One Bonded On, One Embedded In: The Essential Difference Between Surface-Mounted And Interior Rotors in Axial Flux Motors

Axial flux motors are becoming a popular option for electric drive systems. For the same power output, their volume and weight can be reduced by about 50%, their power density can reach four times that of conventional radial flux motors, and their efficiency can exceed 96%. But if you open product manuals from different manufacturers of axial flux motors, you will find a puzzling phenomenon: although they are all "disc-type motors," some rotors look like a disc covered with magnets, while others look like an "iron disc" with metal bars embedded in it. These two rotor types—surface-mounted and interior—what exactly is the difference, and how should one choose?

The answer lies in the "position" of the magnets. And this positional difference determines almost the entire performance divide between the two rotor types in terms of torque density, field-weakening capability, demagnetization risk, and mechanical strength.

Where the Magnets Are Placed Determines How the Magnetic Field Travels

The air gap of an axial flux motor is planar; magnetic flux passes through the air gap in the axial direction, and the stator and rotor are arranged in parallel as discs. Within this "disc," there are two basic options for mounting the permanent magnets.

Surface-mounted rotor is the most intuitive approach: the permanent magnets are bonded directly to the surface of the rotor disc, facing the air gap. The magnets are usually axially magnetized, with adjacent magnets magnetized in opposite directions and arranged alternately. The magnetic circuit of this structure is very "clean"—almost all the flux generated by the magnets passes directly through the air gap into the stator, with little leakage and high permanent-magnet utilization.

Interior rotor embeds the permanent magnets inside the rotor core or back iron, with a layer of rotor pole shoe between the magnets and the air gap. This layer of iron changes the path of the magnetic circuit: the magnetic field generated by the armature reaction mainly passes through the back iron and hardly directly sweeps across the magnet surfaces. More critically, the interior rotor has an asymmetric d-q axis magnetic circuit, which means that in addition to the permanent-magnet torque, the motor can also utilize reluctance torque—the interior rotor produces both permanent-magnet torque and reluctance torque, whereas the surface-mounted type can only use the former.

Torque Density: The "Inherent Advantage" of Surface-Mounted Rotors

If only one metric is considered—how much torque can be produced per unit volume—the surface-mounted rotor is almost the undisputed choice in axial flux motors. The axial flux surface-mounted topology itself is known for high specific torque. In a quantitative comparison of dual-rotor modular-stator axial flux motors, the surface-mounted rotor structure achieved the highest back EMF and output torque. A comparative study of dual-stator axial flux motors also showed that although the surface-mounted rotor cannot utilize reluctance torque and has relatively high harmonic content in the air-gap flux density, its simple structure and high air-gap flux density amplitude still give it an advantage in torque output.

The "secret weapon" of the surface-mounted type lies in its short magnetic path and low magnetic reluctance. The permanent magnets face the air gap directly, so the flux they generate does not need to pass through an additional iron path, and there are fewer leakage paths. With the same amount of permanent magnet material, the surface-mounted structure can provide a larger effective flux than the interior type. In addition, the axial flux motor itself is a configuration that amplifies torque through a "radius lever," so the high air-gap flux density of the surface-mounted rotor is further amplified in this configuration.

The interior rotor is not without means to improve torque density. Through flux concentration—for example, tangentially magnetized magnets concentrating flux into the pole shoes—the interior type can "squeeze" more flux into the air gap, partially offsetting the losses caused by the longer magnetic path. But the price is a more complex rotor structure, greater iron consumption, and increased overall weight and volume.

Field Weakening and Speed Regulation: The Interior Rotor's "Comeback"

The advantages of the surface-mounted rotor are fully displayed at "low speed and high torque," but once the motor needs to run at high speed and requires field weakening for speed extension, the situation completely reverses.

The surface-mounted rotor has small d-axis and q-axis inductances and poor field-weakening capability. The reason is that the surface-mounted magnets face the air gap directly, and the demagnetizing field generated by the armature winding acts on the magnets with almost no obstruction. To weaken the field, a large d-axis current must be applied, and this current does not produce torque—it only generates heat. Worse, under high-speed conditions, a large demagnetizing current can easily cause irreversible demagnetization of the magnets. In axial flux motors, the field-weakening speed range of the surface-mounted topology is very limited, and its applicability in high-speed applications is significantly constrained.

