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  • Journal of Communication and Computer 10 (2013) 178-185

    Power System Design for an Electric Car

    Louiza Sellami, Mathew McIntyre, Linda Yin, Christian Soncini and Amanda Lowery

    Electrical and Computer Engineering Department, US Naval Academy, Annapolis 21402, MD USA

    Received: August 21, 2012 / Accepted: August 22, 2012 / Published: February 28, 2013.

    Abstract: For many years now electrical engineering students at the US Naval Academy have been involved in renewal energy types

    of projects, including electric boats and cars, and have participated in various competitions across the US. Of particular interest to the

    authors is the electric car, since it involves various aspects of electrical engineering. As part of their senior capstone project, and

    under the authors guidance and supervision, four students designed, built, and tested the power generation and distribution system,

    the motor control system for an electric car. This was accomplished by converting an originally gas-powered car into a

    battery-powered car, whereby solar panels are used to recharge the batteries. Towards this end, a mix of off the shelf parts and

    components, and homebuilt circuits were used. The design considerations include selecting an appropriate control system, choosing

    suitable batteries, utilizing solar panels to recharge the batteries, designing the lighting system for the vehicle, and implementing

    several key safety features, including an emergency shut-off switch.

    Key words: Electric cars, batteries, solar panels, DC-DC conversion, DC motor control.

    1. Introduction

    As early as the 1830s, engineers and inventors have

    experimented with utilizing electric motors and

    battery systems as a means of powering vehicles [1-2].

    These vehicles are known to have very low acoustic

    noise as well as zero emissions. Though they have

    existed for a very long time and competed favorably

    with the highly inefficient combustion engine vehicles,

    the latter gained the upper hand because of the

    limitations of the batteries. These included inadequate

    capacity, long charging time, high replacement cost,

    reduced passenger and cargo space, as well as a short

    driving range [3].

    With the ever increasing cost of gasoline, the use of

    modern day road vehicles is constantly becoming an

    economic burden. With most gasoline powered

    vehicles, every mile driven is about 36 cents;

    comparatively the cost of an electric car is under 10

    cents per mile [3]. Nearly 85% of people in the United

    Corresponding author: Louiza Sellami, associate professor,

    research fields: circuits and systems, signal processing, biomedical engineering, power. E-mail: sellami@usna.edu.

    States own and operate motor vehicles. This number is

    projected to increase, along with the number of

    owners in other large countries. Because of this

    increase the world faces many growing economic and

    environmental problems. In the US alone, 18 million

    barrels of oil are consumed daily by driving cars,

    which emit 2.7 billion tons of carbon dioxide each

    year [1]. With dwindling supplies of fossil fuels and

    increasing environmental backlash from greenhouse

    gasses, the world needs to find an alternative to the

    conventional motor vehicle.

    In recent years there has been a resurgence of

    battery and solar powered electric vehicles which was

    brought about by the issues mentioned above, and the

    ensuing government regulations in terms of limiting

    carbon emissions. With this resurgence came great

    advancements in the research and development of

    deep cycle batteries, battery chargers, MOSFET motor

    controllers, DC-DC converters and sensors [4-5]. In

    turn, this led to the inclusion of concepts and designs

    from the green energy field in university curricula,

    including the US Naval Academy.

    This paper presents the design method and

  • Power System Design for an Electric Car

    179

    considerations for building a battery-powered budget

    car, whereby the batteries are recharged by solar

    panels, as well as the test results. Examples of design

    constraints include: The charging time for the batteries

    was limited to an acceptable ratio of charge versus

    discharge. The amount of money spent on the project

    had to be within the prescribed budget of under

    $5,000. The car must be able to attain a reasonable

    mileage and speed. Finally, the design must consider

    the safety of the user during the operation and

    charging of the car. Other design considerations

    include selecting an appropriate control system,

    choosing a suitable battery, utilizing solar cells to

    recharge the batteries, designing the lighting system of

    the vehicle, and implementing several safety features,

    including an emergency shut-off switch.

    2. Considerations and Requirements

    As part of the design, first the requirements (listed

    below) for the car had to be defined. Some of these

    requirements were dictated by the US Naval Academy,

    others by the budget constraints:

    (1) An ideal distance of 40 m per charge;

    (2) Under a budget of $5,000

    (3) Ability to power the lights of the car using the

    power system;

    (4) The max sustainable weight of the chassis is

    400lbs;

    (5) The control system must have an acceleration of

    5 mph/s and a speed limit of 25 m/h;

    (6) The solar cell charge time for 80% of the battery

    capacity of 8 h;

    (7) The wall outlet charge time for 80% of the

    battery capacity of 3 h;

    (8) An emergency shut off switch for safety.

