Warrendale, PA 15096-0001 U.S.A.

Tel: (724) 776-4841 Fax: (724) 776-5760

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1999-01-1149

Design, Modeling and Simulation of an Electric Vehicle System Iqbal Husain and Mohammad S. Islam The University of Akron Copyright © 1999 Society of Automotive Engineers, Inc.

introduction of gasoline powered vehicles. In 1900, 4200 automobiles were sold out of which 40% were steam powered, 38% were electric powered and 22% were gasoline powered. However, the invention of starter motor, improvements in mass production technology of gaspowered vehicles and inconvenience in battery charging led to the disappearance of electric vehicles in the early 1900's. However, environmental issues and reducing the dependence on oil led to the resurgence of interest in EV's in the 1960's. The demand for investing in R&D for EV's is ever increasing in 1990's with the major automobile manufacturers embarking in plans for introducing their own electric or hybrid/electric vehicles.

ABSTRACT The switched reluctance motor is a strong candidate for electric/hybrid vehicle applications primarily because of its high power density, good efficiency and high-speed capability. The motor drive for an EV is desired to have a wide speed range capability to eliminate the gears. A 4:1 speed range of the motor in the constant power region results in good performance of the system. The paper will report the design of an electric vehicle system using a switched reluctance motor drive. A system level simulation tool for electric vehicles using the MatlabSimulink platform will also be presented. The paper will discuss the analytical and simulation models for each of the key system components of an EV, which are the motor, controller, battery and the drivetrain.

This paper presents a modeling and simulation tool based on Matlab/Simulink for electric vehicles with emphasis on the propulsion unit. The efficiency of the motor and controller unit of an electric vehicle can make a significant contribution in enhancing the overall efficiency of electric vehicles over ICE vehicles. A switched reluctance machine (SRM) has been selected as the propulsion unit in this project, because of its high power density and excellent motor-load torque-speed matching characteristics. The advantages of SRMs over other machines will be further discussed in the motor design section. The details of motor, controller and electric vehicle modeling will be presented in the modeling and simulation sections. The developed simulation program will serve as an evaluation tool for SRM based EV designs. Different designs can be analyzed fairly quickly using the methods presented in this paper.

INTRODUCTION Environmental and economical issues are the major driving force in developing electric vehicles for urban transportation. The exhaust emissions of the conventional internal combustion engine (ICE) vehicles is the major source of urban pollution that causes the greenhouse effect leading to global warming [1]. The pollution problem is only going to worsen with the increasing number of automobiles being introduced on the road every year. Statistics show that the number of automobiles in our planet will increase from half a billion to more than a billion from 1992 to year 2000. There is also an economic factor that is inherent in the poor energy conversion efficiency of the combustion engines. Although, when efficiency is evaluated on the basis of conversion from crude oil to tractive effort at the wheels, the numbers for electric vehicles are not significantly higher, it does make a difference. Moreover, efficient power generation at localized plants together with very high motor and controller efficiency and advancements in battery or power source technology within the vehicle, the electric vehicles show immense promise in further overall efficiency improvement. Electric vehicle is the only alternative for a clean, efficient, and environmentally friendly urban transportation system.

The objective of this paper is to present the fundamentals of electric vehicles so that one can easily understand the design criterion, procedure and principles of operation. This will serve the education aspects on electric vehicle, which is very important in arousing the interest among people for developing and using electric vehicles.

EV SYSTEM AND LOAD A. COMPONENTS OF AN EV – The major components of an electric vehicle system are motor, controller, power source, charger and drivetrain. The block diagram of an electric vehicle system is shown in Fig. 1. Lead/acid batteries are typically used as the power source for electric vehicles, although other promising battery technologies

The electric vehicles paved their way into the world as early as in the middle of the 19th century, even before the

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B. ROAD LOAD AND TRACTIVE FORCE – The road load consists of the gravitational force, rolling resistance of the tires and the aerodynamic drag force.

are emerging in the horizon. A model for lead/acid batteries has been developed and added in the simulation program. A microprocessor-based controller is used for vehicle and motor control. Control complications are to be minimized as much as possible to avoid disastrous failures at this early stage of EV development. Insulated gate bipolar transistors with ratings as high as 600A and 1200V are the power devices of choice for the power electronic converter.

FRL = FgxT + Froll + FAD

where xT is the tangential direction along the roadway. The tractive force FTR provided by the main propulsion unit of the vehicle must overcome the road load FRL to propel the vehicle forward at a desired velocity. The forces acting on the vehicle are shown in Fig. 3.

User inputs

T

T

Accel. Brake pedal pedal

Batteries (Source)

Electronic Controller Software Classical Controls Modern Controls Fuzzy Logic Neural Network

Hardware Microcontroller Microprocessor Digital Signal Processor

Devices SCR GTO BJT MOSFET IGBT MCT

Froad

Battery Charger Accel

Power Electronics

Regen

Topology DC/DC DC/AC PWM Resonant Softswitched

FTR xT

yT

FAD

Froll

E

Motor

Design Finite Elements CAD Materials Packaging Mechanical h l

(1)

Roadway

FgxT Types DC Induction PM Brushless SRM Synch. Reluctanc

mg

FgyT

Figure 3. Forces acting on a vehicle.

Instantaneous center of rotation

Figure 1. Major electrical components of an electric vehicle system.

mg Wheel

Motor

Transmission

Froll

Differential

-mg Figure 4. Rolling resistance force of wheels.

Wheel Figure 2. Drivetrain of an electric vehicle.

The gravitational force depends on the slope of the roadway. The force is positive when climbing a grade and is negative when descending a downgrade slope.

The majority of electric vehicles developed so far are based on dc machines, induction machines or permanent magnet machines. The disadvantages of dc machines turned EV developers to look into various types of ac machines. The maintenance-free low-cost induction machines became an attractive alternative to many developers. However, high-speed operation of induction machines is only possible with a penalty in size and weight. The power density of permanent magnet machines together with the high cost of permanent magnets make these machines less attractive for EV applications. The high power density and potentially low production cost of SRMs make it ideally suitable for EV applications. However, the controller and motor must be designed appropriately to eliminate or minimize the disadvantages of SRMs, which include torque-ripple and acoustic noise.

FgxT = mg sin β

(2)

where m is the total mass of the vehicle, g is the gravitational acceleration constant and β is the grade angle with respect to the horizon. The rolling resistance is produced by the flattening of the tire at the contact surface of the roadway. The road reaction force Froad now balances the vertical component at the instantaneous center of rotation on the wheel. The rolling resistance force is the component force of the vertical load mg, which is tangential to the roadway as shown in Fig. 4. The tractive force FTR must overcome tangential force Froll along with the gravitational force and the aerodynamic drag force. The rolling resistance can be minimized by keeping the tires as much inflated as possible. The ratio of the retarding force and the vertical load on the wheel is known as the coefficient of rolling resistance C0. The rolling resistance force is given by

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Froll

sgn[v ]mg (C + C v xT 2 ) if v ≠ 0 xT xT 0 1  = ( FgxT + FTR ) if v xT = 0 and  sgn[ FgxT + FTR ](C 0 mg ) if v xT = 0 and

FgxT + FTR ≤ C 0 mg

A higher motor speed relative to the vehicle speed means a higher gear ratio, larger size and higher cost. Higher motor speed is also desired in order to increase the power density of the motor. Therefore, a compromise is necessary between the maximum motor speed and the gear ratio to optimize the cost. Planetary gears are typically used for electric vehicles with the gear ratio rarely exceeding 10.

(3)

FgxT + FTR > C 0 mg

Typically 0.004