The main difference between electric vehicle and traditional internal combustion engine vehicle is that different power sources are used. It is powered by battery and drives electric vehicle through driving system and motor. The energy supply and consumption of electric vehicles are closely related to the performance of batteries, which directly affects the power performance and driving range of electric vehicles, as well as the cost-effectiveness of electric vehicles.

When the electric vehicle is running, the battery outputs electric energy to the motor to overcome the internal resistance of the mechanical device of the electric vehicle and the external resistance determined by the driving conditions. During the operation of electric vehicle, the driving resistance is changing, and the power transmitted in its main circuit is also changing. The analysis of the force condition and the change of current in the main circuit is the basis of studying the driving performance and economic performance of electric vehicles.

1. Dynamic analysis of electric vehicle

1.1 driving force of electric vehicle

The output torque M of the motor output shaft of the electric vehicle is transmitted to the torque MT on the drive shaft through the reduction gear transmission, resulting in the interaction between the drive wheel and the ground. The wheel and the ground act on a circumferential force F0. At the same time, the ground produces a reaction force ft. ft is equal to F0, the direction is opposite, and the direction of FT is consistent with the forward direction of the drive wheel, which is the external force driving the vehicle forward, It is defined as the driving force of electric vehicles. yes:

The mechanical transmission device of electric vehicle refers to the reduction gear transmission box or transmission, transmission shaft, main reducer and other mechanical devices with kinematic connection with the motor output shaft. The power loss in the mechanical transmission chain includes the friction loss at the meshing point of the gear, the friction loss in the bearing, the friction loss between the rotating parts and the sealing device, and the loss of stirring lubricating oil.

1.2 driving equation and power balance of electric vehicle

The power balance can be used to qualitatively analyze the power problems in the design of electric vehicles. In addition, according to the power balance, the output power of the motor when the electric vehicle is running can be seen, so it is also commonly used in the economic analysis.

1.3 power performance calculation of electric vehicle

Similar to internal combustion engine vehicles, there are three power performance indexes of electric vehicles, namely, maximum speed, maximum acceleration capacity and maximum climbing slope.

The maximum speed of the vehicle refers to the maximum speed that the vehicle can reach on a well leveled hard road under the condition of no wind. Calculation of maximum speed of electric vehicle:

The maximum value satisfying equation (7) is the index vamax reflecting vehicle power performance.

The acceleration ability of a vehicle is expressed by the acceleration ability of the vehicle starting in place and overtaking acceleration ability. Usually, the acceleration time and acceleration distance in the process of vehicle acceleration are used as the indexes to evaluate the acceleration of the vehicle. The acceleration time of electric vehicle is calculated as:

2. Load current analysis of main circuit of electric vehicle

During the driving process of electric vehicle, the required resistance power is changing at any time, and the output power of motor will also change with the change of resistance power. The electric power transmitted in the main circuit of electric vehicle is also changing, but it is always balanced with the required resistance power. Generally, during the operation of electric vehicles, the current in the main circuit changes greatly. The current of the main circuit not only affects the heat dissipation and normal operation of the system, but also directly affects the discharge performance and service life of the battery, as well as the driving range after one-time charging.

When AC induction motor is used, the main circuit of electric vehicle refers to the circuit that provides energy required for electric vehicle driving, that is, the DC circuit from power storage battery to controller and inverter, and the AC circuit between inverter and AC induction motor, as shown in Figure 2.

Assuming that the voltage of the main circuit of the electric vehicle remains unchanged, the load current of the main circuit of the electric vehicle under constant speed driving condition can be calculated according to figure 2.

Similarly, the load current of the main circuit of the electric vehicle when accelerating or when climbing at a constant speed on a ramp can be calculated.

3. Conclusion

From the above analysis, it can be seen that the power performance of electric vehicle is closely related to its energy consumption during driving. Therefore, measures such as improving the performance of power battery and reducing the energy consumption of rolling resistance and air resistance should be taken to improve the power performance of electric vehicle.

In addition, since the current of the main circuit of the electric vehicle directly affects the discharge performance and service life of the battery when driving, and also affects the driving range after one-time charging, factors such as the quality of the battery, the voltage and rated current of the driving motor, acceleration performance, driving range and safety performance should be comprehensively considered when designing the electric vehicle, Through system optimization to improve the performance of electric vehicles and reduce costs.

Responsible editor: GT