Axial Flux Motors: The Future of Custom Electric Powertrains?

Introduction to Axial Flux Motors

Electric motors are at the heart of the ongoing transportation revolution, powering everything from electric vehicles to industrial machinery. As the demand for more efficient and powerful electric powertrains grows, engineers are constantly seeking innovative motor designs. One such innovation that has been gaining attention in recent years is the axial flux motor.

In this comprehensive article, we'll dive deep into the world of axial flux motors, exploring their design, advantages, challenges, and potential applications. We'll compare them to traditional radial flux motors and examine why they might be particularly well-suited for custom and small-scale production runs.

What is an Axial Flux Motor?

An axial flux motor is a type of electric motor where the magnetic flux travels parallel to the motor's axis of rotation. This is in contrast to the more common radial flux motors, where the magnetic flux travels perpendicular to the axis of rotation.

The basic components of an axial flux motor include:

1. Stator: The stationary part of the motor containing the windings
2. Rotor: The rotating part of the motor containing permanent magnets
3. Housing: The outer casing that contains and protects the motor components
4. Cooling system: Often integrated into the housing to manage heat
5. Resolver: A sensor that accurately determines the position of the rotor
6. Electrical connections: For powering the motor

Key Design Features

The axial flux motor design has several unique characteristics:

• Disc-shaped rotor: Unlike the cylindrical rotor in radial flux motors, axial flux motors use a thin, disc-shaped rotor.
• Sandwich configuration: Many axial flux motors use a "sandwich" design with a rotor between two stators, maximizing the active surface area.
• Axial magnetic flux path: The magnetic flux travels axially through the rotor and stator, creating a more direct force path.
• Compact form factor: Axial flux motors tend to be shorter and wider compared to radial flux motors of similar power output.

Axial Flux vs. Radial Flux Motors

To understand the potential advantages of axial flux motors, it's essential to compare them to the more traditional radial flux designs.

Radial Flux Motor Design

In a radial flux motor:

• The rotor is cylindrical and typically contains permanent magnets or electromagnetic windings.
• The stator surrounds the rotor and contains windings.
• Magnetic flux travels radially from the rotor to the stator and back.
• The design is well-established and widely used in various applications.

Axial Flux Motor Design

In contrast, an axial flux motor:

• Uses a disc-shaped rotor with permanent magnets.
• Often employs a sandwich configuration with two stators.
• Magnetic flux travels axially through the rotor and stator.
• Offers potential advantages in torque density and cooling efficiency.

Comparative Advantages

Axial flux motors are often touted for several potential advantages:

1. Higher torque density: The larger surface area between the rotor and stator can theoretically lead to higher torque for a given motor size.
2. Improved cooling: The disc-shaped design allows for better heat dissipation, potentially enabling higher continuous power output.
3. Compact form factor: Axial flux motors can be shorter than equivalent radial flux motors, which may be advantageous in certain applications.
4. Scalability: The disc design lends itself well to stacking multiple motors for increased power output.

Challenges and Trade-offs

However, axial flux motors also face some unique challenges:

1. Axial forces: The strong magnetic attraction between the rotor and stator creates significant axial forces that must be managed by the bearings.
2. Air gap precision: Maintaining a precise and even air gap between the rotor and stator(s) is critical and can be more challenging than in radial flux designs.
3. Structural considerations: The disc-shaped rotor must be designed to withstand high rotational forces without deformation.
4. Manufacturing complexity: Some aspects of axial flux motor production can be more complex than radial flux motors.

Cooling System Design

One of the notable features of the axial flux motor we examined is its innovative cooling system. Effective cooling is crucial for electric motors, as it directly impacts their performance and longevity. The cooling system in this axial flux motor demonstrates several clever design choices:

Water Cooling Channels

The motor uses a water cooling system with the following characteristics:

• Cooling channels are integrated into the motor housing.
• The design keeps water separate from the windings, preventing potential damage from leaks.
• Cool water enters the system and follows a serpentine path around the motor.
• The water exits at a warmer temperature after absorbing heat from the motor.

Balanced Cooling Distribution

The cooling system is designed to provide uniform temperature distribution across the motor:

• The combination of cool incoming water and warmer outgoing water results in a consistent average temperature throughout the motor's circumference.
• This balanced cooling helps prevent hot spots and ensures all windings operate at similar temperatures.

Dual-Sided Cooling

The axial flux design allows for efficient cooling on both sides of the stator:

• In the sandwich configuration, both stators can be cooled effectively.
• This dual-sided cooling approach maximizes heat dissipation from the windings.

The effective cooling system is a significant advantage of this axial flux motor design, potentially allowing for higher continuous power output compared to less efficiently cooled motors.

Components and Assembly

Let's take a closer look at the various components that make up the axial flux motor:

Resolver

• The resolver is a critical component for accurate rotor position sensing.
• It enables precise control of the motor by providing real-time information about the position of the permanent magnets relative to the stator.

Connecting Shaft and Coupler

• These components allow multiple motors to be stacked and connected to a single gearbox.
• The design enables flexible power scaling by using one or two motors in the same setup.

Carbon Brush

• A carbon brush is included to prevent excessive bearing wear.
• It dissipates static electricity that can build up on the plastic rotor during operation.
• This static discharge prevents arcing across the bearings, which could cause premature wear.

Electrical Connections

• The motor features robust electrical connections for the three-phase power.
• Each motor in a dual-motor setup has its own connections, which are then combined for the entire assembly.

