Week-11 Challenge: Braking

BRAKING

Aim:

1. For a defined driving cycle, calculate the energy required for braking.
2. Why electric motor can’t develop braking torque at high speed similar to starting? How electric and mechanical brakes are coordinated? 
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.

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

Driving cycle:

A driving cycle are a set of vehicle speed points versus time it is used to calculate the fuel consumption of a vehicle which is later on is compared with different vehicles. Main use of driving cycle are as follows

To assess the performance of a vehicle (fuel economy, emission test)

To analyse battery state and energy consumption in electrical vehicles

To simulate vehicle model prior to the actual development

Braking:

Road vehicles involves the conversion of kinetic energy into thermal energy (heat) when stepping on the brakes the driver commands a stopping force several times as powerful as the force that puts the car in motion and dissipates the associate kinetic energy as heat

The prosses of braking follows the law of conversation of energy cannot be created or destroyed it can only be converted into another form

E(kinetic)=1/2*m*v^2…

Where

M=mass of the vehicle in kg

V=velocity of the vehicle in m/s

E(kinetic)=kinetic energy in KJ

In our case velocity will be (V-Vi) where V is the speed of the vehicle Vi the speed of the vehicle which is slow down or brought down to rest

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

Regenerative braking:

This are the type kinetic energy recovery system that transfer the kinetic energy of an object into potential or stored energy to slow down the vehicle and as a result to increase the fuel efficiency there are multiple methods of energy conversion in Regenerative braking system including spring fly wheel electromagnetic and hydraulic more recently an electromagnetic flywheel hybrid

They are installed along the drive train or fitted to the drive wheels of a vehicle where they inhibit the motion of the wheels using magnetic fields or mechanical torque these methods of motion inhibition allow energy to be generated under braking as opposed to friction brakes which simply waste away energy to slow the vehicle by turning the kinetic energy into thermal energy

Regenerative braking system can improve consumption and reduce the over all braking load taken on by the vehicles friction brakes reducing the wear on the brake pads. In HEV and EV the transmission of the car is set up such that when drive applies the brakes the electric motor reverses itself and applies a resistance applied to the wheels rather than power this resistance applied to the wheels in then put through the electric motor where it is used to recharge the batteries

Limitation:

There is a limitation due to the maximum recharging rate of the circuit and the capacity of battery the braking force from on electromagnetic is always limited

Why electric motor can’t develop braking torque at high speed similar to starting?

In EV application there is a motor generator and a battery pack to supply power as the electric current travels through the motor within a magnetic field it generates a force the more current applied the more the motor will spin

For regenerative braking a motor will act as a generator the direction of the electric flow is reversed the electric motor will rather than talking in current to spin the armature the armature is spin through the field by the deceleration of the vehicle. The electricity produces is used to recharge the battery and the motor can switch between powering the car and regenerating electricity as fast as the driver can accelerate and lift off

As the motor reverse increase it also  creates “back electro motive force “ or “back EMF “. The faster the motor spins the more back EMF is created reducing the effective voltage it can deliver. But at zero RPM all of the electricity power is applied becomes instant torque without. The higher the revers the more back EMF meaning that instantaneous torque effect diminishes

The reason behind this no matter how much the current loop wants to increase current proportion to torque by increasing the PWM duty cycle of the power stage the V motor is too close to V b emf and therefore current can’t increase more

How electric and mechanical brakes are coordinate?

A regenerative braking system is used to converter waste energy heat into useful energy stored in battery with the help of a motor which acts as a generator when reversed there are certain limitation with RBS the electric motor is unable to provide all the brake torque necessary for large decelerations. In case of braking if the braking torque is applied only one axle there are chances stability and maneuverability  issues can occur. So to avoid such situation regenerate braking is operate in conjunction with a friction mechanical brake system

The RBS and mechanical brake system are used in strategic manner which is called as brake control strategy

1. Series brake control or series RBS:

In this type of control system the mechanical brakes and regenerative (electric) brakes are applied one after the another we can only apply one brake system at a time either RBS or mechanical brakes

The series brake system is further classified:

1. Comfort of the drive
2. Braking according to the energy efficiency

2. Parallel brake control or parallel RBS:

In this type of brake control system both the mechanical brakes and RBS are applied simultaneously

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.

Losses that occur in all types of electric motor:

1. Copper loss:

I^2.R where R is the armature resistance and we can say that torque is proportional to current this copper loss can be consider as kc.T^2 where kc is the resistance of brushes and the flux and its effect

Copper losses = Kc.T^2

2. Iron losses:

Iron losses are occurred due to magnetic losses and it is denoted as ki.w where Ki is the factor which is based on the magnetic field effect. For permanent magnets which has constant magnetic field an this iron losses are based on the value of Ki and this keeps on changing due to variation in speed

Iron losses = Ki.w

3. Windage losses:

this are based on the size & shape of the motor and the amount of windage losses that the motor is going to experience

windage losses = Kw.w^3

4. Constant losses:

This losses are present in the motor constantly and are denoted by C.

Total losses the total losses are consider by combining all the four losses and denote as

Total losses = kc.T^2+ki.w+kw.w^3+c