Week-11 Challenge: Braking

Aim :- Week-11 Challenge: Braking

Objective :-

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.

Solutions :-

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

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Drive Cycle :-

• A driving cycle commonly represents a set of vehicle speed points versus time. It compromises of the Drag and Braking Energy.
• It is used to assess fuel consumption and pollutants emissions of a vehicle in a normalized way, so that different vehicle can be compared.
• Motor is evaluated through a set of motor torque and speed points instead of vehicle speed points.
• There are different kinds of driving cycles, the modal cycles as the European standard NEDC, or Japanese 10-15 Mode, and the transient cycles as the FTP-75 or Artemis cycle.
• Driving range calculation, state of charge estimation, energy consumption prediction, and energy management optimization strategies for EV under the international standard driving cycles can produce large errors.
• The vehicle at many points is brought to rest or decelerated (i.e. 80 Km/hr - 0 km/hr) within 10- 15 sec. Hence high braking energy is required. 

Calculate the energy required for braking :-

Regenerative braking systems (RBSs) are a type of Kinetic energy vehicle that transfers the kinetic energy of an object in motion into a potential energy vehicle to slow the vehicle down, and as a result, increases fuel efficiency These systems are also called kinetic energy recovery systems.

Motor/Generator Unit (MGU): this converts mechanical energy into electricity and vice versa.

Power Control Unit (PCU): controls the switching of electric current between the MGU and the battery.

Storage device: which can be any of those mentioned above (battery, supercapacitor, flywheel).

 The equation for braking energy is given by below formula;

`BE = (1/2)*m*V^2`

where,

m= Gross vehicle weight

V = velocity of vehicle in m/sec

Drive Cycle :- 

The drive cycle is from 1 to 100 sec. 

The drive link of excel sheet is https://docs.google.com/spreadsheets/d/1gSQNk-FAJlLdrxIcVLaaDeqBWB8rn-_2p53kqm2bRvs/edit?usp=sharing

In this drive cycle, vehicle is deaccelerated by using brakes by driver for total 3 nos. of times and accordingly, we have calculated braking energy as under;

• 1st Deacceleration started between 20 - 30 sec. The vehicle speed is dropped from 100 mph to 50 mph. Total braking energy consumed in 1st deacceleration is 54.93 KJ.
• The same is calculated as under;
• 
• 2nd Deacceleration started between 60-80 sec. The vehicle speed is dropped from 109 mph to 70 mph. Total braking energy consumed in 2nd deacceleration is 33.42 KJ.
• The same is calculated as under;
• 
• 3rd Deacceleration started between 87-93 sec. The vehicle speed is dropped from 105 mph to 75 mph. Total braking energy consumed in 3rd deacceleration is 19.78 KJ.
• The same is calculated as under;

• Total Braking energy in KJ = Braking energy consumed during 1st deacceleration + braking energy consumed during 2nd deacceleration + braking energy consumed during 3rd acceleration 
• Total Braking energy in KJ = `54.93 + 33.42 + 19.78`
• Total Braking energy in KJ = 108.13 KJ.
• Hence, total braking energy used in this drive cycle is 108.13 KJ.

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

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• In Vehicle, braking is achieved by using Mechanical brake or Regenerative Brake. Regenerative braking is unique to EVs and enables the vehicle’s kinetic energy to be converted back to electrical energy during braking (deceleration or downhill running).
• The converted electrical energy is stored in energy storage devices such as batteries, to extend the driving range by up to 10%. If the regenerative braking power requirement is more than the charging limit of the battery or the regenerative capacity of the machine, the mechanical brake has to be applied along with ir. In practical, the combination of Mechanical brake and Regenerative Brake.
• In both the cases of AC or DC motor, 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.
• The below figure represents forward motoring (positive torque and speed) and forward braking (negative torque and positive speed).
• We know that torque of the induction motor is inverse proportional to the square of the rotor speed. So at higher speed, the torque will be less. That’s why a high-speed electric motor cannot produce braking torque similar to starting.

