Week-11 Challenge: Braking

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

A driving cycle is a series of data points representing the speed of a vehicle versus time. Driving cycles are produced by different countries and organizations to assess the performance of vehicles in various ways, as for instance fuel consumption, electric vehicle autonomy, and polluting emissions.

Here we consider a drive cycle data and use that to find the energy required for breaking.

We know Energy E=12mv2">E=12mv2E=12mv2

m is the total mass

Here V is the vehicle initial velocity subtracted by the present velocity.

The drive cycle graph is given below

The total mass of the vehicle is considered to be 1000kg

Applying the equation of energy to the drive cycle we use an Excel sheet.

This excel sheet is attached.

The Output plot is given below.

Here the energy required is maximum up to 160J and it is instantaneous in nature.

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

DC motor Torque vs Speed Characteristic

AC motor Torque vs Speed Characteristic

Here for both in case of AC or DC there is no torque at maximum speed, gradually the torque decreases at high speed as the speed tends to be maximum speed. This situation does not occur at the beginning of acceleration because most of the torque is used to increase the kinetic energy of the load (and motor). But when the speed is already high and no extra load is applied it is not possible to increase the torque as well as to develop braking torque. The braking operation happens in the motion which is been distributed as per proportion in the Brake Energy and the regenerative braking.

There are two types of braking :

(i)Mechanical Braking> Mechanical brakes all act by generating frictional forces as two surfaces rub against each other. The stopping power or capacity of a brake depends largely on the surface area of frictional surfaces as well as on the actuation force applied. Mechanical brakes are assemblies consisting of mechanical elements for the slowing or stopping of shafts in equipment drives. They use levers or linkages to transmit force from one point to another. Braking slows or stops the movement of the coupled shafts. There are several types of mechanical brakes.

1)Band brakes>> These are the simplest brake configuration, have a metal band lined with heat and wear resistant friction material.

2)Drum brakes>> These are commonly used on automobile rear wheels, work when shoes press against a spinning surface called a drum.

3)Disc brakes>> These are constructed of brake pads, a caliper, and a rotor. During operation, the brake pads are squeezed against the rotor.

(ii) Electrical Braking> Electric brakes are devices that use an electrical current or magnetic actuating force to slow or stop the motion of a rotating component. They are used in industrial and vehicular braking applications that require fast response times and precise tension control. The magnet in the backing plate has 2 conductor wires which tap directly into the wiring. When electricity is on, it magnetizes the brake magnet. The magnet is attracted to the drum face. When it contacts this area, the friction causes it to rotate, which moves the actuating arm, and pushes the shoes out against the drum. Those shoe have a special brake pad material on them that resists the heat caused by that friction. When the shoes press against the inside of the drum, they prevent the hub, and consequently the wheel that's touching the ground.

A very beautiful electric braking scheme called Regenerative Braking is now being employed for electric braking. Here during the braking action, the motor is operated as generator. As we know that during generating action, a counter torque is produced. So this counter torque opposes the motion of motor and motor comes to halt. Moreover, the brake energy is converted to electrical energy which can be fed back to the source using power converters.

Electric and Mechanical brake coordination>

There are mainly 2 ways by which the Electric Vehicles could be braked.

Those are :

(i) Series- Regenerative braking and mechanical braking both are applied one after another.

(ii) Parralel- Regenerative braking and mechanical braking both are applied simultaneously.

When brake pedal is pressed the hydraulic fluid reaches the brake pistons and created 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.

In Nissan Leaf there is EDIB( Electric Driven Intelligent Brake ), where both mechanical braking and regenerative braking took place in a more efficient way.

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

Here we consider

Loss Coefficients

For Copper Loss (kc)=0.2

For Iron Loss (ki)=0.008

For Windage Loss(kw)=0.0001

and Constant Loss =20.

The Equation of Losses are given below

CopperLoss=kc∗T2CopperLoss=kc∗T2

IronLoss=ki∗ωIronLoss=ki∗ω

Windage Loss=kw∗ω3=kw∗ω3

Total Loss = Copper Loss+Windage Loss + Iron Loss+Constant Loss

Output Power = Torque x Speed

Input Power = Output power +Losses

Efficiency = Output Power/Input Power

The Code used for the plot is given below

clear all
close all
clc
w=linspace(0,1000);
T = linspace(0,300);
kc=0.2;
ki=0.008;
kw=0.00001;
ConL=20;
[X,Y]=meshgrid(w,T);
copper_loss=(Y.^2)*kc;
iron_loss=X*ki;
windage_loss=(X.^3)*kw;
o_p=X.*Y;
input_power=copper_loss+iron_loss+windage_loss+o_p+ConL;
Eff=(o_p)./(input_power);
n=linspace(0.7,0.95,10);
box off
grid off
contour(X,Y,Eff,n)
title('Speed Vs Torque')
xlabel('Speed (Rad/Sec)')
ylabel('Torque (Nm)')

graph :

We can observe Speed(Rad /Sec) in X-axis and Torque(Nm) in Y-axis

Each contour represents Efficiency. Some of the values are displayed with the data cursor in the plot.

CONCLUSION

- Performing calculations for braking energy.

- Understanding of electric and mechanical braking.

- Code for the plotting of torque, speed and efficiency behaviour.