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.

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

Solution: To calculate the energy required for braking for a defined driving vehicle.

Electrical vehicles requires some energy to stop the vehicle at any instant of time. This energy is called Braking energy.

There are two types of braking.

1. Electrical Braking

2. Mechanical Braking

THese two brakings are required to halt the vehicle from a given speed to zero speed

Energy required for braking  = 0.5* mass* velocity^2

Suppose we have NREL Class 3 Electrical vehicle cycle

This graph shows speed with respect to time.

For above driving cycle, it is required to calculate the braking energy. Above data points are converted into a execl format as shown in below figure.

At various speeds the braking energy is calculated by using formula of 1/2*m*v^2.

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

Ans: There are two reasons for a motor developing higher starting torque compare to braking.

1) Due to tranisents

At starting the motor draws large current due to transient. As torque is proportional to square of the current, torque value is also so high at this moment.

when we apply brakes to motor at that time, current value is within the range Therefore torque value is also within the range.

2) Torque Speed Characteristics:

From Torque speed characteristics when speed increase torque should decrease to maintain power constant. At that time starting speed is very low therefore torque value is very high. At that time of braking, speed value is high. At the time of braking, speed value is high therefore torque value will be low. Therefore at the time of braking torque value available will be lower.

In order to stop the vehicle ie requires electrical as well as mechanical brakes.

There are two methods to apply this brakes.

1. Series braking

2. Parallel braking

Series braking: In this mechanical and electrical (Regenerative) brakings are applied oneafter the other.

Parallel Braking: In this both Braking methods are applied at a same time.

At lower speeds Regenerative brakings are used and at higher speeds mechanical brakes are used.

3. Make a MATLAB program which plots contour of given motor speed, torque and efficiency values. Attach the code as a. file attach a screenshot of all the plots.

Ans: To write a matlab program to plot contour for a given speed, torque, efficiency.

Program:

clc;

w= linespace(0,600);           % speed array

T= linespace(0,800);            %torque array

% loss coefficients

kc=0.18;

ki=0.08;

kw=0.0005;

% mesh grid

[X,Y]=meshgrid(w,T);% converting 1d to 2d mesh

coperloss = (Y.^2)*kc;

ironlosee = X.*ki;

windagelose = (X.^3)*kw;

constantloss = 21;

output_power = X.*Y;

inputpower = coperloss + ironloss + windageloss + constantloss + output_power;

effi = output_power./inputpower;

n = linespace (0.7, 0.9, 10);

box off;

contour (X,Y,effi,n);

xlabel ('speed rad/s');

ylabel ('Torque Nm');

Results:

Inner current yellow one indicates higher efficiency compare to outer counter.

As a torque speed values are increases efficiency value decrease from above counter.

For a low speed and low torque values, efficiency of motor is very high.