The below is the Contour Speed Torque Characteristic of one of the DC motor:-

• The plot shows Motor characteristic in two quadrant i.e. 1 and 2. In quadrant 2 the torque is negative. The characteristic of 1st and 2nd quadrant is almost similar.
• An EV accelerate to certain speed in certain time but the reverse of that cannot be achieved. As in constant torque region higher acceleration is possible and then vehicle reaches the point of constant power. This is where the acceleration is achieved. Its easier for motor to achieve acceleration in this manner.
• When it comes to braking, where the torque is negative. When this is replicated in the reverse direction, the braking torque required goes down as the vehicle is in the constant power region lower than the torque which was required during acceleration. So while braking the peak torque is less than the torque required during acceleration. Hence, peak acceleration and peak decceleration torque is not same. Due to this, electric motor can’t develop braking torque at high speed similar to starting.

Types of Brakes used in Automobiles :-

• The electric motor must be controlled to produce the proper amount of braking force to recover as much braking energy as possible and at the same time, the total braking force must be sufficient to meet vehicle deceleration commanded by the driver. For meeting sufficient braking in EV vehicles we use brake control strategies.
• We need both mechanical and electrical braking systems. In emergency braking or parking brake, we needed mechanical braking.
• 
• Mechanical brake:- It is nothing but friction brakes used in a vehicle to slow down the vehicle. The most common type of brakes is drum brake and disc brake.

  i) Drum brake :-

• It is a brake that uses friction caused by a set of shoes or pads that press outward against a rotating cylinder-shaped part called a brake drum.

• The term drum brake usually means a brake in which shoes press on the inner surface of the drum.

                              fig. Drum Brake

• ii) Disc Brake :-

• A disc brake is a type of brake that uses the calipers to squeeze pairs of pads against a disc or a "rotor" to create friction.

• This action slows the rotation of a shaft, such as a vehicle axle, either to reduce its rotational speed or to hold it stationary.

• The energy of motion is converted into waste heat which must be dispersed.

• Hydraulic actuators disc brakes are the most commonly used form of brake for motor vehicles, but the principles of a disc brake are applicable to almost any rotating shaft.

• The components include the disc, master cylinder, caliper (which contains cylinder and two brake pads) on both sides of the disc.

Electrical Braking :-

• It is used to reduce the speed of the motor depending upon the flux and torque. This kind of braking can be done in different methods.
• i) Plugging type braking
• ii) Dynamic braking
• iii) Regenerative braking

(i) Plugging braking:-

• This is the simplest type of electric braking. In this method, the torque of the motor is reversed. It makes the motor and the driven machine standstill condition. A special device is required to cut off the supply as soon as the motor comes to rest.
• The Plugging braking method can be applied to AC and DC motor

(ii) Dynamic braking:-

• In this method of braking, the motor is disconnected from the supply. Then operated as a generator driven by the kinetic energy of the rotor and the load. Thus the kinetic energy of rotation is converted into electrical energy. This electrical energy is dissipated in the external resistance connected across the motor at the braking instant.
• The advantage of dynamic braking is that no energy is required from the supply to brake the motor.
• The Dynamic braking method can be applied to brake DC motors, induction motors, and synchronous motors.

(iii) Regenerative braking:-

• In the Regenerative braking method, the motor is not disconnected from the supply. It remains connected to it and feeds back the braking energy or its kinetic energy to the system. As no energy is wasted in this method and it is supplied back to the system, thus overall energy is saved.
• Thus the regenerative braking is better than Plugging and Dynamic braking.
• This type of braking is used in traction whenever the train runs down.
• The regenerative braking is applied to DC shunt motor, series motor, and 3-phase induction motors.

This Electric and Mechanical brakes are co-ordinated in the following fashion;

(i) Series Brake Control Strategy:-

In case of series brake, we apply Mechanical brake and Regenerative brake one after the another i.e applying one one brake at a time.

(ii) Parallel Brake Control Strategy:-

• In case of parallel brake, we apply Mechanical brake and Regenerative brake at a time i.e both brakes are applied at the same time.
• The parallel braking system is based on a combination of friction-based system and the regenerative braking system, operated in tandem without an integrated control.
• The regenerative braking force is added to the mechanical braking force which cannot be adjusted.The beginning pedal travel is used to control the regenerative braking force only, the normal mechanical braking force is not changed.
• The regenerative torque is determined by considering the motor capacity, battery state of charge SOC, and vehicle velocity.The regenerative braking force is calculated from the brake control unit by comparing the demanded brake torque and the motor torque available.
• Parallel regenerative braking could give an increase of 9-18% in fuel efficiency.
• Both Control Strategy is further classified for two way; either having comfort of the driver or achieving better energy efficiency. This both are not achieved at the same time for either of the brake control strategy. When more energy is required the comfort is compromised. For more comfort, the energy needs to be traded off.

