Week-11 Challenge: Braking

AIM(1):-For a defined driving cycle, calculate the energy required for braking.

• DRIVE CYCLE:-

• A Driving cycle are set of vehicle speed points versus time.It is used to calculate the fuel consumption of a vehicle.
• The drive cycle is given below,

• The above drive cycle is made form excel.The spreadsheet link is uploaded below.

https://docs.google.com/spreadsheets/d/1cr3Ps8dCrHQlM4lqXYiQjr04q_HUOpj8/edit?usp=sharing&ouid=118283153574063949967&rtpof=true&sd=true

• The equation for kinetic energy from braking is given as,

`E=1/2m(V_i-V_f)^2`

• Where,

m=Mass of the vehicle in KG

Vi=Initial speed of vehicle in m/s

Vf=Final speed of vehicle in m/s

• From the drive cycle 5 braking point were obtained.The mass of the vehicle is constant i.e 1400 kg.The energy required for braking is given in the below table,

• Therefore the total energy required for braking is 3285.8KJ.The spreadsheet is given below,

https://docs.google.com/spreadsheets/d/1TQoZaHInYljqem08CpwbinzVlaPHSaDF/edit?usp=sharing&ouid=118283153574063949967&rtpof=true&sd=true

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

• Let us consider the torque-speed characteristics.

• From this curve we have two region, constant torque and constant power region. At the start the motor achieves peak torque till it reaches a certain motor speed range from where it will rapidly drop.
• The reason due to the drop in torque is due to the motor shaft reaching critical speed and the area where the peak torque is constant is known as the constant torque region of the motor.
• The region where the torque decreases is known as the constant power region as the power generated by the motor will be at it's peak.
• Since the torque produced at the constant power region is very low, it is not possible to develop enough braking torque in this region.The braking torque produced increases as the speed decreases.

• How are electric brakes and mechanical brakes co-ordinated?

• In electric vehicles, two types of braking systems are used electric brake system and mechanical brake system.

• Electric Braking System:
• In electrical braking.The winding of the motor is used to produce a braking torque.
• A braking torque is developed during the braking operation. This braking torque opposes the motion of the rotating member or shaft. This is achieved by suitably changing the electrical connections of the motor.
• The motor operates on a speed-torque characteristic, depending upon the method of braking employed.

• Mechanical Braking System:
• In mechanical braking, the frictional force between the rotating parts and the brake drums produces the required braking.
• To achieve this, mechanical equipment, such as brake linings and brake arums, are required.whether electrical or mechanical, the braking of the drive should stop the motor at the specified point of time and Position.

Sr No.	Factors	Electrical Braking	Mechanical Braking
1	Maintenance	Less	Frequently
2	Energy Utilization	Energy of rootating parts can be converted to electrical energy.This can be returned to the mains during braking.	Energy of rotating parts is wasted as heat in friction.Heat is generated during braking.
3	Smoothless of braking	smooth	Not smooth
4	Equipment	Require high rating equipmment	Equipment like brake shoes,brake lining,brake drum are required.

• The electric braking system used in Evs and HEVS is regenerative braking. This method uses the motor as a generator during braking developing a retarding torque, which acts on the running motor and halts it.
• The kinetic energy and potential energy, minus the losses of the motor are returned to the mains and the motor runs at a constant speed.Thus,regenerative braking eliminated the tendency of the load to acceleration the motor.
• Mechanical braking systems used can be either disc or drum brake system. where in case of a disc brake system friction pads rub against a disc when the brake pedal is applied and the kinetic energy is converted to heat and slows the vehicle down.In a drum brake system,a set of brake shoes which are the friction materials press against the a drum.
• Since the braking torque is very less at high speeds, mechanical brakes are applied to slow down the vehicle. As the motor reaches the constant torque region the regenerative braking system takes effect where the motor acts as a generator and the negative torque produced charges the battery pack.

AIM(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 SCRIPT:-

• As shown in above,The speed and torque are defined as 2-D linearly spaced arrays using linspace command.
• The constant of the motor losses is defined.
• The 2-D arrays of the speed and torque are then defined as 3-D arrays using meshgrid command.
• The output power is defined as the product of spedd and torque.
• The equations of motor losses based on the speed and torque are defined.
• The input power is equal to the output power plus losses.
• The efficiency is defined as the ratio of output power and input power.
• In order to map the contour for different efficiency ranges an array is defined which consists of efficiencies from 0.7 to 0.93.
• The graph is ploted using contour command to plot speed,torque and efficiency.

• Contour Graph:-

• From the plot it can be observed that the speed and torque varies as the efficiency increases.This is due to occur the motor losses.
• The dark blue line represent the contour 70% efficiency and the bright yellow line represents the contour 93% efficiency.
• For example at 70% efficiency, the torque reaches it's peak value of 250 N-m when the vehicle is a very low speed but the torque decreases significantly as the speed increases for the same efficiency. And at a motor efficiency of 93% the speed and torque required is much less compared to an efficiency of 70%.
• Greater the motor efficiency, lower speed-torque combination is required to overcome the motor losses.

• CONCLUSION:

• The energy required for braking of a vehicle based on a drive cycleis calculateed.
• The co-ordination between electric brakes and mechanical brakes and why braking torque is less at high speed are explained.
• The MATLAB program to plot the speed,torque and efficiencies contour is written and the graph is plotted.