Week-11 Challenge: Braking

QA:1

Drive cycle :-
A driving cycle is a series of data points representing the speed of a vehicle versus time. Driving cycles are produced by different countries and organizations to assess the performance of vehicles in various ways, as for instance fuel consumption, electric vehicle autonomy and polluting emissions.Driving cycle is a velocity-time profile that describes driving characteristics of specific type of vehicles under. real-world driving condition, which is always used as a standard for the evaluation of vehicles' economy, emission. and driving range in vehicle industry.

From drive cycle, calculating the energy required for braking :-

Drive cycle is UDDS cycle which is urban dynamometer drive schedule. The data is taken for 1370s time for the simplicity of calculation we take 150 sec drive cycle time is considered. Speed which is in miles per hour has been converted into meter per second.

Drive cycle is given below :-

The equation for calculating braking energy :- The energy calculate by equation is given as follows ,
E = (1/2) * M * (Vi - Vf) ^2
Where ,
M = Mass of vehicle in Kg
Vi = Initial speed of the vehicle in m/s.
Vf = Final speed of the vehicle

From above drive cycle, we get 10 braking point. The mass of vehicle is constant is about 1500 Kg then the energy required for braking is calculate as follows:-

RESULT:

From above calculation the energy required for thr braking is about 16.8 KW

QA:2

From above curve , we see the three region first one is constant torque region ,second one is constant power region , third is breakdown torque region. So at start of motor, the motor ahieved the peak torque till the moment at it reaches the certain speed and after that it start to drop rapidly. The reason of dropping the torque of motor is because the motor shaft reaches the critical speed.The area motor achieve the peak torque and constant is called constant torque region.The region where the torque reduce is known as constant power region, the power generated by motor will be at its peak.so the torque produced at constant power region is low this is reason why the motor does not produce braking torque in this region.

The above represents the two quadrants of the motor operation where in the first quadrant both torque and speed are +ve (Forward motoring) and in the second quadrant the speed is +ve and the torque is –ve. (Braking)

At High speed, the torque will be less & operates at a constant power region hence it is difficult to bring the vehicle to rest or decelerate immediately. The peak acceleration torque is greater than the peak deceleration torque. Hence Braking torque of the EV motor is not as high as the acceleration Torque.We can see that, at the time of accelerating vehicle at starting the maximum torque available motor is operating at constant torque region, in that motor will have maximum torque available, but as motor starts deaccelerating motor characteristics are shifted towards braking region, in this we don’t have max torque available for braking as motor is already running at higher speed.Due to this operation electric motor can’t develop braking torque at high speed similar to starting.

We can see that, at the time of accelerating vehicle at starting the maximum torque available motor is operating at constant torque region, in that motor will have maximum torque available, but as motor starts deaccelerating motor characteristics are shifted towards braking region, in this we don’t have max torque available for braking as motor is already running at higher speed.Due to this operation, electric motor can’t develop braking torque at high speed similar to starting.

DC motor Torque vs Speed Characteristic :

Here for both in the case of AC or DC, there is no torque at maximum speed, gradually the torque decreases at high speed as the speed tends to be maximum. 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.

classification of braking:

1.Mechanical braking;

It is mostly used in scooters, motor vehicles, and motorcycles where small power is required. It is essential in manufacturing power transmission applications, material handling, etc. It delivers forces to the axle or a wheel to stop motion. It helps to reduce the speed of the system slowly by the mechanical process when compared to electrical braking. The working of a mechanical brake depends on the pedal. When the pedal is pressed, the brake shoes are pushed outwards and rotates against the drum which is connected to the wheels. Hence the machine or vehicle gets slow down and stopped. And when the pedal is released, it goes to a normal position due to the pullback action of spring shoes.

Types of mechanical braking ;

• Drum braking
• Disk braking
• Band braking
• Pawl and Ratchet braking

* Drum brakes: These are commonly used on automobile rear wheels, which work when shoes press against a spinning surface called a drum.

*Disc brakes: These are constructed of brake pads, a caliper, and a rotor. During operation, the brake pads are squeezed against the rotor.

