Week-11 Challenge: Braking

EV BATCH - 17

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

ANS :

TORQUE SPEED CHARACTERISTICS :

• Peak Acceleration Torque and Peak Deceleration torque are not equal for Electric Vehicle Motor.
• So the Peak Torque during acceleration and the Peak torque during deceleration or braking are not equal.
• Acceleration is usually higher, at low speed we get higher Torque but high speed the value of the torque gets lower.
• So we cannot say that the motor which is producing 7Nm Torque for Acceleration can also produce -7Nm of Braking Torque.
• So beacuse we Operating the Motor in Field Weakening region or we are operating the Motor above base speed we must know that the torque is compromised or the torque value is lower.
• So Electric motor can't develop braking torque at high speed similar to starting, because Braking Torque of an EV Motor is not as high as the Acceleration Torque.
• Therefore Regenerative braking is cannot happen so effectively that we can get all the power back to the battery.
• The Regenerative braking power requirement is more than the charging limit of the battery or more than regenerative capacity of the machine we must apply Mechanical brakes as well.
• In practical case the combination of these two is applied.
• So usually it is combined response of Mechanical brake and Regenerative brake.
• The reason for how these brake force are combined, that particular phenomena is called Brake Control Strategies.

There are two Brake Control Strategies, which are :

Series -

• In case of Series we apply Mechanical brake and Regenerative electric brake one after another.
• We can apply only one brake strategy at a time either apply Regenerative or apply Mechanical brake.
• The Series Brake Control Strategy has further classification, that is based on whether we want comfort of driver or Brake behaviour compared to energy efficiency.

Parallel -

• In case of Parallel we apply both brakes simultaneously. This Particular Parallel braking is going to happen both at a time.

Electric Driven Intelligent Brake (EDIB) -

• It is a combination of Hydraulic brake or Mechanical brake along with a Regenerative brake.
• It is a Parallel braking technique.
• The Control criteria such that at low speed use regenerative braking higher and at high speed or during initial braking period we actually combine the both.

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