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Electric vehicle design and development

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Electric vehicle design and development

This thesis is centered about the cost e ective manufacturing of eccient and reliable control of Electric Vehicle with cheaper solutions as much as possible, because there are many problems associated with conventional and electric vehicles that will be discussed in next chapter. The main parts of EV to be manufactured are:

Motor Drive System

Motor Drive system is basically the heart of Electric Vehicle operations and is responsible for the vehicle control parameters like speed, torque and direction etc. It’s only task is to generate gate pulses for inverter switches to provide required AC output for given xed DC input voltage. The system consists of a three phase inverter that has two main components: rst one is the power stage which converts the DC power from a battery bank into three phase AC supply in order to run the three phase induction motor; a source of propulsion for EV and the second one is the control stage which includes the gate drivers that serve the purpose of isolation of micro-controller from power stage as well as provide the high current logic signal that is sensible for high power operating switches. Low cost and advance micro-controller STM32F407Vg has been utilized for the purpose of di erent tasks like Space Vector PWM signals generation, direction reversal of induction motor for the purpose of forwrad and backward motion of vehicle and breaking of induction motor etc.

Vehicle Control System

For reliable car operations, accelerator and brake pedals are provided. A user interface button provides the facility to reverse the direction of vehicle which reverse the direction of rotation of Space Vector and negative sequence currents ow consequently. Accelerator pedal is linked with potentiometer whose analogue signal is converted into digital signal for speed adjustment of induction motor.

1.3 Vehicle Mechanical Structure

Vehicle mechanical structure is self designed and manufactured through vendor by giving step by step instructions. A Formula-1 based vehicle structure has been followed and shape is pretty much aerodynamic drag eccient with very less aerodynamic drag area in order to minimize acting drag forces. The detailed design and development of car structure will be discussed in Chapter5.

1.4 Block Diagram of the Project

The following block diagram clearly depicts the general image of the project

Motivations and Problem Statement Now a days, electric vehicles are becoming popular because of their number of advantages over other type of vehicles like internal combustion engine vehicles that uses fossil fuels whose cost is increasing day by day because of their shortage and ICE are less eccient than motors that are used for propulsion in EV. Another problem associated with ICEVs is environmental pollution and its hazardous e ects to humanity and other natural life. While EVs have zero carbon emission and less noise pollution as well. Since propulsion source is in electrical form, they can directly be provided with electrical energy from renewable sources without converting into the other form of energy. However, for long range and high torque requirements we can use batteries that causes energy to circulate in electro-chemical form. It is still far more eccient than ICEVs.

Motivation and Scope of the Project

This project has a huge scope as the world is now moving towards the electric vehicles because of the advancement in technology especially in the speed control methods of induction motors and reliable control system via micro-controllers and other embedded systems. Battery technology has been advanced in terms of size and energy density as well. Li-ion batteries have high capacity, high load capability and short charging time with simple algorithms. If EVs are charged via existing utility grid powered by fossil fuel based generation system, even then there is advantage of clean environment. As we know generation systems are far from populated areas while vehicles mostly travel in populated areas. The renewable energy can be utilized in EVs in order to minimize the cost of electricity that will be bene cial for both economy and healthy environment. The traveling cost will be reduced to a huge proportion as compared to that of conventional vehicles. The EVs have very less maintenance expenditures because the induction motors are most rugged and cheaper machines available. There is no gearing mechanism in EVs

Problem Statement

As mentioned earlier, the problem is subjected to the major traveling sources of trans- portation i.e. automobiles. The conventional vehicles have following problems:

Environmental pollution and hazardous e ects to humanity, i.e. smoke and rise in temperature

Less Eccient ICE

High fuel prices

Higher maintenance cost

A well-known solution to this problem is to promote internal combustion engine-free vehicles that have some other sources of propulsion. Electric vehicles also have some following problems associated with them:

Complex control systems

High cost of batteries

High cost of Motor Drive system

Less range

The above mentioned parameters have made the overall cost of electric vehicles too high.

Solutions to Problems

The problem of costly batteries is reducing as the advancement in battery technol- ogy.

