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 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.
Read more about this topic: Dynamic Braking
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