Week-11 Challenge: Braking

AIM:- Braking

OBJECTIVE:-

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.

For a defined driving cycle.

A driving cycle commonly represents a set of vehicle speed points versus time.

Compromises of the Drag and Braking Energy

It is used to assess fuel consumption and pollutants emissions of a vehicle in a normalized way, so that different vehicle can be compared.

Motor and is evaluated through a set of motor torque and speed points instead of vehicle speed points.

There are two kinds of driving cycles, the modal cycles as the European standard NEDC, or Japanese 10-15 Mode, and the transient cycles as the FTP-75 or Artemis cycle.

Driving range calculation, state of charge estimation, energy consumption prediction, and energy management optimization strategies for EV under the international standard driving cycles can produce large errors.

The vehicle at many points is brought to rest or decelerated (i.e. 80 Km/hr - 0 km/hr) within 10- 15 sec. Hence high braking energy is required. 

calculate the energy required for braking.

Regenerative braking systems (RBSs) are a type of Kinetic energy vehicle that transfers the kinetic energy of an object in motion into a potential energy vehicle to slow the vehicle down, and as a result, increases fuel efficiency These systems are also called kinetic energy recovery systems.

Motor/Generator Unit (MGU): this converts mechanical energy into electricity and vice versa.

Power Control Unit (PCU): controls the switching of electric current between the MGU and the battery.

Storage device: which can be any of those mentioned above (battery, supercapacitor, flywheel).

  kinetic energy = 1/2mv^2

OBJECTIVE:- 2

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

Here for both in the case of AC or DC, there is no torque at maximum speed, gradually the torque decreases at high speed as the speed tends to be maximum speed.

This situation does not occur at the beginning of acceleration because most of the torque is used to increase the kinetic 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 as well as to develop braking torque.

The braking operation happens in the motion which is been distributed as per proportion in the Brake Energy and the regenerative braking.

The figure represents forward motoring (positive torque and speed) and forward braking (negative torque and positive speed).

We know that torque of the induction motor is inverse proportional to the square of the rotor speed.

So at higher speed, the torque will be less. That’s why a high-speed electric motor cannot produce braking torque similar to starting.

How electric and mechanical brakes are coordinated?

The electric motor must be controlled to produce the proper amount of braking force to recover as much braking energy as possible and at the same time, the total braking force must be sufficient to meet vehicle deceleration commanded by the driver. For meeting sufficient braking in EV vehicles we use brake control strategies.

Series brake control: Here mechanical brake and the electric brake is done one after another.

Parallel brake control: Here both brakes are applied simultaneously.

We need both mechanical and electrical braking systems. In emergency braking or parking brake, we needed mechanical braking.

Mechanical brake: It is nothing but friction brakes used in a vehicle to slow down the vehicle. The most common type of brakes is drum brake and disc brake.

Drum brake:-

A drum brake is a brake that uses friction caused by a set of shoes or pads that press outward against a rotating cylinder-shaped part called a brake drum.

The term drum brake usually means a brake in which shoes press on the inner surface of the drum.

Disc brake:-

A disc brake is a type of brake that uses the calipers to squeeze pairs of pads against a disc or a "rotor" to create friction.

This action slows the rotation of a shaft, such as a vehicle axle, either to reduce its rotational speed or to hold it stationary.

The energy of motion is converted into waste heat which must be dispersed.

Hydraulic actuators disc brakes are the most commonly used form of brake for motor vehicles, but the principles of a disc brake are applicable to almost any rotating shaft.

The components include the disc, master cylinder, caliper (which contains cylinder and two brake pads) on both sides of the disc.

A drilled motorcycle brake disc

Electrical braking:

It is used to reduce the speed of the motor depending upon the flux and torque. This kind of braking can be done in different methods.

• Plugging type braking
• Dynamic braking
• Regenerative braking

Plugging braking:

This is the simplest type of electric braking

In this method, the torque of the motor is reversed.

It makes the motor and the driven machine standstill condition.

A special device is required to cut off the supply as soon as the motor comes to rest.

