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The advantages and disadvantages of induction / permanent magnet motor, why do d

In our life, there are electric motors in large and small household appliances. Electric motors are essential machines in industry. Because people usually contact more electric motors, the research on electric motors is much more than that on generators, and the classification of electric motors is much more detailed. Generally speaking, motors can be classified according to the working power supply, structure and working principle, different starting and running modes and different purposes. Among them, the motors suitable for driving new energy vehicles mainly include permanent magnet synchronous motor, induction motor and switched reluctance motor. Here, we first use a diagram to summarize the motors classified by structure and working principle.1.jpeg

"Motor family tree" classified by structure and working principle

What is induction motor / permanent magnet motor?

Induction motor is often referred to as AC asynchronous motor. It is a kind of motor type which is driven by electromagnetic torque after the physical interaction between the rotating magnetic field formed after the stator winding and the magnetic field of induced current in the rotor coil winding. The permanent magnet motor mainly refers to the permanent magnet synchronous motor.

Induction motor (AC asynchronous motor)

First of all, because the rotor is always "catching up" with the speed of the stator rotating magnetic field, and in order to be able to cut the magnetic induction line and generate the induced current, the speed of the rotor is always a little slower than the speed of the stator rotating magnetic field, that is, asynchronous operation, so this kind of motor generating the induced current is called the AC asynchronous motor. It is because of asynchrony that there is a slip, which causes the conductor on the rotor to produce current, and then it is forced to rotate in the magnetic field. For this reason, it will be a little more complicated to control. But the induction motor is composed of wound coil and rotor core, so the price is cheap, the overload capacity is strong, the leather is durable and the performance is excellent.

Permanent magnet synchronous motor

When it comes to permanent magnet synchronous motor, it may be better understood literally. The so-called permanent magnet refers to adding permanent magnet to the rotor of motor to further improve the performance of motor. Before that, the performance of magnet was not good, but the stability and wide application of high magnetic performance Nd-Fe-B under the control algorithm increased the reliability and stability of permanent magnet, which can reduce the demagnetization phenomenon. The so-called synchronization means that the speed of the rotor and the current frequency of the stator winding are always consistent.

Rare earth permanent magnet parts

If the current in the rotor winding is not induced by the rotating magnetic field of the stator, but generated by itself, the rotor magnetic field is independent of the rotating magnetic field of the stator, and its pole direction is fixed, then according to the principle of same-sex repulsion and opposite-sex attraction, the rotating magnetic field of the stator will pull the rotor to rotate, and make the rotor magnetic field and the rotor and the rotating magnetic field of the stator "synchronous" rotate. This is how synchronous motors work. There is no doubt that synchronization is easier to control naturally.

Why do domestic car companies prefer permanent magnet motors?

In order to make you more concise and clear about the characteristics of these two motors, we have summarized the relevant knowledge points and made a table for your reference. In the most general terms, induction motor is more suitable for high-performance and high-speed conditions, while permanent magnet synchronous motor is more suitable for frequent start and stop conditions.

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If you pay more attention to domestic new energy vehicles, you may have the impression that Europe and the United States prefer to use induction motors, while most domestic manufacturers prefer to use permanent magnet synchronous motors. Tesla Model s (parameter picture) and model x (parameter picture) from the other side of the ocean are the best examples, but the domestic Weilai es8 and Jiangling e200s are also equipped with induction (AC asynchronous) motors. In addition, it is difficult to find the model of induction motor camp. For example, most pure electric vehicles such as BYD Qin (parameter picture) EV, song (parameter picture) EV, Geely Dihao (parameter picture) EV, Roewe erx5, BMW I3 are equipped with permanent magnet synchronous motor.

VLE es8 with induction (AC asynchronous) motor

Combined with the above table, it seems that the reason why permanent magnet synchronous motor occupies most of the country is very simple. At the same level of technology, in addition to weak magnetic control, permanent magnet synchronous motor performs better in torque density, power density and efficiency than induction motor, so it can improve energy efficiency and increase endurance mileage, which is undoubtedly the most direct impact on the current pure electric vehicle. Big pain. At the same time, the permanent magnet synchronous motor can still ensure a high efficiency of about 95% in the transient state, with small volume and light weight, so it is suitable for frequent starting and stopping conditions and small passenger car layout space. In addition, the permanent magnet motor does not need the induction motor as efficient and complex cooling system, which is conducive to the design and manufacture of the whole vehicle.

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And now that the market is the main direction, there is no difference between the motors of many domestic brands. Except Tesla, BYD and Roewe have their own motors, other manufacturers temporarily choose the motors provided by suppliers, so the channel is relatively single.

