Week-11 Challenge: Braking

AIM: TO STUDY EV BRAKING CHARACTERISTICS

OBJECTIVES:

• For a defined driving cycle, calculate the energy required for braking.
• Why electric motor can’t develop braking torque at high speed similar to starting? How electric and mechanical brakes are coordinated?
• Make a MATLAB program which plots contour of given motor speed, torque and efficiency values.

OBJECTIVE 1:

We can calculate the energy required for braking of a vehicle for a defined driving cycle easily by using MS Excel.

The formula for Braking Energy is as follows:

Braking Energy = (1/2) *M*V^2

Where,

M = Mass (vehicle + load) in kg

V = (initial velocity – final velocity) in m/s

To convert m/s to kmph we have to multiply the value by 3.6 because the units have to be same.

The following is the excel sheet for the calculation of the total Braking Energy consumed.

OBJECTIVE 2:

Why electric motor can’t develop braking torque at high speed similar to starting?

When the motor is at rest and it has to operate from standstill position to the reach the moderate speed, a higher torque is required to bring the motor in operation. Whereas, when the vehicle is already at high speed, the capping or the limit is reached by the motor operation and thus, any further higher torque could not be obtained while braking. The braking operation happens in the motion which is been distributed as per proportion in the BRAKE ENERGY and the REGENERATIVE BRAKING.

How electric and mechanical brakes are coordinated?

In Mechanical braking, which we usually see and experience in all vehicles the brake pedal in the vehicle cabin helps us to engage or disengage the brakes. Currently, the braking technology has done wonders by inventing lifesaving braking technologies such as ABS, EBD, BA and many others. All these technologies play a vital role in stopping the vehicle and also coming back to rest from a high speed. In all these vehicles, when the driver applies the brakes the brake peds come closer to the disc connected with the driving disc. The brake pads usually help in efficient braking.

There are two generic methods by which the vehicle could be braked in the ELECTRIC BRAKES.

Series - The Regenerative Braking or the Mechanical Braking are applied simultaneously i.e., one after the other.

Parallel - The Regenerative Braking or the Mechanical Braking both are applied Parallelly i.e., at a time.

When the brake pedal is pressed the hydraulic fluid reaches the brake piston and creates the braking pressure on the wheels and the same kinetic energy is been used by the Regenerative Braking for generating more power using the Bi-directional motor which operates as a generator while braking.

An example of the proposition of Regenerative Braking and the Mechanical Braking

NISSAN LEAD - EDIB (ELECTRONIC DRIVEN INTELLEGENT BRAKING)

EDIB (Electric Driven Intelligent Brake) controls the regenerative brake and friction brake to support both of these requirements. Further, it also controls the reactive force from the pedal in order to unify the feeling when stepping down on the pedal and the sense of deceleration.

OBJECTIVE 3:

Contour is a graphical technique of representing a 3-dimensional surface.

The following is a MATLAB Program which plots contour for a given Motor Speed-Torque characteristics along its Efficiency.

MATLAB program for obtaining Contour Plots:

clear all
close all
clc 

%Speed Array in rad/s
x = linspace(1,1000); 

%Torque Array 
y = linspace(1,400); 

%Losses Coefficients
kc = 0.16; % For copper losses
ki = 0.010; % For iron losses
kw = 0.00005; %For windage losses

%Meshgrid
[X,Y] = meshgrid(x,y);
C = (Y.^2)*kc; %Copper Losses
I = X*ki; %Iron Losses
W = (X.^3)*kw; % Windage losses
ConL = 20; %constant motor losses

Outputpower = (X.*Y); % Torque x Speed = Power
Inputpower = Outputpower + C + I + W + ConL; %Summation of all losses
Z = Outputpower./Inputpower; %Efficiency

% We now set the efficiencies for which a contour will be plotted
n = linspace(0.7,0.93,10);

box off
grid off
contour (X,Y,Z,n);
xlabel('Speed (rad.s^-^1)');
ylabel('Torque (N.m)');
title('Speed - Torque characteristics');
hold on

OUTPUT:

• Used linspace command to generate values for Torque and Speed.
• We’ve taken user defined values for kc (copper loss coefficient), ki (iron loss coefficient), kw (windage loss coefficient) and ConL (constant motor losses) to calculate respective losses i.e., copper loss, iron loss and windage loss.
• A mesh grid is formed with X axis as Rotational Speed and Y axis as Torque.
• Output Power is the multiplication of Torque and Speed values. And, Input Power is the sum of the losses and output power.
• To form a contour, different variable values are assigned for efficiency. Here, the “n” which contains efficiency values are plotted on w-T plane.