01 What is an axial flux motor?
According to the direction of the magnetic flux path, the motor can be divided into an axial flux motor and a radial flux motor. Although the magnetic flux directions are different, the working principles all follow the basic laws of electromagnetism.
That is, the motor driver controls the magnetic field strength and direction of the coil (rotor) by changing the direction and magnitude of the current passing through the coil (rotor). Since the magnetic poles of the stator are fixed, when the magnetic field direction and strength of the rotor change, the magnetic field interacts to generate the Lorentz force, thereby driving the rotor to rotate.
Schematic diagram of magnetic flux path direction
The magnetic poles of the stator of the axial flux motor are arranged along the axis. The rotor is usually a cylindrical structure. The magnetic flux passes through the central axis of the rotor. When the current passes through the stator winding, the generated magnetic field passes through the rotor along the axis. According to the Lorentz force law, this magnetic field will generate torque in the rotor, thereby driving the rotor to rotate.
The air gap of the axial flux motor is planar, the axial length is short, and it is a thin disk shape, so it is also called a disk motor.
Schematic diagram of axial flux and radial flux motor structures
The topological structure of axial flux motors is flexible and can be combined according to actual application scenarios. According to the stator-rotor combination structure, axial flux motors can be divided into the following four structures:
Single stator/single rotor structure: 1 rotor + 1 stator, simple and compact structure, but large unilateral magnetic pull, large bearing load, large vibration and noise, and the risk of stator-rotor friction, which reduces the life of the motor;
Single stator/dual rotor structure: 2 outer stators + 1 inner rotor, high power density, more suitable for traction systems, aerospace and other fields;
Dual stator/single rotor structure: 2 outer rotors + 1 inner stator, with good symmetry, relatively small unilateral magnetic pull, more suitable for wind power generation systems;
Multiple stators/multiple rotors structure: multiple stators + multiple rotors, suitable for large torque scenarios, such as ship propulsion systems, large wind power generation and hydropower generators.
02 Advantages and limitations of axial flux motors
Advantages of axial flux motors
Small size and light weight: The volume and mass of axial flux motors are 5L and 24kg respectively, while the volume and mass of ordinary radial flux motors for automobiles are 10L and 50kg respectively. Axial flux motors are in line with the trend of lightweight automobiles. At the same time, they have a compact structure, shorter radial length, and high installation freedom.
| Parameter | Axial flux motor | Radial flux motor |
| Volume（L） | 5 | 10 |
| Weight（kg） | 24 | 50 |
| Torque（Nm） | 800 | 400 |
| Power（kW） | 358 | 186 |
| Torque density (Nm/L) | 160 | 40 |
| Power density（kW/kg） | 14.9 | 3.7 |
High torque density & power density: The effective magnetic surface area of axial flux permanent magnet motors is located on the surface of the motor rotor, not on the outer diameter, so they can usually provide more torque within a certain volume, thereby improving torque density and power density.
High efficiency: Thanks to the shorter one-dimensional flux path, the efficiency of axial flux motors is very high, usually over 96%, which is comparable to or better than the best two-dimensional radial flux motors on the market. Axial flux motors save about 15%-25% energy than ordinary radial motors at all power levels and can maintain peak power output for a longer time.
Axial flux motor structure
Low noise and vibration: Due to the uniform distribution of the magnetic field, the axial flux motor generates low noise and vibration during operation, which is particularly suitable for application scenarios with strict requirements on noise and vibration.
Energy saving and carbon reduction: The volume and weight advantages of the axial flux motor can reduce the consumption of copper, iron, permanent magnets and other materials in the production and manufacturing process by about 50% compared with radial permanent magnet motors. If the coreless PCB stator technology is considered, the copper consumption of the axial motor can be reduced to 34% of that of the radial motor.
At the same time, it has a smaller dead weight and higher efficiency. In the application, the motor consumes less power and has stronger driving ability, which is conducive to energy saving and carbon reduction.
Limitations of axial flux motors
Cost issues: Axial flux motors require special materials and manufacturing processes, so the manufacturing cost is usually higher than that of traditional radial flux motors. For example, the use of a yokeless topology and a special stator-rotor design will increase costs.
Complexity of design and production: The design and production of axial flux motors are usually more complicated. For example, the air gap between the rotor and stator needs to be kept uniform. The axial force during operation may cause severe tension on the motor shaft. The manufacturing process and mechanical equipment are not as mature as those of radial motors. These all limit the ability to mass-produce.
Cross-section of an axial flux motor
Heat dissipation challenges: Since the windings of an axial flux motor are located deep inside the stator and between the two rotor disks, heat dissipation is very difficult. Overheating problems will occur when the motor is running under high load, affecting the performance and life of the motor.
