1. What is a motor
A motor is a power device that converts electrical energy and mechanical energy between each other. When electrical energy is converted to mechanical energy, the device operates as a motor. When mechanical energy is converted to electrical energy, it operates as a generator. In new energy vehicles, when the vehicle is driven forward or backward using stored electrical energy, the device functions as a motor. When the accelerator is released or the brake is applied, it functions as a generator.
2. Categories of drive motors
Common drive motors for new energy vehicles today include two main types: permanent magnet synchronous motors and induction (asynchronous) motors. Most vehicles use permanent magnet synchronous motors; only a minority use induction motors.
1. AC motor
An AC motor consists of two main parts: the stator and the rotor. The stator is the outer cylindrical shell, and many windings are mounted on its inner surface. These windings are connected to an external AC power supply, and the stator is fixed to the motor housing, so it does not move. Inside the stator is the rotor, which may be a wound cylinder or a squirrel-cage structure. The rotor is connected to the motor output shaft and rotates at the same speed as the shaft. There is no mechanical contact between rotor and stator, yet when AC is applied to the stator windings, the rotor begins to turn and deliver torque.
2. Induction (asynchronous) motor
The operating principle of an AC motor is that energized windings rotate within a rotating magnetic field. The stator and rotor do not contact each other. When AC is applied to the stator windings, a rotating electromagnetic field is produced. The rotor windings form a closed conductor loop, which cuts the stator's magnetic flux lines as the stator field rotates. According to Faraday's law of electromagnetic induction, a conductor moving through a magnetic field induces an electric current in the closed loop.
According to Lenz's law, the induced current produces effects that oppose the cause of induction. In this case, the induced currents in the rotor act to reduce the relative motion between the rotor conductors and the rotating magnetic field. As a result, the rotor conductors tend to "chase" the stator's rotating magnetic field, causing the rotor to rotate. Because the rotor must be slightly slower than the stator field speed to continually cut flux lines and induce current (typically 2%–6% slower), this operation is asynchronous, which is why such motors are called induction or asynchronous motors.
3. Permanent magnet synchronous motor
In an induction motor, the rotor magnetic field is produced in two steps: first the stator rotating field induces currents in the rotor windings; then those induced currents generate the rotor field. Under Lenz's law, the rotor follows the stator field but never quite reaches synchronous speed. If the rotor field is generated by permanent magnets or by currents not induced by the stator field, the rotor field has fixed poles that are independent of the stator field. By magnetic attraction and repulsion, the stator's rotating field pulls and pushes the rotor field so that the rotor and its magnetic field rotate synchronously with the stator field. That is the principle of synchronous motors.
Permanent magnet synchronous motors offer high power-to-weight ratio, smaller size, lower mass, higher torque output, and favorable maximum speed and braking characteristics. For these reasons, they are widely used in electric vehicles. However, permanent magnets can lose magnetic performance or demagnetize under vibration, high temperature, or overload current, which may degrade motor performance. Motors using rare-earth permanent magnets also depend on rare-earth materials, which can create cost variability.
3. Role of the drive motor
The drive motor, the electronic control system, and the traction battery are the core components of an electric vehicle, often called the "three electrics." The drive motor replaces the internal combustion engine and generator in a conventional vehicle. It can convert electrical energy into mechanical energy to propel the vehicle, and it can operate as a generator to convert mechanical energy back into electrical energy for storage in the traction battery. The motor controller converts the high-voltage DC from the traction battery into high-voltage three-phase AC for the drive motor, producing torque that is transmitted to the wheels through the drivetrain to drive the vehicle.
4. Basic requirements for EV motors
- Compact structure and small dimensions. Packaging constraints require design customized to specific vehicle applications.
- Light weight to reduce vehicle mass. Aluminum housings and high rotational speeds are preferred to reduce overall vehicle mass and improve packaging space and comfort.
- High reliability and controlled failure modes to ensure passenger safety.
- Precise torque control and good dynamic performance.
- High efficiency and high power density. The motor should maintain high efficiency across a wide range of speeds and torques to reduce losses and extend driving range per charge.
- Low cost to reduce overall vehicle production expenses.
- Wide speed range, including constant-torque and constant-power regions. Large constant torque at low speeds is needed for quick starts, acceleration, and hill climbing. At high speeds, the motor should provide constant power over a wide speed range for cruising and overtaking.
- High instantaneous power and overload capacity. The motor should support 4–5 times overload capability for short-term acceleration and maximum grade climbing.
- Good environmental adaptability. The motor must operate reliably across different regional environments, including harsh conditions, with strong high-temperature and moisture resistance.
5. Operating principle of the drive motor
DC from the traction battery passes through the high-voltage distribution box and is converted by the motor controller's DC/AC inverter into three-phase AC for the permanent magnet synchronous motor, which then drives the vehicle. During coasting or braking, the motor controller places the drive motor in generator mode. The motor converts vehicle kinetic energy into three-phase AC, which the controller rectifies via an AC/DC converter into DC to recover energy back to the traction battery.
To prevent overheating during operation, coolant circulating in the motor cooling circuit removes excess heat and keeps the motor within its normal operating temperature range.