    3. Design Overview

    An overview of the chassis structure is shown in

    Fig. 1. The motor used in the vehicle is the D&D

    ES-3336-48 VDC series wound single shaft, which is

    placed in the rear end of the chassis, as shown in

    Fig.2.The motor is powered by ten 12V lead acid

    batteries, which are placed above the motor.

    The batteries are connected to the other components

    of the chassis such as the speedometer, the

    temperature gauge, the controllers, and the lights. In

    order to charge the batteries, two solar panels are

    attached to the batteries at a separate station. The

    system that is used to control the vehicle is the All

    Trax programmable DC motor controller, which is

    shown in Fig. 3. The controller, powered by the

    batteries, has inputs for the accelerator pedal and the

    batteries and an output to the motor. Its function is to

    vary the motor power depending on how hard the

    Fig. 1 Overall view of the budget volt car.

    Fig. 2 Rear of the chassis showing the motor.

  • Power System Design for an Electric Car

    180

    Fig. 3 All Trax programmable DC motor controller.

    acceleration pedal is pressed, which is attached to a

    potentiometer.

    A block diagram for the whole system is shown in

    Fig. 4. The corresponding circuit schematic,

    illustrating the primary and secondary power loops, is

    shown in Fig. 5. The major components of the vehicle

    are the motor, the charging unit, the batteries, the solar

    cells and the controller, which are encased in the

    chassis structure.

    The lead acid battery was chosen as the most

    suitable battery for the vehicle for its light weight and

    affordable cost. It is also the safest option. Other

    design alternatives were nickel metal hydride and

    lithium ion batteries; however, both types presented

    problems for the safety of the car and were not within

    the budget. Lithium-Iron batteries were also not

    chosen because there is a greater chance they will

    become overcharged and start a fire.

    3.1 Chassis

    The original chassis of the car was used. All of the

    components are mounted on the chassis whose

    specifications are shown in Table 1.

    3.2 Motor

    As with any car, propulsion requires a prime mover,

    which in this case is the DC motor. The latter converts

    the electrical energy provided by the batteries into

    mechanical torque. The Motor D&D ES-33 36-48

    VDC series wound 7/8 single shaft, shown in Fig. 6,

    is used. It is 6.7 in diameter and 11.53 long, and

    weighs approximately 57l bs. It can output from 5 Hp

    at 36 V to 7.2 Hp at 48 V and can handle a current of

    135-140 A. However, for the design it was found that

    Fig. 4 Block diagram for the budget volt, illustrating the

    major systems, and the inputs and outputs for those

    systems.

    Fig. 5 Circuit schematic of the power system, distribution,

    and instrumentation.

  • Power System Design for an Electric Car

    181

    Table 1 Chassis specifications.

    Part Dimensions

    Front suspension Dual 12.8 ''shocks

    Rear suspension Dual 15.4'' shocks

    Front brakes Hydraulic disc

    Rear brakes Hydraulic disc

    Front tires 21 7-10 Rear tires 22 10-10

    Capacities / dimensions

    Weight capacity (lbs) 400

    Net weight (lbs) 639

    Gross weight (lbs) 739

    Size Full size

    Overall length 89.4

    Overall width 57.9''

    Overall height 59.5''

    Carton dimension (LWH/in) 90.9'' 45.7 31.1

    Seat height 17.7''

    Wheelbase 65.7''

    Ground clearance 5.9''

    Fuel capacity (gal) 1.26

    Safety / control

    Engine kill switch Yes

    Speed limiter Yes

    Foot brake Yes

    Horn Yes

    Headlights Yes

    Tail lights Yes

    Turn signal indicators Yes

    Fig. 6 D&D ES-33 36-48VDC motor.

    there was no need to go higher than 24 V as

    preliminary experimental results determined that at 16

    V the car moved at the target speed.

    3.3 DC Motor Controller

    A DC motor controller in an electric car can be

    compared to a carburetor or fuel injection system in a

    gas powered car. It is a device that controls the output

    power of the motor by controlling the input power

    drawn from the batteries. By stepping up the amount

    of voltage and current available to the motor it

    controls the speed of the motor and how quickly the

    motor can attain this power [6].

    The All Trax programmable DC motor controller

    used is a durable, high tech DC motor controller. It

    has the advantages of being waterproof,

    corrosion-proof, vibration proof, and is fully

    programmable. Performance characteristics that can

    be programmed include: max output current, throttle

    response profile, acceleration rate, plug-brake current,

    under/over voltage cutback, high pedal disable, and

    throttle input type. The 4855 model whose specs are

    shown in Table 2 was selected.

