Week-11 Challenge: Braking

AIM:-

For a defined driving cycle, calculate the energy required for braking.
Why electric motor can’t develop braking torque at a high speed similar to starting? How electric and mechanical brakes are coordinated?
Make a MATLAB program that plots the contour of given motor speed, torque and efficiency values. Attach the code as a .m file attach a screenshot of all the plots.

Objective 1

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

At first, the drive cycle is determined in a Table and a plot is made. Using the plot, the values from the braking regions of the vehicle are taken. The plot of the drive cycle is given below.

T S

I P

M E

E E

D

0 0
1 1
2 3
3 6
4 9
5 12
6 15
7 18
8 25
9 35
10 50
11 55
12 60
13 50
14 40
15 40
16 41
17 42
18 43
19 44
20 45
21 46
22 47
23 48
24 49
25 50
26 40
27 35
28 30
29 35
30 36
31 37
32 40
33 43
34 47
35 55
36 60
37 63
38 66
39 69
40 70
41 71
42 72
43 73
44 75
45 77
46 78
47 80
48 80
49 79
50 75
51 70
52 65
53 60
54 55
55 50
56 45
57 40
58 30
59 20
60 35
61 50
62 55
63 60
64 50
65 40
66 40
67 41
68 42
69 43
70 44
71 45
72 46
73 47
74 48
75 49
76 50
77 40
78 35
79 30
80 35
81 36
82 37
83 40
84 43
85 47
86 55
87 60
88 63
89 66
90 69
91 72
92 75
93 80
94 60
95 50
96 40
97 30
98 20
99 10
100 0

Objective 2

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

This situation occurs only when the vehicle is at high speeds because the motor will be running at the constant power region. In the constant power region, the torque of the motor reduces and so the motor cannot develop the braking torque which is equal to the torque that occurred during low speed at the constant torque range.

The torque vs speed plot of a vector control induction motor is given below.

rom the above characteristic curves of the motor. that is seen that the torque of the motor remains high at low speeds and reduces as the speed increases.

Since the electric motor cannot develop braking torque at a high speed similar to starting.

Electrical brake and mechanical brake.

Electrical braking

Electrical braking can be done by many methods such as,

- Unbalancing the input points of current to the motor.
- Using transistors in H-bridge arrangement to reverse the motor direction in order to reduce speed.
- Implementing a regenerative braking system by using bi-directional power converters.
- Regenerative braking converts much of the energy to electrical energy, which may be collected for later use.

Mechanical braking

Mechanical brakes commonly use friction between two surfaces pressed together to convert the kinetic energy of the moving object into heat, though other methods of energy conversion may be employed. The friction between the surfaces is performed using pressurized air or pressurized oil which are commonly known as an air brake and hydraulic brake respectively.

Coordination of electrical and mechanical braking in EV's

Two types of brake control strategies are used to organise the electrical brake and mechanical brake they are,

- Series control strategy
- Parallel control strategy
The comparison of the control strategies is shown in the below figure.

In the figure, it is seen that, according to the amount of brake pedal travel of the vehicle,

the mechanical brakes and regenerative brake are employed.

Parallel control strategy

When the brake is applied according to the brake pedal travel the front wheel friction brake, the rear wheel friction brakes and the regenerative brakes are employed simultaneously. The higher the travel of the brake pedal the higher the amount of braking will be done.

The braking energy is distributed in the following practice,

Rear-wheel friction brake ----> Front-wheel friction brake ----> Regenerative brake

Series control strategy

When the brake is applied according to the brake pedal travel the front wheel friction brake, the rear wheel friction brakes and the regenerative brakes are applied one after another. As the braking requirement increases the frictional brakes are used subsequently.

For low speeds of the vehicle, regenerative braking is used but it may be limited according to the SOC (state of charge) of the battery.

As the speed of the vehicle increases the rear wheel frictional brake will be employed if the braking requirement is not done but then the front wheel frictional brake is also used.

The coordination of the series control braking can be done with the choice to the comfort of braking (or) energy storage.

Electric driven intelligent brake (EDIB)

EDIB is a brake control strategy which is developed by NISSAN. It is a type of parallel braking which can be seen in the below image.

The above image shows that the frictional brake and the regenerative brake are used simultaneously at higher speeds of the vehicle.

At low speed the torque of the motor is high and so the regenerative brake has a higher braking part than the frictional brakes.

Brief working of EDIB
When the driver steps on the brake pedal, the stroke sensor detects the operation of the brake. That report is communicated to the ECU and controls the motor. The motor engages the piston through the ball screw and increases the friction brake's pressure (hydraulic pressure). Through optimum control of the friction brake's hydraulic pressure, the energy regeneration result is maximized.

Objective 3

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

Matlab Code:

% A MATLAB PROGRAMM FOR PLOTTING CONTOUR PLOT OF MOTOR SPEED, TORQUE & Efficiency 
clear all
close all
clc

% SPEED ARRAY
w = linspace(0,1000);

% TORQUE ARRAY
T = linspace(0,300);

% LOSS COEFFICIENTS
kc = 0.18;  % COPPER LOSSES
ki = 0.006; % IRON LOSSES
kw = 0.000015; % WINDAGE LOSSES

% MESHGRID
[X,Y] = meshgrid(w,T);
copper_loss = (Y.^2)*kc;
iron_loss = X*ki;
windage_loss = (X.^3)*kw;
K = 20;
output_power = X.*Y;
input_power = (copper_loss)+(iron_loss)+(windage_loss)+(output_power)+K;
Eff = (output_power)./(input_power);
n = linspace(0.7,0.95,10);
contourf(X,Y,Eff,n);
title('SPEED TORQUE CHARACTERISTICS')
xlabel('SPEED rad/s')
ylabel('TORQUE N-m')

Plot Result:-

From the output graph, you could see that only at a particular speed and torque the efficiency is at maximum i.e. 95 %.

Where, Speed x = 151 rad/s @ 24 N-m Torque. As the Speed increases the efficiencies of the motor will be reduced.