Week-11 Challenge: Braking

Aim: -

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

Solution:

The braking system is one of the most critical functions upon which the safety of the entire vehicle depends. The most crucial function of the braking system is to stop the vehicle in the shortest possible distance in case of emergency and to control the vehicle direction.

A brake is a mechanical device that inhibits motion by absorbing energy from a moving system. It is used for slowing or stopping a moving vehicle, wheel, axle, or to prevent its motion, most often accomplished by means of friction.

The most commonly used braking systems are

1. Mechanical braking system: The mechanical braking system powers the hand brake or emergency brake. It is the type of braking system in which the brake force applied on the brake pedal is carried to the final brake drum or disc rotor by the various mechanical linkage like cylindrical rods, fulcrum, springs etc. in order to stop the vehicle
2. Disc brake: -This is a mechanism used for slowing down or stopping the rotation of a wheel from its motion. A disc brake is normally made of cast iron, but in some cases it is also made of carbon or carbon-ceramic Friction causes the disc wheel to stop.

1. Drum brakes:- these are conventional braking system where brakes are applied by friction caused by set of shoes or pads rubbing against the rotating drum called brake drum.

2. Engine braking: - Engine braking occurs when the retarding force within the engine is used to slow down a motor vehicle, as opposed to using additional external braking mechanism such as friction brakes or magnetic brakes. Traffic regulation in many countries require trucks to always drive with an engaged gear, which in turn provides a certain amount of engine braking when no accelerator is applied.
3. Vacuum braking : The term engine braking refers to the braking effect that occurs in gasoline engine when the accelerator pedal is released. This causes fuel injection to cease and the throttle valve to close almost completely, greatly restricting forced airflow from eg,. Turbo. The latter of which causes a strong manifold vaccum for which the cylinders have to work against- sapping much of the potential energy out of the system over time and producing the majority of the engine braking effect.
4. Compression release brake: - Diesel engines in personal car provided little engine braking as they are not equipped with a throttle body and thus cannot draw a vaccum in the intake manifold. A compression release brake is the type of brake most commonly confused with real engine braking. It is mainly used in diesel truck and works by opening the exhaust valve at the top of compression stroke, so the large amount of energy stored in that compressed air is not returned to the crankshaft but is released into the atmosphere.

Commonly used braking system in Electric vehicle:

1.Electromagnetic braking system:

This type of braking system can be found in many modern and hybrid vehicles. The electromagnetic braking system uses the principle of electromagnetism to achieve frictionless braking. This serves to increase the life span and reliability of brakes. Also traditional braking system are prone to slipping while this is backed with quick magnetic brakes. So without friction or the need for lubrication, this technology is preferred in hybrids. Also it is quiet modest in size compared to the traditional braking systems. It is mostly used in trams and trains. To make these brakes work, a magnetic flux is passed in a direction perpendicular to the rotating direction of the wheel. We see rapid flux flowing in a direction opposite to the rotating wheel.

2.Dynamic braking: This conversion of mechanical energy of a rotating shaft to electrical energy to decrease reduce the speed of the vehicle is called dynamic braking. The dynamic braking slows down a moving vehicle or object by converting its kinetic energy into a form that can be either used immediately or stored until needed. In this mechanism the electric motor uses vehicle’s momentum to recover energy that would otherwise be lost to the brake disc heat. The interaction of armature winding with a moving external magnetic field, with the armature connected to an electric circuit with either a power supply or power receptor. Since the role of the electrical converting device is determined by which interface provides or receives energy, the same device can full fill the role of either generator or a motor. In dynamic braking the traction motor is switched into role of generator by switching from a supply circuit to a receptor circuit while applying electric current to a field coils that generate magnetic field.

Different types of braking are

1. Rheostatic braking
2. Regenerative braking

Drive cycle:

A driving cycle is a series of data points representing the speed of a vehicle versus time. Driving cycle are produced by different countries and organization to assess the performance of vehicle in various ways, as for instance fuel consumption, electric vehicle autonomy, and polluting emissions.

The drive cycle used is a driver_glider drive cycle containing values for time and velocity.

Braking energy:

The braking energy requirement can be expressed in terms of the change in the kinetic energy of the vehicle. Kinetic energy of a vehicle is the energy possessed due to the motion of the vehicle.

Ke=1/2 m*v^2

Ke is the kinetic energy of a vehicle in joules

m is the mass of the object

v is the velocity of the object in m/s.

Brake energy can be expressed as change in the kinetic energy

delta ke is the change in kinetic energy in joules.

Vf is the final speed in m/s

Vi is the initial speed in m/s

m is the mass of vehicle in kg

Question 2: Why electrical motor cant develop braking torque at high speed similar to starting? How electric and mechanical brakes are coordinated?