The Plugging braking method can be applied to AC and DC motor

Dynamic braking

In this method of braking, the motor is disconnected from the supply

Then operated as a generator driven by the kinetic energy of the rotor and the load.

Thus the kinetic energy of rotation is converted into electrical energy

This electrical energy is dissipated in the external resistance connected across the motor at the braking instant.

The advantage of dynamic braking is that no energy is required from the supply to brake the motor.

The Dynamic braking method can be applied to brake DC motors, induction motors, and synchronous motors.

Regenerative braking:-

In the Regenerative braking method, the motor is not disconnected from the supply.

It remains connected to it and feeds back the braking energy or its kinetic energy to the system.

As no energy is wasted in this method and it is supplied back to the system, thus overall energy is saved.

Thus the regenerative braking is better than Plugging and Dynamic braking.

This type of braking is used in traction whenever the train runs down.

The regenerative braking is applied to DC shunt motor, series motor, and 3-phase induction motors.

Electrical and mechanical braking systems can are coordinated by using a Brake strategy subsystem.

1) Serial Braking Strategy

2) Parallel Braking Strategy

Serial regenerative braking:-

Serial regenerative braking is based on a combination of friction-based adjustable braking systems with a regenerative braking system that transfers energy to the electric motors and batteries under an integrated control strategy.

The overall design is to estimate the deceleration required by the driver and distribute the required braking force between the regenerative braking system and the mechanical braking system.

Serial regenerative braking could give an increase of 15-30% in fuel efficiency. It requires a brake-by-wire system and has a more consistent pedal feel due to good torque blending capability.

Parallel regenerative braking:-

The parallel braking system is based on a combination of friction-based system and the regenerative braking system, operated in tandem without an integrated control.

The regenerative braking force is added to the mechanical braking force which cannot be adjusted.

The beginning pedal travel is used to control the regenerative braking force only, the normal mechanical braking force is not changed.

The regenerative torque is determined by considering the motor capacity, battery state of charge SOC, and vehicle velocity.

The regenerative braking force is calculated from the brake control unit by comparing the demanded brake torque and the motor torque available.

Parallel regenerative braking could give an increase of 9-18% in fuel efficiency.

OBJECTIVE:-3

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.

clear all
close all
clc
x = linspace(1,1000);
y = linspace(1,250);

% copper loss contant
kc = 1.5
% iron loss constant
ki = 0.1
kw = 0.00001
conl = 30%
[x,y] = meshgrid(x,y);
pout = (x.*y);
b = kc*(y.^2);
c = ki*x;
d = kw*(x.^3);
pin = pout + b + c+ d + conl;

%efficency
eta = pout./pin;
v=[0.5,0.6,0.7,0.75,0.8,0.85,0.88];
box off
grid off
contour(x,y, eta, v);
xlabel('speed in rad/s')
ylabel('torque in Nm');

In this MATLAB script, we create arrays for the speed and the torque of the motor using the linspace command.

For the speed, consider a range of values from 0 to 2000 radians per second whereas for the torque, consider a range of values from 0 to 300Nm.

These arrays are then used to create 2D grid coordinates using the mesh grid command.

These coordinates will help in the creation of the contour plot. In order to find the efficiency of the motor, we need to consider the various losses that occur in the motor’s operation. These are:

Copper losses

Iron losses

Windage losses

Conduction losses

These various losses are dependent on the speed and the torque of the motor.

They each also have a characteristic constant of proportionality which are all assumed for the following calculations and represented as 'K'.

In each calculation, the speed and the torque are represented respectively as ω and T.

Copper losses = KCu⋅ω2

Iron losses =KFe⋅T2

Windage losses =Kw⋅T3

The values of coefficients are taken as general values. Conduction losses are assumed to be 18 Joules.

Finally, we plot the contour with the 'contour' command.

In addition, an array for the efficiency values we want to plot for is created called 'W' and is assigned for values ranging from an ef

RESULT:-

From this contour plot, we can see that the highest level of efficiency arises in the innermost contour plot and this efficiency decreases as the contours increase in size from the innermost contour.