1.？ Brief introduction of axial flux motor

2.？ Advantages and disadvantages of axial flux motor

3.？ Hub power axial flux motor

Introduction of axial flux motor

1.？ The first generator in the world was an axial flux motor.

183 1 year, the British physicist Faraday invented the world's first real motor-Faraday disk generator, which is the prototype of axial flux motor. This kind of generator is mainly composed of disc, brush and horseshoe magnet. Faraday put a rotatable metal disk in the magnetic field of a magnet, and measured the current between the edge of the disk and the shaft with an ammeter. Experiments show that when the disk rotates, the ammeter deflects, which proves that there is current in the loop, that is, mechanical energy is converted into electrical energy.

Faraday axial flux motor

Due to the limitation of materials, structure and technology, and the late demand of large-scale application of mobile electrical equipment such as new energy vehicles, axial flux motors have not been industrialized for nearly 200 years. The reason is that electromagnetic calculation is difficult, material selection is difficult, technology is difficult, materials, structure and technology are coupled with each other, and the difficulty is doubled.

2. What is an axial flux motor?

Axial flux motor is also called "disk motor" because of its flat shape and short axial dimension. The air gap is planar, and the air gap magnetic field is distributed along the axial direction.

The magnetic flux paths of axial and radial motors are basically the same, which are all emitted by N-pole permanent magnets, pass through air gap, stator, air gap, S-pole and rotor core, and finally return to N-pole to form a loop. However, the directions of their magnetic flux paths are different. The direction of the whole flux path of the axial flux motor is first through the axial direction, then through the circumferential closure of the stator yoke, then through the axial direction to the S pole, and finally through the circumferential closure of the rotor disk, forming the whole loop. The flux path direction of radial flux motor is first closed along the radial direction, then closed along the circumferential direction of stator yoke, then closed along the radial direction to S pole, and finally closed along the circumferential direction of rotor core, forming the whole loop.

Axial motor flux path

Radial motor flux path

The work of axial flux motor is to drive the rotor to rotate through the interaction between permanent magnet (rotor) and electromagnet. When the stator coil is electrified to become an electromagnet, N and S poles will appear, while the N and S poles of the rotor are fixed. According to the principle that like poles repel each other and opposite poles attract each other, the S pole of the rotor will be attracted by the N pole of the stator, and the N pole of the rotor will be repelled by the N pole of the stator, thus forming a tangential force component, thus driving the rotor to rotate. By energizing the coils at different positions to form a stable tangential force, the rotor can also obtain a stable torque output.

3. Structure of axial flux motor

The main components of axial flux motor and radial flux motor are composed of stator (stator core and winding) and rotor (permanent magnet and rotor core). The stator core and rotor core of axial flux motor have the same outer diameter and inner diameter, but different axial lengths, and the stator part and rotor part are assembled relatively along the axial direction; The rotor part of radial flux motor is assembled in the stator part, and the axial lengths of the stator core and rotor core of this structure are the same.

Axial and radial flux motor models

According to the number of rotors, relative position and main magnetic circuit, the structure of axial flux motor can be divided into four categories: single stator and single rotor structure, double stator and single rotor structure, single stator and double rotor structure and multi-disk structure.

2. Advantages and disadvantages of axial flux motor

1. Advantages

The core technical advantage of axial flux motor is that the rotating rotor is located on the side of the stator (not inside the stator), and then the rotor diameter is larger, and the torque = force × radius, so higher torque output can be obtained under the same force. This means that under the premise of providing the same permanent magnet material and copper wire material, higher torque output can be obtained, which is very suitable for high-performance vehicles.

Axial flux motor

Compared with radial magnetic field motor, axial magnetic field motor has larger double-sided air gap under a certain size, higher magnetic field utilization efficiency and higher output power.

Under the same output torque, speed and power, the axial flux motor is compared with the radial flux motor:

The axial dimension of (1) is shortened by more than 50%, which is more suitable for occasions with high space requirements.

(2) The weight is reduced by about 50%, which can improve the mobility of the equipment and realize light weight.

The smaller size of axial flux motor also enriches the freedom of automobile design. For example, the axial motor can be installed on all four wheels as required, thus forming the torque vector distribution of the four wheels, which makes the vehicle have better handling.

Constantine Ness, head of research and development of Mercedes-Benz gearbox and electric drive, said: "Compared with traditional motors, axial flux motors use less materials and have a simpler structure. When the number of suppliers in this market segment increases, they can control costs well. " In this regard, we can speculate that if the cost is low, the axial motor will not be exclusive to AMG models.

