Last time, we looked at Dynamic Braking and the principals of how it works. In this blog, we are going to look at Regenerative Braking as a whole and on Electrical Multiple Units (EMUs). Once again, I turned to Railway Technical Webpages to help in my explanation.
Another name for Dynamic Braking is Rheostatic Braking. As mentioned before, it is a braking system used by locomotives and trains fitted with electric traction motors where the motors become generators and the current developed is fed to on-board resistor grids. The energy is dissipated as heat as the grids cool. Some grids have to be force-ventilated to dispose of the heat quickly enough. Rheostatic braking is useful for diesel-electric locomotives with heavy freight trains on long down grades.
Dynamic braking can be used on electric railways to convert the energy of the train back into usable power by diverting the braking current into the current rail or overhead line. This is known as regenerative braking. It is used in the same way as rheostatic braking but the energy can be used by other trains requiring power. The power developed by a braking train may not be accepted by the line if no other trains are drawing power so trains equipped with regenerative braking will usually have resistor grids as well to absorb the excess energy. The balance between regenerated current and rheostatic current is also controlled electronically.
The major drawback with the regenerative braking system is that the contact wire is not always able to accept the regenerated current. Some railways had substations fitted with giant resistors to absorb regenerated current not used by trains but this was a complex and not always reliable solution. As each train already had resistors, it was a logical step to use these to dispose of the generated current. The result was rheostatic braking. When the driver calls for brake, the power circuit connections to the motors are changed from their power configuration to a brake configuration and the resistors inserted into the motor circuit. As the motor generated energy is dispersed in the resistors and the train speed slows, the resistors are switched out in steps, just as they are during acceleration. Rheostatic braking on a DC motored train can be continued down to less than 30km/h when the friction brakes are used to bring the train to a stop.
Before the advent of power electronics, there were some attempts to combine the two forms of what we now call “dynamic braking” so that the generated current would go to the power supply overhead line or third rail if it could be absorbed by other trains but diverted to on-board resistors if not.
In the case of diesel-electric locomotives, dynamic braking is restricted to the rheostatic type. Racks of resistors can often be seen on the roofs of heavy-haul locomotives for which dynamic braking is a big advantage on long downhill grades where speed must be maintained at a restricted level for long periods.
Since the DC motor and a DC generator are virtually the same machine mechanically, it was immediately realised that a train could use its motors to act as generators and that this would provide some braking effect if a suitable way could be found to dispose of the energy. The idea formed that if the power could be returned to the source, other trains could use it. Trains were designed therefore, which could return current, generated during braking, to the supply system for use by other trains. Various schemes were tried over many years with more or less success but it was not until the adoption of modern electronics that reliable schemes have been available.
This is a system, used on modern, regenerative braked EMU vehicles and some locomotives, to ensure that air and regenerative braking acts in co-ordination. An electronic signal from the regenerative brake indicating the brake effort achieved is compared with the brake effort demanded by the Driver and will then call up additional braking from the air brake system if required.
A typical set-up on a coach will comprise a brake control unit (BCU) which contains electronic controls and electro-pneumatic valves. Various inputs are processed in the BCU which then generates electronic or pneumatic outputs as necessary.
Brake Demand: When a brake demand is requested by the Driver, it is transmitted along a train wire to the BCU on each coach. The signal can be digital or analogue providing a message either in steps or infinitely variable. The demand is then matched to a load compensation signal provided by the car suspension system. The greater the weight, the greater the brake demanded.
The brake effort demand is now converted into air pressure signal and the brake is applied by sending air into the brake cylinders until it matches the signal. At the same time, a matching demand is sent to the regenerative brake controller and the traction control system will initiate regenerative braking. The system will send a “regenerative brake effort achieved” signal to the brake controller which will subtract it from the air brake demand signal and so reduce the brake cylinder pressure accordingly.
Preference is given to the regenerative brake to save wear on brake blocks (shoes) or pads and air braking is added if necessary to achieve the braking rate required. Typically, at 15km/h, the regenerative brake becomes ineffective and the air brakes take over completely.
Trailer Cars: Most types of EMU comprise a mixture of motor coaches and trailer coaches. As trailer coaches have no motors, they do not have their own regenerative braking. They can, however, use regenerative braking on motor coaches in their braking effort if that is available. In the case of a four-coach unit, for regenerative demand, the motor coaches’ BCU will add the trailer coach demand to the motor coaches’ demand. The resulting regenerative brake achieved may be sufficient to match all of the motor coaches’ demand and have some extra for the trailer. In this case, the motor coaches’ BCU sends a message to the trailer coach to say how much of the trailer coach demand has been fulfilled by the regenerative brake. The trailer’s air brake pressure can be reduced accordingly.
There will be some limit on the total regenerative brake possible because of adhesion limits and this will be incorporated into the brake control calculations. If the regenerative brake is reduced for any reason, the trailer coach air brakes will be reapplied first followed by the motor coach brakes.
Smoothing: A feature of modern brake control is the “inshot”. This is a small amount of air injected into the brake cylinders immediately when brake is called for so that the build-up time is reduced. Braking systems are also “jerk limited”; smoothed out as they are built-up so that the passengers don’t feel the coaches snatch as the brake is applied. This is particularly important in the case of regenerative brakes which, if not jerk limited, have a tendency to apply sharply if the train is at speed.
Fade: Once the brake is applied, the regenerative portion will have a tendency to fade as the speed, and thus the current generated by the motors, reduces. Some systems have a pre-fade control; a signal sent by the traction controller to indicate the brake is about to start fading. This gives a smoother changeover into air braking.
I hope you have a fuller understanding of Dynamic/Regenerative Braking. It is one of the best ways a railway can save on wear and tear and energy as a whole. Good drivers know how to optimize their train control using dynamic braking.
In Part 3, we will look at a procedure for drivers of the South African Class 6E/6E1 3kV Locomotive using Regenerative Braking. Thanks to Bennie for giving me his old Text Book.