Knowledge of driving motors for electric cars

The function of the drive motor is to convert the electrical energy of the power supply into mechanical energy, and drive the wheels and working devices directly through the transmission device or directly. At present, the drive motors used in electric cars include DC, AC, permanent magnet and switched reluctance.
The electric car motor speed control device is set for the speed change and direction change of the electric car, and its function is to control the voltage or current of the motor, and complete the driving of the motor to shorten and rotate, and control the direction.

1.Electric car motor operation mode
The electric motor of an electric car has two operating modes: electric and power generation.
(1) Electric mode
In the electric mode, the motor converts electrical energy into mechanical energy.
① The inverter obtains electric power from the battery, and the battery discharges.
②The motor obtains electric power from the inverter.
③The motor outputs mechanical energy, and the motor torque is in the same direction as the speed, and the motor drives the car.
(2) Power generation mode
In power generation mode, the motor converts mechanical energy into electrical energy.
① The car drives the motor, the motor torque and speed are reversed, and mechanical energy is input to the shaft.
②The motor outputs electric energy.
③The inverter outputs direct current and the battery is charged.

2.The difference between electric car drive motor and industrial motor
The drive motors used in electric cars are different from conventional industrial motors. The drive motor of electric cars usually requires frequent start/stop, acceleration/deceleration, high torque at low speed or climbing, low torque at high speed, and a wide range of speeds; while industrial motors are usually optimized for rated work point. Therefore, the drive motors of electric cars are unique and should be categorized separately, and they have special requirements in terms of load, technical performance, and working environment. The difference between an electric car drive motor and an industrial motor is as follows.
① The electric car drive motor needs to have 4 to 5 times the overload to meet the requirements of short-term acceleration or climbing; while the industrial motor only requires 2 times the overload.
② The maximum speed of the electric car drive motor is required to reach 4 to 5 times the basic speed when cruising on the highway, while the industrial motor only needs to reach 2 times the basic speed when the power is constant.
③The electric car drive motor needs to be designed according to the car model and the driver’s driving habits, while the industrial motor only needs to be designed according to the typical working mode.
④Electric car drive motors require high power density and good efficiency maps (higher efficiency in a wide range of speed and torque), which can reduce car weight and extend driving range; industrial motors usually Comprehensive consideration of power density, efficiency and cost, and optimization of efficiency near the rated operating point.
⑤Electric car drive motors require high work controllability, high steady-state accuracy, and good dynamic performance; while industrial motors have only certain specific performance requirements.
③The drive motor of the electric car is installed on the car, with a small space, and works in harsh environments such as high temperature, bad weather and frequent vibration. However, industrial motors usually work in a certain fixed position.

3.Types of electric car drive motors
Electric cars are driven by electric motors, which are key components of electric cars. To make electric cars have good performance, the drive motor should have a wide speed range, high speed, large enough starting torque, small size, light weight, high efficiency, and strong dynamic braking. And the performance of energy feedback. At present, the motors that have been applied and have application prospects in electric cars include DC motors, induction motors, permanent magnet motors and switched reluctance motors. There are also many research institutions that are studying the application of superconducting motors in electric cars.

4.The main performance parameters of the motor
The main performance parameters of the drive motor include rated power, rated voltage, rated current, rated frequency, rated speed, rated efficiency, rated power factor, insulation class, specific power, overload capacity, etc.
①Rated power: refers to the mechanical power at the output end of the motor when the motor is running under the rated conditions specified by the manufacturer. The unit is generally kW.
②Rated voltage: refers to the line voltage applied to the stator winding when the motor is running under rated conditions, in V. It is generally stipulated that the working voltage of the motor should not be higher or lower than 5% of the rated value. When the working voltage is higher than the rated value, the motor is prone to heat; when the working voltage is lower than the rated value, it will cause the output torque to decrease, the speed to drop, the current to increase, and the winding will overheat.
③Rated current: refers to the line current of the stator winding when the motor is at the rated voltage and rated output power, the unit is A.
④Rated frequency: The frequency of my country’s power grid is 50Hz, so except for products sold, the rated frequency of domestic motors is 50Hz.
⑤Rated speed: refers to the speed of the rotor when the motor has a rated power output at the output terminal under the rated voltage and rated frequency. The unit is r/min. The rotation speed of induction motors used in electric cars is generally 8000~12000r/min.
⑥Rated efficiency: refers to the efficiency of the motor when operating under rated conditions, and is the ratio of the rated output power to the rated input power. The maximum efficiency of the motor in other operating conditions is the peak efficiency, and the higher the overall efficiency, the better. Electric cars also need to realize energy recovery when the car is decelerating and braking. The energy recovered by regenerative braking can generally reach 10% to 15% of the total energy.
⑦Rated power factor: For AC motors, the stator phase current lags behind the phase voltage by an angle φ, and the cosφ response is the power factor of the asynchronous motor. The power factor of the three-phase asynchronous motor is low, 0.7~0.9 at rated load, and lower at light load and no load. Therefore, the capacity of the motor must be selected correctly to prevent the phenomenon of “big horse-drawn carts” and strive to shorten the no-load time.
⑧Insulation grade: The insulation of the motor is graded according to the allowable limit temperature of the insulating material used in the motor windings, which is called the insulation grade. The so-called limit temperature refers to the maximum allowable temperature of the hottest spot in the motor insulation structure. The corresponding relationship between insulation class and limit temperature is shown in Table 1.

⑨ Specific power: refers to the output power of a unit mass motor, the unit is kW/kg, the larger the specific power, the better.
⑩Overload capacity: refers to the ability of the motor to work under conditions exceeding the rated load (power, torque, current, etc.). The electric car motor should have a large starting torque and a large speed regulation performance, which can make the car have good starting and acceleration, so as to obtain the required power and power for starting, accelerating, driving, decelerating, braking, etc. Torque.
⑪Other indicators: In addition to the performance parameters mentioned above, the motor also requires good reliability, good humidity and moisture resistance, low operating noise, low vibration, and can work for a long time in a harsh environment, with a simple structure and suitable for large Mass production, convenient use and maintenance, high cost performance.