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Ultimate Guide to Axial Flux Motors
Everything you need to know Axial Flux Motors
This guide will help you understand all the characteristics of an axial flux motor and how to select the right one for your application.
Axial flux motors are a type of permanent magnet synchronous motor where magnetic flux is parallel to the axis of rotation, resulting in a very low-profile design. Though recent trends in the electrification of vehicles have led to an uptick in axial flux motor interest, this technology has been around since the emergence of electric motors over 200 years ago.
Due to their compact design and options for zero-cogging torque, modularity, and integrated electronics, axial flux motors are the optimal solution for a range of applications from gimbals and pointing systems, to large scale medical scanning systems (CT and MRI). As demand grows for lightweight and power dense solutions, axial flux motors are quickly becoming an important factor in modern robot and machine design.
Figure 1. Allient SA Series Axial Flux Motor
What is an axial flux motor?
An axial flux motor is a 3-phase permanent magnet synchronous motor and abides by the same operating principles as a radial flux motor – converting electrical energy to mechanical energy. The motor consists of a rotor with permanent magnets and a stator with windings. Current applied to the stator generates torque and rotation of the rotor.
An axial flux motor is unique in that the direction of the magnetic field is parallel to the axis of rotation. The disk-shaped motor also offers zero-cogging torque if smooth motion is needed. For increased torque capacity, iron cores can be added to the stator or multiple rotors or stators can be stacked coaxially.
How do Axial Flux Motors Work?
When three-phase current is applied to the stator windings from a motor driver, a rotating magnetic field is generated. This rotating field interacts with the permanent magnetic field of the rotor magnets. A Lorentz force is generated on the rotor, causing rotation. For highest system efficiency the motor should be paired with a servo drive that utilizes sinusoidal current control (e.g., space-vector pulse width modulation) and a precise position feedback device. However, operation with a trapezoidal or sensorless drive is possible. Axial flux motors can also be designed with integrated hall-effect sensors that indicate change in position from changing magnetic fields for commutation or redundancy.
Axial Flux vs. Radial Flux Motors
There are a few key differentiators between axial flux and radial flux motors:
1. ARCHITECTURE
Axial flux motors feature a flat, disk shape where rotor and stator are stacked length wise along the axis of rotation. In contrast, radial flux motors have a cylindrical design with rotor inside stator for in-runners and stator inside rotor for out-runners. The flat winding layout of axial flux motors requires less tooling for assembly and is therefore easier to scale and modify than radial flux motors.
2. PERFORMANCE
While air-core axial flux motors have a lower torque density than radial flux motors, they offer a lower-profile and lightweight design. Iron core axial flux motors are competitive with radial flux motors in terms of torque. When multiple rotors or stators are stacked, axial flux motors can reach higher torque density than radial flux designs. In short, motor performance is highly dependent on the axial flux motor configuration which is driven by the unique requirements of the application.
Axial Flux vs. Transverse Flux Motors
Axial flux motors and transverse flux motors differ primarily in their direction of magnetic flux. In axial flux motors, the magnetic flux flows parallel to the motors axis of rotation, resulting in a compact, disc-like shape with moderate torque density. In contrast, transverse flux motors have a magnetic flux that flows perpendicular to the axis of rotation but with a more complex mechanical structure than radial flux motors. Iron is used in three dimensions with a toroidal coil to generate an electromagnetic field. This unique design enables high torque density but is more difficult to manufacture and tends to be more expensive than axial flux motors. While axial flux motors are ideal for applications that require a lightweight, cog-free or large-scale solutions, transverse flux motors are preferred for industrial, heavy-duty applications that need high torque.
Understanding cogging torque in axial flux motors
Cogging torque is a form of torque ripple that occurs in permanent magnet iron core motors as the rotor magnets move across the stator teeth. While the iron teeth in the stator increase the magnetic flux, the discontinuities between teeth causes a change in magnetic reluctance and therefore oscillations in torque.
