Regenerative Braking

In Regenerative Braking, the power or energy of the driven machinery which is in kinetic form is returned back to the power supply mains. This type of braking is possible when the driven load or machinery forces the motor to run at a speed higher than no load speed with a constant excitation.

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Under this condition, the back emf Eb of the motor is greater than the supply voltage V, which reverses the direction of motor armature current. The machine now begins to operate as a generator and the energy generated is supplied to the source.

Regenerative braking can also be performed at very low speeds if the motor is connected as a separately excited generator. The excitation of the motor is increased as the speed is reduced so that the two equations shown below are satisfied.

regenerative-braking-eq-1

The motor does not enter into saturation on increasing excitation.

Regenerative braking is possible with shunt and separately excited motors.In compound motors, braking is possible only with weak series compounding.

Applications of Regenerative Braking

  • Regenerative braking is used especially where frequent braking and slowing of drives is required.
  • It is most useful in holding a descending load of high potential energy at a constant speed.
  • Regenerative braking is used to control the speed of motors driving loads such as in electric locomotives, elevators, cranes and hoists.
  • Regenerative braking cannot be used for stopping the motor. It is used for controlling the speed above the no-load speed of the motor driving.

The necessary condition for regeneration is that the back EMF Eb should be greater than the supply voltage so that the armature current is reversed and the mode of operation changes from motoring to generating.

Regenerative Braking in DC Shunt Motors

Under normal operating conditions the armature current is given by the equation shown below.

regenerative-braking-eq-2

When the load is lowered by a crane, hoist or lift causes the motor speed to be greater than the no-load speed. The back EMF becomes greater than the supply voltage. Consequently, armature current Ia becomes negative. The machines now begin to operate as a generator.

Regenerative Braking in DC Series Motors

In case of DC Series Motor an increase in speed is followed by a decrease in the armature current and field flux. The back EMF Eb cannot be greater than the supply voltage. Regeneration is possible in DC Series Motor since the field current cannot be made greater than the armature current.

Regeneration is required where DC Series Motor is used extensively such as in traction, elevator hoists etc. For example – In an Electro-locomotive moving down the gradient, a constant speed may be necessary. In hoist drives the speed is to be limited whenever it becomes dangerously high.

One commonly used method of regenerative braking of DC Series Motor is to connect it as a shunt motor. Since the resistance of the field winding is low, a series resistance is connected in the field circuit to limit the current within the safe value.

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