Principle of operation
– Conversion of electrical energy to mechanical energy is achieved through interactions of armature windings and a moving external magnetic field.
– The same device can function as both a motor and a generator.
– Dynamic braking switches the traction motor into the role of a generator.
– The braking power is determined by the strength of the magnetic field and the rotation of the wheels.
– The amount of braking power is controlled by varying the strength of the magnetic field through the current in the field coils.
Types of braking
– Rheostatic braking: Electrical energy produced by the motors is dissipated as heat in a bank of resistors. Cooling fans are necessary to protect the resistors from damage. Thermal monitoring is implemented to prevent excessive temperatures. When the temperature becomes excessive, the braking reverts to friction only. Rheostatic braking is not suitable for traction applications.
– Regenerative braking: Regenerative braking feeds the current produced during braking back into the power supply system. Both regenerative and rheostatic braking are typically incorporated in electrified systems. If the power supply system cannot absorb the current, rheostatic mode is used for braking. Yard locomotives with onboard energy storage systems can recover wasted energy. Modern passenger locomotives with AC inverters can use regenerative braking to power onboard systems.
– Blended braking: Dynamic braking is used in conjunction with the regular air brake to stop a locomotive. Dynamic braking alone is not sufficient below a certain speed. Blended braking combines dynamic and air braking to provide the same braking force as air brakes alone. The dynamic brake portion is maximized, and the air brake portion is automatically regulated. Dynamic braking provides a significant portion of the braking force during blended braking.
– Hydrodynamic braking: Diesel locomotives with hydraulic transmission can be equipped for hydrodynamic braking. The torque converter or fluid coupling acts as a retarder. Braking energy heats the hydraulic fluid. The heat is dissipated through a heat exchanger connected to the engine cooling radiator. The engine is idling during braking, minimizing heat production.
Self-load test
– Brake grids can be used as a form of dynamometer to test the power output of a locomotive.
– The main generator output is connected to the grids instead of the traction motors.
– The grids can absorb the full engine power output.
– Engine power output is calculated from the main generator voltage and current.
– This test is performed with the locomotive stationary. Source: https://en.wikipedia.org/wiki/Dynamic_braking
This article relies largely or entirely on a single source. (September 2023) |
Dynamic braking is the use of an electric traction motor as a generator when slowing a vehicle such as an electric or diesel-electric locomotive. 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 reduces wear on friction-based braking components, and regeneration lowers net energy consumption. Dynamic braking may also be used on railcars with multiple units, light rail vehicles, electric trams, trolleybuses, and electric and hybrid electric automobiles.
