Challenge: 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.

DESCRIPTION:

                Brake is a mechanical device. From a moving system, it absorbs energy and inhibits motion. It is used for reducing the speed of a wheel or axle. It works by means of friction. The maximum decelerating effect obtained is called peak force, which is the main characteristic of the braking system. The temperature of brakes gets high when they are typically used and this may lead to failure of the system.

1) FOR A DEFINED DRIVING CYCLE, CALCULATE THE ENERGY REQUIRED FOR BRAKING:

BRAKING:

                 A device for arresting or preventing the motion of a mechanism usually by means of friction apply the brakes took his foot off the brake, something used to slow down or stop movement or activity use interest rates as a brake on spending.

As we assume the values of time and speed we have created a graph in excel.

CODE:

% mass of the vehicle in kg
mass = 1000;
% time period 
t = drivecycle(:,1);

% velocity data in
v = drivecycle(:,2);
for i=1:48
    
    if v(i)>v(i+1)
        % BE stands for brake energy
        BE(i) = 0.5*mass*(v(i)-v(i+1))^2;
    end
end
fprintf('Total brake energy is %d',sum(BE))
fprintf('Average brake energy is %d',mean(BE))

% plotting the results
figure(1)
subplot(2,1,1)
plot(t,v,'linewidth',3,'color','g')
xlabel('Time(s)')
ylabel('Velocity(m/sec)')
title('Drivecycle')

subplot(2,1,2)
plot(BE,'linewidth',3,'color','r')
xlabel('Time(s)')
ylabel('Braking Energy')

By comparing the excel plot and matlab plot the drivecycle remains same.

GRAPHS:

2) WHY ELECTRIC MOTOR CAN'T DEVELOP BRAKING TORQUE AT HIGH SPEED SIMILAR TO STARTING? HOW ELECTRIC & MECHANICAL BRAKES ARE CO-ORDINATED:

 WHY ELECTRIC MOTOR CAN'T DEVELOP BRAKING TORQUE AT HIGH SPEED SIMILAR TO STARTING?

Sometimes 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 Vbemf and therefore current can't increase more. Whenever Friction torque + Additional loads = Motor Torque, the load remain in equilibrium (does not move, or moves under constant speed). Whenever Friction torque + Additional loads < Motor Torque, the load accelerates (Motor torque - Friction torque - Additional loads = Acceleration torque > 0). Al high speeds, the BEMF is so close to Vbus that the applicable Motor torque gets limited. Then, the bigger the Friction torque + Additional load is, the smaller the Acceleration torque becomes.  This can also occur if acceleration or speed is limited by some reason.  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. 

Reasons why the torque actual is NOT the real torque:

• If commutation, phasing, is not correct, an important part of the current is not useful torque, it just creates electrical and magnetic loses. 
• If current loop is not well configured or unstable, the current will be higher and with ripple, the real "useful mechanical torque" will be indeed lower.
• Torque constant can vary due to several factors, like motor construction, temperature of the magnets (that affects its magnetic properties)

MAXIMUM ACHIEVABLE TORQUE DEPENDING ON MOTOR SPEED:

To prevent this;

• Increase DC bus voltage. 
• Unlimit system acceleration.
• Use a motor with less resistance and lower back EMF proportion. 

HOW ELECTRICAL & MECHANICAL BRAKES ARE CO-ORDINATED:

TYPES OF BRAKING SYSTEMS:

 MECHANICAL BRAKING:

• Drum braking
• Disc braking
• Band braking
• Pawl & Ratchet braking

 ELECTRICAL BRAKING:

• Plugging type braking
• DC injection type braking
• Eddy current braking
• Dynamic resistor type braking
• Regenerative braking
• Sharing DC bus type braking                                              

MECHANICAL BRAKING SYSTEM:

Mechanical braking mostly used in scooters, motor vehicles, and motorcycles where small power is required. It is essential in manufacturing power transmission applications, material handling, etc. It delivers forces to the axle or a wheel in order to stop motion. It helps to reduce the speed of the system slowly by the mechanical process when compared to electrical braking. The working of a mechanical brake depends on the pedal. When the pedal is pressed, the brake shoes are pushed outwards and rotates against the drum which is connected to the wheels. Hence the machine or vehicle gets slow down and stopped. And when the pedal is released, it goes to normal position due to the pullback action of spring shoes.

ELECTRICAL BRAKING SYSTEM:

                                                   Electrical braking is used to reduce the speed of the machine depending on flux and torque. This type of braking is mainly used for functional braking to control the speed of the machine. It is easy to handle and comfortable. But it cannot be used for emergency braking and parking braking. The working of electrical braking depends on the electromagnetic force (EMF) acting on the brake shoes. The battery is used to generate an electric current which helps to energize the electromagnet mounted on the backplate. This results in activating the cam and expanding the brake shoes. Hence the vehicle or machine is stopped by braking the wheel.

REGENERATIVE BRAKING SYSTEM:

                                          It is one of the types of electrical braking system. When the speed of the motor is increased than the synchronous speed, then the regenerative braking is used. When the rotor rotates higher than the speed of the synchronous speed, then the motor acts as a generator and the directions of current flow and the torque are reversed. Hence the generator is stopped by braking. The main disadvantage is, when the motor exceeds the synchronous speed, it is possible of mechanical and electrical damage. So, regenerative braking can be done at the sub-synchronous speed only when the variable frequency source is applied. An inverter is used to return excess energy back to the three-phase supply rather than the dissipation of energy in the resistor. To drive the variable frequency systems, an inverter is connected in parallel to the rectifier. The regenerative braking is mainly used in electric vehicles.

