Electric braking in AC and DC traction drives

Electric braking stops or slows a traction motor by making it act as a generator, so the kinetic energy of the train is turned into electrical energy that is either wasted in resistors or returned to the supply. It reduces wear of brake shoes and gives smooth, controllable braking; mechanical brakes are still needed to hold the train at rest.

1. Plugging (reverse current braking)

• The connections of the armature (dc motor) or two supply phases (induction motor) are reversed, so the motor produces torque opposite to motion.
• A resistor is inserted to limit the very large current.
• Braking is quick but wasteful: energy from both the supply and the train is lost as heat. Used only rarely in traction (for example, emergency stopping).

2. Rheostatic (dynamic) braking

• The motor is disconnected from the supply and its armature is connected across a braking resistor.
• DC series motor: the field connections are reversed relative to the armature so the machine self-excites as a generator; two motors may be cross-connected (each excites the other's field) to share load.
• Induction motor: the stator is disconnected from ac and fed with dc (dc injection), creating a stationary field; the rotor currents produce braking torque and energy is lost in the rotor resistance.
• Simple and does not depend on the line, but energy is wasted as heat.

3. Regenerative braking

• The motor runs as a generator and returns energy to the supply line. This happens when the back emf exceeds the supply voltage (e.g. going downhill).
• DC series motors cannot regenerate directly because their field becomes weak, so they are switched to separate (shunt) excitation during braking. With chopper drives, regeneration is done by the chopper.
• Induction motor: regeneration happens naturally when the rotor runs above synchronous speed; with VVVF inverter drives the frequency is lowered so the motor runs super-synchronously.
• Most efficient (about 20–40% energy saving in hilly or frequent-stop routes), but needs a receptive supply or other trains to absorb the energy.

 Supply ===+=== line
           |
       [Motor as generator] --> energy back to line
           |                    (regenerative)
           +--[ R ]--           (rheostatic: heat)