The motors used in cars are divided into two main categories: asynchronous AC motors and permanent magnet synchronous motors. In the past, a few brands of vehicles used asynchronous motors, but now synchronous motors are the main type. So what are the advantages and disadvantages of these two types of motors, and what are the differences?

As the name suggests, synchronous motors always have something that runs in "synchronization," or in other words, rotates synchronously; whether synchronous or asynchronous motors, they both have a stator and a rotor; the stator is the fixed part of the motor, while the rotor is the part that can rotate. The stator is divided into two types: winding and iron core, which have the same function. When the three-phase alternating current is connected to the stator, it generates a rotating magnetic field, as shown in the figure below. (This is just a concept / it won't be that simple.)

The rotational speed of the stator's magnetic field is variable, and the frequency of the power supply determines the "speed" of the magnetic field. The rotor of a synchronous motor can be understood as a permanent magnet (including an electromagnet), and the rotor naturally has magnetic poles, that is, N/S. Therefore, the rotating magnetic field of the stator can attract the magnetic field of the rotor to make it rotate. A permanent magnet synchronous motor means that the speed of the stator's magnetic field is the same as the speed of the rotor, and they are the same, which is why it is called a synchronous motor.

Conversely, there are asynchronous motors, but asynchronous motors are not that simple either. The rotor of an asynchronous motor has a short-circuited winding and does not have magnetic poles. After the stator is energized, the rotating magnetic field generated will cut through the rotor winding, inducing current in the winding. When the rotor has current, it can be accelerated by electromagnetic force. However, as the rotor's rotational speed increases, once the rotor's speed equals the speed of the stator's magnetic field, the two become relatively stationary; at this point, the rotating magnetic field and the rotor winding no longer have a cutting effect, and the rotor will lose electromagnetic force (which can be understood as losing power or attraction) and decelerate, which can even be understood as "instant stall."

After the rotor decelerates, and the rotational speeds of the rotor and stator magnetic field are different again, there will be relative motion between the two—deceleration followed by acceleration, synchronization followed by deceleration, and acceleration after deceleration. This is the asynchronous motor, where the rotor and magnetic field speeds are different.

After clarifying the differences between the two, the next thing to understand is their advantages and disadvantages.

The disadvantages of asynchronous motors are quite prominent. Their starting performance is worse than that of permanent magnet synchronous motors because, during startup, current must be generated on the rotor (short-circuited winding) through the rotation of the magnetic field before it can start operating. Synchronous motors can interact with the rotor as long as the stator is energized. Another disadvantage is that the speed regulation performance is relatively poor, and the core reason is precisely the "asynchronous" nature, which seems not to be ideal as a drive motor, with emphasis on "seems."

Secondly, asynchronous motors have a low power factor, and during operation, they need to absorb useless power from the circuit system. The concept of power factor is the ratio of active power to apparent power in an AC circuit; the higher this value, the better. Useless power is not the concept of "power that has no use"; drive motors operate based on the principle of electromagnetic induction, and an alternating magnetic field must be established to convert and transfer energy. Establishing an alternating magnetic field and inducing magnetic flux requires useless power. Although this power cannot be directly converted into mechanical energy (power), without useless power, it cannot operate. Therefore, the disadvantages of asynchronous motors are quite prominent. So what are the disadvantages of synchronous motors?

Synchronous motors have a relatively prominent disadvantage, or rather, an absolutely prominent and unsolvable disadvantage.

The manufacturing cost far exceeds that of asynchronous motors; apart from that, it is basically all advantages. Its advantages include a high power factor, high operating power, and constant speed; operating at a constant speed increases the usage scenarios of the motor due to the stability of its function. However, in reality, the issue of manufacturing costs has made asynchronous motors the mainstream option in industrial and agricultural production. Synchronous motors are capable of generating and absorbing reactive power, and can operate stably to achieve N = ns = 60f/p (grid frequency / number of motor pole pairs). As long as the grid frequency is stable, it can operate stably, while instantaneous changes in frequency can cause the rotor of the motor to change instantly, making it relatively more "agile" than asynchronous motors. It can be said that permanent magnet synchronous motors are more suitable for use as engines in passenger cars and are also excellent as generators.

The generation method of permanent magnet synchronous motors is very simple. When the stator is energized, the prime mover drives the rotor to rotate, and the three-phase winding of the stator cuts through the rotor's magnetic field to generate electromotive force for power generation. In contrast, the stator of an asynchronous motor is a three-phase winding, while the rotor is a short-circuited winding without a magnetic field. Therefore, for an asynchronous motor to generate electricity, a magnetic field must first be established, which means that the stator must be energized first, and then current can appear in the rotor. But how exactly does it generate electricity?

The premise for an asynchronous motor to generate electricity is to first energize the stator, and then the prime mover must pull the rotor's speed to exceed the speed of the stator's magnetic field to start generating electricity. This means that during driving, it must reach an instantaneous "constant speed" and continue to accelerate at the moment it starts to decelerate to generate electricity. Here, the so-called prime mover refers to the wheels, and the power comes from the vehicle's inertia during coasting. This method of generating electricity is called kinetic energy recovery, and theoretically, the recovery effect of synchronous motors should be higher.