  fig. Parallel Braking flowchart

Electric Driven Intelligent Brake:-

• After stepping on the brake pedal, the system produces natural and adequate braking force that corresponds to the operation. The amount of energy regeneration also needs to be increased as much as possible. EDIB controls the regenerative brake and friction brake to support both of these requirements. Further, it also controls the reactive force from the pedal in order to unify the feeling when stepping down on the pedal and the sense of deceleration.
• It can be noted that the combination is the sum of friction brake and regenerative brake which is desired by the user. The magnitude of this brake can vary, but their sum will not vary, it will remain same.

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.

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These coordinates will help in the creation of the contour plot. In order to find the efficiency of the motor, we need to consider the various losses that occur in the motor’s operation. These are:

The equations of types of losses that takes place in a motor are;

i. Copper Loss = `Kc*T^2`

ii. Iron Loss = `Ki*omega`

iii. Windage Loss = `Komega* omega^2`

iv. Constant Loss in the motor = `C`

Matlab Code :-

%Program to plot Efficiency speed-torque characteristics contour of motor
clear all;
close all;
clc;

%Constant Coefficient values of Motor Losses
kc = 0.2;     %Copper loss constant coeficient        
ki = 0.008;  % iron loss constant coeficient
kw = 0.00001; % windage loss constant coeficient
ConL = 20;

%Creating Speed array in rad/sec
s = linspace(1,1000);  

%Creating Torque array
t = linspace(1,250);

%Creating Meshgrid
[S,T] = meshgrid(s,t);
outputpower = (S.*T);
copper_loss = (T.^2)*kc;       %Copper Loss
iron_loss = S*ki;              %Iron Loss
windage_loss = (S.^3)*kw;      %Windage Loss
Inputpower = outputpower + copper_loss + iron_loss + windage_loss + ConL;   %Constant Loss   
Z = outputpower./Inputpower;

%Efficiency to plot the contour
V = [0.70, 0.80, 0.85, 0.90, 0.91, 0.92, 0.925, 0.93];
box off;
grid off;
contour(S,T,Z,V);
xlabel('Speed (rad/s)');
ylabel('Torque (N.m)');
title('Efficiency Contour plot of an Electric Motor');
hold on;

Explanation of the code:-

• Initially, we have defined the values of copper loss constant, iron loss constant, windage loss constant, conduction loss.
• Thereafter, we have created an array for speed of motor ranging from 1 to 1000 in rad/sec. Also, we have created an array for torque value ranging from 1 to 250 N-m.
• These arrays are then used to create 2D grid coordinates using the mesh grid command.
• These coordinates will help in the creation of the contour plot. In order to find the efficiency of the motor, we need to consider the various losses that occur in the motor’s operation such as Copper losses, Iron losses, Windage losses and Conduction losses. These various losses are dependent on the speed and the torque of the motor.
• After consideration of constant coefficient values of all the losses occured in the motor has been calculated. Further, we have calculated input power and output power with the help of the calculated losses and 2-D array co-ordinates.
• Thereafter, we have created an array for efficiency values and finally, we plot the contour with the 'contour' command.

Output :-

• From this contour plot, we can see that the highest level of efficiency arises in the innermost contour plot and this efficiency decreases as the contours increase in size from the innermost contour.

Google drive link of Matlab code :-

https://drive.google.com/file/d/1ZxVnbQDqhKH_FY3J9jPueIrq6BOpnv56/view?usp=sharing

Reference :-

• https://www.mechanicalbooster.com/2018/06/types-of-braking-system.html
• https://studentlesson.com/automotive-braking-system-definition-functions-working/
• https://doc.ingeniamc.com/wiki/motion-wiki/why-sometimes-i-can-t-reach-100-motor-torque-when-is-already-turning
• https://www.researchgate.net/figure/Efficiency-contours-of-the-motor-at-different-levels-of-torque-and-speed-The-dashed_fig1_262413935
• https://itectec.com/matlab/matlab-how-to-draw-efficiency-map-contour-of-a-motor/