There are two types of axial flux motors – air core and iron core. As the name suggests, iron core axial flux motors experience cogging torque due to iron cores in the stator that can be especially problematic at low speeds. Air core axial flux motors, on the other hand, do not have an iron core and eliminate cogging torque – offering extremely smooth motion. Most axial flux motors are air core, but iron cores can be used for applications that require a low-profile design and moderate torque.
While some applications may find value in the increased torque density from iron core stators, others may require zero cogging torque for smooth positioning particularly at low speeds.
Figure 2. Nuclear Imaging Application
Benefits of axial flux motors
Axial flux motors offer an ultra-compact and lightweight design ideal for space and weight limited applications. While axial flux motors tend to be less torque dense than radial flux motors of a similar frame size due to their low-profile, they are easily scalable to larger sizes making quick-turn customizations possible.
Modularity is a key benefit for large format axial flux motors - both stator and rotor can be constructed in segments and pieced together upon installation for safe and simple handling. Additionally, multiple rotors or stators can be added to the stack for higher torque.
Voltage and power operation of axial flux motors
Operating voltages and power ranges can vary significantly depending on the application and size of the motor. However, typical bus voltages range from 12VDC to 300VDC with the upper limit determined by the insulation rating of the motor. Axial flux motors designed for robotic applications tend to fall under a few kilowatts of output power. Those designed for e-vehicles, on the other hand, can reach over 100 kilowatts of output power. If standard motor options do not meet the application requirements, customizations are available. The motor design can be tailored around the customer’s bus voltage and current limit to achieve highest efficiency at the application operating point.
Design and Construction of Axial Flux Motors
Stator
i. Stator Core:
- High-grade magnetic steel laminations are bonded together to reduce eddy currents and hysteresis loss.
- Alternatively, soft-magnetic composites can be used to further reduce these losses.
- The stator core can be customized to include customer-specified mounting features.
ii. Windings:
- Three phases of enameled copper magnet wire are interconnected to form either a ‘Wye’ or ‘Delta’ configuration.
- The wire gauge and number of turns is dependent on the electromagnetic design.
- Windings are typically varnished or encapsulated for rigidity and insulation.
iii. Stator PCBA/Coil Assembly:
- Responsible for connecting individual coils to complete stator winding. “Wye” configuration is typical.
- Can also house integrated electronics (see next section).
iv. Optional Integrated devices: Hall sensors.
- Temperature sensors.
- Incremental encoder.
Figure 3. SA Series Axial Flux Motor - Stator and Rotor
Rotor
i. Permanent Magnets:
- Neodymnium (NdFeB) or Sumarian Cobalt (SmCo) magnets are typical.
- There are various magnet grades that offer thermal or performance advantages and are selected based on application requirements.
- Pole counts vary based on the electromagnetic design but generally increase with size. An equal number of north and south poles are used to create a specific number of electrical cycles per revolution.
ii. Windings:
- Three phases of enameled copper magnet wire are interconnected to form either a ‘Wye’ or ‘Delta’ configuration.
- The wire gauge and number of turns is dependent on the electromagnetic design.
- Windings are typically varnished or encapsulated for rigidity and insulation.
Axial Flux Motors Efficiency and Performance Metrics
Motor efficiency is dependent on the operating point for a given application – torque and speed. Efficiency is equal to the output power divided by the input power. When the output power is low, due to low torque or speed, the efficiency is low. The efficiency peaks near the ‘knee’ in the torque-speed curve where torque and speed are maximized before the motor is voltage-limited. This behavior is the same in a radial flux motor.
The losses in the motor are driven by two factors – resistive copper losses and speed-dependent losses due to eddy currents and hysteresis in the stator core. While copper losses are unavoidable, speed-dependent losses can be circumvented by using an air-core axial flux design without a stator back iron. Applications that require highest efficiency such as battery-powered systems, can benefit from this customization. However, it should be noted that removing the iron core, reduces the flux density and, therefore, the torque output of the motor.
Axial Flux Motor Customization and Scalability in Design
There are many customization options available with an axial flux design.
SIZE:
Axial flux motors are easily scaled in size to easily integrate into unique system designs with outer diameters from 30mm to over 1 meter. Radial cross-sections can also be minimized for applications that require a large aperture such as gimbals and optical pointing systems.
