If the train starts to descend a grade, the speed increases because the (reduced) drag is less than the torque. With increased speed, the internally generated back-EMF voltage rises, reducing the torque until the torque again balances the drag. Because the field current is reduced by the back-EMF in a series wound motor, there is no speed at which the back-EMF will exceed the supply voltage, and therefore a single series wound DC traction motor alone cannot provide dynamic or regenerative braking.