Week-11 Challenge: Braking

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

Objective: To calculate the enregy required for braking for a defined driving cycle.

Explanation: Electrical vechiles requires some energy to stop the vechile at any instant of time. This energy is called Braking energy.

There are two types of braking

1: Electrical braking

2: Mechincal braking

These two brakings are required to halt the vechicle from a given speed to zero speed

Energy required for a braking =0.5 * mass*velocity^2

Suppose we have NREL Class 3 Electric Vechicle Cycle

This graph shows speed with respect to time.

For above driving cycle, it is required to caliculate Braking energy. Above data points are converted into excel format as shown in below figure.

At various speeds the braking energy is calculated using formula of 1/2*m*v^2 .

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

Ans: There are two major reasons for a motor devloping higher starting torque compare to braking.

1) Due To Transients:

At starting the motor draws a large current due Transient . As torque is proportional to square of current, therefore torque value is also high at this moment.

When we applies brakes to motor at that time current value is within the range therfore Torque values is also within the range.

2) Torque speed characteristic:

From Torque speed characteristic when speed increases torque should decrease to maintain power constant. At the time of starting speed is very low therfore Torque value is very high. At the time of braking, speed value is high therfore torque value will be low. Therfore at the time of braking torque value availabe will be lower.

In order to stop the vechicle it requires electrical as well as mechincal brakes.

There are two methods to apply this brakes:

1: Series braking

2: Parallel braking

Series braking: In this mechincal and electrical(Regenerative) brakings are applied oneafter the other.

Parallel Braking: In this both Braking methods are applied at a same time.

At lower speeds Regenerative brakings are used and at higher speeds mechanical brakes are used.

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.

Ans: To write a matlab program to plot contour for a given motor speed, torque, efficiency.

Program:

clc;
w= linspace(0,600);% speed array
T= linspace(0,800); %torque array
% loss coffiecients
kc=0.18;
ki=0.08;
kw=0.0005;
% mesh grid
[X,Y]=meshgrid(w,T);% converting 1d to 2d mesh
coperloss = (Y.^2)*kc;
ironlosee = X.*ki;
windagelose = (X.^3)*kw;
constantloss = 21;
output_power = X.*Y;
inputpower = coperloss+ ironlosee + windagelose + constantloss + output_power;
efii= output_power./inputpower;

n = linspace(0.7,0.9,10);
box off;
grid off;
contourf(X,Y,efii,n);
xlabel('speed rad/s');
ylabel('Torque Nm');

Results:

Inner counter yellow one indicates higher efficiency compare to outer counter.

As a torque speed values are increases efficiency value decrease from above counter.

For a low speed and low torque values, efficeincy of motor is very high.