PLUGGING TYPE BRAKING SYSTEM:

                                                          It is also one of the types of electro braking system. In this type, the pedal is used for braking the vehicle. When the pedal is pressed, the speed of the electric vehicle is reduced by changing the polarity and direction of the motor. The direction of the motor gets reversed and inturn causes braking the wheel. In generators, the use of the plugging type braking system results in decreased speed due to the reversal of terminals of the supply, reversal of torque, and restriction of rotation of the motor . An external resistor is used to limit the current flowing through the plugging circuit. The more power is wasted during the plugging.

DYNAMIC BRAKING SYSTEM:

                                              It is also known as dynamic resistor braking or dynamic rheostat braking. In this type, the resistance is provided to the motor by the rheostat connected to the circuit is capable of acceleration or deceleration of the vehicle. This resistance helps to reduce the speed and stops the electric vehicle. The resistor or rheostat in the circuit dissipates excess energy on the capacitor by connecting a resistor in parallel with the capacitor. When the motor acts as a generator, reverse current flows through the circuit, and the torque changes and causes braking. The resistance in the circuit can be removed to maintain the constant torque while braking the motor.

HYDRAULIC BRAKING SYSTEM:

                                                 The hydraulic braking system uses liquid as a pressure for the movement or force or to increase the force. The pressure applied on a liquid can be called as hydraulic pressure. This type of braking system works on the principle of pascal’s law. In this type, when the force is applied on the pedal, it is converted to hydraulic pressure by using a master cylinder/liquid. This hydraulic pressure helps in braking the vehicle by transferring pressure to the final brake drum or disc rotor via braking lines. It ensures that the braking effect is the same on all the four/two wheels. Instead of using brake fluids, hydraulic brakes are used to accelerate or stopping the vehicle. It is mostly used in all types of bikes and cars because of their effectiveness, the highest brake generating force capacity.

Comparison between Electrical and Mechanical Braking Systems:

a.)Electrical braking systems are used for functional braking (controlling the speed; bringing the load to standstill). Mechanical braking systems are used for emergency stopping or for parking.
b.)Electrical braking is usually very smooth and comfortable. Mechanical braking is usually rough and uncomfortable.
c.) No wear results from electrical braking. Mechanical braking on the other hand causes wear in the braking components and requires regular maintenance.
d.) It is possible (but not always feasible) in electrical braking systems to return the regenerated energy back to main supply. This is not possible in mechanical braking systems and the energy is always lost as heat, noise and wear.
e.) The electrical braking system cannot be used as a safety device. Most
systems will require a mechanical braking system as a backup safety device.

3) MAKE A MATLAB PROGRAM WHICH PLOTS CONTOUR OF GIVEN MOTOR SPEED, TORQUE & EFFICIENCY VALUES:

The different types of losses occuring in motor are:

• Copper losses
• Iron or core losses
• Mechanical losses

i) COPPER LOSSES:

ARMATURE COPPER LOSSES:

                                              These losses occur in armature and field copper windings. Copper losses consist of Armature copper loss, Field copper loss and loss due to brush contact resistance. This loss contributes about 30 to 40% to full load losses. The armature copper loss is variable and depends upon the amount of loading of the machine.

SHUNT FIELD COPPER LOSSES:

                                                  In the case of a shunt wounded field, field copper loss is practically constant. It contributes about 20 to 30% to full load losses. Brush contact resistance also contributes to the copper losses. Generally, this loss is included into armature copper loss.

SERIES FIELD COPPER LOSSES:

                                                   In a series machine, the copper loss in the series windings is I²_seR_se, where, I_se is the current through the series field windings, and R_se is the resistance of the series field windings. In a Compound machine, both the shunt and the series field losses occur.

ii) IRON OR CORE LOSSES:

WINDAGE LOSSES:

                             Windage loss is the reduction in efficiency due to windage forces. For example, electric motors are affected by friction between the rotor and air. Large alternators have significant losses due to windage.

CODE:

clear all
close all
clc


w = linspace(0,1000);
T = linspace(0,250);

% motor constants
kc = 0.2; % for copper loss
ki = 0.008; % for iron loss
kw = 0.00001; % for windage loss
conL = 20; % for constant motor losses

% making mesh
[X,Y] = meshgrid(w,T);
Outputpower = (X.*Y) ; % Torque * Speed = Power
B = (Y.^2)*kc; % copper loss
C = (X.^3)*kw; % winding loss
D = X*ki; % Iron loss
InputPower = Outputpower + B + C + D + conL;
Z = Outputpower./InputPower;

% Set the efficiencies for which the control will be plotted
V = [0.70,0.75,0.80,0.82,0.87,0.89,0.90,0.92];
box off
grid off
contour(X,Y,Z,V)
xlabel('Speed(rad/s)')
ylabel('Torque(N.m)')
hold on
V = [8000,10000];
contour(X,Y,Outputpower,V)

 GRAPH:

RESULTS:

               1) 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.