Dynamic braking is the use of the electric traction motors of a vehicle as generators when slowing. It is termed rheostatic if the generated electrical power is dissipated as heat in brake grid resistors, and regenerative if the power is returned to the supply line. Dynamic braking lowers the wear of friction-based braking components, and additionally regeneration reduces energy consumption.
Principle of operation
During braking, the motor fields are connected across either the main traction generator (diesel-electric locomotive, hybrid electric vehicle) or the supply (electric locomotive, electric vehicle) and the motor armatures are connected across braking grids (rheostatic) or the supply (regenerative). The rolling wheels turn the motor armatures and when the motor fields are excited, the motors act as generators.
During dynamic braking, the traction motors, which are now acting as generators, are connected to the braking grids of large resistors which put a large load on the electrical circuit. When a generator circuit is loaded down with resistance, it causes the generators to resist rotation, thus slowing the train. By varying the amount of excitation in the traction motor fields and the amount of resistance imposed on the circuit by the resistor grids, the traction motors can slow the train to about 5 mph (8 km/h) (for a direct current "transmission" system; for an alternating current "transmission" system, the traction motors can slow the train to nearly 0 mph (0 km/h)).
Locomotives with a direct current "transmission" system always use series-wound traction motors as these motors produce their maximum tractive effort at "stall", or zero mph, thereby easily starting almost any train.
For permanent magnet motors, dynamic braking is easily achieved by shorting the motor terminals, thus bringing the motor to a fast abrupt stop. This method, however, dissipates all the energy as heat in the motor itself, and so cannot be used in anything other than low-power intermittent applications due to cooling limitations. It is not suitable for traction applications.
The electrical energy produced by the motors is dissipated as heat by a bank of onboard resistors or "braking grid". Large cooling fans are necessary to protect the resistors from damage. Modern systems have thermal monitoring, so if the temperature of the bank becomes excessive, it will be switched off, and the braking will revert to pure friction mode.
Regenerative braking feeds the current produced during braking back into the power supply system, instead of being wasted as heat.
With electrified trains, it is normal practice to incorporate both regenerative and rheostatic braking. If the power supply system is not "receptive", i.e. incapable of absorbing the current, the system will default to rheostatic or pure friction mode in order to provide the braking effect.
In a hybrid-electric vehicle, the electric current is used to recharge the electrical cells (batteries). This allows the energy to be reused later, often in acceleration.