An attempt has been made to recover the problems of complex control system of vehicle.

Also to design a cost e ective motor drive system using inverter technology.

The problem of range can be reduced by introducing highly eccient control algo- rithms and regenerative braking system.

Main Components of EV

Some major components of EV include the battery system, motor and the system used for motor drive.

Battery System

Battery bank is the source of energy for Electric vehicles in the form of electricity. There are many kinds of rechargeable batteries are being used for EVs like Li-ion batteries, lead-acid batteries, Ni-CAD batteries etc. Their characteristics so far achieved are as follows.

Li-ion Batteries

Drawbacks of this battery are listed below:

Lead-acid Batteries

Invented by the French physician Gaston Plante in 1859, lead acid was the rst recharge- able battery for commercial use. Despite its advanced age, the lead chemistry continues to be in wide use today. There are good reasons for its popularity; lead acid is dependable and inexpensive on a cost-per-watt base. There are few other batteries that deliver bulk power as cheaply as lead acid, and this makes the battery cost-e ective for automobiles, golf cars, forklifts, marine and uninterruptible power supplies (UPS).

The grid structure of the lead acid battery is made from a lead alloy. Pure lead is too soft and would not support itself, so small quantities of other metals are added to get the mechanical strength and improve electrical properties. The most common additives are antimony, calcium, tin and selenium. These batteries are often known as lead-antimony and leadcalcium.

Lead acid is heavy and is less durable than nickel- and lithium-based systems when deep cycled. A full discharge causes strain and each discharge/charge cycle permanently robs the battery of a small amount of capacity. This loss is small while the battery is in good operating condition, but the fading increases once the performance drops to half the nominal capacity. This wear-down characteristic applies to all batteries in various degrees.

Depending on the depth of discharge, lead acid for deep-cycle applications provides 200 to 300 discharge/charge cycles.

Deep-cycle Battery

The deep-cycle battery is built to provide continuous power for wheelchairs, golf cars, forklifts and more. This battery is built for maximum capacity and a reasonably high cycle count. This is achieved by making the lead plates thick. Although the battery is designed for cycling, full discharges still induce stress and the cycle count relates to the depth-of-discharge (DoD). Deep-cycle batteries are marked in Ah(ampere-hour) or minutes of run-time. The capacity is typically rated as a 5-hour and 20-hour discharge. Battery has some following advantages and disadvantages.

Advantages Limitations

Inexpensive and simple to manufac- ture; low cost per watt-hour.

Low speci c energy; poor weight-to- energy ratio.

Low self-discharge; lowest among rechargeable batteries.

Slow charge; fully saturated charge takes 14-16 hours.

High speci c power, capable of high discharge currents

Must be stored in charged condition to prevent sulfation.

Good low and high temperature per- formance.

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Limited cycle life; repeated deep- cycling reduces battery life.

Flooded version requires watering. it’s not environment friendly

Ni-Cad Batteries

Nickel-based batteries are more complex to charge than Li-ion and lead acid. Lithium- and lead-based systems are charged with a regulated current to bring the voltage to a set limit after which the battery saturates until fully charged. This method is called constant current constant voltage (CC/CV). Nickel-based batteries also charge with constant cur- rent but the voltage is allowed to rise freely. Full charge detection occurs by observing a slight voltage drop after a steady rise.

Full-charge Detection by Temperature

Full-charge detection of sealed nickel-based batteries is more complex than that of lead acid and lithium-ion. Low-cost chargers often use temperature sensing to end the fast charge, but this can be inaccurate. The core of a cell is several degrees warmer than the skin where the temperature is measured, and the delay that occurs causes over-charge. Charger manufacturers use 50C (122F) as temperature cut-o . Although any prolonged temperature above 45C (113F) is harmful to the battery, a brief overshoot is acceptable as long as the battery temperature drops quickly when the ready light appears.

Advanced chargers no longer rely on a xed temperature threshold but sense the rate of temperature increase over time, also known as delta temperature over delta time, or dT/dt. Rather than waiting for an absolute temperature to occur, dT/dt uses the rapid temperature increase towards the end of charge. Battery has some following advantages and limitations.

