Week-6 Challenge: EV Drivetrain
1) Which types of power converter circuits are employed in electric and hybrid electric vehicle?
Power Electronic Converters: -
In Electric and Hybrid Electric Vehicles (EV/HEV), the power electronic converters are considered as the key elements that interface their power sources to the drivetrain of the EVs. In order to design highly efficient converters for the EV’s power system, advanced DC/DC and DC/AC converters are required to adapt the output voltage and current levels with high power quality.
The power electronic devices are a combination of converter and controller where the converter functions according to the control signal generated by the controller. A power electronic converter is composed of power semiconductor devices controlled by integrated circuits.
The 3 types of converters which are widely used in an Electric Vehicle are:
1) AC – DC Converter (On Board Charger):
There are generally 2 methods to charge the EV High voltage battery. Ie AC charging and DC charging.
In AC charging , Input is an AC signal And output given to battery is DC. So in the on board charger we use a Power converter called AC-DC converter also called rectifier which convert AC signal to DC signal.
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.
During Regenerative braking to charge HV battery we use AC to 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 , weight, maximum torque, and power profile (peak power, continuous power).
3)DC-AC converter(Inverter):The DC/AC converter is required to convert the dc power to a suitable ac power for the electrical machine. The power output from battery pack (DC current) is converted to AC power using inverter to drive the Traction motor
Please refer below attached image to find the all 3 types of Power converter in an electric vehicle. Refered from Tata electric bus.
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:
Where;
Tv= Vehicle Torque (N-m)
Vv = Vehicle voltage (Volts)
ω= steady state speed of vehicle (rps)
K∅= Motor constant
Ra= Armature Resistance
Duty cycle =70%=0.7
Vehicle output voltage is calculated using formula= Armature voltage * Duty cycle= 72*0.70=50.04
Now Vehicle output voltage(Vv)=50.04V
To find the Vehicle Torque we have to use below formula:
Tv=((Vv−ω.K∅)*K∅)/(Ra).
Tv=((50.04-0.7*w)*0.7)/0.5
Tv=70.056-0.98w
Now substituing Tv value in the above equation.
24.7+0.0051w^2=70.056-0.98w
0.0051w^2+0.98w-46.94.
After solving the quadratic equation , the roots are
w= 38.85 rad/s and w = -230.95 rad/s . Considering positive value , the steady state speed is 38.85 rad/sec.
Develop a mathematical model of a DC Motor for the below equation using Simulink.
ω= (V/Kϕ -(Ra/Kϕ^2) *T.)
Considering Torque input as ramp signal:
Output w=61.28rad/sec
Considering Torque input as step signal:
Output w=70.47rad/sec
4) Refer to the blog on below topic:
Induction Versus DC brushless motors by Wally Rippel, Tesla
Explain in brief about author’s perspective.
Solution: Going through the arcticle, The author has explained about his view on evolution/journey of the vehicle propulsion system from IC engine to Electric vehicles
Initiallty he has described on Internal combustion engine types, and also existance of Electrical vehicles using Lead battery and now it has been replaced by lithium and DC brush motor by either DC brushless or induction.
Then he explained about Brushless motors working.
Next the author has done comparison between Brushless motors and induction motor technologies and mentioned some advantages and disadvantages of each.Unlike the DC brushless rotor, the induction rotor has no magnets. while 3-phase induction motors have great utility,they also have some severe limitations. They cannot operate from DC; AC is a must.
Both DC brushless and induction drives use motors having similar stators. Both drives use 3-phase modulating inverters. The only differences are the rotors and the inverter controls. And with digital controllers, the only control differences are with control code. (DC brushless drives require an absolute position sensor, while induction drives require only a speed sensor; these differences are relatively small.)With DC brushless, as machine size grows, the magnetic losses increase proportionately and part load efficiency drops. With induction, as machine size grows, losses do not necessarily grow. Thus, induction drives may be the favored approach where high-performance is desired;peak efficiency will be a little less than with DC brushless, but average efficiency may actually be better.
Induction machines are more difficult to control. The control laws are more complex and difficult to understand. Achieving stability over the entire torque-speed range and over temperature is more difficult with induction than with DC brushless.
His conclusion is 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 electrics.