What Are Braking Resistors? -
Their Use In Dynamic Braking / Regenerative Braking / Blended Braking
In general, resistors consume heat. By doing this, they can be used to stop or slow down a mechanical system. This type of resistor is called a dynamic braking resistor and the process is called dynamic braking. When kinetic energy is transformed back into electrical energy you can slow down or decelerate an electric motor. This energy is dissipated by using a power resistor. The brake resistors usually have a high power rating and relatively low ohmic values. A suitable solution is the "Wirewound Resistor." Usually, they have a ceramic core, are fully welded, and are encased in a frame to establish a safe distance from other parts.
Dynamic braking is the use of an electric traction motor as a generator when slowing a vehicle such as an electric or diesel-electriclocomotive. 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, and electric and hybrid electric automobiles.
Advantages - Dynamic braking resistors versus Friction braking
- Lower wear of components.
- Control motor voltage within safe levels.
- Faster braking of AC and DC motors.
- Less service required and higher reliability.
Principle of Operation
When braking, the motor fields are connected across either the main traction generator (diesel-electric locomotive) or the supply (electric locomotive) and the motor armatures are connected across either the brake grids or supply line. The rolling locomotive wheels turn the motor armatures, and if the motor fields are now excited, the motors will act as generators.
During dynamic braking, the traction motors, which are now acting as generators, are connected to the braking grids (large resistors), which put a large load on the electrical circuit. When a generator circuit is loaded down with resistance, it causes the generators to slow their rotation. By varying the amount of excitation in the traction motor fields and the amount of resistance imposed on the circuit by the resistor grids, the traction motors can be slowed down to a virtual stop (approximately 3-5 MPH).
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. It is not suitable for traction applications.
The electrical energy produced by the motors is dissipated as heat by a bank of onboard resistors. Large cooling fans are necessary to protect the resistors from damage. Modern systems have thermal monitoring, so, if the temperature of the bank becomes excessive, it will be switched off, and the braking will revert to friction only.
In electrified systems the similar process of regenerative braking is employed whereby the current produced during braking is fed back into the power supply system for use by other traction units, instead of being wasted as heat. It is normal practice to incorporate both regenerative and rheostatic braking in electrified systems. If the power supply system is not "receptive", i.e. incapable of absorbing the current, the system will default to rheostatic mode in order to provide the braking effect.
Yard locomotives with onboard energy storage systems which allow the recovery of some of this energy which would otherwise be wasted as heat are now available. The Green Goat model, for example, is being used by Canadian Pacific Railway, BNSF Railway, Kansas City Southern Railway and Union Pacific Railroad.
On modern passenger locomotives equipped with AC inverters pulling trains with sufficient Head End Power loads braking energy can be used to power the train's on board systems as a form of 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
Dynamic braking alone is insufficient to stop a locomotive, as its braking effect rapidly diminishes below about 10 to 12 miles per hour (16 to 19 km/h). Therefore, it is always used in conjunction with the regular air brake. This combined system is called blended braking. Li-ion batteries have also been used to store energy for use in bringing trains to a complete halt.
Although blended braking combines both dynamic and air braking, the resulting braking force is designed to be the same as what the air brakes on their own provide. This is achieved by maximizing the dynamic brake portion, and automatically regulating the air brake portion, as the main purpose of dynamic braking is to reduce the amount of air braking required. This conserves air, and minimizes the risks of over-heated wheels. One locomotive manufacturer, Electro-Motive Diesel (EMD), estimates that dynamic braking provides between 50% to 70% of the braking force during blended braking.
Frequently Asked Questions
What is a brake resistor for an inverter?
Dynamic braking resistors (DBR's) for inverters and DC drive systems. A drive motor can also act as a generator. ... All the energy is used in heating the resistor; some is dissipated at once, the rest after the stop while the resistor cools.
How does a dynamic braking resistor work?
Dynamic braking is the use of the electric traction motors of a railroad vehicle as generators when slowing the 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.
What is the use of DBR?
Dynamic braking resistors (DBRs) produce braking torque and absorb the high amounts of energy generated by stopping electric motors. They are used in variable-speed drive systems such as elevators, cranes, and trains.
How does a braking resistor work?
The purpose of a dynamic braking resistor is to slow down or to quickly stop a motor by draining excess voltage and keeping it within safe tolerances. Our rheostatic resistors dissipate the excess voltage as heat.
What is a regenerative resistor?
Regenerative resistors are usually a part with servo systems to absorb returned energy from decelerating or braking servo axis. Servo drive with motor can act two ways: energy supply and energy generator.
How do the brakes on a train work?
A fully charged brake pipe is typically 70–90 psi (4.8–6.2 bar; 480–620 kPa) for freight trains and 110 psi (7.6 bar; 760 kPa) for passenger trains. The brakes are applied when the engineer moves the brake handle to the "service" position, which causes a reduction in pressure in the train pipe.
What are the advantages of the regenerative braking system?
A regenerative brake is an energy recovery mechanism which slows a vehicle or object by converting its kinetic energy into a form which can be either used immediately or stored until needed.
What is regenerative braking system in hybrid cars?
They understand that in a hybrid or all-electric vehicle the word "regenerative," in terms of regenerative braking, means capturing the vehicle's momentum (kinetic energy) and turning it into electricity that recharges (regenerates) the onboard battery as the vehicle is slowing down and/or stopping.
What cars use regenerative braking?
This system is called regenerative braking. At present, these kinds of brakes are primarily found in hybrid vehicles like the Toyota Prius, and in fully electric cars, like the Tesla Roadster. In vehicles like these, keeping the battery charged is of considerable importance.
What is the KERS?
A kinetic energy recovery system (often known simply as KERS, or kers) is an automotive system for recovering a moving vehicle's kinetic energy under braking. The recovered energy is stored in a reservoir (for example a flywheel or high voltage batteries) for later use under acceleration.