Development of Transmission Specifications for an Electric Vehicle
At present, a great deal of efforts are on to develop personal modes of transportation such as an electric vehicle (EV) with potentially a higher degree of sustainability as compared to a conventional IC (internal combustion) engine powered vehicle. A transmission system with torque amplification capabilities is essential for an IC engine-based vehicle as, at low rpm, such engines develop extremely low torques which are not sufficient to move a vehicle from rest. However this is not the case for a DC (direct current) motor powered electric vehicle in which the motor intrinsically generates a high torque at rest conditions and delivers maximum power over a wide range of speed which can be well within the operational requirement of a vehicle. Thus,by varying the armature speed of a DC motor with an electronic controller and maintaining a uniform gear ratio, its drivability can be achieved. As most small EVs using DC traction motors are equipped with a single gear transmission system, it may appear that multi-speed gears are redundant for such vehicles. The current study dispels this myth by showing that a two-speed gear box can actually improve the functionality of a DC traction motor-based EV by increasing the rated top speed, initial acceleration and gradeability. In the process,a systematic methodology is developed for estimating traction requirements when a vehicle is propelled into motion from rest and remains in accelerated or steady state conditions subsequently. Together with this, a step-by-step approach has been devised for arriving at the specifications for a two-speed gear box taking into account speed-dependent motor torque and power characteristics.