In addition, in addition to some factors of the above-mentioned technology market, there is also a big reason that cannot be ignored. The above mentioned permanent magnet materials such as neodymium, iron and boron for permanent magnet synchronous motor are important rare earth resources. For countries with rare earth resources lacking or rare earth industry underdeveloped, this is very embarrassing. We know that the technical resources of vehicle power motor are the core of a country. Competitiveness, the surrounding are not easy to spread. China has 70% of the world's rare earth resources, and the total output of NdFeB magnetic materials has reached 80% of the world's total. Due to the advantage of rare earth resources in China, it seems that it is natural to choose high efficiency permanent magnet synchronous motor. Japan is a big country in rare earth industry, but for European and American automobile enterprises, if they want to use permanent magnet synchronous motor, they must consider a series of factors such as the supply guarantee of imported rare earth.

Rare earth mineral rare earth is the general term of 17 metal elements in the chemical cycle table, including lanthanide, scandium and yttrium

There is no exact difference between induction motor and permanent magnet motor

Although the market share of permanent magnet synchronous motor is very large, does it mean that induction motor is inferior to permanent magnet synchronous motor? The answer is No. From the current technical advantages, permanent magnet synchronous motor has become the mainstream. It is worth noting that although induction (AC asynchronous) motor is not dominant in weight and volume, it has a wide range of speed, low cost, simple process, reliable and durable operation. As for the influence of weight on the endurance mileage, if the battery endurance mileage is greatly increased in the future and the volume optimization of induction motor is well done, it may be better to compete with permanent magnet motor in the market performance.

It can be said that at present, Tesla Model s and model x standing at the top of the pure electric vehicle "pyramid" are still using induction motor, because the power of induction motor is larger, it is easy to achieve faster acceleration of 100 kilometers, and it will not appear demagnetization phenomenon when it operates continuously under long-term high power and high temperature. In order to build a high-performance electric vehicle, we naturally choose the same induction (AC asynchronous) motor as Tesla.

Tesla Model s and model 3

Although the cost of induction motors is low, Tesla is not considering saving money. It is worth mentioning that Nikolai Tesla, as the Grandmaster of electric power, invented three-phase alternating current (power electricity) and three-phase alternating current asynchronous motor, which is the ancestor of the power heart of Tesla. Today's Tesla Motors are named after this, which is actually a tribute to this outstanding scientist. Tesla's early products used induction (AC asynchronous) motors, mainly for performance, and they had enough confidence in their own battery management system (BMS). Moreover, in the early overseas, Tesla had not mastered the permanent magnet synchronous technology in place, plus the newly established The technical team is good at induction motor. In addition, the product positioning is sports car, so we chose induction motor along the river.

Induction motors are usually squirrel cage shaped. Invented by scientist Nicolas Tesla in 1887

Interestingly, Tesla's latest mid size model 3 has been converted to a permanent magnet motor. Because the model 3 chassis is smaller than the model s, even if the 2170 battery with higher energy density is used, the total energy is still lower than the model s, and the endurance can only be extended by improving the efficiency. It is also the use of permanent magnet synchronous motor, model 3 with 75 kwh battery capacity in the range of 100 kwh model s 100D. When it comes to model 3, it is undeniable that model 3 has no way to install large induction (AC asynchronous) motors because of the platform structure.

Tesla factory, USA

In general, compared with induction motor, permanent magnet synchronous motor has obvious advantages. However, at present, the range of pure electric vehicle is bound to be an important index. The high efficiency of PMSM can improve the range better. Moreover, the successful development of NdFeB permanent magnet with high thermal and magnetic properties and the further development and improvement of power electronic components make the development of rare earth permanent magnet synchronous motor further perfect. But as far as the development trend is concerned, PMSM seems to have a better future.

However, Tesla's technological breakthrough in the induction (AC asynchronous) motor copper core rotor technology also gives this kind of motor greater advantages in efficiency, while obtaining a win-win situation of low cost and high efficiency. For example, with the development of AC speed regulation system, the speed regulation performance and economy of the current asynchronous motor suitable for wide speed regulation are comparable to that of DC motor.

It has to be said that in the era of rapid iteration of technical level, if the power battery technology of pure electric vehicle makes a major breakthrough, the AC asynchronous motor may also share the world equally. However, the so-called Yangtze River wave is pushing forward the wave behind. The switched reluctance motor (SRM), which is now widely used in commercial vehicles, is a kind of motor with great development potential. Its advantages and disadvantages are distinct, but the disadvantages are not hidden. If the torque fluctuation can be improved, it is possible to replace the asynchronous and permanent magnet synchronous motor perfectly.


 
 
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