Solution :

The electric motor cannot produce higher torque at a higher speed similar to starting torque because in order to achieve higher speed the motor is operated in the field weakening region, where the flux density is reduced resulting in lower torque from the motor at higher speeds. Thus torque produced at higher speed is less compared to torque at lower speeds.

While developing a high brake torque at high speed the rotor is made to rotate in a stationary magnetic flux of high density thus the motor acts as a generator. Due to high rate of change of flux inside the rotor a high magnitude of current is generated and is dissipated as heat and some amount of current is used to charge the battery. Due to the limiting charging rate, limiting charging current and cooling limitation the heat produced can damage the windings. Hence for safe working condition it is not practical to develop high braking torque. While at lower speeds the rate of change of flux inside the rotor is less thus developing higher brake torque at lower speeds.

Coordination of mechanical and electrical brakes

The electrical brake alone cannot meet the braking torque requirement thus in electric vehicle and hybrid electric vehicle mechanical braking system is incorporated in order to achieve the desired control over the vehicle. Considering that regenerative braking is a key requirement the control and coordination of both mechanical and electrical braking are essential.

The two major factors affecting the design of EV braking system are:

1. Braking power requirement
2. Braking energy requirement

Based on the braking requirement the braking system technique for control and coordination of braking system are designed.

1. Series braking: Series braking allows independent modulation of the mechanical brake torque of each axle according to the regenerative torque
2. Series braking- optimal feel

The series braking system with optimal feel consists of a braking controller that controls the braking force on the front and rear wheels. The control objective is to minimize the stopping distance and optimize the driver’s feel

1. Series braking optimal energy recovery

The principle of the series braking system with optimal energy recovery is to recover the braking energy as much as possible in the condition of meeting the total braking force demanded for the given deceleration

2. Parallel braking system

The parallel braking system includes both an electrical brake and mechanical brake, which produce braking force is parallel and simultaneously.

Nissan-electric driven intelligent brake

The EDIB allows and controls optimization of the regenerative brake and regular friction brake . the electric driven intelligent brake, utilizes the vehicle’s electric motor to directly operate the brake cylinders. When the driver steps on the brake pedal, the necessary braking force is maintained while the hydraulic preassure of the friction brake is controlled to help maximize the energy regeneration of the hybrid motor. Because of its simple structure, in which the motor directly operates the brake cylinder.

Q3. Make a MATLAB programme that plots the contour of the given motor speed, torques and efficiency values. Attach the code as a .m file, attach screenshot of all the plots.

Solution:

The efficiency of an electric motor can be defined as the ratio of the output mechanical power to the electrical power.

The two type of losses that occur in a three phase induction motor. These losses are:

1. Constant or fixed losses

Constant losses are those that are considered to remain constant over the normal working range of the motor. The fixed losses can be easily obtained by performing a non-load test.

Efficiency=mechanical power output/electric power input

Efficiency can be also written as

These losses are further classified as

1. Iron core losses

These losses are further classified as hysteresis losses and eddy current losses. Eddy current losses are minimized by using lamination on the core. Since by lamination the core, area decreases and hence resistance increases, which results in a decreases in eddy current. Hysteresis losses are minimized by using high grade silicon steel. The core losses depend upon the frequency f and the frequency of the rotor is slip times the supply frequency which is always less than the stator frequency. For stator frequency of 50 Hz, rotor frequency is about 1.5 hzsince uncer normal condition slip is of the order of 3%. Hence the rotor core loss is very small as compared to stator core loss and is usually neglected in running condition.

1. Mechanical and brush friction losses

Mechanical losses occur at the bearing and brush friction losses occur in the wound rotor induction motor. These losses are zero at the start and with an increase in speed these losses increase.

1. Variable losses

These losses are also called copper losses. These losses occur due to the current flowing in stator and rotor winding. As the load changes,the current flowing in rotor and stator winding also changes , and hence these losses also change. Therefore these losses are called variable losses. The copper losses are obtained by performing blocked rotor test.

Matlab code for contour plot

clear all 
close all
clc

x=linspace(1,750);
y=linspace(1,750);

kc=0.2;
ki=0.008;
kw=0.00001;
c=20;
[W,T]=meshgrid(x,y);
output_power=(x.*y);
q=(T.^2)*kc;
i=W.*ki;
w=(W.^3)*kw;
In_power= output_power+q+i+w+c;
Z=(output_power./In_power)*100;
V=[50,55,60,65,70,75,80,85,90,95];
box off
grid off
[q,h]=contour(W,T,Z,V);
clabel(q,h)
title('Speed Vs Torque Vs efficiency')
xlabel('Torque in Nm')
ylabel('Speed in rad/s')
hold on

Contour plot

A contour line of a fuction of two variable is a curve along which the function has a constant value so that the curve joins points of equal value. It is a plane section of the three dimensional graph of the function(x,y) parallel to the (x y )plane.

The bottom part of the diagram show some contour lines with a straight line running through the location of the maximum value. The curve at the top represent the value along the straight line.