Week-11 Challenge: Braking

1.For a defined driving cycle, calculate the energy required for braking.

Let us consider a Vehicle.

Mass*(m) of vehicle=120kgs

Drive cycle of vehicle: Speed Vs Time

X-Axis: Time(Sec)

Y-Axis:velocity(Kmph)

V*(5/18) m/s

Let 3 points of braking is considered

Energy Loss for braking= `1/2m(V_f-V_i)^2`

1.At t=112sec to 122sec

energy loss(E)=0.5*120*(18.9-0)^2

=21.43 KJ for 10 secs from 68kmph to 0kmph

2.At t=245sec to 265sec

E=0.5*120*(10-0)^2

=6KJ for 20 sec from 36kmph to 0kmph

2.At t=428sec to 432 sec

E= 0.5*120*( 7.23)^2

=3.13KJ for 4sec from 56kmph to 30kmph

Total Energy loss for braking :

`E_(brake)`=21.43+6+3.13

=30.56 KJ.

2. why electric motor can’t develop braking torque at high speed similar to starting? How electric and mechanical brakes are coordinated?

• Braking torque in an electric motor is not as high as acclerlation at high speed.Due to the following reasons.
• Rate of Discharge is greater than the charging rate of battery (i.e., Discharge=2*charging rate).
• Power inverter used in EV are bidrectional As it is used to convert regenerative power from braking to DC source(battery). By this Inverter efficiency is decreased.
• At Low speeds, regenerative braking is very high, As speed increases efficiency of regenerative braking decreases. By this baraking torque also decreased.
• At high speeds both Hydraulic brakes(mechanical) and regenerative braking are used.

There are two methods of control strategies for coordinating the Hydraulic and regenerative braking.

1.Series : The braking is applied one after the other either of Hydraulic and regenerative braking alternatively.

2.Parallel: Both brakes are applied at the same time.

Coordination between hydraulic and electric braking is important as charging rate of current for battery is Limited. So, while braking current towards the battery is limited by applying electric brake along with hydraulic brake. .

When brake pedal is applied the fluid in hydraulic disc is comes out to slow down the vehicle. And by decreasing accleration apllied the required amount of electric brake is applied to supply power to the source.

At high speeds as Amount of braking required is high. so, hydraulic braking is used, When speed is decreased to an amount torue of vehicle is increased then electric brake comes into picture to reversal energy to battery efficiently

The ECU system with a feedback finds the high torque region to coordinates the both brakes as follows.

• Here we can clearly observes that at high speeds amount of frictional brakes are applied is higher than the electric brakes. As speed decreases role of regenerative brake increases.

3.Make a MATLAB program which plots contour of given motor speed, torque and efficiency values. Attach the code as a .m file attach a screenshot of all the plots.

MATLAB Program for contour plots of motor Speed, torque and efficiency.

close all
clc
x=linspace(1,1000);
y=linspace(1,250);
kc=0.2;
ki=0.08;
kw=0.00001;
ConL=20;
[X,Y]=meshgrid(x,y);
outputpower=(X.*Y);
B=(Y.^2)*kc;
C=X*ki;
D=(X.^3)*kw;
Inputpower=outputpower+B+C+D+ConL;
Z=outputpower./Inputpower;
V=[0.70,0.80,0.85,0.90,0.91,0.93,0.94,0.96];
box off
grid off
contour(X,Y,Z,V);
xlabel('speed/rad.s^-^1'),ylabel('Torque/N.m');

• Here all losses constants are taken and calculated. Later its output power is calculated and efficiency is determined.
• Then contour is plotted.

Contour plot:

• Here the various Efficiencies are Plotted in the Contour plot.