2. Disadvantages:

People have a thorough understanding of radial motors, and their manufacturing methods and mechanical equipment are readily available, but axial flux motors are facing some severe challenges in design and production.

For the dual-rotor axial flux motor, heat dissipation is very difficult because the winding is located deep in the stator and between the two rotor disks. The cooling structure of the traditional axial motor is located at the radial sides of the casing, which has good cooling effect on the stator core and winding, but poor cooling effect on the rotor (permanent magnet) and bracket. Permanent magnet is the most sensitive component affected by temperature, and overheating is likely to lead to failure.

air gap

The gap between stator and rotor is called air gap, which is an important part of motor magnetic circuit. The axial flux rotor is placed flat on the stator, and the air gap area is obviously large, which requires extremely high surface smoothness and gap uniformity. This means that the requirements for manufacturing accuracy and assembly accuracy are very high, which brings extremely high manufacturing cost and extremely low production flexibility. The high manufacturing difficulty also leads to insufficient production scale of axial flux motors, small market capacity, and the matching of parts and components cannot keep up. Not only the axial flux motor itself, but also the axial movement caused by the load at the rear end after connecting the transmission shaft will seriously affect the magnetic force level between the rotor and stator of the axial flux motor. Once the critical point is broken, the rotor will be attracted to the stator, which will be a disaster. These shortcomings in turn lead to difficulties in the application of axial flux motors, resulting in fewer employees in this field, low product maturity and lack of research and development verification system.

3. Hub Power Axial Flux Motor

Axial flux motor has the advantages of small size, light weight and high efficiency, but it has not been applied in large-scale industrialization at home and abroad due to technical problems in materials, technology and structure. The hub power lasted for 5 years, and the research and development of special materials, processes and equipment was completed in 2020, taking the lead in realizing industrial application in the field of axial flux motors. Under the same power output, the hub power axial magnetic field motor product has a weight reduction of 50% and an axial dimension reduction of more than 50% compared with the ordinary permanent magnet motor product, and the efficiency diagram can be double 90, thus realizing a more efficient application of resources and energy.

As shown in the following figure, the three technical routes of hub power axial flux motor cover different requirements from 350W to 900KW, and the technical route and product line layout are becoming more and more mature.

Comparison between hub axial flux motor and traditional motor;

* traditional motor:

(1) silicon steel adopts punching process, while copper wire adopts ordinary embedding process, and the end of copper wire is longer.

(2) Using impregnating varnish technology. Heat dissipation capacity, fixed strength and insulation capacity are average.

And (3) adopting a common stamping and embedding process. Rotor safety factor 1.3. Heavy rotor, large moment of inertia and high iron loss.

(4) Single channel design, there is no heat dissipation on the contact surface of the reducer, or an oil cooling unit needs to be added. ?

(5) Protection grade IP67. Under normal temperature and pressure, it can still be used normally after soaking in the water depth below 1 m for more than 0.5 hours.

(6) Generally, the water channel can't dissipate heat to the gearbox, which easily leads to the rapid aging of the oil seal, so it needs to be replaced every 2~3 years.

* On the contrary, hub axial motor:

(1) stator silicon steel adopts integral forming stamping winding process, and copper wire adopts centralized winding process, which has high integration, shorter copper wire end and reduced copper loss by 15%.

(2) Adopting special composite materials and surface mounting magnetic steel technology. The rotor safety factor reaches 5; The weight is only 50% of that of ordinary motor rotor, and the moment of inertia is reduced by 30%.

(3) Independent research and development of special potting materials, special potting technology and equipment, and overall packaging to improve insulation and heat dissipation. Heat dissipation is increased by 4 times and insulation resistance is increased by 25 times.

(4) Double-sided sandwich waterway design is adopted, and the motor also has heat dissipation at the contact surface of the gearbox.

(5) The motor protection grade is IP68. It can still be used normally after being soaked in water with a depth of 1 m for more than 36 hours. ?

With the continuous maturity of production technology and manufacturing process, the power mass density advantage of axial flux motor is becoming more and more obvious. At present, the power density of mass-produced axial flux motor with hub power has reached 8. 17kW/kg, and the energy efficiency level has completely exceeded IE5 standard, reaching the world leading level. Under the same power output, the axial flux motor with hub is smaller and lighter, as shown in the following figure.