Relative Merits

Advantages Limitations Fast and simple charge even after pro- longed storage. Relatively low energy density . High number of charge/discharge cy- cles if properly maintained, the NiCad provides over 1000 charge/discharge cycles.

Memory e ect the NiCad must period- ically be exercised to prevent memory. Good load performance the NiCad al- lows recharging at low temperatures. Environmentally unfriendly the NiCad contains toxic metals. Some countries are limiting the use of the NiCad bat- tery.

Long shelf life in any state-of-charge. Has relatively high self-discharge needs recharging after storage Simple storage and transportation most airfreight companies accept the NiCad without special conditions. Good low temperature performance & lowest cost battery in terms of cost per cycle.

Electric Motor

Electric motor is the main source of thrust in EV. A number of types of electric motors are being used depending upon speci c applications. In electric vehicles, two kinds of motors are used mostly one is induction motor while the other is BLDC motor. Both have their own advantages and limitations as explained below.

Induction Motor

Induction motors are the famous AC motors. More than 90% motors in industry are the induction motors. There are basically 2 types of induction motors depending upon the type of input supply:

Or they can be divided according to type of rotor

Working Principle

In an induction motor only the stator winding is fed with an AC supply.

Alternating ux is produced around the stator winding due to AC supply. This alternating ux revolves with synchronous speed. The revolving ux is called as Rotating Magnetic Field (RMF).

The relative speed between stator RMF and rotor winding causes an induced emf in the rotor winding, according to the Faraday’s law of electromagnetic induction. The rotor winding are short circuited, and hence rotor current is produced due to induced emf. That is why such motors are called as induction motors. (This action is same as that occurs in transformers, hence induction motors can be called as rotating transformers.)

Now, induced current in rotor will also produce alternating ux around it. This rotor ux lags behind the stator ux. The direction of induced rotor current, according to Lenz’s law, is such that it will tend to oppose the cause of its production.

As the cause of production of rotor current is the relative velocity between rotating stator ux and the rotor, the rotor will try to catch up with the stator RMF. Thus the rotor rotates in the same direction as that of stator ux to minimize the relative velocity. However, the rotor never succeeds in catching up the synchronous speed. This is the basic working principle of induction motor of either type, single phase or 3 phase.

Advantages

Disadvantages

Brushless DC Motor

As their name implies, brushless DC motors do not use brushes. Because the coils are not located on the rotor. Instead, the rotor is a permanent magnet; the coils do not rotate, but are instead xed in place on the stator. Because the coils do not move, there is no need for brushes and a commutators.

Since the rotor is a permanent magnet, it needs no current, eliminating the need for brushes and commutators. Current to the xed coils is controlled from the outside. With a BLDC motor, it is the permanent magnet that rotates; rotation is achieved by changing the direction of the magnetic elds generated by the surrounding stationary coils. To control the rotation, we need to adjust the magnitude and direction of the current into these coils.

Advantages

Disadvantages

After all the comparisons of electric motors, the squirrel cage three phase induction motor is best suited for our application because of its ruggedness and low cost features.

Advantages of EVs over Conventional Vehicles

  1. A battery electric vehicle (BEV) has far fewer moving parts than a conventional gasoline-powered vehicle. There’s no need for liquid fuels or oil changes. There’s no transmission or timing belt to fail when one least expects it. In fact, most of the maintenance costs that are associated with an internal combustion engine, are eliminated.
  2. Even if vehicle is to be fueled exclusively by electricity generated by coal, EVs have a better emissions pro le than internal combustion vehicles. Not only its overall emissions content is lower, but the location and timing of the emissions are also better. While a gasoline-powered vehicle emits pollution during peak driving hours in the middle of the city, the emissions from fueling an EV generally happen during o -peak driving hours at power plants in remote locations.
  3. By introducing renewable technology like EVs, threat of electricity generated by coal even reduced.
  4. In EVs, a compact and quiet motor yields surprising acceleration: 100% torque at zero rpm, without the negative emissions associated with most cars on the road.
  5. The overall cost of EV ownership is actually lower than gasoline-powered cars. The savings are primarily on fuel bills, maintenance costs, oil changes, replacing belts, and transmission systems.
  6. The electricity that powers an EV can come from many sources, which include low- emission sources like natural gas and zero emission sources like wind, solar, hydro and nuclear power, which enable EVs to dramatically reduce the gaseous emissions.

