Week-11 Challenge: Braking

BRAKING CHALLANGE

OBJECTIVES :

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

• Whenever a force is applied to a vehicle’s brakes, there is work done due to the friction between the brakes and wheel. This reduces the vehicle’s kinetic energy, slowing it down and causing the temperature of the brakes to increase.
• The braking energy is the energy generated when the vehicle travelling at a certain speed comes down to low/zero speed. The braking power can be calculated when time taken for deceleration is also considered.
• To calculate the energy required for braking first, the driving cycle is defined in an excel sheet and plot is obtained.
• The plot for defined drive cycle is given below:-

From the plot we can see that, this driving cycle consists of six deceleration region i.e. six braking regions. The initial and final values of velocities along with time required are given as:

• The initial and final velocities are taken in kmph for driving cycle. So these values are converted in m/s for braking energy calculation.
• Kinetic energy is the force that keeps vehicle moving. This energy is provided by the engine in order to accelerate the vehicle from standstill to desired speed. Kinetic energy is dissipated as heat during braking.
• Thus kinetic energy of a vehicle during braking is given by:

Where;

m = mass of a vehicle in kg

Vi = initial velocity of a vehicle in m/s.

Vf = final velocity of a vehicle in m/s.

B.E = braking energy in Joules.

• For calculation we consider, the mass of vehicle = 1000kg.
• Now calculate deceleration & braking energy of the vehicle using excel calculator.

∴ Deceleration = change in velocity / time

= (Final velocity – Initial velocity) / time.

• The excel calculations are as given as:-

Thus the total energy braking required is 0.148 KWh.

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

Why electric motor can’t develop braking torque at high speed similar to starting?

As the motor will have rated speed when the vehicle speed increases beyond the rated speed the torque decreases and cannot attain the required brake torque demand as it was given at starting speed.So, the electric motor cannot develop braking torque at high speed because wheel lock-up can happen which is not recommended as it is harmful to the vehicle. In such a situation, mechanical braking can be assisted to reach the required brake torque demand. So, there will need of both electrical braking and mechanical braking at high speed.

in more details,

Electric motor cannot develop braking torque at high speed similar to starting because braking torque is applied in the opposite direction of the rotation. Also, there are two major reasons for a motor developing high starting torque compared to braking:

1. Due to transients:-

At stating the motor draws a large current due to transients. As torque is proportional to a square of current, therefore torque value is also high at the starting of motor. When vehicle is running at high speed and we apply brakes, at this time current value is less therefore torque values are also less.

2. Torque speed characteristics:-

From torque-speed characteristics, when speed increases the torque should decrease to maintain power constant. At the time of starting, speed is very low therefore torque is high. When vehicle is running at high speed and we need to apply brake at that time we need high braking torque. But it is not possible to have high torque in constant power region. Thus at the time of braking, speed value is high therefore torque will be lower. Electric motors are designed for high starting torque. So motor will not produce high braking torque at high speed than during starting.

How electric and mechanical brakes are coordinated?

by two method(Strategy) we can coordinate electric and mechanical brake

1. SERIES BRAKE
2. PARALLEL BRAKE

1. Series Brake :

• Here we can apply mechanical brake & regenerative brake one after another. Also only one brake strategy is applied at a time i.e. either apply regenerative or apply mechanical brake.
• The regenerative braking system could restore kinetic energy & potential energy to a battery or any other forms of power during braking; it is a special system for EV & HEV. However, large current produced by generator during braking would affect battery performance and shorten battery life.
• A hybrid power system is an excellent energy storage system, in which ultra capacitor bank & battery packs are connected through a DC/DC power converter.
• The ultra-capacitor in this system can sore instantaneous large currents quickly, which is able to protect battery from large currents. This suggest that the ultra-capacitor is able to restore regenerative energy very quickly which will improve starting & acceleration performance, extend driving range and increase fuel economy.
• For low speeds of vehicle, regenerative braking is used but it may be limited acc. to SOC of battery. As speed of vehicle increases the rear wheel frictional brake will be applied, if braking requirement is not satisfied yet then front wheel frictional brake is also used

2. Parallel Brake :

• Here we can apply both mechanical brake & regenerative brake simultaneously.
• The system restores the kinetic & potential energy used during start and accelerating into battery through electrical machine.
• When brake is applied according to the brake pedal travel; front wheel friction brake, rear wheel friction brakes & regenerative brakes are applied simultaneously. Higher the travel of brake pedal the higher the amount of braking will be done.
• The parallel brake strategy is not only safe enough but also can restore largest amount of brake energy.
• So, whenever higher braking torque is required at high speed, mechanical brakes is used more than electrical brakes. Similarly, at low speed, high braking torque is sufficiently provided by Electric brakes itself.

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.

The four main types losses accure in EV Motors

• Copper losses: caused by the electrical resistance of wires of the motor which dissipates heat.
• Iron loss: These are caused by magnetic effects in the iron of the motor, particularly in the rotor.
• Friction and windage losses: these are caused by the friction torque in the bearings and brushes of the motor and the rotor will have wind resistance. The friction force will be constant but windage will be changing with the square of the speed
• Constant losses,C: these losses occur when the motor is stationary and with speed or torque.

Motor efficiency, Eff_motor=output power/input power

=output power/output power+ Total Losses

Using the Contour function, we can plot a three-dimension graph of efficiency in torque speed plot.

MATLAB CODE :

clear all
close all
clc

%for x and y axis,speed and torque
%speed in rps
x = linspace(1,1000);
%torque T in Nm
y = linspace(1,250);
%motor losses constants
kc = 0.2;  %copper losses
ki = 0.008;  %iron losses
kw = 0.00001;  %windage losses
conL = 20;   %motor losses
%creating mesh
[X,Y] = meshgrid(x,y);
outputpower = (X.*Y);       %power output
B = (Y.^2)*kc;              %copper losses
C = X*ki;                   %iron losses
D = (X.^3)*kw;              %windage losses
inputpower= outputpower+B+C+D+conL;
Z = outputpower./inputpower;

%efficiences for countour
V = [0.70,0.75,0.80,0.83,0.85,0.90,0.91,0.92,0.93,0.95];
box off
grid off
contour(X,Y,Z,V,'showtext','on')
xlabel('speed rps')
ylabel('Torque Nm')
hold on
v = [8000,10000];
contour(X,Y,outputpower,v)

OUTPUT :

Observation :

• Innermost contour is the highest efficiencies amongst all as the height is maximum as the height decreases the efficiencies also decreases.
• two yellow lines represent the power output lines between 8kW and 10kW.
• Here we can observe that all contours are a group of efficiencies.