Week-11 Challenge: Braking

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

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

STUDY#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.

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

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

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

There are two types of braking

1: Electrical braking

2: Mechanical braking

This two braking’s are required to halt the vehicle from a given speed to zero speed

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

Suppose we have NREL Class 3 Electric Vehicle Cycle

This graph shows speed with respect to time.

For above driving cycle, it is required to calculate 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 .

STUDY#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 developing 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 therefore 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 therefore Torque value is very high. At the time of braking, speed value is high therefore torque value will be low. Therefore at the time of braking torque value available will be lower.

In order to stop the vehicle it requires electrical as well as mechanical brakes.

There are two methods to apply this brakes:

1: Series braking

2: Parallel braking

Series braking: In this mechanical and electrical (Regenerative) braking’s are applied one after the other.

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

At lower speeds Regenerative braking’s are used and at higher speeds mechanical brakes are used.

STUDY#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:

% A MATLAB PROGRAMM FOR PLOTTING CONTOUR PLOT OF MOTOR SPEED, TORQUE & Efficiency

clear all

close all

clc

 

% SPEED ARRAY

w = linspace(0,17000);

 

% TORQUE ARRAY

T = linspace(0,25000);

 

% LOSS COEFFICIENTS

kc = 0.18;  % COPPER LOSSES

ki = 0.006; % IRON LOSSES

kw = 0.000015; % WINDAGE LOSSES

 

% MESHGRID

[X,Y] = meshgrid(w,T);

copper_loss = (Y.^2)*kc;

iron_loss = X*ki;

windage_loss = (X.^3)*kw;

K = 20;

output_power = X.*Y;

input_power = (copper_loss)+(iron_loss)+(windage_loss)+(output_power)+K;

Eff = (output_power)./(input_power);

n = linspace(0.7,0.95,10);

contourf(X,Y,Eff,n);

title('SPEED TORQUE CHARACTERISTICS')

xlabel('SPEED rad/s')

ylabel('TORQUE N-m')

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 value are increases efficiency value decrease from above counter.

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