The interior rotor naturally has asymmetric d-q axis magnetic circuits, with the d-axis inductance much larger than the q-axis inductance, producing a significant saliency effect. This means the motor can utilize both permanent-magnet torque and reluctance torque. Under field-weakening conditions, the demagnetizing effect of the d-axis current is more "efficient"—the same current can produce a greater field-weakening effect without being entirely converted into heat. A comparative study of interior tangential-rotor YASA axial flux motors clearly pointed out that while maintaining the advantages of the surface-mounted type, the interior rotor has significantly enhanced field-weakening capability, lower permanent-magnet eddy current loss, and stronger demagnetization resistance.

This difference is directly reflected in engineering metrics. A 100-kW-class interior axial flux motor can achieve a maximum speed of over 18,000 r/min, a peak power density of 10 kW/kg, and a maximum efficiency of 97.8%. Such a combination of speed and power density is difficult for a surface-mounted rotor to achieve in an axial flux configuration.

Demagnetization and Mechanical Strength: Two Different "Safety Lines"

Demagnetization is the failure mode that permanent-magnet motors must guard against most. The magnets of a surface-mounted rotor are directly exposed to the air-gap magnetic field, and both the armature reaction field and temperature rise act directly on the magnets. Once the local temperature exceeds the irreversible demagnetization temperature of the magnet, or the demagnetizing field strength exceeds the coercivity of the magnet, the magnetic performance will permanently decline.

The interior rotor has a much larger "safety margin" in this respect. The rotor pole shoe acts as a physical barrier; the armature reaction field mainly passes through the pole shoe and back iron, and the demagnetizing field strength borne by the magnets is greatly reduced. The stronger resistance of the interior rotor to irreversible demagnetization has been verified in multiple comparative studies.

But mechanical strength is the other side of the interior type. The magnets of a surface-mounted rotor require a sleeve or banding to resist centrifugal force during high-speed rotation, but the structure is relatively simple and the stress path is clear. The magnets of an interior rotor are embedded in core slots, and the core itself provides mechanical constraint, but slot machining—especially sector-shaped slots—is difficult, and the design of magnetic bridges and flux barriers requires repeated trade-offs between flux leakage and mechanical strength.

There Is No "Better," Only "More Suitable"

Putting these two rotor types together, the selection logic is actually quite clear.

Applicable scenarios for surface-mounted rotors: applications that are sensitive to torque density and cost and operate at low speeds. Small- and medium-power servo motors, in-wheel motors with extreme requirements for axial length compression, and direct-drive applications that do not require a wide speed-regulation range—the simple structure and low manufacturing cost of the surface-mounted rotor still have irreplaceable appeal.

Applicable scenarios for interior rotors: applications requiring a wide speed-regulation range, high speed, or a high demagnetization resistance margin. The main drive motor of an electric vehicle needs to switch between low-speed high torque and high-speed cruising, and the field-weakening capability and reluctance torque of the interior rotor exactly match this need. High-speed servo motors, fan and pump loads requiring constant-power operation, and any occasion that may face high temperature or high-current demagnetization risk—the interior rotor is a more reliable choice.

It is worth mentioning that axial flux motors and radial flux motors do not share the same "technology generation." In radial flux motors, interior rotors have already become mainstream because of mature manufacturing processes and speed-regulation advantages; but in axial flux motors, surface-mounted rotors have long been the default choice, and the manufacturing processes for interior axial flux rotors are still evolving rapidly. This means that when selecting a motor, one cannot simply apply the experience of radial flux motors—the same "interior" label may differ substantially in implementation difficulty, cost, and performance benefits in an axial flux configuration compared with a radial flux motor.

The ultimate question is not "which rotor is better," but "at what speed, with how much current, and for how long does your motor need to operate." Torque density, field-weakening capability, demagnetization resistance, and mechanical strength—these four metrics show an almost opposing distribution between the two rotor structures. Once you understand the physical logic behind magnet position, rotor selection is no longer a multiple-choice question, but a matching problem.