Sunday, 3 July 2016

Regenerative Braking


In the last post "Need for Electrical Braking in Electrical Drives", we discussed the various reasons which call for Electric Braking. Brakes are used to reduce or cease the speed of motors. Braking of Motor can be classified into three types:
  • Regenerative Braking.
  • Plugging type Braking.
  • Dynamic Braking.

In this post we will focus on Regenerative Braking and its application aspect. Regenerative Braking systems are used on most electric and gas-electric hybrid vehicles.



In a Regenerative Braking system, the objective is to recapture the energy that results when the brakes are applied. In electric or hybrid automobiles, the electric motor that drives the car's wheels plays a major part during braking. When the brake pedal is pressed, the regenerative braking circuit switches the motor so that it now operates in reverse to counter the direction of the wheels. This reversal actually makes it perform like a power generator that produces electrical energy. The electricity developed is routed towards the car's storage batteries to recharge them. 

At higher speeds, regenerative brakes still require the assistance of traditional brake systems to be applied as a backup. The efficiency of regenerative braking systems in use today has improved significantly. Some existing systems are able to capture and store as much as 70 percent of the energy that would otherwise have been lost.

What exactly is Regenerative Braking?


For understanding Regenerative Braking Principle, consider the figure below.



For Regenerative Braking to take place, the Back emf of the DC Motor should be greater than the Supply voltage i.e.

E>V and the direction of Armature Current Ia shall be negative which means the Motor is now working as a generator.

As we know that Back emf E = KaØWm

But as the field flux cannot be increased beyond the rated as the core will saturate, therefore, for a fixed supply voltage, Regenerative Braking is only possible at a speed higher than the rated speed but if we have variable supply voltage then Regenerative Braking at a speed below rated speed can also be achieved.

Mind that Regenerative Braking is not possible for DC Series Motor. Why?


In case of DC Series motor, as the speed of Motor increases, the armature current nd hence the field flux will decrease and therefore Back emf E can never be greater than the supply voltage V. Therefore, Regenerative Braking is not possible in DC Series Motor.

It should always be ensured that load is available for consuming the Regenerated power. If no load is there to consume the regenerated power the voltage of the line will increase due to capacitance of line which will definitely damage the insulation. Hence Regenerative Braking shall only be used when there are enough loads to absorb the regenerated power.
In case the source of supply is battery, the regenerated power can be used to charge the Battery as shown in the example of automobile.

In the next post we will discuss the remaining two methods of Electric Braking. So be there…..


Thank you!

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