Week-6 Challenge: EV Drivetrain

 

1 WHICH TYPES OF POWER CONVERTER CIRCUITS ARE EMPLOYED IN ELECTRIC AND HYBRID ELECTRIC VEHICLE?

AIM:

POWER CONVERTER:

 The task of a power converter is to process and control the flow of electric energy by supplying voltages and currents in a form that is optimally suited for user loads     

The large number of automobiles in use around the world has caused and continues to cause serious problems of environment and human life. Air pollution, global warming, and the rapid depletion of the earth’s petroleum resources are now serious problems. Electric Vehicles (EVs), Hybrid Electric Vehicles (HEVs) and Fuel Cell Electric Vehicles (FCEVs) have been typically proposed to replace conventional vehicles in the near future.

                                

An Electric Vehicle is a vehicle that uses a combination of different energy sources, Fuel Cells (FCs), Batteries and Super capacitors (SCs) to power an electric drive system. In EV, one or more energy storage devices assist the main energy source. Thereby the system cost, mass, and volume can be decreased, and a significant better performance can be obtained. Two often used energy storage devices are batteries and Super capacitors

                               

The main components of an Electric Vehicle are

DC-AC Inverter

DC – DC Converter

Battery

MOTOR

Electric Vehicle comprises of 2 main power converter components they are

DC – DC Converter

DC – AC Inverter

 

DC-DC CONVERTER:

DC-DC converters can be used to interface the elements in the electric power train by boosting or chopping the voltage levels.

The different configurations of EV power supply show that at least one DC/DC converter is necessary to interface the FC, the Battery or the Super capacitors module to the DC-link.

The amount of power flow between the input and the output can be controlled by adjusting the duty cycle (ratio of on/off time of the switch). Usually, this is done to control the output voltage, the input current, the output current, or to maintain a constant power.

They are used in battery charging and Regenerative braking

 DC/DC converter topologies can be used as bi-directional. A bi-directional converter can move power in either direction, which is useful in applications requiring regenerative braking.

                                 

 

The most common DC/DC converters can be grouped as follows

 Non-isolated converters

The non-isolated converters type is generally used where the voltage needs to be stepped up or down by a relatively small ratio and when there is no problem with the output and input having no dielectric isolation. These are main types of converter in this non-isolated group, usually called the buck, boost, buck-boost and Cuk  converters.

 The buck converter is used for voltage step-down,

 The boost converter is used for voltage step-up.

 The buck-boost and Cuk converters can be used for either step-down or step-up.

 Isolated converters

Usually, in this type of converters a high frequency transformer is used. In the applications where the output needs to be completely isolated from the input, an isolated converter is necessary. There are many types of converters in this group such as Half-Bridge, Full-Bridge, and Fly-back, Forward and Push-Pull DC/DC converters. All of these converters can be used as bi-directional converters and the ratio of stepping down or stepping up the voltage is high.

 

Electrical Vehicle Converter Requirements:

Small volume,

Low electromagnetic interference,

Low current ripple drawn from the Fuel Cell or the battery,

The step up function of the converter,

High efficiency

 

INVERTERS (DC-AC)

It Converts direct current (DC) from the battery to alternating current (AC) to be used by other devices such as the traction motor

Inverter can used to control speed of the motor by adjusting the frequency of the Alternating current

Inverters have capability to change the torque power by adjusting the alternating current parameters

                                 

NEED OF INVERTER IN ELECTRIC CARS:

It inverts DC charge to AC charge for driving the motor and then the opposite during regenerative braking. An equally important job of the inverter is to control the switching of the static coils in the motor to create a revolving magnetic field, like a merry-go-round, to spin the motor's rotor.4

 

2) An Electric Vehicles s powertrain with 72-V battery pack is shown in the diagram below. The duty ratio for acceleration operation is ‘d1’ and for the braking Operation, the duty ratio is ‘d2’

What is EV steady state speed if duty cycle is 70%?

                               

MOTOR AND CONTROLLER PARAMETRS:

           Rated Armature voltage= 72 V

           Rated Armature current= 400 A

            Ra= 0.5 Ω

            KØ= 0.7 Voltage Seconds

           Chopper Switching frequency= 400 Hz

         Vehicle Speed Torque equation is given as:

            Tv: 24.7+ (0.0051)w^2

 

AIM:

    Given Duty cycle = 70 % = 0.7

     Voltage = Rated Armature Voltage * Duty Cycle

                   = 72*0.7 = 50.4 Volts

    T-motor Torque󠅶

    V-Voltage

    Ra-Armature Resistance

    KØ-Motor constant

     w  -Angular Speed of Motor

  Torque-Speed characteristics of DC motor:

       Tm = (V KØ/Ra)-(( (KØ)^2/Ra)*w

       Tm = ((50.4)*0.7)/0.5 – ((0.7)^2/(0.5))*w

       Tm = 70.56 – 0.98 w   _____EQ (1)

       Tv = 24.7 + (0.0051) w^2_______EQ (2)

      Equating the above the equations (1) & (2)

       24.7 + (0.0051)w^2 = 70.56 – 0.98

       So we get quadratic equation 

         0.0051 w  ^2 + 0.98 w   – 45.86 = 0

        In order to find roots to Solve the above equation which is in form of quadratic equation

        ax^2 + bx + c = 0 by using these formule:

            

          Substituting the equations values in above formula

          w = 0.98± ((√ (0.98)^2 – 4*0.0051 * 45.8) / 2*0.0051

         Here we get w = 38.92 rad/sec or 231.02 rad /sec

         Steady state of the angular speed is 38.92 rad /sec

3. Develop a mathematical model of a DC Motor for the below equation using Simulink.   

      ω=   V/Kϕ -Ra/Kϕ^2   

  AIM:

 

 

4) INDUCTION MOTOR VS BLDC

Induction Versus DC Brushless Motors

 

His prospective about electric vehicle future technology cannot be predicted easily. The gas powered vehicles engines are not all same and there is no best engine type people choose according to their requirements such as price and performance these also comes under electric vehicles but in nearby future EV drives may get stability.

The author said that both induction motor and DC brushless motor have advantages as well as disadvantages and he explained about basic construction of both the motors

Induction motor does not have permanent magnets and it is connected to 3-phase utility power, torque is produced at the outset the motor has ability to start under load so the inverter is not needed so it has ability to compatible with the conventional vehicles.  If induction motor size increases the losses will not increase. With the help of induction motor drive we can achieve required high performance.

Limitations of induction motor is it requires source power as AC it will not operate in DC. This motor has limited starting torque and limited running peak torque capabilities as shaft speed is proportionate to line frequency so in order to overcome such draw backs it requires an inverter and feedback signal and speed variation depends upon the frequency input. It is hard to control

In DC brushless motors the strength of magnetic field is adjusted When maximum torque is required, especially at low speeds, the magnetic field strength should be maximum – so that inverter and motor currents are maintained at their lowest possible values. This minimizes the I² R losses

Where DC brushless machines requires permanent magnets which is cost efficient. Construction of DC brushless motor is costly compared to induction motor because of magnets 

He finally said that DC brushless motors will dominance in HEV and PHEV

And Induction motor is highly preferred in PURE EV