HIGHER TORQUE:
Motors can be designed with an iron-core stator for increased flux density. This can be advanced with either a dual stator or dual rotor design.
INTEGRATED SENSORS:
Hall Effect sensors, temperature sensors and incremental encoders can all be integrated in the stator design. Reduce component count and assembly footprint with an all-in-one motor kit.
MOUNTING FEATURES:
Stator and Rotor hubs can be designed with customer-specified mounting features for easy installation.
Applications and Use Cases for Axial Flux Motors
Easily scalable, axial flux motors can suit a wide range of applications with frame sizes from 30 millimeters to 1.5 meters and beyond.
While axial flux motors have long been used in the automotive industry for hybrid-electric drivetrains, they hold a valuable place in high-tech robotic applications that require an ultra-low profile and smooth rotary motion. Key applications include camera gimbals and pointing systems for defense or aerospace vehicles, precision rotary stages for semiconductor test equipment, and wafer handling systems.
Applications that require motors on a very large scale, such as MRI and CT scanning machines, can benefit from the modularity of axial flux motors. Stator and rotor segments can be pieced together upon installation for simpler and safer handling.
Figure 4. Mobile Satcom Motion Application
Figure 5. Oncology Screening & Treatment Application
Figure 6. Wafer Handling System Application
Figure 7. Camera Gimbal Application
Advances in Axial Flux Motor Technology
Robotics OEMs are striving for ways to scale down their systems; making compact machines that weigh less and consume less of a footprint. When motors are designed to fit a system rather than designing a system around a motor, OEMs can achieve ultra-compact and lightweight systems. Axial flux scalability, as described above, is a key advantage for these space-sensitive applications.
Redundancy is another emerging trend in autonomous robotic applications. This safety feature places a burden on machine designers to include back-up components in case of failure. While this could go as far are requiring redundant windings, in most cases it requires dual position feedback devices per axis. Axial flux motors can be designed with both hall effect sensors and incremental chip encoders in an ultra-compact package. This allows machine designers to meet safety standards while reducing overall machine footprint.
Addressing Challenges and Limitations of Axial Flux Motors
As axial flux motors are most used in the electrification of the automotive industry, there are only a few motor manufacturers who have released standard offerings for small low-power motors designed for robotic applications. OEMs can partner with motor manufacturers, like Allient, to design the right solution for their application.
Axial Flux Motor Maintenance and Reliability Considerations
Long-term performance of axial flux motors depends on the thermal management of the system. Motors are typically rated up to a winding temperature of 130°C. This temperature is limited by various components of the motor such as magnets, magnet wire varnish, and electronics. Overheating the motor beyond the maximum winding temperature degrades these components and can lead to premature motor failure. With sufficient cooling methods in place, such as using high-conductivity materials or forced air/liquid cooling and ensuring that the motor is operating within the continuous torque-speed rating, overheating can be avoided.
Axial Flux Motor Integration with Modern Motor Control Systems
For efficient motor control, axial flux motors should be driven by 3-phase sinusoidal FOC drives. For motors with low inductance, a high PWM frequency (50kHz and up) should be used to prevent excessive heating due to high RMS current. Many modern robotic control systems already utilize this level of technology so integration of an axial flux motor should be relatively seamless. For high-speed applications, the motor pole count should be low enough to ensure the electrical frequency is less than the sample frequency of the current servo loop sample rate (25-50kHz is typical for higher-end servo drives). High precision position feedback devices should also be used for efficient servo loop control.
Figure 8. SA Series Axial Flux Motor Stator
Future Outlook and Emerging Applications for Axial Flux Motors
As OEMs continue to push the limits of size and performance, motors must become more efficient and consume less footprint in the overall machine design. Axial flux motors are sure to assist designers in this progression. With a lightweight design and ultimate flexibility for scaling and component integration, these motors are fit for an array of applications from space vehicles and optical pointing systems to industrial automation joints and expansive medical scanning machines. With the right partner and axial flux motor design, high tech robotic and automation companies can meet critical performance requirements.
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