Why has permanent magnet synchronous motor become mainstream?

The electric motor can convert electrical energy into mechanical energy and transmit the mechanical energy to the wheels through the transmission system to drive the car. It is one of the core drive systems of new energy vehicles. At present, the commonly used drive motors for new energy vehicles are mainly permanent magnet synchronous motors and AC asynchronous motors. Most new energy vehicles use permanent magnet synchronous motors, and representative car companies include BYD and Ideal Auto. Some vehicles use AC asynchronous motors, and representative car companies include Tesla and Mercedes-Benz.

Classification of commonly used motors suitable for electric drives

The asynchronous motor is mainly composed of two parts: a stationary stator and a rotating rotor. When the stator winding is connected to an AC power supply, the rotor will rotate and output power. The main principle is that when the stator winding is energized (AC power), a rotating electromagnetic field will be formed, and the rotor winding is a closed conductor that continuously cuts the stator's magnetic induction lines in the stator's rotating magnetic field. According to Faraday's law, a closed conductor cutting magnetic induction lines will generate current, and current will generate an electromagnetic field. At this time, there are two electromagnetic fields: one is the stator electromagnetic field connected to the external AC power, and the other is the rotor electromagnetic field generated by cutting the stator electromagnetic induction lines. According to Lenz's law, the induced current always resists the cause of the induced current, that is, it tries its best to prevent the conductor on the rotor from cutting the magnetic induction lines of the stator's rotating magnetic field. The result is that the conductor on the rotor will "catch up" with the stator's rotating electromagnetic field, that is, the rotor chases the stator's rotating magnetic field, and finally the motor starts to rotate. In the process, the rotor rotation speed (n2) and the stator rotation speed (n1) are not synchronized (the speed difference is about 2-6%), so it is called an asynchronous AC motor. Conversely, if the speed is the same, it is called a synchronous motor.

Above left: Schematic diagram of the structure of a single-phase asynchronous motor

Above right: Schematic diagram of the working principle of asynchronous motor

Permanent magnet synchronous motor is also a kind of AC motor. Its rotor is made of steel with permanent magnets. When the motor is working, the stator is energized to generate a rotating magnetic field to drive the rotor to rotate. "Synchronous" means that the rotation speed of the rotor is synchronized with the rotation speed of the magnetic field during steady-state operation. Permanent magnet synchronous motors have a higher power-to-weight ratio, smaller size, lighter weight, greater output torque, and better limiting speed and braking performance. Therefore, permanent magnet synchronous motors have become the most widely used motors in electric vehicles today. However, when permanent magnet materials are subjected to vibration, high temperature and overload current, their magnetic conductivity may decrease, or demagnetization may occur, which may reduce the performance of permanent magnet motors. In addition, rare earth permanent magnet synchronous motors use rare earth materials, and the manufacturing cost is not very stable.

Left: Schematic diagram of the structure of a permanent magnet synchronous motor

Above right: Audi's permanent magnet synchronous motor structure

Compared with permanent magnet synchronous motors, asynchronous motors need to absorb electrical energy for excitation when working, which will consume electrical energy and reduce the efficiency of the motor. Permanent magnet motors are more expensive due to the addition of permanent magnets.

Models that choose AC asynchronous motors tend to prioritize performance, taking advantage of the performance output and efficiency advantages of AC asynchronous motors at high speeds. The representative model is the early Model S. Main features: When the car is driving at high speed, it can maintain high-speed operation and efficient use of electric energy, while maintaining maximum power output and reducing energy consumption;

Models that choose permanent magnet synchronous motors tend to prioritize energy consumption, using the performance output and efficient operation of permanent magnet synchronous motors at low speeds, and are suitable for small and medium-sized vehicles. The characteristics are small size and light weight, which can increase endurance. At the same time, it has good speed regulation performance and can maintain high efficiency when facing repeated starts and stops, acceleration and deceleration.

