Week-11 Challenge: Braking
Week-11 Challenge: Braking
1.For a defined driving cycle, calculate the energy required for braking.
Braking is the process of controlling the velocity of an object by inhibiting its motion. An object in motion possesses kinetic energy and to bring the object to a stop this kinetic energy must be removed. Removing the kinetic energy can be accomplished by dissipating the energy to the atmosphere through friction or by converting it into another form of energy. The most common type of braking is a mechanical brake which inhibits motion through friction brake pads. A mechanical brake applies a friction force to convert the kinetic energy of the vehicle into thermal energy which then dissipates into the atmosphere.Braking systems which don't use friction are referred to as regenerative braking systems (RBSs). In RBSs, the kinetic energy is converted into other forms of useful energy, which can be stored for later use, increasing fuel efficiency.
The braking energy is the energy generated when the vehicle (travelling at a certain speed) comes down to zero speed. The braking power can also be calculated when the time taken for deceleration is also considered.The duty cycle selected for our challenge is NEDC(New European Drive Cycle) which came into existence in 1970. This drive cycle is designed to assess the emission levels of car engines and fuel economy in passenger cars.
It consists of four repeated urban driving cycles (UDC) and one Extra-Urban driving cycle (EUDC).
For this challenge, we only consider the UDC and neglect the Extra-Urban cycle EUDC. Each UDC has 3 regions where
Region 1 has a top speed of 15kmph,
Region 2 with a peak speed of 32kmph,
Region 3 has a peak speed of 50kmph and travels at 35kmph for some time.
But for the purpose of calculations, we assume the deceleration to be from 50 to 0kmph.
Formula
E=0.5*m*v^2
where m is the total mass of the vehicle(kerb weight + payload)
V is the velocity of the vehicle given by (Vf−Vi)
Vf is the final velocity
Vi is the initial velocity
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Since the UDC has a similar cycle for four times, the average braking energy obtained is the same for all four cycles and the total braking energy for the entire UDC is the sum of all the braking energy for individual cycles which is found out to be 231.41 KJ.
2.Why electric motor can’t develop braking torque at high speed similar to starting? How electric and mechanical brakes are coordinated?
The braking torque in an electric motor is not as high as that of the acceleration torque at higher speeds. This can be attributed to many different reasons.The rate of discharge of the battery is higher than the charging rate. It can be said that the discharge rate can be twice as that of the charging rate.There are other limitations to the motors in the form of power converters too. This power converter is customised 3-phase inverters which are bidirectional as it needs to convert regenerative braking energy back to DC current for the battery and this reduces the efficiency of the power converter.At low speeds, regenerative braking is very high and as speed increases, the regenerative braking reduces as the effectiveness of inverter plays an important role and the overall efficiency reduces resulting in reduced braking torque.Hence, for effective braking at high speeds, we use a combination of Hydraulic(mechanical) brake and the regenerative(electric) brake.
There are different ways of coordinating between the hydraulic brakes and regenerative brakes. One of the brake control strategies involves series connection where both brakes are connected together(one after another) which implies that in this scenario both brakes are used together and the next one in the parallel connection where either of the two brakes is operated individually without relying on one another depending on the braking requirements.
This coordinated use of the two brakes are due to various reasons such as electromagnetic torque oscillation in low speed, the requested braking torque higher than the power absorption capacity of the electric source (including the converters) and emergency situations. Also, below a certain speed limit, the kinetic energy is very low, there is no enough regenerative braking and there it is proposed to use a mechanical brake with the help of brake pedal making sure that required braking torque is obtained.This coordination between mechanical and electrical brakes results in braking that complies with driver input on the brake pedal.
The idea of coordinating control is to take full advantage of the characteristics of the braking systems. The hydraulic system can provide large and stable torque, and the motor braking system’s torque response is quick and accurate, reducing the fluctuation of the vehicle.
Types of Control Strategy
a) Series - The braking is applied one after the other. Either of regenerative & mechanical braking is applied alternatively.
b) Parallel - The both the brakes are applied. The regenerative & mechanical are applied in parallel.
When the brake pedal is applied it supplies the hydraulic fluid to the disc to slow the tires at the same time when the acceleration pedal is lifted a certain amount of electric brakes are applied. At high speed, since the braking force required is a high large amount of hydraulic brakes are applied & once the speed is reduced and the torque value is also high so the electric brakes become predominate so the battery charging occurs efficiently. The ECU system with a feedback system to find the torque region & regenerative braking predominates in the high torque region
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 speed-torque characteristics of the induction motor and BLDC(Brushless DC) motor have been studied already in the previous challenges. However, the variation of the speed-torque characteristic with respect to efficiency has not been performed until now. In this challenge, we are analysing the speed-torque variations with respect to efficiency for a PMDC (Permanent Magnet DC) motor in MATLAB using contours.For designing an EV to be sold in the market, one key aspect the customer expects is that the vehicle he purchases should perform well in their respective driving conditions. In other words, the vehicle should be highly efficient and for the EV to perform well, the selected motor should have good efficiency. We've understood that Vector Control is the best method to control speed and it is also highly efficient.To have better overall efficiency, it is absolutely crucial that all components involved in driving a vehicle should perform well and the motor is the most important of them all. So, we've created a MATLAB code that plots the speed-torque-efficiency which gives us an understanding of the performance of the motor.
% a program for efficiency contours for electric motors.
x=linspace(1,1000);
y=linspace(1,250);
%allocate motor loss constants
Kc=0.2; %copper loss
Ki=0.008; %iron loss
Kw=0.00001; %windage loss
Constant=20; %other losses
%now make mesh
[W,T]=meshgrid(x,y);
Output_power=(W.*T);
B=(T.^2)*Kc;
C=W*Ki;
D=(W.^3)*Kw;
Input_power=Output_power+B+C+D+Constant;
Z=Output_power./Input_power;
%we now set the efficiencies for which a contour will be plotted
V=[0.70,0.80,0.85,0.90,0.91,0.93,0.94,0.96];
box off
grid off
contour(W,T,Z,V);
xlabel('speed/rad.s^-^1');
ylabel('torque/N.m');
hold on
%now plot a contour of the power output
%the array output power has already been calculated we draw contours
%at 8and 10kw
V=[8000,10000];
contour(W,T,Output_power,V);
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