3.3 Applications

Electric vehicles are just like other vehicles except their propulsion gets through electric motors instead of internal combustion engine. Therefore, it nds applications in every eld where other vehicles are being used.

They are almost noiseless and can be used in speci c areas for speci c purposes like hospitals, Armed forces, travels and Parks etc.

Electric Motors have tendency to carry heavy load because of high torque capability in many kinds of motors like DC series motor etc.

These vehicles are of great interest in the western areas where oil is rarely found. The designed drive can also nd application in submarines as it requires no oxygen unlike internal combustion machines.

They can provide shuttle services in large complexes.

Used for air-side operations at airports including aircraft tugs and baggage handling vehicles.

Induction Motor Drive

Introduction

The hardware that’s used to control the speed and torque of induction motor is known as induction motor drive.

Squirrel-cage induction motors are the most rugged motors available in this era. But they operate at speci c speed, at designed nominal frequency and rated voltage. Their speed can be controlled by various methods that are categorized in two major categories:

  1. Scalar Control
  2. Vector Control

Scalar control of IM includes the speed variation of motor by changing the applied voltage or applied frequency or changing both at same time. While vector control includes the torque parameter such as a control variable and current sensor and some other sensors like voltage, speed and position sensors that are required in order to control the torque of IM.

Three phase Inverter

Almost in all type of AC drives inverter technology is being used which is the backbone of motor drives. Inverter is a device or a system that takes a DC voltage as an input and provides single phase or 3 phase AC voltage of required magnitude and frequency at the output. Inverter can be divided into two major parts as explained beloInduction Motor Drive 14

Power Stage

Power stage consists of a speci c semiconductor transistor switches combination and their on-o states controlled by control stage via micro-controller. This switching scheme decides the output parameters of inverter on which IM behavior is dependent. Various types of switches are available depending upon the various operating conditions. In our case the targeted IM is of 5 horse power so we have found IGBT modules(CM150-DY 24E) suitable for our application. The ratings and other speci cations of this IGBT switch can be seen in appendix. The circuit diagram of power stage is shown below: Figure 4.1: Inverter Power Stage topology In the above diagram the combination of 6 IGBTs makes a power stage through which main power ows from battery bank through DC link capacitor and switches to the IM load converted from DC to three phase AC supply but their gate signal is given by the control card that controls the state of IGBTs in a speci c manner. DC link Capacitor bene ts

A series combination of 1000uF Capacitors has used for smooth operations of in- verter.

All the high frequency switching harmonics will get short circuit path through DC link Capacitors.

Major advantage in our application is that when the reverse power will ow from motor towards the batteries the capacitors will store charge and safe battery bank from abrupt change in voltage and harmonics that reduce battery life. [5]

Induction Motor Drive 15

Control Stage

This portion is responsible to give the logic signals to IGBTs for proper control of 3-phase output voltage. Basically logic signals are generated through microcontroller embedded systems that run at very low voltages and power as compared to the power stage so microcontroller need isolation. We have used a gate driver IC HCPL-3120 for this purpose.

Here,

R12 = 150 ohm = STM pulses current limiting resistance.

R10 = 220 ohm = IGBT gate resistance for proper turn on delay.

Importance of Gate driver IC

It provides high current output up to 2A that is one of the necessary conditions in order to operate the power switches. Because STM output current is not enough to be sensed by IGBTs running for high power applications. It provides STM with physical isolation from power stage i.e. to do the job of opt-coupler as well that is necessary condition for the secure operation of micro- controller.

Demerit Observed in Gate driver usage We have observed one disadvantage of directly using gate driver in our application. As we have to use this inverter in a movable standalone vehicle where AC supply can’t be used from WAPDA directly. We have attached separate batteries for giving the DC supply to the gate driver that needs a switch that should be opened when inverter is not to be operated because HCPL consume 45mW power constantly even no gate pulses are present.

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