Week-11 Challenge: Braking
WEEK-11
BRAKING
QUESTIONS:
- For a defined driving cycle, calculate the energy required for braking.
ANSWER:
Driving cycle:
A driving cycle will have some reference parameters. Minimum speed, Maximum speed, Average speed, Speed Acceleration Probability Distribution (SAPD), and Speed Acceleration Frequency Distribution (SAFD) which are three-dimensional plots. Driving Cycles can be constructed combining the selected micro-trips.
example of driving cycle in xlsx file,
driving cycle is the ratio of time and velocity.
Braking energy,
Deceleration is just same as acceleration is called braking.
To calculate deceleration we can use the same equation used to calculate acceleration,
Energy = 1/2 * m*v^2
where,
m = mass of the vehicle
v = velocity of the vehicle
v = vi - vf
Therefore above equation can be writtem as
E = 1/2 * m * (vi - vf)^2
Example of braking calculations,
Mass = 1000kg/m^3
Initial velocity = 120 kmph
Final velocity = 0 kmph
so, the braking energy
E = 1/2 *m*(vi - vf)^2
the above equation calculate to the excel file ,
so, the acceleration power is 83kw
Energy required for braking is 0.23kwh
2. Why electric motor can’t develop braking torque at high speed similar to starting? How electric and mechanical brakes are coordinated?
ANSWER:
In speed-Torque charateristics of motor, the rated speed that marks the break point of the constant torque line the voltage remains constant but the flux is weakened.The motor torque will decrease hyperbolically with speed.This region bounded by the hyperbola is the constant power region.The line of constant power connects the rated speed to maximum allowable rotational speed of the motor. The maximum rotational speed of a PM motor is roughly 2-3 times the rated speed. The magnitude of regenerative braking torque is determined as a function of motor speed.
When a brake torque of 800 N.m is requested by the driver the 30kw electric motor is unable to provide sufficient torque above a motor rotational speed of approxmately 40 rad/sec. when the electric motor is connected to the driveshaft, without any gear ratio, the brake toque request 800 N.m cannot be met above a vehicle velocity of 45kmph. for higher velocity a friction brake torque must be added to meet the brake torque request of 800 N.m.
Brake force distribution
They are of twp types:
1. Electrical braking
2. Mechanical brake
brake force is distributed is
1. Front axle
2. Rear axle
brake control strategies,
There are 2 brake controls,
1. Series : In series we apply mechanical brakes are or regenerative braking one after another.
2. Parallel: we apply both mechanical and regenerative brakes simultaneously.
At low speed Regenerative brakes are used.
At high speed Both regenerative and hydralic brakes are used.
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.
ANSWER:
SCRIPT FILE:
RESULTS:
