Dynamic braking

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.

The electrical energy produced by the motors is dissipated as heat by a bank of onboard resistors, referred to as the braking grid.

Modern systems have thermal monitoring, so that if the temperature of the bank becomes excessive it will be switched off, and the braking will revert to being by friction only.

On modern passenger locomotives equipped with AC inverters pulling trains with sufficient head-end power (HEP) loads, braking energy can be used to power the train's on board systems via regenerative braking if the electrification system is not receptive or even if the track is not electrified to begin with.

The HEP load on modern passenger trains is so great that some new electric locomotives such as the ALP-46 were designed without the traditional resistance grids.

It is the most rapid form of electric braking, but comes at the disadvantage of applying significant transient stresses to motors and mechanical components.

After zero speed is reached, plugging must cease to avoid the drive running in reverse, and this function may be provided automatically, by a 'plugging relay'.

[3][4][5] It is possible to use the brake grids as a form of dynamometer or load bank to perform a self-load test of the power output of a locomotive.

With the locomotive stationary, the main generator (MG) output is connected to the grids instead of the traction motors.