*Band brakes: These are the simplest brake configuration, have a metal band lined with heat and wear-resistant friction material.

2.Electrical Braking

It is used to reduce the speed of the machine depending on flux and torque. This type of braking is mainly used for functional braking to control the speed of the machine. It is easy to handle and comfortable. But it cannot be used for emergency braking and parking braking.

• Plugging type braking
• DC injection type braking
• Eddy current braking
• Dynamic resistor type braking
• Regenerative braking
• Sharing DC bus type braking

Electrical and mechanical braking systems can are coordinated by using a Brake strategy subsystem;

1) Serial Braking Strategy

2) Parallel Braking Strategy

1) Serial braking strategy

Once the brake pedal is pressed and depending on the level of depression, brake torque is sent. Now, this brake torque passes into the brake control strategy block. Here, firstly it is checked if the battery state-of-charge is below 80% which is set as the maximum permissible level of charge. If that condition is satisfied, then the battery is recharged. The brake torque requested is compared to the maximum regenerative torque limit of the motor-generator. If the overall brake torque requested is less than or equal to the maximum regenerative torque, then the vehicle brakes purely on regenerative torque, and the battery is charged. The amount of charge ultimately put in depends also on the motor-generator operation efficiency and the battery charging efficiency. If the overall brake torque requested is greater than what the motor-generator can provide, then the friction brakes kick in. Now since the vehicle is front-driven, it is checked if the total brake torque requested from the front is less than or equal to the maximum regenerative torque capability of the motor-generator. If yes, then front braking is provided purely by regeneration and rear (non-driven axle) by friction. Once the total brake torque request increases further, then front friction brakes (driven axle) comes into play in combination with rear friction brakes and front regenerative braking, to satisfy the overall brake torque demand. This has been represented in a flow chart manner

2) parallel braking strategy

Once the brake pedal is applied and a desired overall brake torque request is sent from the driver block. This braking torque on passing through the brake control strategy gets divided between regeneration and friction braking at all times. The strategy is so modeled with the comfort and brake pedal feel in mind. As parallel regenerative braking is an add-on to conventional systems, it is devoid of any integrated control. So to make the transition and eco-performance of regenerative braking with friction smooth, it is assigned in such a way that, there is a clear division between the amount of brake torque that will be achieved from regeneration and friction braking. On application of brakes and in keeping with the design of the motor-generator, it is designed to operate the generator in a constant generator torque region. Motor-generator can provide a maximum of 322 Nm of generator torque at or under the base speed. The generator is scaled such that the base speed of the generator corresponds to a velocity of 124 [kmph], which is the maximum speed limit on most highways. Hence regeneration works, as long as the vehicle is braking from a speed of under 124 [kmph] and the requested brake torque on the front-driven wheels, after the front-rear brake split is greater than or equal to maximum generator torque. If that condition is satisfied, then regenerative and friction braking operates in tandem else it’s just the friction brakes at the driven and non-driven axle. This has been represented in a flow chart manner

QA:3

MATLAB code;

clear all

close all

clc

w = linspace(0,1000);

T = linspace(0,250);

% motor constants

kc = 0.23; % for copper loss

ki = 0.009; % for iron loss

kw = 0.00001; % for windage loss

k = 20; % for constant motor losses

% Losses calculation

[X,Y] = meshgrid(w,T);

OutputPower = (X.*Y); % Torque x Speed = Power

B = (Y.^2)*kc; % copper loss

C = (X.^3)*kw; % winding loss

D = X*ki; % Iron loss

InputPower = OutputPower + B + C + D + k;

Z = OutputPower./InputPower;

% Mapping contour of efficiency

%n = [0.7,0.8,0.85,0.9,0.91,0.92,0.925,0.93];

n = linspace(0.7,0.95,10);

contourf(w,T,Z,n);

title('Speed-Torque Characteristics','color','r')

xlabel('Speed in [Rad/S]')

ylabel('Torque in [Nm]')

PLOT:

RESULT:

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.