Automobile motors fall into two main categories: asynchronous AC motors and permanent magnet synchronous motors. While a few brands used to use asynchronous motors, synchronous motors are now the dominant type. So what are the advantages and disadvantages of these two types of motors, and what are their differences?
As the name suggests, a synchronous motor always has something that operates "synchronously," or rather, rotates synchronously. Whether synchronous or asynchronous, it has a stator and a rotor; the stator is the fixed part of the motor, while the rotor is the rotating part. The stator consists of two parts: windings and an iron core, which have the same function. When three-phase AC current is applied to the stator, it generates a rotating magnetic field, as shown in the diagram below. (This is just a conceptual explanation/it's not that simple.)
The rotational speed of the stator magnetic field is variable; the frequency of the power supply determines the "rotation speed" of the magnetic field. The rotor of a synchronous motor can be understood as a permanent magnet (including electromagnets). The rotor naturally has magnetic poles, i.e., N/S. Therefore, the rotating magnetic field of the stator can attract the magnetic field of the rotor, causing it to rotate. A permanent magnet synchronous motor is one in which the rotational speed of the stator magnetic field is the same as the rotational speed of the rotor; because they are the same, it is called a synchronous motor.
Conversely, there's the asynchronous motor, but it's not that simple. The rotor of an asynchronous motor has short-circuited windings and no magnetic poles. When the stator is energized, the resulting rotating magnetic field cuts the rotor windings, inducing a current in them. This current in the rotor allows it to receive electromagnetic force and accelerate. However, as the rotor's rotational speed increases, once the rotor's speed equals the stator's magnetic field speed, the two become relatively stationary. At this point, the rotating magnetic field and the rotor windings no longer cut each other, and the rotor loses its electromagnetic force (which can be understood as losing power or attraction) and decelerates, even experiencing a momentary stall.
After the rotor decelerates, the rotor and stator magnetic field speeds become different again, and the two will then move relative to each other—decelerating and then accelerating again, synchronizing and then decelerating again, and then accelerating again. This is the asynchronous motor, where the rotor and magnetic field speeds are different.
After understanding the differences between the two, the next step is to understand 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 they require the rotation of a magnetic field to generate current in the rotor (short-circuited windings) before they can start running. Synchronous motors, on the other hand, can interact with the rotor as soon as the stator is energized. Another disadvantage is their relatively poor speed regulation performance, the core reason being "asynchronous," which makes them seem less than ideal as drive motors. Note the emphasis on "seems."
Secondly, asynchronous motors have a low power factor, requiring the absorption of "useless power" from the circuit system during operation. The power factor is the ratio of active power to apparent power in an AC circuit; a higher value is better. "Useless power" is not simply "power that is not used." Drive motors operate based on the principle of electromagnetic induction, requiring the establishment of an alternating magnetic field for energy conversion and transfer. This establishment of the alternating magnetic field and induced magnetic flux requires "useless power." While this power cannot be directly converted into mechanical energy (power), operation is impossible without it. Therefore, asynchronous motors have significant disadvantages. So, what are the disadvantages of synchronous motors?
Synchronous motors have a rather prominent drawback, or rather, an absolutely prominent and unsolvable drawback.
Aside from its significantly higher manufacturing cost compared to asynchronous motors, it boasts advantages in almost every aspect. These advantages include a high power factor, high operating power, and constant speed. Operating at a constant speed expands its application scenarios due to functional stability. However, in reality, manufacturing costs have made asynchronous motors the mainstream choice in industrial and agricultural production. Synchronous motors, on the other hand, can both generate and absorb reactive power, achieving steady-state operation with N=ns=60f/p (grid frequency/number of pole pairs). As long as the grid frequency is stable, it can operate stably, and instantaneous frequency changes can cause instantaneous rotor changes, making it more "flexible" than asynchronous motors. It can be said that permanent magnet synchronous motors are more suitable for use as passenger car engines, and are also excellent as generators.
The power generation method of a permanent magnet synchronous motor is very simple: after the stator is energized, the prime mover drives the rotor to rotate, and the three-phase windings of the stator cut the rotor's magnetic field to generate electromotive force and produce electricity. However, the stator of an asynchronous motor has three-phase windings, while the rotor has short-circuit windings and no magnetic field. Therefore, for an asynchronous motor to generate electricity, a magnetic field must first be established. In other words, just like when driving a motor, the stator must be energized first, and then current can flow into the rotor. But how exactly does this generation work?
The prerequisite for generating electricity with an asynchronous motor is to first energize the stator, and then the prime mover pulls the rotor speed to a speed exceeding that of the stator magnetic field before it can start generating electricity. In other words, it can generate electricity only when it reaches a momentary "constant speed" during driving and then accelerates again at the instant it begins to decelerate. The so-called prime mover here is the wheel, and the power comes from the inertial force of the vehicle when it is coasting. This method of generating electricity is called kinetic energy recovery. Theoretically, the recovery effect of a synchronous motor should be even higher.
Do asynchronous motors have no advantages?
The answer is, of course, no. It has an absolute advantage, and an advantage that can only be realized when used in electric vehicles.
Permanent magnet synchronous motors (PMSMs) have high operating efficiency at high speeds and low torque due to their low manufacturing cost. However, their efficiency decreases in the ultra-high speed range, meaning they consume more electricity. This is one reason why electric vehicles consume more electricity at high speeds. However, by improving the efficiency of synchronous motors and increasing their power to achieve lower speeds at high vehicle speeds, electricity consumption can be reduced. But using asynchronous motors with the same technical standards can further reduce electricity consumption in the high-speed range. Therefore, the optimal solution is to use both types of motors in combination. For example, a full-time four-wheel drive vehicle with front and rear dual motors can be built using a "synchronous + asynchronous" combination. Currently, it seems that two passenger car brands have used this combination: e3.0 and a certain brand.
In summary, these are the advantages and disadvantages of synchronous and asynchronous motors. Theoretically, a combination of multiple synchronous motors can achieve ultra-high performance and a sufficiently high driving quality; however, using asynchronous motors alone is somewhat inferior. Combining the two may bring some interesting changes, and we'll have to see what surprises the e3.0 platform can bring.
Inexpensive asynchronous motors may still have room to be used in electric vehicles.
