Regenerative braking
A regenerative brake is a mechanism that reduces vehicle speed by converting some of its kinetic energy into electrical energy. This electrical energy is then stored for future use or fed back into a power system for use by other vehicles.
Regenerative brakes in electric railway vehicles feed the generated electricity back into the supply system. In battery electric and hybrid electric vehicles, the energy is stored in a battery or bank of capacitors for later use.
Regenerative braking should not be confused with dynamic braking, which dissipates the electrical energy as heat.
Traditional friction-based braking is still used with regenerative braking for the following reasons:
- The regenerative braking effect rapidly reduces at lower speeds.
- The amount of electrical energy capable of dissipation is limited by either the capacity of the supply system to absorb this energy or on the state of charge of the battery or capacitors. No regenerative braking effect can occur if another electric vehicle on the same supply system is not currently drawing power or if the battery or capacitors are already charged. For this reason, it is normal to also incorporate dynamic braking to absorb the excess energy.
- For these reasons there is typically the need to control the Regenerative Braking and to match the friction braking and Regenerative braking to produce the desired total braking output. the GM EV-1 was the first commercial car to do this. Engineers Abraham Farag and Loren Majersik were issued 2 patents for this 'Brake by Wire' technology.
The motor as a brake
Regenerative braking utilizes the fact that an electric motor can also act as a generator. The vehicle's electric traction motor is reconnected as a generator during braking and its output is connected to an electrical load. It is this load on the motor that provides the braking effect.
An early example of this system was the Energy Regeneration Brake, developed in 1967 for the Amitron. This was a completely battery powered urban concept car whose batteries were recharged by regenerative braking, thus increasing the range of the automobile.
Electric railway vehicle operation
During braking, the traction motor connections are altered to turn them into electrical generators. The motor fields are connected across the main traction generator (MG) and the motor armatures are connected across the load. The MG now excites the motor fields. The rolling locomotive or multiple unit wheels turn the motor armatures, and the motors act as generators, either sending the generated current through onboard resistors (dynamic braking) or back into the supply (regenerative braking) provides the braking load.
When rail operator c2c's began using regenerative braking with a fleet of Bombardier Class 357 EMUs, monitoring over the first two weeks showed an immediate energy saving of 15%. Savings of 17% are claimed for Virgin Trains Pendolinos. There is also less wear on friction braking components.
For a given direction of travel, current flow through the motor armatures during braking will be opposite to that during motoring. Therefore, the motor exerts torque in a direction that is opposite from the rolling direction.
Braking effort is proportional to the product of the magnetic strength of the field windings, times that of the armature windings.
Comparison of dynamic and regenerative brakes
Dynamic brakes ("rheostatic brakes" in the UK), unlike regenerative brakes, dissipate the electric energy as heat by passing the current through large banks of variable resistors. Vehicles that use dynamic brakes include forklifts, Diesel-electric locomotives and streetcars. If designed appropriately, this heat can be used to warm the vehicle interior. If dissipated externally, large radiator-like cowls are employed to house the resistor banks.
The main disadvantage of regenerative brakes when compared with dynamic brakes is the need to closely match the generated current with the supply characteristics. With DC supplies, this requires that the voltage be closely controlled. Only with the development of power electronics has this 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).
A small number of mountain railways have used 3-phase power supplies and 3-phase induction motors. This results in a near constant speed for all trains as the motors rotate with the supply frequency both when motoring and braking.
Use in motor sport
The governing body of international motor sport, the FIA, has allowed the use of 60 kW "Kinetic Energy Recovery Systems" (KERS), in the regulations for the 2009 Formula One season.