Regenerative braking in an electric vehicle (2009)

Electric vehicles have been attracting unprecedented attention in light of the volatile market prices and prospect of diminishing supplies of fuel. Advances in battery technology and significant improvements in electrical motor efficiency have made electric vehicles an attractive alternative, especially for short distance commuting. This paper describes the application of Brushless DC (BLDC) motor technology in an electric vehicle with special emphasis on regenerative braking. BLDC motors are being encountered more frequently in electric vehicles due to their high efficiency and robustness; however a BLDC motor requires a rather complex control to cope with the reversal of energy flow during the transition from motoring regime to regenerative braking. In an electric vehicle, regenerative breaking helps to conserve energy by charging the battery, thus extending the driving range of the vehicle. There is a number of different ways to implement regenerative braking in a BLDC motor. This paper describes the Independent Switching scheme for regenerative braking [1] as applied to a developmental electric vehicle at the University of South Australia. 1. Electric vehicles and regenerative braking In recent times, electric vehicles (EVs) have received much attention as an alternative to traditional vehicles powered by internal combustion engines running on non-renewable fossil fuels. This unprecedented.focus is mainly attributable to environmental and economic concerns linked to the consumption of fossil-based oil as fuel in internal combustion engine (ICE) powered vehicles. With recent advances in battery technology and motor efficiency, EVs have become a promising solution for commuting over greater distances. Plug-in EVs utilise a battery system which can be recharged from standard power outlets. Since performance characteristics of electric vehicles have become comparable to, if not better than those of traditional Internal Combustion Engine (ICE) vehicles, EVs present a realistic alternative. Regenerative braking can be used in an EV as a way of recouping energy during braking, which is not possible to do in conventional ICE vehicles. Regenerative braking is the process of feeding energy from the drive motor back into the battery during the braking process, when the vehicle’s inertia forces the motor into generator mode. In this mode, the battery is seen as a load by the machine, thus providing a braking force on the vehicle. It has been shown that an EV, which uses regenerative braking can have an increased driv ing range of up to 15% compared with an EV, which only uses mechanical braking [2]. A rare case when regenerative braking can not occur is when the battery is already fully charged [3]. In such a case, braking needs to be effected by dissipating the energy in a resistive load. Mechanical braking is still required in EVs for a number of reasons. At low speeds regenerative braking is not effective and may fail to stop the vehicle in the required time, especially in an emergency. A mechanical braking system is also important in the event of an electrical failure. For example, if the battery or the system controlling the regenerative braking failed, then mechanical braking becomes critical. It is common in electric vehicles to combine both mechanical braking and regenerative braking functions into a single foot pedal: the first part of the foot pedal controls regenerative braking and the final part controls mechanical braking. This is a seamless transition from regenerative braking to mechanical braking, akin to the practice of ‘putting the brakes on’ in a conventional ICE vehicle.