China permanent magnet manufacturer: www.rizinia.com

# Why do BYD use permanent magnet synchronous motor, while Tesla and BMW prefer AC asynchronous motor?

When the media reports the power information of BYD electric vehicles, they often use the phrase “equipped with a permanent magnet synchronous motor, the maximum power XX ……” because most of the products are equipped with a motor that is “permanent magnet synchronous motor “, many readers will interpret this sentence as “equipped with a maximum power of …… motor”
But friends who are concerned about Tesla may know that Tesla is equipped with “AC asynchronous motor”, while the new Netflix ES6 performance version is equipped with “front 160 kW permanent magnet synchronous motor, after 240 kW induction motor” what is this thing?
So let’s tell you the difference between synchronous and asynchronous motors.

## Forget about synchronous asynchronous first, to recall the motor.

The motor is something I believe we are not unfamiliar with, recall the high school physics DC brush motor, the principle is the same for thousands of years, a permanent magnet (magnet) as the stator, a coil as the rotor, after energized to produce a magnetic field, according to the principle of “same-sex repulsion, opposites attract”, he will rotate around the stator.

But right, after 180 degrees, you will find that the N pole of the stator is next to the S pole of the rotor, and the two of them start to attract each other, not to turn what? So this time you need to change the direction of the current through the rotor, but you can not change the direction of the power supply current, so this time you need to commutator and brush out.

The main role of the brush is to conduct the current, its positive and negative pole is connected to the positive and negative battery, the rotor every 180 degrees, the brush and commutator connection to turn, the original positive pole of the coil into negative, negative pole into positive, so alternating cycle, to achieve electromagnetic force to push the rotor coil continues to rotate. However, because the brushes have been brushing around on the commutator, it is bound to produce wear and tear, and bad luck will also produce sparks.

The brushes will wear and even spark, so most cars now use brushless motors, we are discussing the permanent magnet synchronous motors and AC asynchronous motors are brushless motors, the following finally get to the point, to tell you the difference between the two.

## Synchronous and asynchronous refers to what?

It refers to whether the rotor speed and the speed of the magnetic field generated by the stator are the same. To fully understand this issue, we need to understand the principle of these two types of motors.
Let us first look at the permanent magnet synchronous motor.
It is important to note that the permanent magnet synchronous motor is different from the textbook motor we used as an example above, in that its coil acts as the stator and the permanent magnet acts as the rotor, the opposite of the textbook one.
Most of the permanent magnets are made of neodymium iron boron material, which is called “king of permanent magnets” and is the highest magnetic material. This material is called “King of permanent magnetism” and it is the most magnetic material.
China’s rare earth reserves account for 71.1% of the world’s total, but currently they account for less than 23% of the world’s total. China has also taken on the role of exporting rare earths in the past few years, and Mr. Bang’s hometown is very good at handling rare earths, even opening a special college for rare earths.

The reason for so much nonsense here is to tell you that rare earths are very expensive and rare, so it leads to permanent magnets are very expensive and permanent magnet DC motors are also very expensive ……

Okay, a little bit off-topic, let’s go on to discuss the permanent magnet synchronous motor. Because there is no brush, so the commutation must be “electronic” way, the following is the structure of the permanent magnet synchronous motor schematic.

The outer circle of gray is fixed in the body of the stator housing, the above interval wound with red, green, blue three groups of six-turn coil, diagonal for the same group. And the middle white wheel turns for the rotor, connected with the output shaft to the tire.
Suppose we first energize the green coil, then it will produce a magnetic field similar to that of a bar magnet. If we energize all three sets of coils, the magnetic field of the three sets of coils will be equivalent to three bar magnets placed inside the stator in a “meter” shape, which will produce a rotating magnetic field, and then the rotor will be rotated by the magnetic force generated by the stator.

Note that the bar magnet in the above picture is only to help you understand the direction of the magnetic field of the energized coil, not that there is a magnet placed inside the motor!

In this case, after the change of the energized coil, the rotor can be immediately stressed, that is, the rotor speed is the same as the speed of the magnetic field generated by the stator, so it is called synchronous motor.

In addition, the rotor here is not simply a magnet, if so, it will lead to uneven force due to the single direction of the magnetic poles, so it is generally more magnets on the stator to form a stator magnetic group, so that the formation of a SN pole interval permanent magnet rotor.
This makes the rotor rotate steadily as there are magnets in any direction in the magnetic field. Of course, the coil is controlled by the rotor position sensor and the switch, and the three of them are not simply “you go on and I go off”, but a more complex on/off rule, so I won’t go into details here.

Next, we talk about AC induction motors.

The stator of AC induction motor is composed of iron core, winding and seat, while the rotor is composed of iron core and winding, the rotor winding has squirrel cage and wire winding type, long like this.

