The Electric Vehicle Architecture

Hey Guys,

Last time we looked at the fall and rise of electric vehicles. I realized that there is a decent interest amongst the student community to learn more about electric vehicles. I am also very glad that there are groups in BITS-Pilani Goa campus working on electric car prototypes and I am sure other campuses would also be making such contribution. I strongly believe that such experience in college will definitely benefit students who are interested in pursuing career/ higher studies in this domain or even in fields like power electronics/ motor controls and firmware for automotive powertrain systems.

For the people out there who are not really up to speed but are super interested and would like to get into this, today I would like to give a brief architectural overview of electric vehicles. The major difference between gasoline and electric vehicles is that there is no internal combustion engine in electric vehicles. This is replaced by an electric motor (AC or DC) and this motor is charged by a battery on the car. So, as gasoline engine cars require petrol or diesel as a fuel, these cars need charge or current. People who know the complexity of an engine would instantly realize the simplicity of electric car.

As you can see in the figure below the electric car consists of 4 major components.

A Battery: A battery is the heart of the car. This is the energy storage device (analogous to fuel tank in normal car). This stores DC voltage when it’s charged. The voltage levels are generally in 100s of volts (~400V). This battery has certain number of cells in series and parallel. The number of cells in series and parallel is decided on how much voltage (~range) and how much current (~power) you want in your car respectively.

A Charger: (not shown in the figure) It is used to charge the battery up to the desired voltage. This takes in AC voltage from the wall, rectifies it and then charges the battery. The charger is rated at certain KW which determines how much current it can consume which in turn dictates how much time you will need to charge the battery. You will study the concepts used in charging in ES1.

A Motor: Usually a 3ph AC induction motor is used in electric cars (due to efficiency/ reliability reasons).It takes in 3ph AC current and produces torque due to magnetic induction across the stator and rotor. The shaft of the motor is connected to the halfshaft of the car through a reduction gear box. You guys generally study these in ES2.

An Inverter: Inverter is a power electronics device that is needed in between a battery and the motor to convert the DC current that the battery produces to AC current that the motor consumes. It has IGBTs as switches which switch at high frequencies like 10KHz while converting DC to AC.

Having said this there are a couple peculiarities of electric vehicles:

Regenerative Braking: When you apply brakes you are essentially applying negative torque to the motor to speed it down. That means the motor consumes negative current which you can visualize as a current going from motor to battery through the inverter. This results in charging the battery. This phenomenon makes electric vehicles very attractive in the sense that you can recharge the battery using the energy spent in braking.

Max torque at zero speed: If you see the motor torque speed characteristic as shown below you will realize that we can extract maximum torque from the motor even at zero speeds. This means that we will not need a gear box to shift to the right gear to attain right amount of torque from the motor at various speeds (as we require in an engine). Hence you can achieve high amounts of accelerations/ power from the vehicle from dead stop positions.

Below is a very interesting link of a drag race between BMW M5 (known for its acceleration) and Tesla ModelS (electric sedan company that I work for :)). See how Tesla gets a head start due to full torque at zero speed!!

Drag Racing: Tesla ModelS vs BMW M5!!

The Basics of Controls and State Space Behavior

Hey Guys.. I will try to share some tools that I learnt during the course of my Masters and in the industry. In my last post I mentioned a few things related to controls that you will learn during your 3^rd or 4^th year. Today I will discuss a few things that are important and always needed in industry. These may or may not be covered in the class and hence I felt I could share these.

In this field you will be given a task of modeling a particular system and then designing a controller for it which will help you attain desired behavior. If you consider a black box model your entire work could be represented by the block diagram shown below. Let’s discuss the parts of this which will result as a perfect segway into state space (the most important tool to design systems).

Block Diagram

1. The block “A” is called the plant. This is defined by the laws of physics and is the guy that we  have to control. E.g. This could be a car going uphill at constant speed, a tap filling water in a tank, a line follower robot, a hydro electric power plant etc. All you need to remember is.. the behavior of this plant is defined by physical laws and is constant. You don’t have any control over its definition.
2. The block “B” is your controller. You design this so that you can control “A” as you want. E.g You will have a flexibility to design a controller “B” that will maintain the speed of your car going uphill at 50Km/Hr or maintain the robot on the black line all the time. 
3. The block “C” is your output that you can measure or observe and then use it in your controller. E.g It can be the speed of the car or velocity of the line follower. 
4. The signal “u” is the control. This is the signal that is generated from your controller which is fed to the plant to control it as you want. 
5. And finally, this controller will be responsible to control a particular “state” of your system. Your system can have many states. One of these many states is your “output” that you observe/ measure with the help of block “C”.

So, the ingredients of the any damn system are: the Plant (A), the Controller (B),the control input (u), the state (x) and the output that you measure (y). And mathematically, any damn system can be represented as

Believe me, with this representation, it becomes super easy to analyze the system compared to writing huge differential equations by drawing free  body diagrams as we all learn in Math 3 and Physics! This representation is called state space and next time we will  see how we generate this state space from any given damn system.

State Space should be your friend and  you should be comfortable in playing with it!