Week-11 Challenge: Braking

Objective:

The main objectives are :

1. For a defined driving cycle, calculate the energy required for braking.
2. Why can't electric motors develop braking torque at a high speed similar to starting? How are electric and mechanical brakes coordinated?
3. Make a MATLAB program that plots contours of given motor speed, torque, and efficiency values. Attach the code as a .m file attach a screenshot of all the plots.

Theory:

Braking:

A brake is a mechanical device that inhibits motion by absorbing energy from a moving system. It is used for slowing or stopping a moving vehicle, wheel, axle, or to prevent its motion, most often accomplished by means of friction.

Need of a braking system:

In an automobile vehicle braking system is needed –

• To stop the moving vehicle.
• To de accelerate the moving vehicle.
• For stable parking of a vehicle either on a flat surface or on a slope.
• As a precaution for accidents.
• To prevent the vehicle from any damage due to road conditions.

Types of Braking system:

1. Mechanical Brakes
2. Hydraulic Brakes
3. Electrical Brakes

Regenerative Braking:

It is the type of electrical braking in which at the time of braking the motor which is the main power source of the vehicle becomes the generator i.e. when brakes are applied, the power supply to the motor cuts off due to which the mechanical energy from the wheels becomes the rotating force for the motor which in turn converts this mechanical energy into the electric energy which is further stored in the battery.

• Regenerative braking saves energy and is widely used in today’s electric vehicles.
• Tesla Model-S provides the most effective regenerative braking.

In a battery-powered electric vehicle, regenerative braking (also called regen) is the conversion of the vehicle’s kinetic energy into chemical energy stored in the battery, where it can be used later to drive the vehicle. It is braking because it also serves to slow the vehicle. It is regenerative because the energy is recaptured in the battery where it can be used again.

Procedure:

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

Ans.:

The energy required for braking:

• For the Energy required for the braking, we need to calculate the Braking Energy

Braking energy = `1/2 times mtimes (vunderset(f)()-vunderset(i)())^2`

Where,

m = Mass

`vunderset(f)()` = Final Velocity

`vunderset(i)()` = Initial Velocity

• The above formula is used in an excel sheet stating the different velocities at different times is taken and made into the plot and the plot is observed.

Q.2. Why can't electric motors develop braking torque at a high speed similar to starting? How are electric and mechanical brakes coordinated?

Ans.:

• The Torque Speed characteristic of a motor is not the same in the braking and motoring region.
• At low starting torques in EV, motors have high torque producing capability. With the increase in speed, the torque value decreases.
• During the Starting Stage, the speed values are low and thus torque produced is much higher.
• In the Braking stage, the speed of the motor has high values and the braking torque has low values.
• Braking power is limited by the motor’s power rating and the charging power limit of the battery pack.

Electric and Mechanical Braking Coordination:

During the Initial braking time, mechanical brakes are used in larger proportion than regenerative braking.

To ensure directional stability and enough efficiency in the process of braking, ECE R13 braking regulations constituted by the United Nations economic commission for Europe put forward explicit requirements for front and rear braking force of the dual-shaft vehicle. Under various loading conditions, we must ensure that the braking strength is satisfied.

There are two different brake control strategies namely, series and parallel control. The series-parallel regenerative braking control strategy of hybrid electric vehicles is designed according to the basic idea of parallel braking control strategy as well the characteristics of a regenerative braking system of hybrid electric vehicles and the working characteristics of key components of regenerative braking system such as driving motor and power battery.

Series:

We apply mechanical brakes and electric brakes one after the other.

Parallel:

We apply both mechanical and electric brakes simultaneously. The mechanical braking of the parallel braking system can distribute the braking force of the front and rear wheels in a fixed proportion and the regenerative braking supplement the braking force insufficient on the supplementary driving wheel, which forms the distribution curve of braking force required by the driver. The mechanical friction braking force applied on the front and rear axle is in direct proportion with the hydraulic pressure in the brake master cylinder. Regenerative braking force is a function of the hydraulic pressure in the main cylinder. Regenerative braking force is a function of the hydraulic pressure in the brake master cylinder, which is also a function of the deceleration speed of vehicles braking.

Electric Driven Intelligent Brake:

After stepping on the brake pedal, the system produces a natural and adequate braking force that corresponds to the operation. The energy regenerated needs to be increased. EDIB controls the regenerative brake and friction brake support both of these requirements.

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

Ans.:

• Initially, we open the MATLAB and click Ctrl+ N to open the Editor tab.
• Now, give the input torque and Input speed values.

%Inputs
Speed = linspace(0,1000,500);
Torque = linspace(0,250,500);

• Convert the torque and speed from 3D to the 2D array.

%To convert the 3D and 2D array
[v,T] = meshgrid(Speed,Torque);

• Give the coefficient of Losses values such as Copper loss, Iron loss, Windage (Wind Resistance) loss, and Constant.

%Losses
kc = 0.25;     %Copper loss constant 
ki = 0.008;    %Iron loss constant
kw = 0.0001;   %Windage loss constant
C = 20;        %Constant loss

• Now give the Equations for each of the losses.

%Equation for losses
Copper_loss = kc*(T.^2);
Iron_loss = ki*(v);
Windage_loss = kw*(v.^3);

• Give the Output and Input power equations.

%Output Power Equation
P_out = v.*T;

%Input Power Equation
P_in = P_out + Copper_loss + Iron_loss + Windage_loss + C;

• Give the Efficiency Equation.

%Efficiency
effi = P_out./P_in;

• Now, plot the Speed torque curve with the efficiency of a motor and Efficiency Values at different points of a motor graph.

Code:

clear all
close all
clc

%Inputs
Speed = linspace(0,1000,500);
Torque = linspace(0,250,500);

%To convert the 3D and 2D array
[v,T] = meshgrid(Speed,Torque);

%Losses
kc = 0.25;     %Copper loss constant 
ki = 0.008;    %Iron loss constant
kw = 0.0001;   %Windage loss constant
C = 20;        %Constant loss

%Equation for losses
Copper_loss = kc*(T.^2);
Iron_loss = ki*(v);
Windage_loss = kw*(v.^3);

%Output Power Equation
P_out = v.*T;

%Input Power Equation
P_in = P_out + Copper_loss + Iron_loss + Windage_loss + C;

%Efficiency
effi = P_out./P_in;

% Plotting
figure(1)
box off
grid off 
contourf(v,T,effi);
colorbar
xlabel('Rotor-Speed(RPM)')
ylabel('Torque(N-m)')
zlabel('Efficiency')
title('Speed Torque Curve with Efficiency of an Induction Motor')
saveas(1, 'Speed Torque Curve with Efficiency of an Induction Motor.png')

figure(2) 
surf(v,T,effi)
colorbar
shading interp
view(0, 90)
xlabel('Rotor-Speed(RPM)')
ylabel('Torque(N-m)')
zlabel('Efficiency')
title('Efficiency Values at different points of a motor')
saveas(2, 'Efficiency Values at different points of a motor.png')

Plots:

Result:

1. For the Brake Energy Calculations, at the peak speed of 50 KMPH, we get the braking energy as 96.45KJ.
2. The Braking torque and starting torque differences and the conditions are explained.