Recently, the first batch of Tesla Model 3 vehicles were delivered to owners, once again bringing electric vehicles into the spotlight. In terms of drive motors, the two most representative types currently used in small cars are Tesla's AC asynchronous motors and BYD's rare-earth permanent magnet motors . Today, we'll briefly introduce some knowledge about rare-earth permanent magnet motors.
The Development History of Rare Earth Permanent Magnet Motors
The world's first electric motor appeared as early as the 1820s, and its rotor was a permanent magnet used to generate an excitation magnetic field. However , the permanent magnet material used at that time was natural magnetite (Fe3O4), which had a very low magnetic energy density . Motors made from it were bulky and were soon replaced by electrically excited motors.
With the development of technology, there have been many choices for permanent magnet materials. Among them, rare earth materials are the best. Therefore, motors that use rare earth permanent magnet materials are also called rare earth permanent magnet motors.
Mechanism and Principle of Rare Earth Permanent Magnet Motor
From a structural perspective, rare earth permanent magnet synchronous motors are mainly composed of components such as rotor, end covers, and stator.
Generally speaking, the stator structure of a permanent magnet synchronous motor is very similar to that of an ordinary induction motor, but the unique structure of the rotor is what distinguishes it from other motors.
The biggest difference between it and the AC induction asynchronous motor used by Tesla is that it has high-quality permanent magnet poles (rare earth) on the rotor.
Because there are many options for placing permanent magnets on the rotor, permanent magnet synchronous motors are generally divided into three main categories : embedded, surface-mounted, and plug-in.
Their working principles are roughly the same. Three-phase current is passed through the stator winding of the fixed motor. After the current is passed through, a rotating magnetic field is formed in the stator winding of the motor. Since permanent magnets are installed on the rotor, and the magnetic poles of the permanent magnets are fixed, according to the principle that like poles attract and unlike poles repel, the rotating magnetic field generated in the stator will drive the rotor to rotate.
Technical bottlenecks of rare earth permanent magnet motors
1. Control issues
Permanent magnet motors can maintain their magnetic field without external energy after being manufactured, but this also makes it extremely difficult to adjust and control their magnetic field from the outside. Permanent magnet generators are difficult to adjust their output voltage and power factor from the outside, and permanent magnet DC motors can no longer adjust their speed by changing the excitation.
2. Cost issues
Because rare-earth permanent magnets are currently relatively expensive, the cost of rare-earth permanent magnet motors is generally higher than that of electrically excited motors. This higher cost needs to be compensated by their high performance and lower operating costs. Therefore, permanent magnet motors are suitable for low-power applications.
3. Demagnetization problem
Rare earth permanent magnet motors have stringent requirements for the working environment . Rare earth permanent magnet materials will experience irreversible demagnetization and failure when the temperature exceeds 180°C. They are prone to breakage under severe vibration or large temperature differences. The materials are easily oxidized and corroded , and surface coating is required before use. Rare earth permanent magnet motors are very sensitive to overload , and overload will cause demagnetization of the permanent magnet materials.
Meanwhile, rare-earth permanent magnet motors have very high electromagnetic loads, and their magnetic fields are difficult to adjust after fabrication, making their power control systems far more complex than those of induction motors. Traditional motor design theories, calculation methods, and motor control systems are inadequate for the development requirements of high-performance motors.
In conclusion, the development of rare-earth permanent magnet motors faces numerous challenges. However, just as one cannot see a rainbow without experiencing storms, the tireless efforts of automotive engineers in the face of air disasters will eventually break through the bottlenecks, allowing the public to benefit from the convenience brought by technology.
Differences between mainstream new energy vehicle drive motors
The drive motors for new energy vehicles mainly fall into three categories: permanent magnet synchronous, AC asynchronous, and switched reluctance. Each type has its own application scenarios due to its different characteristics.
Rare earth permanent magnet synchronous motor
In principle, it is possible to achieve small size, light weight, high power density, high reliability, high speed regulation accuracy, and fast response speed. Due to the high power density of permanent magnet synchronous motors, their working efficiency can reach up to 97% , enabling them to output maximum power and acceleration for vehicles. Therefore, they are mainly used in new energy passenger vehicles with the highest requirements for energy-volume ratio.
The disadvantages are lower maximum power and higher cost. AC asynchronous motors are inexpensive and reliable; however, their inherent limitations include low power density, complex control, and limited speed range. Their price advantage makes them widely used in new energy buses.
Induction (Asynchronous) Motor
It can withstand significant temperature variations during operation. Conversely, significant temperature variations can damage permanent magnet motors. Induction motors, on the other hand, can adjust their output torque over a wide range, do not require a heat dissipation mechanism, are lightweight (about the size of a watermelon, weighing only 52 kg), and can operate at speeds from 0 to 12,000 rpm, thus eliminating the need for additional transmission mechanisms. Therefore, its weight is a significant advantage among new energy vehicles.
The disadvantages are that the rotor rotation speed is difficult to control, energy consumption is high, power factor is lagging, structure is complex, and the control system of the induction (asynchronous) motor is complex, with high technical requirements and high manufacturing cost.
Switched reluctance motor
It is inexpensive, has a simple and reliable circuit, and a wide speed range; however, it generates significant vibration and noise, has a complex control system, and produces a large pulse current to the DC power supply. It is used in large buses and BYD electric vehicles.
Among them, Japanese and Korean cars mostly use permanent magnet motors, which have relatively high speed range and efficiency, but require the use of expensive system permanent magnet materials such as neodymium iron boron; European and American cars mostly use AC induction motors, mainly due to the scarcity of rare earth resources and the consideration of reducing motor costs. Their disadvantages are mainly a small speed range and low efficiency, requiring a higher-performance speed controller to match the performance.
