Week-11 Challenge: Braking
EV BATCH-17
2)Why electric motor cant develop braking torque at high speed similar to starting?How electric and mechanical brakes are coordinated?
A) Regenerative braking allows electric vehicles to use the motor as a generator when the brakes are applied to pump vehicle energy from the brakes into an energy storage .Regenerative braking is an effective approach to extend the driving range of EV.
The regenerative braking torque cannot be made large enough to provide all the the required braking torque of the vehicle as it might lead to the deterioration in stability of the vehicle also as compared to the torque values of acceleration torque of motor is much higher than decceleration torque.
At low speed : Regenrative brakes are used
At high speed : Both regenrative and hydraulic brakes are used.
At high speed,the motor might be in its constant power region which causes less values of torques produced and this limit the amount of braking torquethat is used for regenration.Hence the peak value of acceleration torque and braking torque are not equal.
Electrical and mechanical co-ordination:
The regenerative braking system may not be used under may conditions,such as with a high state of charge (SOC) or a high temperature of the battery.In these cases ,the mechanical braking system works to cover the required total braking torque.
Thus ,co-operation between the mechanical braking system and the regenerative braking system is a main part of the design of the EV braking control strategy and is known as torque blending.
There are 2 different control strategies for braking combinations :
1.series control strategy
2.parallel control strategy
series control strategy :
A combination of friction based mechanical braking system with a regenrative braking sysytem that transfers energy to the battery with an integrated control,which distributes the required braking force between regenerative braking and the mechanical braking system.
Series strategy could give an increase of 15-30% in fuel efficiency and has more consistent braking due to good torque blending capability.
Parallel control strategy :
. Combination of friction based mechanical and the regenerative braking system,operated simultaneously without an integrated control
. The regenerative braking force is added to the mechanical braking force which cannot be controlled
. The regenerative torque is determined by considering the motor capacity battery soc and vehicle velocity.The regenerative braking force is calculated from the brake control unit by comparing the demand brake torque and the motor torque available.
Application of braking combination :EDIB(Electric Driven Intelligent Brake) in NISSAN LEAF
.EDIB allows and controls optimization of the renerative brake and regular friction brake(hydraulic brake)
.The system produces an adequate braking force keeping energy regenaration increased as much as possible.
. At higher speeds, acombination of renerative and friction braking is applied.
. At lower speeds, regenerative braking is given a higher proportion for maximised energy conversion.