Week-6 Challenge: EV Drivetrain
Question 1) Which types of power converter circuits are employed in electric and hybrid electric vehicle?
Power Electronic Converters:
In EV’s and HEV’s, the power electronic converters are considered as the key elements that interface their power sources to the drivetrain of EVs. In order to design highly efficient converters for EV power system, advanced AC/DC and DC/DC converters are required to adapt the output voltage and current levels with high power quality.
The 3 types of converters which are widely used in an electric vehicle are:
AC – DC Converter:
AC to DC convertors are most commonly used in PHEVs and BEVs. The battery needs to charge with DC current so AC to DC converter is used.
DC – DC Converter(Chopper):
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 converter one 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 kmph, weight, maximum torque and power profile
The main dc-dc converter changes dc power from an on-board 200-800V high voltage battery into lower dc voltages (48V or 12V) to power headlights, interior lights, wiper and window motors, fans, pumps and many other systems within electric vehicles (EV) and hybrid electric vehicles (HEV).
DC - AC Converter(Inverter):
The DC/AC converter is required to convert the dc power to a suitable ac power for the electric machine. The power output from the battery pack(DC current) is converted using inverter to be used in electric motor(AC current).
Question 2) An Electric Vehicle's powertrain with 72V battery pack in shown in the diagram below. The duty ratio for acceleration operation is 'd1' and for the braking operation the duty ratio is 'd2'.
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
What is EV steady state speed if duty cycle is 70%?
Solution:
Vehicle output voltage (V) is:
V = Rated voltage * Duty cycle
V= 72*0.070
Vehicle voltage = 50.4V
Now, equation of torque is T =
Where, T= Vehicle torque(Nm)
V = vehicle voltage
ω = steady state speed of vehicle(rad/s)
Kϕ = Motor constant
Ra = Armature resistance
Substituting all values in eqn of torque
`T = ((50.04-0.7*omega)*0.7)/0.5`
T = 70.56-0.98ω ...........(1)
Vechicle torque speed characteristic is
comaparing with eqn 1
`70.56 - 0.98*ω = 24.7 + 0.0051*ω^2`
we get quadratic equation
`0.0051ω^2 + 0.98ω - 46.94 = 0`
So ω = 38.85 and -230.95
take positive value so, ω = 38.85 rad/s
Question 3. Develop a mathematical model of a DC Motor for the below equation using Simulink.
ω =V/Kϕ - Ra/Kϕ^2*T
Solution:
Consider V = 220V
Kϕ = 0.7
Ra = 0.5
Simulink Model Screenshot:
Scope Result:
Question 4) Induction Versus DC brushless motors by Wally Rippel, Tesla. Explain in brief
Wally Rippel is a long-time proponent of electric vehicles. Prior to joining Tesla Motors, he was an engineer at AeroVironment, where he helped develop the EV1 for General Motors and was featured in the documentary movie, Who Killed the Electric Car? Wally has also worked for the Jet Propulsion Laboratory on electric vehicle battery research, among other projects. In 1968, as a Caltech undergraduate student, he built an electric car (a converted 1958 Volkswagen microbus) and won the Great Transcontinental Electric Car Race against MIT.
In the beginning author describes the dominance of IC engines in the world of automobile industry and also discuss types of engines.He also speaks about the induction motor and brushless DC motor and presents his veiw that these two technologies will be widely used for the coming years.
Then he explains the working of both 3 phase induction motor and a brushless DC motor.
He did comparison between these two technologies and stated some advantages and disadvantages of each.
Induction motor have some advantages like:it has no magnets.The motor has the ability to start under load.No inverter is needed. Some disadvantages of Induction motors are: They cannot operate from DC; AC is a must. Shaft speed is proportionate to line frequency. Hence, when used with utility power, they are constant speed machines. When operated from utility power, they have limited starting torque and some what limited running peak torque capabilities.
when compared to DC type machines Brushless DC motor have some advantages like: much less rotor heat is generated with the DC brushless drive. The peak point energy efficiency for a DC brushless drive will typically be a few percentage points higher than for an induction drive Some disadvantages of Brushless DC motor are: as machine size grows, the magnetic losses increase proportionately and part load efficiency drops. Permanent magnets are expensive and difficult to handle due to very large forces that come into play when anything ferromagnetic gets close to them.
Finally the author couldn't choose which drive is better than the other but makes it clear that DC brushless drives will likely continue to dominate in the hybrid and coming plug-in hybrid markets, and that induction drives will likely maintain dominance for the high-performance pure electric vehicles.