    3.4 Contactor

    Because a large amount of power is used by the

    electric motor and the accessories there is a need for

    a safe and reliable way of turning on the electrical

    system without either leaving the system running or

    reconnecting the batteries to the system. Unsafe

    conditions can occur as a result of a high

    current which can cause damage to the system,

    hence the use of a contactor in the design. The latter

    is an electromagnetic relay that, when a voltage of

    12 V is applied across the solenoid end, causes a

    large metal plate to shift up and create an

    instantaneous connection between the batteries and

    the rest of the electrical system. This acts as a dead

    mans switch as it stops the car by interrupting

    energy flow if anything goes wrong. The

    white Rogers 586 shown in Fig. 7 was selected,

    which is capable of carrying up to 200 Aa

    current that is below the current at which the car

    operates.

  • Power System Design for an Electric Car

    182

    Table 2 AXE controller specifications.

    AXE model 4855 Specs

    Battery voltage 24-48 V

    Current limit:

    2 Minute rating 650 A

    5 Minute rating 400 A

    60 Minute rating 250 A

    Voltage drop @ 100 A < 0.08 V

    Fig. 7 White Rodgers 586 on budget volt.

    3.5 Solar Panels and Solar Controller

    Solar panels and solar controller serve as the

    charging station for the car. The two available models

    are the MSX120 and the SX80V. Each solar panel

    model specifications are shown in Table 3. The

    Solarex SX-80 photovoltaic module is constructed of

    36 polycrystalline silicon solar cells. The cells can be

    utilized in configurations directly with DC loads or in

    an inverter-equipped system for AC loads [7]. The

    overall dimensions of the rain-tight structure are

    19.75 in by 57.31 in. Each solar cell is rated at a

    maximum power output of 80 watts. The voltage at

    maximum power is 16.8 V and the current is rated at

    4.75 A. The guaranteed minimum power output is

    75 W. The short circuit current is rated at 5.17 A and

    the open circuit voltage is rated at 21.0 V. The solar

    cells are utilized in configuration with the DC load of

    the 24 V battery array, which is used to power the

    motor and the auxiliary electrical systems in the car.

    The solar controllers used in this project are the

    SunSaver SS-20L-12V and the SS-20L-24V whose

    specifications are listed in Table 4. The solar

    Table 3 Solar panel specifications.

    Model MSX120 SX80V

    Pmax 125.8 80

    V (V) 17 16.8

    I (I) 7.41 4.75

    Voc 21..4 21

    Isc 8.39 5.17

    Table 4 Solar controller specifications.

    Model SS-20L-12V SS-20L-24V

    Solar rating 20 A 20 A

    Load rating 20 A 20 A

    System voltage 12 V 24 V

    controller utilizes switching pulse width modulation to

    charge batteries.

    3.6 Batteries

    Batteries are portable sources of electrical energy

    which the DC motor converts to mechanical. There

    are many new types of batteries that are suitable for

    electric vehicle application. These include lead acid,

    various types of nickel-based (iron, cadmium, and

    metal hydride), lithium-based (polymer and iron), as

    well as sodium-based (sulfur and metal chloride) [8-9].

    However, at the present time deep cycle lead acid

    batteries are still the most commonly used. They have

    the advantage of being fully recyclable and their cost

    is significantly lower. As a result, the use of a

    standard 12 V lead acid battery was the best option.

    In order to design the power system, it is important

    to find the power needs of each of the components.

    Obviously, the motor uses the majority of the power,

    but the peripherals such as lights are also important to

    take into consideration. Comparing current capacity,

    weight and cost, a pack of 10 lead acid batteries

    (model: CSB-GP-12260 shown in Fig. 7) each with a

    weight of 18.6 lbs and a current capacity of 26 Ah.

    They are connected in five branches with two batteries

    in series per branch in order to provide 24 V each.

    This was done to reduce the amount of current draw

    from each battery, since the capacity is only 26 Ah. A

    is a total current of 18A is drawn, and the car runs for

    an hour and a half before it needs to be recharged.

  • Power System Design for an Electric Car

    183

    3.7 DC-DC Power Conversion and Instrumentation

    A DC-DC converter is necessary in order to power

    the lights and the other car accessories, since the latter

    operate at 12 V whereas the battery pack provides 24

    V. The Pyle PSWNV720 shown in Fig. 8 was chosen

    due to its ability to sustain a 360 W output, which low

    enough so that it does not reduce traction power or

    range of the car. It has a converse efficiency of 87%,

    and has several safety built-in features.

    To fully implement a working set of turn signals

    and hazard lights that blink, the use of the LM555

    timer is essential. Two identical timers were...

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