Week-11 Challenge: Braking
- For a defined driving cycle, calculate the energy required for braking.
we use this formula to calculate energy required for braking
we need to create a excel sheet for a drive cycle-
2.Why electric motor can’t develop braking torque at high speed similar to starting? How electric and mechanical brakes are coordinated?
According the graphs attached above -
we have two region, constant Torque and constant power region .
For electric motor , a high torque is available at starting , which is rated torque of motor . motor occurs maximum torque until it reach at base speed or max speed or we can say certain speed range after base speed torque will going to start decrease.The area where peak torque is constant called the constant torque region and the region where torque start decreases is known as the constant power region.
hence in constant power region at high speed there is very less torque , its not possible to develop high braking torque in tis region. if speed will decrease then only the condition when torque increases.
or
power formula --
power = torque * speed
we cannot inecrease torque and speed together becaues if we increase both then the power will also increasess rapidly then its drain the battery very fastly..
How electric and mechanical brakes are coordinated?
Braking systems are necessary for the correct and safe operation of electric vechile. Most functional braking is achieved using electrical braking, which is made possible by the addition of a variable speed drive system (e.g., variable frequency drive system). Electrical braking is efficient and ensures smooth operation and allows the designer to recover the mechanical energy. However electrical braking cannot be used as a safety braking system for the cases of emergency braking and parking braking. It is for these reasons that mechanical braking systems are still employed today, in order to act as emergency and parking brakes.
The Rationale for Braking Systems ,In any motion control system, braking is necessary for the following reasons:
- In order to stop the system in cases of emergency.
- In order to ensure that the system remains stationary when not in operation.
Two types of braking systems are employed in most drive systems: electrical andmechanical.
Types of Electrical Braking Systems
There are six methods of electrical braking in drive systems, listed below:
1. Plugging: The three phase voltage sequence is reversed at the stator terminals of the induction motor. The equivalent form of plugging in dc drive systems is the reversal of the polarity of the armature voltage of a dc motor.
2. DC Injection Braking: A d.c. voltage is applied to any two of the three terminals of the induction motor.
3. Eddy Current Braking: A separate eddy current braking device is coupled to the shaft of the motor. Eddy current braking results when electromagnets areactivated and they induce eddy currents in the rotating metallic disk. The eddy currents produce magnetic fluxes that oppose the flux produced by the electromagnets and thus result in a braking torque on the motor. The energyis dissipated as heat in the rotating metallic disk.
4. Dynamic Resistor Braking: In variable frequency drive systems, when the motor is braking and acting as an induction generator, it returns power back to the d.c. link capacitor and the voltage of the d.c. link increases. A resistor can
thus be used to dissipate the excessive charge on the capacitor, whereby an IGBT is controlled to proportionately switch the resistor in parallel with the capacitor (using pulse width modulation switching). The excessive energy is thus dissipated in the resistor.
5. Regenerative Braking: Rather than dissipating the excessive charge in the resistor, it is possible to return the excessive energy back to the three phase supply by using a dedicated inverter, placed in parallel with the uncontrolledrectifier that is used in the variable frequency drive system.
6. Sharing the DC Bus: Another possible method of removing the excessive charge on the capacitor is to share the d.c. between a number of different variable frequency drive systems. The drive that is braking can supply energyto the drive that is motoring. In such a case, there is no need to dissipate the excessive energy into a resistor or to return it to the three phase powersource.
In the first two methods, the excessive energy is dissipated as heat in the motoritself. In the Eddy current method, the excessive energy is dissipated as heat in the metallic disk. In the fourth method, the excessive energy is dissipated as heat in theresistor. In the fifth method the energy is returned to the ac suppy. In the sixthmethod, the energy circulates between the different motors (from the one that isacting as a generator to the one that is acting as a motor).
In cases where it is possible to recover the energy, the following can be done:
1. It can be stored in batteries.
2. It can be stored in a flywheel (a flywheel that has an integrated permanent
magnet dc alternator).
3. It can be returned to the ac supply using a regenerative drive.
Mechanical Braking Systems
There are four types of mechanical brakes used in drive systems:
1. Drum brakes.
2. Disk brakes.
3. Band brakes.
4. Pawl and Ratchet brakes: The ratchet is linked to a spring loaded friction
plate. The ratchet is lifted using a solenoid.
The drum and disk brakes are either electromagnetically lifted or hydraulically lifted,but always spring applied. They must be spring applied for safety reasons, so thatwhen the power supply is lost, the brake will apply and safely bring the load to a
standstill.
Comparison between Electrical and Mechanical Braking Systems-
The following is a 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 usedfor 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 inmechanical braking systems and the energy is always lost as heat, noise andwear.
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.
There are two ways of applying the combination of brakes -
SERIES- regenerative braking and frictional braking are apply one by one.
Parallel- regenrative braking and frictional are applied at the same time.
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.
Efficiency is calculated from losses-
here to get efficiency we calculaed - iron losses(constant loss),copper losses(variable losses ) , windage losses, friction losses
clear all
close all
clc
speed= linspace (1,1500);
torque= linspace(1,350);
Kc=0.2;
Ki= 0.008;
Kw=0.00001;
con =20;
[X,Y]= meshgrid(speed,torque);
% calculating output power = torque*speed
Outputpower = (X.*Y);
%Calculating copper losses
B= (Y.^2)*Kc;
%Calculating iron losses
C=X*Ki;
%Calculating windage losses
D= (X.^3)*Kw;
Inputpower = (Outputpower +B+C+D+ con);
% Calculating efficiency (Z)
Z= Outputpower./Inputpower;
% the efficiencies for which the contour will be plotted
V= [0.7,0.8,0.85,0.90,0.91,0.92,0.925,0.93];
box off
grid off
contourf(X,Y,Z,V);
xlabel=('speed');
ylabel= ('Torque');
From the countor plot the observations are -
contour output plot is a group of efficiencies where inner most will have highest efficiency and the height is maximum as we keeep on going outside or outward the height reduces and efficiency is reduces