Week-6 Challenge: EV Drivetrain

In the past decades, power device technology has made a tremendous progress. These power devices have grown in power rating and performance by an evolutionary process. Among existing power devices, including the thyristor, gate turn-off thyristor (GTO), power bipolar-junction transistor (BJT), power metal-oxide field-effect transistor (MOSFET), insulated-gate bipolar transistor (IGBT), static-induction transistor (SIT), static-induction thyristor (SITH), MOS-controlled thyristor (MCT),

and MOS turn-off thyristor (MTO), the IGBT is almost exclusively used for modern EVs. Nevertheless, the power MOSFET has also been accepted for those low-voltage low-power EVs.

The evolution of power converter topologies normally follows that of power devices,aiming to achieve high power density, high efficiency, high controllability, and high reliability. The power converter topologies depend on the motors to be driven. The

selection criteria of motor drives, including the motors and their power converters, for EVs can be divided into the mandatory requirements and the preferable requirements. The mandatory ones are that the motor drive can offer the torque-speed requirements of the EV driving profile without involving variable gearing or gearbox, and the motor drive can provide the capability of bidirectional power flow to recover the regenerative braking energy. In general, the DC, AC or SR motor drives can offer the desired torque-speed requirements under proper motor design. The preferable requirements of EV motor drives are low cost, high efficiency, high power density, high controllability and maintenance-free operation.

The demand of electrical power in vehicular application is growing rapidly as the mechanical components are being replaced by the electrical and electronic components . It is expected that the power demand in electric vehicular system (EVS)

could reach 2 to 3 times of the current demand. In order to accomplish the growing power demands of EVs in desired manner, integration of power electronic components with electrical and mechanical loads of vehicle becomes crucial. The integration of power electronic converter (PEC) not only improvesthe overall performance and fuel economy but also reduces the emission as well as weight and size of the vehicle.

For electric vehicular system, there is a need of highly reliable, flexible and fault tolerant electrical powerprocessing system on board to deliver high quality of power based on vehicle demands. At present, this responsibility have been taken care by the available PEC that includes DC/DC converters, rectifiers (DC/AC), inverters (AC/DC) and battery charger composed of AC/DC and or DC/DC converters . Individually or combination of these converters can be taken to serve the purpose;

however, operation of each PEC isentirely different from the other. PECs perform some of the critical tasks like ON/OFF switching of various loads; power conditioning and voltage/current modulation to create compatibility among the energy source system (ESS), traction motors and auxiliary loads. PECs not only serve thepurpose of converting electrical powerfrom one form to another (DC/DC, DC/AC and, AC/DC) but also help to step up or step down the system voltage level. In the last decade, significant advancement in converter topologies dealing with battery charger, voltage source inverter for motor drives and DC/DC conversion has been achieved. Regardless of this topological advancement, still conventional PEC topologies are being used in modern electrified vehicles.

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. By introducing DC/DC converters one can choose the voltage variation of the devices and the power of each device can be controlled. The power of the DC/DC converter depends on the characteristics of the vehicle such as top speed, acceleration time from 0 to 100 km/h. weight, maximum torque, and power profile (peak power, continuous power).

AC-DC Converter (On Board Charger): -

AC to DC converters are most commonly used in Plug-in Hybrid Electric Vehicles (PHEV) & Plug-in Electric Vehicles (PEV). These vehicles get their propulsion power from an external electricity source (charging stations) which is alternating current. The battery needs to be charged using a direct current source. For this purpose, AC to DC converter is employed in a PHEV & PEV.

DC-AC CONVERTER(INVERTER):

This converter is required to convert the DC power into a suitable ac power for the electrical machine. The power output from battery pack (DC current) is converted using inverter to be used in electric motor (AC current).

#ANSWER 2:

An Electric Vehicle's powertrain with 72V battery pack in shown in the diagram below.

The other parameters of the electric vehicle is given below,

Motor and Controller Parameters:

Rated Armature voltage= 72 V

Rated armature current= 400 A

Ra= 0.5Ω, KΦ= 0.7 Volt second

Chopper Switching frequency= 400 Hz

The vehicle speed-torque characteristics are given by the below equation

the motor torque equation is

the voltage is = dutiratio * battery pack voltage= 0.7*72= 50.4 volts.

substituting all values in motor torqe equtation we get

motor torque = 70.56-(0.98w)

at equilibrium

the vehicle torque is equal to motor torque so

24.7−(0.0051w^2)=70.56-(0.98w)

by solving the above quadratic equation we get

w = 38.9 rad/sec and w = -231rad/sec

w can never be negative so

the ev speed at 70% of duty cycle is 38.9 rad/sec

# ANSWER 3:

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

ω= ( V/Kϕ) -(Ra/Kϕ^2 ).T

#ANSWER 4:

The author writes this blog to explain the differences between the two commonly used motor drives in the automobile industry. The DC brushless motor drive and the Induction motor drive. Rippel goes into detail about the differences in construction, working, and cost between the two drive types. However, with what he has written, he has not arrived at the conclusion that one type is better than the other. In the last lines of the opening paragraph, Rippel writes that "there is no one best engine type" and that it is all a matter of personal preference when it comes to choosing an engine. It is in this same regard that he ends the blog stating that there is still 'no winner' when it comes to motor drive selection either. In spite of both motor drive types having similarities, they also have their benefits and drawbacks.

For DC brushless drives, we are told that their construction implements 2 or more permanent magnets at the rotor to generate a DC magnetic field. It also consists of a stator core (made up of thin, stacked laminations). It also uses an inverter for the current control, meaning that it cannot run on a DC power supply as the current direction has to be changed repeatedly to maintaining constant torque.

For Induction drives, their rotor has no magnets but instead stacked steel laminations with buried peripheral conductors which form a 'shorted structure. It is the current in the stator windings that generates a rotating magnetic field that interacts with the rotor.

Induction motors can be used directly with power from the mains and do not require an inverter to start the drive and generate the starting torque. It has the ability to start under the mains load applied. However, DC brushless drives cannot do the same. They cannot generate a starting torque when connected to a mains power supply. They require an inverter to do this.

Induction motor drives cannot run on a DC power supply and are solely dependent on AC power supply. Also, when they are connected to a 3 phase mains supply, the rotor rotates at a constant speed and also they have a limited starting torque as well as peak torque. In order to rectify this an inverter can be used to operate the induction drive from a DC power supply or a battery. Also, if the inverter frequency is varied, then the constant speed is no longer an issue - it can be varied.