As you can see, AC asynchronous motors do not have permanent magnets, its rotor and stator are energized to produce a magnetic field.
Since three-phase alternating current is used, the coils are arranged symmetrically within the stator, one unit every 120 degrees, for a total of three units. The three individual windings are combined to form a 360-degree cylindrical stator.

Since the frequency and direction of the alternating current are constantly changing, suppose we pass an alternating current to winding A, it will produce a magnetic field with changing direction and size, and the same is true for the other two windings, similar to the following diagram.

This diagram only shows that the three windings will produce a magnetic field of changing size and direction, and does not represent the true size and direction of the relationship!

At this point we need some knowledge of vector synthesis, the three magnetic fields with size and direction combine to form a magnetic field with changing size and direction. Then we adjust the order of energization: phase A first energized, and then phase A AC voltage waveform swing a third of the cycle to phase B energized, and then phase B voltage swing a third of the cycle to phase C energized.

This time the synthesized magnetic field becomes this: the size is fixed, the direction changes regularly.

You can simply understand it like this~

OK, with this magnetic field, the rotor can finally be happy to rotate ~ but because the rotor of the AC asynchronous motor is squirrel cage type, it will do in the stator generated by the magnetic field we are familiar with junior high school physics “conductor cutting magnetic induction line motion”, resulting in an induced current.

Electromagnetic induction phenomenon

This current in the rotating magnetic field and the role of the amperage force, so that the conductor rotation, the superposition of these two rotations lead to the rotor speed and the stator generated magnetic field speed is not the same, that is, the meaning of “asynchronous”. See the following diagram for details.

The advantages of a synchronous motor are obvious: at the same weight and volume, a permanent magnet synchronous motor can deliver higher power and torque. Similarly, for the same power and torque, it weighs less and is smaller, leaving enough space and mass for another extremely important component, the power cell. In addition, permanent magnet synchronous motors have the feature of being less noisy.
As for the disadvantages, as I said earlier, permanent magnets rely on rare earths, if you want a large power, you need a large permanent magnet, the cost is high; and although it is called “permanent magnet”, but it is easy to demagnetize in high temperatures and other harsh environments.
As for the asynchronous motor, the cost is very low, simple process, reliable operation, easy maintenance and other characteristics, can work in complex working environment, but also for the surrounding working temperature changes have a relatively strong adaptability.
However, at the same power and torque, the size and weight required for an asynchronous motor is much larger than that of a permanent magnet synchronous motor.

## Why does Tesla like to use asynchronous motors?

In fact, not only Tesla, many European brands of electric vehicles like to use asynchronous motors, such as BMW i3, but Chinese and Japanese brands are not so, for permanent magnet synchronous motor or love deeply.
As for Tesla and other foreign brands to give up the obvious advantages of synchronous motors, there are several inferences, one is that the asynchronous motor is more adaptable to the harsh environment, and Tesla such higher positioning models may be more than the family car to experience intense driving and other conditions, then the asynchronous motor is not easy to demagnetize the characteristics of the very they fit.
The second issue is the core material of permanent magnet synchronous motor, rare earth, for our country and Japan, our country has good rare earth resources, and Japan is a big country of rare earth industry, the top three NdFeB companies in the world: Sumitomo Special Metals, Shin-Etsu Chemical Industries and TDK Group are all Japanese companies, the strength can be seen. European and American countries may not be able to use rare earth materials as they like.
The third is that Tesla has its own patents on asynchronous motors, circumventing the shortcomings of asynchronous motors, such as the technology of copper-core rotor, which is briefly introduced to you as follows.
Most of the current asynchronous motor rotor, that is, the squirrel cage, the material is aluminum, aluminum has a better electrical conductivity and lower melting point (660.4 ℃) cost also has advantages, but its efficiency is limited, it is difficult to go further, if the use of higher electrical conductivity of copper to make the squirrel cage, the efficiency of the motor will be significantly improved.
However, the high melting point of copper (1083°C) makes it difficult to manufacture copper-core rotors; and the cost of welding copper is also high, and the requirements for the strength of the welded joints are also very high.

 Material Temperature t/°C Resistivity p/X10-8 Ω.M Resistance temperature coefficient Silver 20 1.586 0.0038  (20°C) Copper 20 1.678 0.00393 (20°C) Gold 20 2.4 0.00324 (20°C) Aluminium 20 2.6548 0.00429 (20°C) Calcium 0 3.91 0.00416 (0°C)

▲Different metal resistivity
Tesla’s patented RotorDesignforAnElectricMotor, which uses a welded copper process, first inserted the copper bar into the rotor slot in the same way as the welded squirrel cage technical solution, after which the end rings were supposed to be welded, but Tesla manufactured a set of copper wedges with a silver-plated surface and inserted these wedges into the gap at the end of the copper bar, and after inserting the wedges, the wedges After inserting the wedges, welding is performed between the wedges and the copper bar, and this welding requires less cost and difficulty. After the welding is completed, the end rings are inserted.

The real thing looks like this.