Week-6 Challenge: EV Drivetrain

ANS 1 :

1. Which types of power converter circuits are employed in electric and hybrid electric vehicle?

Power Electronic Converters :

In Electric and Hybrid Electric vehicle (EV/HEV), the power electronic convertors 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 EVs power system, advanced DC/DC and DC/AC converters are required to adopt the output voltage and currents levels with high power quality.

The 3 types of converters which are widely used in an Electric Vehicle are :

AC/DC Converter(On Board Charger/Rectifier) :

AC to DC converters are most commonly used in Plug-in-Hybrid Electric Vehicle (PHEV) and 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/DC converter is employed in a PHEV and PEV.

DC/DC Converters(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 converters one can choose the voltage variation of the devices and the power such as top speed, acceleration time from 0 to 100km/h, weight, maximum torque and power profile (peak power, continuous power).

DC/AC Convetrs (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 using inverter to be used in electric motor (AC current).

ANS 2 :

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%?

Vehicle Output Voltage (Vv) is given by :

Vv = Battery Voltage*Duty Cycle;

Vv= 72*0.70;

i.e Vv= 50.4 V;

Now, Equation for Vehicle torque is given as :

Tv = (Vv-(w*K)).K/Ra; (1)

Where,

Tv = Vehicle Totque (N-m);

Vv=Vehicle Voltage (Volts);

w=Steady State Speed of Vehicle (rps);

K= Motor constant;

Putting all these values in equation (1), we get

Tv= (50.4-(0.7*w))*0.7/0.5;

Tv= 70.56-(0.98*w); (2)

and, Tv=24.7+(0.0051*(w0)^2); (3)

Solving equation (2) and (3), we get:

0.0051*(w)2+(0.98*w)-46.86=0;

Solving, this quardratic equation, we get:

w=38.82 rad/s and w= -230.98 rad/s;

we take the positive value, so:

Steady state value of vehicle = 38.82 rad/s.

ANS 3 :

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

ω= V/Kϕ -Ra/Kϕ^2 .T

Mathematical model of a DC Motor :

Output :

ANS 4 :

4. Refer to the blog on below topic:

Induction Versus DC brushless motors by Wally Rippel, Tesla

Explain in brief about author’s perspective.

1. In the beginning author describes the dominance of ICE in the worldof automobile industry and also discuss different types of engines. He explains that all engines were similar and it was up to select according to their requirement. He also speaks about motor and brushless DC motors and presents his view that these two technologies will be widely used for the coming years.

2. Then he explains the working of both 3-phase induction motor and a brushless DC motor.

3. Now the author has done comparison between there two technologies an dstated some advantages and disadvantages of each.

4. Induction motor has 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 can not operate from DC, AC is must. Shaft speed is propotionate 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 somewhat limited running peak torque capabilities when compared to DC type machines.

5. 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 DC motor drive will typically be a few percentage points higher than for an induction drive. Some disadvantage of Brushless DC motor are: as machine size grows, the magnetic losses increase propotional and part load efficiency drops. Permanent magnets are expensive and difficult to handle due to very large forces that came into play when anything ferromagnetic gets close to mthem.

6. Finally the author could not 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 electrics.