- Power system design
Basic requirements for vehicle design
The parameters of the vehicle design usually include the curb weight of the vehicle, the total weight of the vehicle at full load, the wheelbase, the windward area requirements, the air damping coefficient, the rolling damping coefficient, the acceleration time, the mileage, etc., as shown in Table 9-1.
- The power demand of the vehicle and the selection of the motor
(1) Determination of rated power and maximum power of Lunyi electrode.
According to automobile theory and automobile design, the relationship of automobile power balance should meet the requirements of formula (9-1), namely
Among them, P is the power required by the vehicle, kW; η is the efficiency of the transmission system, and the transmission efficiency of the Lunyi motor is selected in this section as 1; μ is the vehicle speed, km/h; C is the air damping coefficient; f is the automobile rolling damping coefficient; A is the windward area of the car m².
The maximum power demand of the car should meet the maximum power demand of the car in starting acceleration, climbing and at the highest vehicle speed, so the calculation of the required power under the three conditions is carried out.
Maximum speed μ vehicle power requirements:
Maximum gradeability i=0.2, vehicle gradeability a (a=arctan) climbing speed μ=15km/h, the power required by electric vehicles is
When accelerating to 100 kilometers, the power demand of the car at the end of acceleration:
Among them, δ is the rotation mass conversion factor, which is taken as 1.1; X is the fitting coefficient, which is generally taken as 0.5.
Through formula calculation, the maximum speed, maximum gradeability and power required for acceleration of 100 kilometers can be obtained. From the above calculation, it can be seen that the motor has the highest efficiency near the rated power, so the rated power of the car is usually determined according to the maximum speed of the electric car, and a certain amount of backup power should be reserved. Therefore, when selecting a motor, the rated power should be greater than P, and the maximum power should be greater than 2P.
(2) Selection of motor voltage.
In order to protect the power supply system and keep the operating current of the control bus within a certain range, a certain high-voltage design is required. In this way, the motor output will be shorter and the power will be larger, and the vehicle dynamics performance will be better. However, the maximum voltage of the DC bus cannot be too high, and the voltage should be within a certain range. The recommended voltage levels in the national standard are 120V, 144V, 168V, 192V, 216V, etc.
- Determination of the basic parameters of the power supply system
(1) Nominal voltage and voltage range of power supply system
According to the voltage of the motor, the working voltage of the available electrodes does not exceed 120% of the rated voltage and not less than 80% of the rated voltage. The battery cell application range is 3.2-4.2V, and the ternary nominal voltage is 3.7V, which requires 38 in series. ~56 battery cells, select 48 battery cells in series, the nominal voltage of the power system is 172.8V, and the voltage range of the system is 153.6V~201.6V, which can ensure that the motor works within the normal voltage range.
(2) Determination of the maximum output power and current of the power supply system.
The maximum power of a single hub motor is 19.7kW, and the power of the two motors is 39.4kW. Assuming that the conversion efficiency of the motor and the efficiency of the controller are 0.90 and 0.95, respectively, the maximum power required by the entire power supply system is
Among them, Pb is the maximum power required by the entire power system; P per is the sum of the power of all motors; ηc is the efficiency of the controller: ηm is the conversion efficiency of the motor.Considering that the voltage drop of the battery system at maximum power is large, it is generally calculated as 90% of the nominal voltage. When the battery outputs the maximum power, the maximum current of the battery system is
Among them, μ is the nominal voltage of the system.