Regenerative brakes
A regenerative brake is an apparatus, a device or system which allows a vehicle to recapture part of the kinetic energy that would otherwise be lost to heat when braking and make use of that power either by storing it for future use or feeding it back into a power system for other vehicles to use.
It is similar to an electromagnetic brake, which generates heat instead of electricity and is unable to completely stop a rotor.
Brakes as an electrical generator
Regenerative brakes are a form of dynamo generator, originally discovered in 1832 by Hippolyte Pixii. The dynamo's rotor slows as the kinetic energy is converted to electrical energy through electromagnetic induction. The dynamo can be used as either generator or brake by converting motion into electricity or be reversed to convert electricity into motion.
Using a dynamo as an regenerative brake was discovered co-incident with the modern electric motor. In 1873, Zénobe Gramme attached the wires from two dynamos together. When one dynamo rotor was turned as a regenerative brake, the other became an electric motor.
It is estimated that regenerative braking systems in vehicles currently reach 31.3% electric generation efficiency, with most of the remaining energy being released as heat; the actual efficiency depends on numerous factors, such as the state of charge of the battery, how many wheels are equipped to use the regenerative braking system, and whether the topology used is parallel or serial in nature.
<<< draft of proposed edit: "Current regenerative systems are unable to optimally meet the high demands of energy recovery (electrical generation) under braking conditions (31.3% efficiency was the figure previously mentioned). Most generators (and motors) typically operate most efficiently within a narrow range of speeds. Regenerative braking systems, by nature, need to operate during (and in fact to affect) a condition where the speed is continuously changing from a given rotational velocity, all the way down to 0. A conventional friction-based brake system, however, is 0% efficient in terms of recovering the kinetic energy lost in deceleration. Other factors need to be considered to determine whether this efficiency enhancement is worth it. A RBS is more complex and thus more expensive than the simpler, friction-based system. Higher system complexity often translates into higher maintenance costs, in end applications (more stuff that can break). At least one part of a conventional friction brake system will “break” (and hopefully brake) regularly because it is designed that way. The contact surfaces are not able, and/or not designed to quickly and completely dissipate the braking energy as heat. Thus they are designed to fail slowly as a result of dissipating some the braking energy mechanically. This necessitates a regular, maintenance cost. Such a design can also make the parts more susceptible to catastrophic failure than components in other designs, where the dynamic interactions do not require physical contact.">>> The system is no more efficient [[[<--requires clarification or correction]]] than conventional friction brakes, but reduces the use of contact elements like brake pads, which eventually wear out. Traditional friction-based brakes must also be provided to be used when rapid, powerful braking is required.
Dynamic and regenerative electrical brakes
Electric brakes have been used in vehicles with electric motors since the early-20th century. The Warner Electric Brake Corporation used electric brakes in 1927, although earlier use is probable.
Dynamic brakes (rheostatic brakes in the UK) convert the electric energy back into heat by passing the current through large banks of variable resistors. Vehicles that use dynamic electrical brakes include forklifts, diesel-electric locomotives, and streetcars. If designed appropriately, this heat can be used to warm the vehicle interior. When the energy is meant to be dissipated externally, large radiator-like cowls can be employed to house the resistor banks.
Regenerative brakes in electric railway vehicles feed the generated electricity back into the grid. In battery electric and hybrid electric vehicles, the energy is stored in a battery or bank of capacitors for later use.
The main disadvantage of regenerative brakes when compared with dynamic brakes is the need to closely match the electricity generated with the supply. With DC supplies this requires the voltage to be closely controlled and it is only with the development of power electronics that it has been possible with AC supplies where the supply frequency must also be matched (this mainly applies to locomotives where an AC supply is rectified for DC motors).
It is usual for vehicles to include a 'back-up' system such that friction braking is applied automatically if the connection to the power supply is lost. Also in a DC system or in an AC system that is not directly grid connected via simple transformers special provision also has to be made for situations where more power is being generated by braking than is being consumed by other vehicles on the system.
A small number of mountain railways have used 3-phase power supplies and 3-phase induction motors and have thus a near constant speed for all trains as the motors rotate with the supply frequency both when giving power or braking.