Locomotives that use Dynamic Braking with DC traction motors excite the field coils of the motors and turn them into energy producing generators. AC induction motors don't have that option since there are no windings on the rotating armature and there are no slip rings. When used as a traction motor, there are short circuited windings on the rotating armature that produce magnetic fields when rotated in magntic flux fields produced by three phase rotating fields in the outer windings. It would seem that for these traction motors to produce electricity that they must have additional windings on the rotating armature or permanent magnetics installed. Other articles on AC locomotive dynamic braking say that an induction motor is used with no slip rings. If so then how can the traction motor be turned into an alternator?
If the load were to make the motor operate above 100% speed, such as could occur when an electric locomotive is going down hill, the motor operation moves into the red part of the curve. As operation moves from the green part to the red part of the curve, the torque produced by the motor drops to zero and becomes negative.
A low voltage DC is applied to winging after motor is stopped which creates the magnetic Field applied on rotor to stop it
