Editing Dynamic braking (section)

==Principle of operation==
Converting [[electrical energy]] to the [[mechanical energy]] of a rotating shaft (electric motor) is the inverse of converting the mechanical energy of a rotating shaft to electrical energy (electric generator).  Both are accomplished through the interactions of [[Armature (electrical)|armature]] windings with a (relatively) moving external magnetic field, with the armature connected to an electrical circuit with either a power supply (motor) or power receptor (generator).   Since the role of the electrical/mechanical energy converting device is determined by which interface (mechanical or electrical) provides or receives energy, the same device can fulfill the role of either a motor or a generator.  In dynamic braking, the traction motor is switched into the role of a generator by switching from a supply circuit to a receptor circuit while applying electric current to the [[field coils]] that generate the magnetic field ([[Excitation (magnetic)|excitation]]).

The amount of resistance applied to the rotating shaft (braking power) equals the rate of electrical power generation plus some efficiency loss.  That is in turn proportional to the strength of the magnetic field, controlled by the current in the field coils, and the rate at which the armature and magnetic field rotate against each other, determined by the rotation of the wheels and the ratio of power shaft to wheel rotation.  The amount of braking power is controlled by varying the strength of the magnetic field through the amount of current in the field coils.  As the rate of electrical power generation, and conversely braking power, are proportional to the rate at which the power shaft is spinning, a stronger magnetic field is required to maintain braking power as speed decreases and there is a lower limit at which dynamic braking can be effective depending on the current available for application to the field coils.

The two main methods of managing the electricity generated during dynamic braking are rheostatic braking and regenerative braking, as described below.

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, such as in cordless power tools. It is not suitable for traction applications.