# Parameter Design of Drive Motor for Pure Electric Vehicle

Drive motor parameters mainly include rated speed and maximum speed, rated power and peak power, rated torque and peak torque, and rated voltage.

1. The speed of the drive motor
The relationship between the maximum speed of the motor and the maximum speed of a pure electric vehicle is
nmaxmaxit/0.377r

In the formula, nmax (r/min) is the maximum speed of the motor: νmax (km/h) is the maximum speed of a pure electric vehicle: it is the transmission ratio of the pure electric vehicle transmission system: r (m) is the wheel radius.
The maximum speed of a pure electric vehicle refers to the highest average speed of a pure electric vehicle that can go back and forth over a distance of more than 1km.
The rated speed of the motor is
ne=nmax
In the formula, ne (r/min) is the rated speed of the motor: β is the coefficient of expanding the constant power zone of the motor.
The larger the β value, the greater the torque the motor can obtain in the low-speed range, which is beneficial to improve the vehicle’s acceleration and climbing performance, and has good stable running performance; but too large a β value will increase the working current of the motor. At the same time, the power loss and size of the power converter will increase, so the β value should not be too high. The β value is usually 2-4.

2. Power of drive motor
The drive motor is the only power source for pure electric vehicles, which has a direct impact on the power of the entire vehicle. The greater the power of the selected motor, the better the power of the vehicle, but if the power is too large, the mass and volume of the motor will increase, and the working efficiency of the motor is not high, so that the limited on-board vehicle cannot be fully utilized. Energy, thereby reducing the driving range.
1) The peak power of the drive motor
The peak power of the drive motor is determined by the design goal of the entire vehicle, and the peak power should reach the maximum power requirements corresponding to the maximum vehicle speed, maximum climbing degree and acceleration time respectively.
(1) Determine the peak power of the drive motor according to the maximum speed of the pure electric vehicle.
The power of the drive motor required for a pure electric vehicle to drive on a flat road at the highest speed is
Pm1max/3600ηt[mgf＋(CDAν²max)/21.15]
In the formula, Pm1 (kW) is the power consumed by the pure electric vehicle when running at the highest speed; m (kg) is the test quality of the pure electric vehicle: f is the tire rolling resistance coefficient; CD is the wind resistance coefficient of the car; A (m²) It is the windward area of ​​a pure electric vehicle: ηt is the efficiency of the pure electric vehicle transmission system.
(2) Determine the peak power of the drive motor according to the maximum gradeability of the pure electric vehicle.
The power of the driving motor required for a pure electric vehicle to climb the maximum slope at the climbing speed is
Pm2p/3600ηt[mgfcosαmax＋mgfsinαmax＋(CDAν²p)/21.15]

In the formula, Pm2 (kW) is the power consumed by the pure electric vehicle to climb the maximum gradient at the climbing speed; vp (km/h) is the climbing speed of the pure electric vehicle; amax (°) is the maximum gradient angle.
The climbing speed of an electric vehicle refers to the highest average speed at which a pure electric vehicle can maintain a driving range of more than 1km on a slope with a given slope.
For pure electric vehicles, the climbing speed of the vehicle through 4% gradient is not less than 60km/h; the climbing speed of the vehicle through 12% gradient is not less than 30km/h; the maximum climbing degree of the vehicle is not less than 20%
(3) Determine the peak power of the drive motor based on the acceleration capability of the pure electric vehicle.
The acceleration capability of a pure electric vehicle refers to the shortest time required for a pure electric vehicle to accelerate from a certain speed to another speed.
The motor power required for a pure electric vehicle to meet the acceleration capability is
Pm3=1/1000ηt[(2/3)mgfνf＋(1/5)ρaCDAν²f＋(δm/2ta)(ν²f＋ν²b)]

In the formula, Pm3 (kW) is the power required by the pure electric vehicle to meet the acceleration capability; vf (m/s) is the vehicle speed after acceleration: vb (m/s) is the vehicle speed corresponding to the rated speed of the drive motor: ρa is the air Density: tα(s) is the expected acceleration time; δ is the rotation mass conversion coefficient.
The first item in the brackets in formula Pm3 represents overcoming the tire rolling resistance; the second item represents the average power of air resistance; the third item represents the ability to accelerate the vehicle.
For pure electric vehicles, the acceleration time of 0→50km/h and 50→80km/h should not exceed 10s and 15s, respectively.
The peak power of the drive motor should be able to meet the requirements of the pure electric vehicle for the maximum speed, maximum gradeability and acceleration capability at the same time, so the peak power of the pure electric vehicle drive motor is
Pemax=max{Pm1, Pm2, Pm3}
In the formula, Pemax (kW) is the peak power of the drive motor.
2) Rated power of the drive motor
It is very important to correctly choose the rated power of the drive motor. If the rated power is selected too small, the motor often runs under overload; on the contrary, if the rated power is selected too large, the motor often runs under underload, and the efficiency and power factor decrease, which not only wastes electric energy but also increases the capacity of the power battery. Economic benefits have declined. The rated power of the motor should make the motor work in a high efficiency area as much as possible.
The rated power of the motor should meet the maximum speed requirements of pure electric vehicles, and the overload requirements of the motor should be considered.
The rated power of the pure electric vehicle drive motor is
Pe=max{Pm1,(Pemax/λ)}
In the formula, Pe (kW) is the rated power of the drive motor; λ is the overload factor of the motor.

3. Torque of drive motor
The rated torque and peak torque of the drive motor are respectively
Te=9550Pe/ne
Temax=9550Pemax/ne
In the formula, Te (N·m) is the rated torque of the drive motor: Temax (N·m) is the peak torque of the drive motor.
After the drive motor parameters are initially determined, it is necessary to verify whether the maximum gradeability at a certain speed and the maximum speed of the vehicle are met, that is,
[mg/Temaxηt)(fcosαmax＋sinαmax＋(CDAν²p/21.15)]≤it/r≤0.377nmaxmax

4. Rated voltage of drive motor
The determination of the voltage level of the drive motor is closely related to the voltage level of the power battery pack. Under the same output power condition, the current will decrease as the voltage becomes higher, which reduces the requirements for components such as switches and wires. If the voltage is too high, it will increase the number of single batteries in series, which will increase the quality of the whole vehicle. And the cost increases, the dynamics decreases and the layout is difficult. The rated voltage of the motor is often determined by the parameters of the motor and is proportional to the rated power of the motor. That is, the greater the rated power of the motor, the higher the rated voltage of the motor. At the same time, the rated voltage of the motor should be selected in accordance with the voltage specified in the standard series.