Week-11 Challenge: Braking
1) For a defined driving cycle, calculate the energy required for braking.
Answer:-
Braking-
Road vehicle involved the conversion of kinetic energy into thermal energy (heat). when stepping on the breaks, the driver command stop for several times as powerful as the force that put the car in motion dissipated the associated kinetic energy as heat.
The process of breaking follow the law of conversion of energy that is energy cannot be created or destroyed it can only be converted into the another form.
Braking Energy = (1/2)*Mass*(Final velocity-Initial velocity)^2
`E=(1/2)*M.(Vf-Vi)`
where-
M= Mass of the vehicle in kg
V= velocity of vehicle in m/s
Vf= final velocity - Hear the maximum velocity obtained after breaking is consider as final velocity.
Vi= initial velocity - Here the maximum velocity obtained before breaking is considered as a initial velocity.
Before calculating the breaking energy of a particular vehicle we need to create an excel sheet for a drive cycle-
Calculation-
Here, in this drive cycle there are total 3 times the braking is applied. So we need to calculate braking energy for different drive period and then calculate total braking energy.
Here, we consider mass of vehicle is 800 kg.
1) for 50 kmph to 22 kmph = 13.88 m/s to 6.11 m/s
`E1=(1/2)*800*(6.11-13.88)^2`
`E1=24149J`
2) for 80 kmph to 30 kmph = 23.61 m/s to 8.33 m/s
`E2=(1/2)*800*(8.33-23.61)^2`
`E2=93391J`
3) for 62 kmph to 0 kmph = 17.22 m/s to 0 m/s
`E3=(1/2)*800*(0-17.22)^2`
`E3=118611J`
Total Braking Energy
`E=E1+E2+E3`
`E=24149+93391+118611`
`E=236151 J`
Energy required for Braking of Vehicle is 2,36,151 J.
2) Why electric motor can’t develop braking torque at high speed similar to starting? How electric and mechanical brakes are coordinated?
Answer:-
The situations 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 reduced and so the motor can not develop the braking torque which is equal to the torque that occurs during low speed at the constant torque range.
The torque vs speed plot of vector control induction motor is given below-
From the above characteristics curve of the motor. That is seen that the torque of the motor remains high at low speed and reduced as the speed increases.
Since electric motor cannot develop braking torque at the high speed similar to starting.
Electric brake and Mechanical brake.
Electric Braking-
Electric braking can be done by many methods such as-
- Unbalancing the input point of current to the motor.
- using transistor in H-Bridge arrangement to reverse the motor direction in order to reduce speed.
- implementing a regenerative braking Systems by using bi-directional power converter.
- regenerative braking convert much of energy to electrical energy which may be collected for later use.
Mechanical Braking
Mechanical brakes commonly use friction between two surfaces are pressed together to convert the kinetic energy of the moving object into heat, through other methods of energy conversion may be employed. The friction between the surface is performed using pressurized air or pressurized oil which are commonly known as Air brake and Hydraulic brake respectively.
Coordination of electrical and mechanical braking in EV
Two types of brake control strategy are used to organise electric brake and mechanical brakes together they are-
- series control strategy
- parallel control strategy
The comparison of the control strategy is shown in a below figure.
In this figure, it is seen that according to the amount of brake pedal travel of the vehicle, the mechanical brakes and regenerative brakes are employed.
Parallel control strategy
when the brakes is applied according to the brake pedal travel, the front wheel friction brake, the rear wheel fiction brakes and the regenerative brakes are employed simultaneously. The higher the travel of brake pedal the higher the amount of breaking will be done. The breaking energy is distributed in following practice
rear wheel friction brake ----> front wheel friction break ----> regenerative brake
Series control strategy
when the brake is applied according to the brake pedal travel of the vehicle, the front wheel friction brake, the rear wheel friction brakes and the regenerative brakes are applied one after another. As the breaking a requirement increase the frictional brake are used subsequently.
For low speed of vehicle, regenerative braking is used but it may be Limited according to the SOC (state of charge) of battery.
As the speed of the vehicle increases the real will friction break will be employed. If the breaking requirement is not done but then the front wheel friction break is also used. The coordination of the series control breaking can be done with the choice to the comfort of break energy storage.
Electric drive 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 a below image.
The above image shows that the frictional brake and the regenerative brakes are used simultaneously at high speed of the vehicle. At low speed, the torque of the motor is high and so the regenerative brake has a high breaking part than the frictional break.
Brief working of EDIB
When the driver step on the brake pedal, the stroke sensor detect the operation of the brake, that report is communicated to the ECU and control the motor. The motor engage the Piston through the ball screw and increase the friction brake pressure (hydraulic pressure). Through optimum control of friction brakes hydraulic pressure, the energy regeneration result is maximizeg.
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.
Answer:-
To plot contour line for a given motor. Motor have various type of losses happening inside. The various types of losses are-
1.Copper losses- `I^2R` Where are is armature resistance and we say that torque is proportional to current, this copper loss can be considered as `Kc*T^2` where Kc is to take care of resistance of brushes and also take care of flux and it is effect.
Copper losses=`Kc*T^2`
2.Iron losses- Iron losses are nothing but magnetic losses which is denoted as `Ki*w`. where Ki is factor which based on the magnetic field effect. For permanent magnet have constant magnetic field and these iron losses is based on the value of Ki and this keeps on changing due to variation in speed.
Iron losss= `Ki*w`
3.Windage losses- This are based on size and shape of the motor and the amount of windage losses that the motor is going to experiance.
Windage losses= `Kw*w^3`
4.Constant losses- This losses are present in the motor constantly and are denoted by C
Total losses= `Kc*T^2+Ki*w+Kw*w^3+C`
clc
clear all
close all
Torque= linspace(1,250,250); %input torque
Speed= linspace(1,1000,250); %input speed
[W,T]= meshgrid(Speed,Torque);
Kc=0.3; %copper loss constant
Ki=0.009; %iron loss constant
Kw=0.0005; %windage loss constant
C=30; %constant loss
Pout=W.*T; %output power
C_loss=Kc*(T.^2); %coper loss
I_loss=Ki*W; %iron loss
W_loss=Kw*(W.^3); %windage loss
Pin= Pout+C_loss+I_loss+W_loss+C;
eff=Pout./Pin;
contour(Speed,Torque,eff)
title('countor for efficiency of induction motor');
xlabel('Speed (rpm)');
ylabel('Torque (Nm)');
