Week-11 Challenge: Braking

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

BRAKING

The function of the braking system is to retard the speed of the moving vehicle or bring it to rest in the shortest possible distance whenever required

Brakes are mechanical devices for increasing the frictional resistance that retards the turning motion of the vehicle wheels

It absorbs either kinetic energy or potential energy or both while remaining in action and this absorbed energy appears in the form of heat.

While moving down a steep gradient the vehicle is controlled by the application of brakes. In this case, brakes remain in action for a longer period making it imperative to dissipate the braking heat to the atmosphere as rapidly as possible.

Automobiles are fitted with two brakes; the service or foot brake and the emer­gency or hand brake.

The foot brake is used to control the speed of the vehicle and to stop it, when and where desired, by the application of force on the brake pedal.

The hand brake, applied by a lever, is used to keep the vehicle from moving when parked. Hand brakes are called emergency brakes because they are applied when the service brake fails.

Virtually all vehicles are now equipment with 4-wheel brakes. The front brakes must operate without interfering with the steering action.

Before calculating the Braking Energy of a particular vehicle ,we need to create an excel sheet for a drive cycle.

Formula:

braking energy: (1/2) *m*((Vi-Vf)^2)

Where,

m=Mass of the vehicle(kg)

Vi=Initial velocity(m/s)

Vf=Final velocity(m/s)

total braking enrgy=102500 J

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

Ans:The electric motor can't develop braking torque at high speed similiar to starting because once the electric supply is pulled off from the motor,it acts as a generator and due to the flux weakening effect of the electric generator,the generator produces low torque as expected.

Regerative Braking:

Regenerative braking is an energy recovery mechanism that slows down a moving vehicle or object by converting its kinetic energy into a form that can be either used immediately or stored until needed. In this mechanism, the electric traction motor uses the vehicle's momentum to recover energy that would otherwise be lost to the brake discs as heat. This contrasts with conventional braking systems, where the excess kinetic energy is converted to unwanted and wasted heat due to friction in the brakes, or with dynamic brakes, where the energy is recovered by using electric motors as generators but is immediately dissipated as heat in resistors. In addition to improving the overall efficiency of the vehicle, regeneration can significantly extend the life of the braking system as the mechanical parts will not wear out very quickly.

"Electric motor can't develop braking torque at high speed" - Reason:

Now Speed torque characteristics of motor is shown below,

Here it is clearly shown in the first graph that till the motor reaches the critical speed it has constant torque and once it reaches the critical speed, the curve is having a down slope which is a constant power region. And this graph denotes the accelerating operation of the motor. For the deceleration or braking operation of the motor, this graph is mirrored down which will be having the speed (+ve) and torque (-ve).

For example,

we can say that an EV can achieve 0-100 kmph in 10sec. Acceleration operation goes this way, with low speed and high torque Constant torque region is achieved which has accelerating power and by reaching the critical speed it shifts to constant power region and thus it easy for an EV to reach 0-100 kmph.

As said the breaking operation graph will be th mirrored image of above plot. So we cannot say that the motor producing 7Nm torque for acceleration can also produce -7Nm for breaking. Because we are operating in the field weakening region, and operating above the base speed. Thus Peak Acceleration torque is greater than the Peak Deceleraion torque. Thus for acceleration it has high torque with low speed and for deceleration it has low torque for high speed.

Electric and mechanical brakes are coordination:

There are two brake control strategies namely, SERIES & PARALLEL.

SERIES:

Brakes will be applied one after the another so which they are connected in series. Either one brake stragety at a time either Regenerative braking or Mechanical braking at a time.

PARALLEL:

In case of parallel both the brakes are applied simultaneously, Both at a time.

ELECTRIC DRIVEN INTELLIGENT BRAKE:

And Nissan has developed a practical brake strategy called ELECTRIC DRIVEN INTELLIGENT BRAKE developed by Nissan leaf.

3)To make a MATLAB program which plots contour of given motor speed, torque and efficiency values.

MATLAB CODE;-

speed = linspace(1,1500); % motor's speed parameter
torque = linspace(1,250); % motor's torque parameter
volt = linspace(0.9,2,40); % voltage parameter
% linspace command used to obtain value from its x1 value to x2 values

Kcl = 0.05; % Assigning copper losses of motor
Kil = 0.10; % Assigning Iron losses of motor
Kwl = 0.00001; % Assigning windage losses of motor
Kct = 20; % Assigning constant losses of motor

[speed,torque] = meshgrid(speed,torque); % plotting in 3D axis using meshgrid command
powerout = (speed.*torque); % defining output power

cl = (torque.^2)*Kcl; % copper loss
il = (speed.*Kil); % iron loss
wl = (speed.^3)*Kwl; % windage loss

powerin = powerout+cl+il+wl+Kct; % defining input power
efficiency = powerout./powerin; % defining efficiency

contour(speed,torque,efficiency,volt); % plotting contour of all parameters
xlabel('SPEED(RAD/SEC)');ylabel('TORQUE(N-M)'); % labelling the x and y axis respectively
title('PLOTTING CONTOUR OF SPEED,TORQUE,EFFICIENCY'); % assigning title to the plot
hold on

OUTPUT;-

CONCLUSIONS:

1. For a defined driving cycle, the energy required for braking is calculated.
2. Why electric motor can’t develop braking torque at high speed similar to starting and how electric and mechanical brakes are coordinated were dicussed.
3. A MATLAB program which plots contour of given motor speed, torque and efficiency values is coded.