Characteristics of asynchronous induction motors and permanent magnet synchronous motors

Permanent magnet synchronous motors dominate. According to the statistics of the "New Energy Vehicle Industry Chain Monthly Database" released by Gaogong Industry Research Institute (GGII), from January to August 2022, the installed capacity of domestic new energy vehicle drive motors was about 3.478 million sets, a year-on-year increase of 101%. Among them, the installed capacity of permanent magnet synchronous motors was 3.329 million sets, a year-on-year increase of 106%; the installed capacity of AC asynchronous motors was 1.295 million sets, a year-on-year increase of 22%.

Permanent magnet synchronous motors have become the main drive motors in the pure electric passenger car market.

From the perspective of the motor selection of mainstream models at home and abroad, the new energy vehicles launched by Shanghai Auto, Geely Auto, Guangzhou Auto, Beijing Auto, and Denza Auto all use permanent magnet synchronous motors. The domestic market mainly uses permanent magnet synchronous motors, firstly because permanent magnet synchronous motors have good low-speed performance and high conversion efficiency, which are very suitable for the complex working conditions of frequent start-stop in urban traffic, and secondly because the NdFeB permanent magnet materials in permanent magnet synchronous motors require the use of rare earth resources, and my country has 70% of the world's rare earth resources, and the total output of NdFeB magnetic materials reaches 80% of the world, so China is more keen on using permanent magnet synchronous motors.

Tesla and BMW abroad are developing permanent magnet synchronous motors and AC asynchronous motors in synergy. From the perspective of application structure, permanent magnet synchronous motors are the mainstream choice for new energy vehicles.

Some types of new energy vehicle motors at home and abroad

Permanent magnet material costs account for about 30% of the cost of permanent magnet synchronous motors. The main raw materials for manufacturing permanent magnet synchronous motors are neodymium iron boron, silicon steel sheets, copper and aluminum, among which permanent magnet material neodymium iron boron is mainly used to manufacture rotor permanent magnets, accounting for about 30% of the cost; silicon steel sheets are mainly used to make stator and rotor cores, accounting for about 20% of the cost; stator winding costs account for about 15%; motor shaft costs account for about 5%; and motor housing costs account for about 15%.

Cost Analysis of Permanent Magnet Synchronous Drive Motor

Why are permanent magnet motors more efficient?

The permanent magnet synchronous motor is mainly composed of a stator, a rotor and a housing. Like ordinary AC motors, the stator core is a laminated structure to reduce iron loss due to eddy current and hysteresis effects during motor operation; the winding is usually a three-phase symmetrical structure, but the parameter selection is quite different. The rotor part has various forms, including permanent magnet rotors with starting squirrel cages, and embedded or surface-mounted pure permanent magnet rotors. The rotor core can be made into a solid structure or laminated. The rotor is equipped with permanent magnet material, which is usually called magnetic steel.

When the permanent magnet motor is working normally, the rotor and stator magnetic fields are in a synchronous state, there is no induced current in the rotor part, no rotor copper loss, hysteresis, and eddy current loss, and there is no need to consider the problem of rotor loss and heating. Generally, permanent magnet motors are powered by special inverters and naturally have a soft start function. In addition, permanent magnet motors are synchronous motors, which have the characteristics of synchronous motors that adjust the power factor by the strength of the excitation, so the power factor can be designed to a specified value.

From the starting perspective, due to the fact that the permanent magnet motor is started by a variable frequency power supply or a matching inverter, the starting process of the permanent magnet motor is very easy to implement; similar to the starting of a variable frequency motor, it avoids the starting defects of ordinary cage-type asynchronous motors.

In short, the efficiency and power factor of permanent magnet motors can be very high, the structure is very simple, and the market has been very hot in the past decade.

However, demagnetization failure is an unavoidable problem for permanent magnet motors. When the current is too large or the temperature is too high, the motor winding temperature will rise instantly, the current will increase sharply, and the permanent magnet will lose magnetism quickly. In the control of permanent magnet motors, an overcurrent protection device is set to avoid the problem of the motor stator winding being burned, but the resulting demagnetization and equipment shutdown are inevitable.

Compared with other motors, permanent magnet motors are not very popular in the market. Both motor manufacturers and users have some unknown technical blind spots, especially when it comes to matching with frequency converters, which often lead to serious discrepancies between design values ​​and test data and must be repeatedly verified.