Week-11 Challenge: Braking

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

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

kinetic energy = ½ mv^2

weight of vehicle = 500kg,

velocity of vehicle during initial moment and during destination reached moment.

Vf = final velocity,

Vi = initial velocity,

For final velocity, Vf = 40 km/h....

For initial velocity, Vi = 0 km/h....

Power required to move electric vehicle of mass from 0 km/h to 40 kmh:-

Braking power (P);

P = [ ½ m ( Vf - Vi )^2 ]

P = [ ½ 500 ( 40 - 0 )^2 ]

P = [ ½ 500 ( 40^2 )]

P = [3200000]watt...

P = (400) k-watt

Energy required to move electric vehicle of mass from 0 km/h to 40 kmh in time period of T = 20sec:-

Energy = power/time:

Energy = 400/20:

E = 20kwh...

Braking energy required for braking de-acceleration of vehicle of weight = 500kg, from initial speed of 40 km/h to 0 km/h, in T = 20sec, is 20KWH...

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

ANS: Electric Motor Braking:-

Eectric motor can’t develop braking torque at high speed is simila to starting, because Braking torque is applied in the opposite direction of the rotation. We can use a mechanical brake to produce a braking torque. We can also do electrically.

Regenerative Braking: It is a form of braking in which kinetic energy of the motor is return to the power supply system.

• Regenerative braking takes place whenever the speed of the motor is exceeds the synchronous speed. This baking method is called regenerative baking because here the motor works as generator and supply itself is given power from the load, i.e. motors. The main criteria for regenerative braking is that the rotor has to rotate at a speed higher than synchronous speed,only then the motor will act as a generator and the direction of current flow through the circuit and direction of the torque reverses and braking takes place.
• Sometime after accelerating you can't reach the maximum torque. The reason is that no matter how much the current loop wants to increase current (proportional to torque) by increasing the PWM duty cycle of the power stage, the Vmotor is too close to V_emf and therefore current can't increase more.This situation does not occur at the beginning of an acceleration because most of the torque is used to increase the kinetical energy of the load (and motor). But when the speed is already high and no extra load is applied it is not possible to increase the torque.
• Peak Acceleration Torque and Peak Deceleration torque are not equal for Electric Vehicle Motor. So the Peak Torque during acceleration and the Peak torque during deceleration or braking is not equal. Acceleration is usually higher, at low speed we get higher Torque but high speed the value of the torque gets lower. So we cannot say that the motor which is producing 7Nm Torque for Acceleration can also produce -7Nm of Braking Torque. So beacuse we Operating the Motor in Field Weakening region and we are operating the Motor above base speed we must know that the torque is compromised or the torque value is lower. So Electric motor can’t develop braking torque at high speed similar to starting, because Braking Torque of an EV Motor is not as high as the Acceleration Torque. Therefore Regenerative braking is cannot happen so effectively that we can get all the power back to the battery.

Coordination of Electric and Mechanical Brake :-

In a electric Motor the Total Brake distribution is in two aspects which are Electrical Brake which is called Regenerative Brake and Mechanical Brake or Classical Brake with hydraulic cylindrical system. Brake Force distribution must be uniform and If the Regenerative braking power requirement is more than the charging limit of the battery or more than regenerative capacity of the machine we must apply Mechanical brakes as well. In practical case the combination of these two is applied. So usually it is combined response of Mechanical brake and Regenerative brake. The reason for how these brake force are combined, that particular phenomena are called Brake Control Strategies. There are two Brake Control Strategies, which are

Series :- In case of Series we apply Mechanical brake and Regenerative electric brake one after another. We can apply only one brake strategy at a time either apply Regenerative or apply Mechanical brake.

The Series Brake Control Strategy have further classification, that is based on whether we want comfort of driver or Brake behaviour compared to energy efficiency.

Parallel :- In case of Parallel we apply both brakes simultaneously. This Particular Parallel braking is going to happen both at a time.

Electric Driven Intelligent Brake (EDIB) is a combination Hydraulic brake or Mechanical brake along with a Regenerative brake. It is a Parallel braking technique. The Control criteria such that at low speed use regenerative braking higher and at high speed or during initial braking period we actually combine the both.

OBJECTIVE 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: MATTLAB Code:

%contour for Efficiency
x = linspace(1,50000);%speed
y = linspace(1,50000); %torque
kc = 0.2;
kw = 0.00001;
ki = 0.008;
conl = 20;
[x,y] = meshgrid(x,y);
outpower = (x.*y);
copper_loss = (y.^2)*kc;
iron_loss = (x.*ki);
windage = (x.^3)*kw;

input_power = outpower + copper_loss + iron_loss + windage + conl;
E = (outpower./input_power);
% efficinecy values
v = linspace(0.65,0.95,10);
box off
grid off
contour (x,y,E,v);
xlabel('speed'),ylabel('torque');

PLOTS:

The above plot shows motor speed vstorque characteristics efficiency plots

The innermost contour line is having highest ever efficiency

Contour show the maximum height and point in that same group of efficiencies

With increase in efficiency the contour also grow inside and similarly the contour showing outermost line is having lowest efficiency