Fasteners and Assembly Considerations

While examining the motor, some observations were made regarding the use of threaded fasteners:

• The motor uses numerous threaded fasteners in its assembly.
• There may be room for optimization in this area, potentially reducing complexity and assembly time.
• The use of many fasteners may be a result of the motor's focus on custom, small-run applications rather than mass production.

Stator Design and Manufacturing

One of the most exciting aspects of this axial flux motor is its innovative stator design and manufacturing process. This approach offers significant advantages, particularly for custom and small-scale production runs.

Traditional Stator Manufacturing

In conventional electric motors, the stator is typically one of the most expensive components to tool up for production:

• Traditional stators are often made from stamped steel laminations.
• The stamping process requires expensive dies and large presses.
• This high tooling cost makes traditional stators less economical for small production runs.

Innovative Wound Stator Design

The axial flux motor we examined uses a novel wound stator design:

• The stator is created from a continuous strip of steel.
• The steel strip is notched at regular intervals to form the stator slots.
• The notched strip is then wound like a slinky to create the circular stator shape.

Notching Process

The key to this design is the notching process:

• A relatively inexpensive notching machine (around $110,000) is used.
• The machine operates similarly to an industrial sewing machine.
• It punches small notches into the steel strip at precise intervals.

Clever Slot Alignment

To ensure proper alignment of the slots as the stator diameter grows:

• The gap between notches is incrementally increased by 0.1mm every 36 slots.
• This gradual adjustment ensures that the slots line up perfectly as the strip is wound.
• The result is straight, evenly spaced slots for the windings.

Winding Process Advantages

This stator design offers significant advantages in the winding process:

• The open slot design allows for easy insertion of pre-formed windings.
• Windings can be dropped directly into the slots.
• This is much simpler than the complex winding processes used in some radial flux motors.

Cost and Flexibility Benefits

The innovative stator design and manufacturing process offer several benefits:

• Lower tooling costs compared to traditional stamped stators.
• Increased flexibility for producing custom stator designs.
• Economical production of small batches or one-off custom motors.
• Faster turnaround times for new designs.

This manufacturing approach is particularly well-suited to Omni Powertrains' business model of producing custom motors for specialized applications like motorsports and heavy-duty machinery.

Applications and Market Potential

The unique characteristics of axial flux motors, combined with the innovative manufacturing process we've discussed, open up exciting possibilities for various applications:

Motorsports

• The high torque density and compact form factor make axial flux motors attractive for racing applications.
• The ability to quickly produce custom motors allows for rapid iteration and optimization for specific racing categories.

Heavy-Duty Machinery

• The scalability of axial flux motors through stacking makes them suitable for high-power applications.
• Custom designs can be tailored to the specific needs of different types of machinery.

Low-Volume Production Vehicles

• Specialty vehicle manufacturers can benefit from custom-designed motors without the high tooling costs associated with traditional motor production.

Robotics and Automation

• The compact form factor and customizability of axial flux motors could be advantageous in robotic applications where space is at a premium.

Renewable Energy

• Custom axial flux generators could be designed for specific wind or hydroelectric turbine applications.

Aerospace

• The high power-to-weight ratio potential of axial flux motors could make them attractive for electric aircraft propulsion.

Current State of Axial Flux Technology

Despite the theoretical advantages of axial flux motors, it's important to note that the technology is still evolving:

• Current production axial flux motors often have comparable, rather than significantly superior, torque and power density to the best radial flux motors.
• The Lucid Motors' radial flux motor design currently holds the lead in production motor torque and power density.
• However, ongoing research and development in axial flux technology may lead to further improvements and breakthroughs.

The Future of Electric Motor Design

As we look to the future of electric powertrains, several trends and possibilities emerge:

Continued Innovation

• Both axial and radial flux motor designs will likely see continued improvement.
• Advances in materials science, manufacturing techniques, and control systems will drive performance increases.

Specialization

• Different motor designs may find niches in various applications based on their specific strengths.
• Axial flux motors may become dominant in areas where their unique characteristics offer clear advantages.

Hybrid Designs

• We may see the development of motor designs that combine elements of both axial and radial flux topologies to maximize performance.

Integration with Power Electronics

• Closer integration of motors with inverters and other power electronics could lead to overall system optimizations.

Sustainability Considerations

• Future motor designs may place greater emphasis on recyclability and the use of sustainable materials.

Conclusion

Axial flux motors represent an exciting development in electric motor technology. While they may not yet have fully realized their theoretical potential, they offer unique advantages that make them particularly well-suited for custom and small-scale production applications.

The innovative stator design and manufacturing process we've examined demonstrate how creative engineering can open up new possibilities in motor production. This approach allows for the economical production of custom motors, enabling rapid iteration and optimization for specific applications.

As electric powertrains continue to evolve and expand into new areas, the flexibility and customizability of axial flux motors may prove to be a significant advantage. Whether in high-performance motorsports, specialized industrial machinery, or emerging technologies like electric aircraft, the ability to quickly design and produce application-specific motors could be a game-changer.

While radial flux motors currently maintain an edge in mass-production scenarios, the ongoing development of axial flux technology suggests that we may see further breakthroughs in the coming years. The competition between these two motor topologies will likely drive innovation in both fields, ultimately benefiting a wide range of industries and applications.

As we move towards an increasingly electrified future, the story of axial flux motors serves as a reminder of the importance of continued innovation and the value of specialized solutions. By pushing the boundaries of what's possible in electric motor design, engineers are laying the groundwork for more efficient, powerful, and versatile electric powertrains that will shape the technologies of tomorrow.

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