Week-11 Challenge: Braking

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

The energy of traction of wheels mainly compromises of drag and braking energy. where braking energy contribute the highest. We can notice that when a standard driving cycle undergoes changes in speed, acceleration and deceleration. The vehicle at many point is brought to rest or decelerated (i.e. 80 km/hr - within 10-15 sec).hence a high braking energy is required. hence the braking energy is also increases with deceleration of vehicle.

Braking: Braking is a condition to slow down or stops a moving vehicle as required. There are several techniques to apply brakes, which can be mechanical or electrical. the most commonly used technique is by applying friction. It consumes energy . The consumed kinetic energy can be derived as following equation.

Braking Energy = (1/2)*m*v^2

Where, braking energy is the kinetic energy in unit of joule. m= total mass of the vehicle (vehicle mass+pay load)in unit of Kg.

v= (initial velocity-final velocity)in unit m/s.

For the example if initial speed of a vehicle is 20m/s and final speed is 10m/s. the mass of vehicle is 500kg . then the required kinetic energy of braking is (1/2)*500*(20-10)^2=25000 J = 25KJ.

please refer the below mention files.

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 characteristics

here for both case of AC or DC there is no torque at maximum speed , gradually the torque decrease at high speed as the speed tends to be maximum speed. this situation does not occurs 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:

(1) Mechanical braking:

Mechanical brakes all act by generating friction forces as two surface rub against each other. the stopping power or capacity of a brake depends largely on the surface area of frictional surface 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 drive . they use levers or linkages to transmit force from one point to another. braking slows or stop the movement of the coupled shaft. 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:

There are commonly used on automobile rear wheels, work when shoes press against a spinning surface called a drum.

3). DISC BRAKES:

There are constructed of brake pads, a caliper and a rotor . during operation the brake pads are squeezed against the rotor.

(2).Electrical braking:

Electrical brakes are device 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 application that require fast response times and precise tension control. the magnet in the braking plate has 2 conductor wire which tap directly into the wiring. When electricity is on, it magnetized the brake magnet. the magnet is attracted to the drum face. When it contacts this area, thw friction causes it to rotate ,which moves the actuating arm ,and pushes the shoes out against the drum. those shoes have a special brake pad material on them that resist the heat caused by the 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 cloud be braked:

1. series - regenerative braking and mechanical braking both are applied one after another.

2. Parallel - 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.

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.

matlab code:

clear all

close all

clc

% SPEED ARRAY

w = linspace(0,1000);

% TORQUE ARRAT

T = linspace(0,300);

% LOSS COEFFICIENT

kc = 0.20; % COPPER LOSSES

ki = 0.008; % IRON LOSSES

kw = 0.000020; % 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')

output:

From the output graph you could notice that only at particular speed and torque the efficiency is at maximum i.e. 91% where speed x= 60.6061 rad/sec @ 24.2424 Nm torque. As the speed increase the efficiency of the motor will be reduced.

Hence, to attain maximum efficiency the motor has to constantly run at around 151 rad/sec speed @54.5455 Nm.