Week-11 Challenge: Braking

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

Ans:

Under the condition of braking, some amount of energy is required to do the sam as that of energy required for accelerating the vehicle. Hence to calculate energy, use the acceleration energy equation,

Ek =1/2 mv²

Ek = kinetic energy (J)

m = mass of object (kg)

V= speed of the object (m/s)

Calculating the energy required for braking for a defined drive cycle:

From the drive cycle, there are 4 different situations of decceleration of which the energy has to be calculated.

First Braking = 24-15kmph = 6.67-4.17m/s

Second Braking = 35-30kmph = 9.72-8.33m/s

Third braking = 45-23 kmph = 12.5-6.39m/s

Fourth braking = 63-5 kmph= 17.5-1.39m/s

We consider the total mass of vehicle is 1750 kg

We know that the energy is given by

Ek = ½ m (Vi - Vf)

- For 1^st braking

Ek = ½ * 1750 * (6.67-4.17) = 2187.5 J

-For 2^nd braking

Ek = ½ 1750 * (9.72-8.33) = 1216.25 J

-For 3^rd braking

Ek = ½ * 1750 * (12.5-6.39) = 5346.25 J

-For 4^th braking

Ek = ½ * 1750 *(17.5-1.39) = 14096.25 J

Total Energy Required For Braking = 22846.25 J

Total Energy Required For Braking = 22.84625 KJ

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

Ans:

Regenerative braking allows electric vehicles to use the motor as a generator when the brakes are applied, to pump vehicle energy from the brakes into an energy storage device. Regenerative braking is an effective approach to extend the driving range of an EV.

- The regenerative braking torque cannot be made large enough to provide all the required braking torque of the vehicle as it might lead to the deterioration in stability of the vehicle. Also as compared to the torque values of acceleration torque of motor is much higher than decceleration torque.

- At high speed, the motor might be in its constant power region which causes less values of torques produced and this limits the amount of braking torque that is used for regeneration. Hence the peak vlaue of accleration torque and brakingtorque are not equal.

Electric and Mechanical brakes Co-ordination:

The regenerative braking system may not be used under many conditions, such as with a high State of Charge (SOC) or a high temperature of the battery. In these cases, the mechanical braking system works to cover the required total braking torque

Thus, cooperation between the mechanical braking system and the regenerative braking system is a main part of the design of the EV braking control strategy and is known as torque blending

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.

Ans:

Code;

omega=linspace(0,1500);

T=linspace(0,300);

kc=0.25;

ki=0.0075;

kw=0.0001;

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

copper_loss=(Y.^2) *kc;

iron_loss=X.*ki;

windage_loss=(X.^3) *kw;

con=20;

o_p=X.*Y;

input_power=(copper_loss+iron_loss+windage_loss+o_p);

eff=(o_p)./(input_power);

n=linspace(0.65,0.95,10);

contour (X, Y, eff,n)

xlabel('rpm')

ylabel('troque')

title('Efficiency')

Arrays are formed for rotational speed and torque using linspace command • In a motor there are four types of losses

1. Copper loss(kc)- these are generated as a result of the armature rsistance, T proportional to I-kc*T^2
2. Iron loss(ki)-change according to speed, as speeds are changed back emf of different values are generated (magnetic field effect)= ki w
3. Windage losses- due to size and shape of motor =kw*w^3
4. 4other losses are in constant (c)

-Ameshgrid is formed with X as rotational speed and Y as torque

- Output power at all these points(x,y) is found by T*W

-Total losses is sum of all above losses

- Input power is sum of output power and inputpower

-To form a contour different variable calues are assigned for efficiency. Here the V which contains efficiency values are plotted on w-T plane