I. What is an electric motor?
An electric motor, as the name suggests, is an electrical component that converts electrical energy into mechanical energy and vice versa. When electrical energy is converted into mechanical energy, the motor exhibits the working characteristics of an electric motor; when mechanical energy is converted into electrical energy, the motor exhibits the working characteristics of a generator. In the context of new energy vehicles, when a new energy vehicle is in a discharged state and driving the vehicle forward or backward, it exhibits the characteristics of an electric motor; when the accelerator pedal is released or the brake pedal is pressed, it exhibits the characteristics of a generator. II. Classification of Drive Motors Currently, the commonly used drive motors in new energy vehicles include two types: permanent magnet synchronous motors and AC asynchronous motors. Most new energy vehicles use permanent magnet synchronous motors, with only a small number using AC asynchronous motors.
1. AC motor
An AC motor mainly consists of two parts: the stator and the rotor. The stator is the outermost cylinder, with many windings wound around its inner side. These windings are connected to an external AC power source. The entire cylinder is connected to the frame and remains stationary, hence the name "stator." Inside the stator, there are either cylinders with many windings or cage-like cylinders, which are connected to the motor's output shaft and rotate at the same speed; these are called the "rotor." There is no direct connection or contact between the rotor and the stator, but when the windings on the stator are connected to an AC power source, the rotor immediately rotates and outputs power.
2. Asynchronous motor
The working principle of an AC motor: A current-carrying winding rotates in a rotating magnetic field. The stator and rotor of the motor are not in contact, so why does the rotor rotate when AC current is applied to the stator winding? Its working principle utilizes two major electromagnetic laws: the law of electromagnetic induction and Lenz's law. When AC current is applied to the windings wound on the stator, due to the characteristics of AC current, the stator windings generate a rotating electromagnetic field. The windings on the rotor are closed-loop conductors, and being in the rotating magnetic field of the stator is equivalent to them constantly cutting the magnetic field lines of the stator. According to Faraday's law of electromagnetic induction, when a part of a closed conductor moves in a magnetic field, cutting magnetic field lines, a current is generated in the conductor.
According to Lenz's law, the effect of induced current always opposes the cause that induced current, meaning it tries to prevent the conductors on the rotor from cutting the magnetic field lines of the stator's rotating magnetic field. As a result, the conductors on the rotor "chase" the rotating electromagnetic field of the stator, causing the rotor to follow the stator's rotating magnetic field, ultimately causing the motor to start rotating. Because the rotor is always "chasing" the rotational speed of the stator's rotating magnetic field, and in order to cut the magnetic field lines and generate induced current, the rotor's speed is always slightly slower than the stator's electromagnetic field speed (2%~6%), meaning it operates asynchronously. Therefore, this type of motor that generates induced current is called an asynchronous motor.
3. Permanent magnet synchronous motor
In an asynchronous motor, the formation of the rotor magnetic field involves two steps: first, the stator's rotating magnetic field induces a current in the rotor windings; second, this induced current generates the rotor magnetic field. Under Lenz's law, the rotor follows the stator's rotating magnetic field, but it "can never catch up," hence the name asynchronous motor. If the current in the rotor windings is generated independently of the stator's rotating magnetic field, then the rotor magnetic field is independent of the stator's rotating magnetic field, and its magnetic pole direction is fixed. According to the principle of like poles repelling and unlike poles attracting, the stator's rotating magnetic field will push and pull the rotor to rotate, causing the rotor magnetic field and the rotor itself to rotate "synchronously" with the stator's rotating magnetic field. This is the working principle of a synchronous motor.
Permanent magnet synchronous motors (PMSMs) have a high power-to-weight ratio, are smaller and lighter, and have greater output torque. They also offer superior limiting speed and braking performance, making them the most widely used motors in electric vehicles today. However, the magnetic permeability of permanent magnet materials may decrease or demagnetize when subjected to vibration, high temperatures, and overload currents, potentially reducing the performance of the motor. Furthermore, rare-earth PSMs require rare-earth materials, leading to less stable manufacturing costs.
III. Function of the drive motor
The drive motor, electronic control system, and power battery are the core components of an electric vehicle, collectively known as the "three electrics." In an electric vehicle, the drive motor replaces the engine and generator in a traditional car. Traditional cars typically convert chemical energy into mechanical energy to drive the vehicle, while the drive motor can both convert electrical energy into mechanical energy to drive the car and act as a generator to convert mechanical energy into electrical energy, storing it in the power battery. The motor controller converts the high-voltage direct current from the power battery into high-voltage three-phase alternating current for the drive motor, enabling the drive motor to generate torque. This torque is then transmitted to the wheels through a transmission system, propelling the vehicle.
IV. Basic Requirements for Electric Vehicle Motors
1. The motor has a compact structure and small size, with limited package dimensions, requiring special design based on specific products. 2. Lightweight to reduce the overall weight of the vehicle. Aluminum alloy housings should be used whenever possible, along with high speeds to reduce overall vehicle weight, increase motor-vehicle compatibility, expand usable space, and improve passenger comfort. 3. High reliability and controllable failure modes to ensure passenger safety. 4. Precise torque control and good dynamic performance. 5. High efficiency and high power density. High efficiency must be maintained across a wide speed and torque range to reduce power loss and increase driving range per charge. 6. Low cost to reduce overall vehicle production costs. 7. Wide speed range. Includes constant torque and constant power regions. High constant torque output at low speeds to meet the requirements of rapid vehicle start-up, acceleration, and load climbing; constant power output at high speeds with a wide speed range to meet the requirements of high-speed driving on flat roads and overtaking. 8. High instantaneous power and strong overload capacity. The vehicle must have a load capacity of 4-5 times its rated capacity to meet the requirements of short-term acceleration and maximum climbing. 9. Good environmental adaptability. It must be able to adapt to different operating environments, functioning normally even in harsh conditions, and possess good resistance to high temperatures and humidity.
V. Working principle of the drive motor
The DC power from the power battery passes through a high-voltage distribution box, where it is converted into AC power by a DC/AC converter in the motor controller. This AC power is then supplied to the permanent magnet synchronous motor, which in turn drives the vehicle. When the vehicle is coasting or braking, the motor controller activates the drive motor to generate electricity. The drive motor uses the vehicle's kinetic energy to generate electricity, which is then rectified into DC power by the AC/DC converter in the motor controller, recovering the energy and storing it in the power battery.
To prevent the drive motor from overheating during operation, the coolant in the motor cooling circulation pipe can carry away excess heat and keep it within the normal operating temperature range.
