Week-11 Challenge: Braking

1)

1. For a defined driving cycle, calculate the energy required for braking.
2. Why electric motor can’t develop braking torque at high speed similar to starting? How electric and mechanical brakes are coordinated?
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.

Aim:

To calculate the energy required for braking for a defineed ddriving cycle.

Simulink model:

Methodology:

1)

Using Drivecycle source block to input the speed vs time as input.

Select the desired drive cycle.

E= ( m (Vi - Vf)^2)/2

m - Mass of the vehicle (Kg)

Vi - initial veloicity (m/s)

Vf - final velocity(m/s)

E - Braking Energy (J)

While braking, the speed of the vehicle is less than that of the previous time step. In order to check this, delay block and relational operator is used.

If this condition is true, then the above formula E= ( m (Vi - Vf)^2)/2 is being simulated. This is done by switch block.

Else, the previous value is fed back using delay block.

The output is being added with previos value to get the total energy required in braking.

Before simulating, the stop time is entered as the last data of the drive cycle

Output:

2)

The speed - torque curves for positive and negative voltages are given in the above figure. In the positive sequence voltage, the operation above synchronous speed gives the regenerative braking operation (BAE)Similarly, with a negative sequence voltage across the motor terminals, regenerative braking is obtained for speeds above the synchronous speed in the reverse direction (portion bae).

• When brake is applied, power supply to the motor is being cut off and now the motor will be in the rotation due to it inertia. As a result of rotation in the magnetic field , a Back Emf is generated which produces a negative Torque and that used as the brakes to reduce the vehicle speed. Also the current flows in the reverse direction, Using a Bi-directional Inverter we can store the current back into the battery.
• During initial raise in speed, the Torque is high that the motor operates in the constant Torque region. During accelartion, after reaching a certain speed the torque gradually decreases which is known as the constant Power region.At high speed , when brake is applied , the torque is low and a negative torque produced by the motor is less.
• At higher speeds the motor operates in constant power region and additional load could not be imposed, the torque could not be increased

• Besides this, Battery Limitations plays a role in which, due to which we cannot get High braking torque also the rate of discharge is always higher than the rate of charging due to the battery's chemical compositions.

Electric and mechanical brakes co-ordination :

The braking stability of electric vehicles with electro-hydraulic hybrid braking systems can be influenced by braking force distribution between hydraulic braking force and regenerative braking force

Electric cars have regenerative braking and this does substantially reduce the need for hydraulic braking. But there are some limitations like the maximum deceleration with the engine alone is far smaller than with brake assistance So you still need an ordinary Fractional brake system to make the car stop instantly if required.. When the regenerative capacity is less than the actual braking required, mechanical brakes are applied at those times.

• During low brake pedal force, only the regenerative braking torque is applied on the driving wheels and is proportional to the pedal pressing force. The braking torque on the non-driving wheels is always due to hydraulic braking which is also proportional to the pedal pressing force.
• When the pedal force is beyond a certain limit, the maximum regenerative braking torque is applied on the driving wheels, and the hydraulic braking torque is simultaneously applied on the driving wheels to top up the desired braking torque. Meanwhile, the maximum regenerative braking torque is kept constant to fully recover the kinetic energy

There are two ways of applying the combination of brakes and they are classified as,

1. Series - Regenerative braking and Frictional braking are applied one after another

Serial regenerative braking is basically based on the combination of friction-based adjustable braking system that transfers energ to the electric motors and batteries under an integrated control strategy. The overall design of serial regenerative braking is to estimate the deceleration required and distribute the required braking force between the regenerative braking system and the mechanical braking system. The serial braking system requires brake by wire system and has a more consistent pedal feel due to good torque blending capability. It can increase the efficiency by 15-30%.

2. Parallell - Regenerative braking and Frictional braking are applied at the same time . A parallel braking system is basically based on the combination of a friction-based system and the regenerative braking system operated in tandem without an integrated control. The regenerative braking force is calculated from the brake control unit b comparing the requested brake torque and the motor torque available. A parallel regenerative braking system requires more work in achieving good torque blending. This system can increase the efficiency by 9-18%.

We see that both the Regenerative Brakes and the Frictional brakes are applied at the same time. After braking the speed reduces from higher speed values and at this moment the % of the composition of the combined braking changes. So after brake application, the speed is reduced to low speeds. Here Torque will be high and the motor will be operating in a constant Torque region. Now at this point, the % of Frictional brake application is reduced and Regenerative Braking is Increased because at this point more Negative Torque can be generated and thus higher electric power generated out of this braking can also be fed back to the battery. Thus using this technology in Braking an electric driven vehicle

3)

Copper Losses - `I^2*R`Where R is armature resistance and we say Torque is proportional to current, this copper loss can be considered as. where is to take care of resistance of brushes and also take care of ux and its effects.

copper losses = `Kc*T^2`

Iron losses - Iron losses are nothing but magnetic losses, which is denoted as, where the factor is based on the magnetic eld effect. for permanent magnets have constant magnetic eld and iron losses is based on the value and this is going to change according to the speed.at various speeds have a different value of back EMF and that is going to affect the iron losses.

Iron losses = `Ki*omega`

Windage losses - This is a mechanical or friction of windage losses this is based on the size of the motor and the number of windage losses that the motor is going to experience

Constant losses -

this denoted as C

Total losses are considering the above four losses and denoted as,

Total losses = `Kc*T^2 +Ki*omega+K*omega^3+ C`

All the motors are model as per the above equation of losses. This above equation is not only applicable for permanent magnet DC motor but other as well like induction motor

%motor charactersistcs 
%speed_vs_torque_vs eficciency

clear all
close all
clc

w = linspace(0,1000);   %speed of the motor
t= linspace(0,250);     %torque 

%Motor Losses 
%for copper loss
k_c = 0.2;
%for iron loss
k_i = 0.008;
%for winding loss
k_w = 0.00001;
% constant loss
cl = 20;

% Making 2d matrix
[W,T] = meshgrid(w,t);

% Output power = torque*Speed
Out_power = (T.*W);
% Copper Loss
b = (T.^2)*k_c;
% Winding Loss
c = (W.^3)*k_w;
% Iron Loss
d = W*k_i;

%  Input Power
In_power = Out_power +b +c +d +cl;
% Efficiency
E= Out_power./In_power;
%Efiiciency levels for contour
e=[0.70,0.75,0.80,0.82,0.87,0.89,0.90,0.92];

%plotting the graph 
%Creating Contour
[C,h]=contourf(T,W,E,e);
clabel(C,h,'labelspacing',300); % plotting the effeciency values and its scpacing 
xlabel('Speed');
ylabel('Torque');
title('Motor speed,Torque and Efficiencies')

Result:

Thus the motor characteristics are plotted successfully: