For a regenerative braking with DC series motor it's field windings are.... .... excited

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.