A dynamic brake is a type of brake that reconnects a motor as a generator. It uses a railroad vehicle’s electric traction motors as generators to slow the vehicle. A dynamic brake system takes advantage of the idea that an electric motor works the same as a generator. So if electrical energy can be converted into mechanical energy to power a train, the excess mechanical energy can also be converted into electrical energy to slow the train down and put it on a halt.
Types of Dynamic Braking
Dynamic braking can be rheostatic or regenerative. Rheostatic dynamic braking is when the electrical power generated is dissipated in brake grid resistors as heat. Regenerative dynamic braking is when the power is returned to the supply line. The advantages of using dynamic brakes include lowered wear of friction braking components and lowered energy consumption.
How Do Dynamic Brakes Work?
Like traditional friction brakes, dynamic brakes convert energy to heat. The difference lies on how it is done. Dynamic brakes transform energy by passing the produced current through the resistors, which dissipate the energy. If used appropriately, the generated heat can even be used to warm the interior of the vehicle.
So how do dynamic brakes work? During braking, the motor fields connect to the main traction generator or the supply. At the same time, the motors armatures connect to the supply line or the brake grids. The rolling locomotive wheels turn these motor armatures. The motors act as generators when the motor fields are excited. The current flow through the motor armatures when braking is opposite to the current flow during motoring. Ergo, the motor produces torque in a direction opposite the rolling direction. The effort needed for braking must be proportional to the product of the magnetic strength of the armature windings and the field windings. For permanent magnet motors, it works by shorting the motor terminals. This brings motor to a fast abrupt stop.