Due to increases in fuel price and the environmental pollution caused by conventional petrol vehicles, electric vehicles have attracted more and more attention. A power battery is used as a power source of an electric vehicle, and the electric vehicle is driven by an electric motor, thus avoiding limitation of an engine and reducing pollution.
Conventionally, the electric vehicle may be driven by a driving motor and a reducer with a constant transmission ratio. The driving motor has a high output torque and a low efficiency at a low rotational speed. With the increasing of the rotational speed of the driving motor, the efficiency is increased gradually, but the output torque is much lower. With the electric vehicle including the reducer with a constant transmission ratio, for the reducer with a high torque, the efficiency is increased slowly with the increasing of the rotational speed of the driving motor, and consequently the power consumption of the vehicle is quite large at a high speed; and for the reducer with a high rotational speed, the efficiency is increased quickly with the increasing of the rotational speed of the driving motor. However, due to the low transmission ratio of the reducer, the torque transmitted to the wheels is too low to satisfy the requirement of the vehicle when it is starting or climbing. As a result, the power consumption of the vehicle is quite large at a low rotational speed. For example, the electric bus may need frequent parking, starting or acceleration, which may cause the low efficiency of the driving motor. Therefore, the driving system of the conventional electric vehicle is difficult to meet the work conditions for low speed and high speed simultaneously, thus the driving motor has a low efficiency, which may shorten the driving range of the electric vehicle if the power storage of the battery is not changed. As a result, the conventional electric vehicle may not be applied to complicated driving conditions.