Week-11 Challenge: Braking

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

ans.

At first drive cycle is determined in a table and a plot is made. Using the plot the values from the braking regions of a vehicle are taken. The plot of the drive cycle is given below:

for total values see drive cycle.xls file attached below

Drive cycle

vehicle weigth - 1000kg

Total braking energy required (Wh) - 148.9

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

ans.

This situation occurs only when a vehicle is at high speeds because the motor will be running at a constant power region. In the constant power region the top of the motor reduces and so the motor cannot develop the braking torque which is equal to the torque that occur during low speed at the constant torque range.
The torque vs speed plot of a vector control induction motor is given below.

From the above characteristic curves of the motor that is seen that the torque of the motor remains high at low speed and reduces as the speed increases.
Since the electric motor cannot develop braking torque at a high speed similar to starting.

Electrical brake and mechanical brake

Electrical braking

Electrical braking can be done by many methods such as:

• Unbalancing the input points of current to the motor
• using transistors in H bridge arrangement to reverse the motor direction in order to reduce speed
• Implementing a regenerative braking system by using bi-directional power converters
• Regenerative braking converts much of the energy to electrical energy which may be collected for later use

Mechanical Braking

Mechanical brakes commonly used friction between two surfaces are pressed together to convert the kinetic energy of the moving object into heat, do other methods of energy conversion may be employed. The friction between the surfaces is performed using pressurized air or pressurized oil which are commonly known as an air brake and Hydraulic brake respectively.

Coordination of braking in EVs

Two types of brake control strategies are used to organise the electrical brake and mechanical brake they are

• Series control strategy
• Parallel control strategy

The comparison of the control strategies is shown in the below figure
In the figure it is seen that it according to the amount of brake pedal travel of the vehicle the mechanical brakes and regenerative brakes are employed.

Parallel control strategy

When the brake is applied pedal travel the front wheel friction brakes and the regenerative brakes are employed simultaneously. The higher the travel of the brake pedal the higher the amount of breaking will be done.
The breaking energy is distributed in the following practice
Rear wheel friction brake----------> front wheel friction brake-------> regenerative brake

Series control strategy

When the brake is applied according to the brake pedal travel the front wheel fiction break the rear wheel friction brakes and the regenerative brakes are applied one after another. As the braking requirement increases the friction brakes are used subsequently.

For low speeds of the vehicle regenerative braking is used but it may be Limited according to the SOC (state of charge) of the battery.
As the speed of the vehicle increases the real wheel fiction will be employed if the breaking requirement is not then but then the front wheel friction brake is also used.
The coordination of series control braking can be done with the choice to comfort of breaking or energy storage.

Electric driven intelligent brake(EDIB)

EDIB is a brake control strategy which is developed by Nissan. It is a type of parallel braking which can be seen in below image.

The above image shows that the fictional Break and the regenerative brake are used simultaneously at higher speeds of the vehicle.

At low speed the torque of the motor is high and so the regenerative brake has a higher braking part than the fictional breaks.

Brief working of EDIB

When the driver steps on the brake pedal the stroke sensor detects the operation of the brake. That report is communicated to the ECU and controls the motor. The motor engages the Piston through the ball screw and increases the friction brakes pressure (hydraulic pressure). Optimum control of the friction brakes hydraulic pressure the energy regeneration result is maximized.

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.

Matlab code

%A matlab program for plotting contour plot of motor speed, torque and
%efficiency
clear all
close all
 
%speed array
w = linspace(0,1000);

%torque array
T = linspace(0,300);

%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')

Plot results

From the output graph you could see that only at a particular speed and torque the efficiency is at maximum ie 95%.
Where, Speed x = 151rad/s @ 24Nm torque. As the speed increases the efficiencies of the motor will be reduced.

I have attached contour.m file below.