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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 braking grids of large resistors which limit the current flow and dissipate the converted energy as heat in the resistors instead of the motor. Brake intensity can be controlled by varying the excitation of the traction motor field and the resistance of the resistor grid. A direct current system can slow the train to about ; an alternating current system can slow the train to nearly a full stop. 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. Dynamic braking can also be achieved by shorting the motor terminals, thus bringing the motor to a fast abrupt stop. This method causes an enormous current surge through the motor itself, dissipating all the energy as heat, and can only be used in low-power intermittent applications due to cooling limitations. It is not suitable for traction applications. Permanent magnet motors do not require an excitation field, this field is provided by the permanent magnets. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Dynamic braking」の詳細全文を読む スポンサード リンク
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